ELECTROLUMINESCENT DEVICES

Chapter 6

ELECTROLUMINESCENT DEVICES THE first Section of this Chapter deals with the construction and characteristics of electroluminescent cells. In the remainder we are concerned with their applications. The simplest of these make use of electroluminescent panels of normal structure. Others demand composite structures which involve the association of electroluminescent layers and various control elements. Most of these systems are still in a state of development. An exhaustive account would thus be premature, but a brief review can be usefully given. Additional information can be found in the patent literature to which a bibliography prepared by Ivey (1959:29 and 1961:7) forms an extensive and valuable guide.

6.1 ELECTROLUMINESCENT L I G H T SOURCES 6.1.1

CONSTRUCTION OF TEST CELLS AND PANELS

Electroluminescent cells for test purposes or practical applications can be constructed in the manner shown on Fig. 611.1. There is a great deal of scope for variation (1959:44). One of the important decisions concerns the medium in which the powder phosphor is dispersed. The electric fields prevailing across the layer during operation may well be of the order of 105 V/cm and a high dielectric strength is thus required. The material should also be reasonably translucent; it should have a low power factor and should be chemically inert, even when the cell heats up during operation. It should have a high dielectric constant so as to sustain only a small fraction of the applied field, the remainder being effective across the phosphor grains. (See also Sub-section 2.8.1.) In an attempt to satisfy these needs, Mager (1951:2) used nitrocellulose lacquer plasticized with castor oil, camphor or dioctyl-phthalate. The

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lacquer with its phosphor suspension (about 50 per cent by volume) can be sprayed on to a supporting glass plate which carries a previously applied transparent conducting layer (Fig. 611.1a). It must (a)

ψ^ί^ΜΜΜ^Μ&Μ^Μ^ΙΜΙ/ΜΜ^^ΜΜΜ^ί ####)j^

M a i n s t r u c t u r a l__ member ; glass

Light

protection-

Main structural m e m b e r ; glass

emission .Lead dioxide electrode

p*#>/>#a>^

-Phosphor ^Transparent conducting layer ; tin o x i d e

emission

(0

Phosphor suspension in vitreous ceramic

Main structura I member ; s t e e l layer"""

Protective

varnish '

reflector

Yw/mmmmMmmMMMm/wm/mtmMtmMmtMi!£^\tonrt*

Light

Reflecting

electrode

^Phosphor ^Transparent conducting layer

(b) Wax

-Metal

Transparent -conducting layer tin oxide

:^::v/;;;///;^//:^;;^:;;r:^p/;:^'-;,';;^///:^!z Light

;

emission

(d) Main structural member ; glass Phosphor-

;■^:;;^^\::;,yl·^::'^^\:?^^^,::^^.'l,y■'■·^■^

'ψ/Μ/^λ^λνψΜ^Μ/λ'λψ/λ^/λ Light

^--/Transparenf VY

layers

;

tin

conducting oxide

emission

FIG. 611.1. Electroluminescent light sources—I. Types of construction. (Thicknesses not to scale.) (a) After Mager (1951:2) and Gungle and Cleary (1955:93). (b) After D . H. Mash (personal communication). (c) After Rulon (1955:1).

then be carefully dried. An alternative method is to spread the phosphor layer by centrifugal spinning. The top electrode is applied by vacuum evaporation. A weak alkali agent (e.g. sodium bicarbonate) must be used to neutralize the plasticized lacquer. Panels made in this way i8

260

ELECTROLUMINESCENT

DEVICES

have been successfully operated at between 25 and 2500 V, depending on the thickness of the phosphor layer. Piper and Johnson (1955:72) have described a similar structure in which zinc fluoride is used as a phosphor instead of zinc sulphide. A substantially higher brightness level is claimed for such cells but confirmatory information does not appear to be available. The zinc fluoride may be applied by vacuum evaporation and it may contain additions of manganese, thallium, cerium or lead as activators, to the extent of between 1 and 6 per cent by weight. Some electroluminescent cells are based on mixed phosphors, e.g. 80 per cent zinc sulphide and 20 per cent zinc selenide, with copper as the activator (1957:4). One such type, described by Roberts (1952:1) was made by dissolving polymethylmethacrylate in ethylene dichloride until a highly viscous mixture was obtained to which the phosphor was then added. The resulting suspension was then divided into small portions, each specimen being compressed between hardened steel plates with a definite and constant spacing and allowed to dry. It was found possible to produce uniform sheets of about 100 μ thickness in this way. These films were then applied to a conducting glass surface, using a very thin layer of viscous silicone oil as adhesive. Aluminium foil served as a top electrode. Other preparatory techniques have been described by Siddall (1959:44). It is evident that the optimum thickness of the phosphor layer is governed by conflicting requirements. To achieve a high field, the layer should be very thin. On the other hand, to achieve uniform light emission, it must always be thick compared with the grain size of the phosphor used. Other things being equal, the grain size is related to the electrical performance in a complicated way, as described in Sub-section 6.1.3. A compromise thickness is usually adopted. It is between 0Ό01 and 0-004 in. for an operating voltage of 100-600 V. Such a layer has a considerable capacitance, for which Destriau and Ivey (1955:85) quote about lOO/x/xF/cm2. The capacitance is important since its existence may lead to an additional and undesirable voltage drop along the conducting electrodes, unless the electrode resistance is very low. Unless this condition is satisfied, the brightness of the cell is very non-uniform since the intensity of emission is a sensitive function of the local average field. The conducting glass electrodes now employed have resistances of 100 to 500 ohms per square and optical transmissions of the order of 80 per cent or more. Such layers can be made by spraying a solution of stannous chloride in acetone on to a glass plate heated to just below the melting point.

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Alternatively, the heated glass surface may be exposed to the vapours of silicon, tin or titanium chloride and the resulting coating cooled in a reducing atmosphere (1956:6). Fischer (1955:19) used coatings of Sn0 2 + l-5%Sb 2 0 3 . It has been found (1955:93) that important improvements in the surface brightness and efficiency of electroluminescent panels can be achieved by a process of electro-forming. For this process the panel is heated to a temperature just above the softening point of the dielectric (but, of course, below its decomposition point) and a voltage (d.c. or a.c.) applied. As a rule, the dielectric constant or the plastic is smaller than that of the phosphor. Under these conditions the field causes the phosphor particles which are normally elongated to align in a direction perpendicular to the plane of the panel by movement in the viscous medium. After subsequent cooling, the particles remain in their new positions. The precise mechanism by which the alignment gives rise to the observed advantages in operation is not yet understood. The use of a metal layer as the top electrode is not in all respects advantageous. Such a layer has a high conductivity, but offers no protection to the system against electrical breakdown. Mashf has used a thin layer of lead dioxide in place of the metal, sufficiently semiconducting to ensure efficient operation of the device. In the event of dielectric breakdown occurring in the phosphor layer, the lead dioxide in the region concerned is heated and tends to revert to the stoichiometric form which is an insulator. The system is thus self-healing. Panels of this form can be further improved by the introduction of a reflecting layer between the phosphor and the metal or semiconductor electrode. Barium strontium titanate has been used for this purpose, since it combines the desirable properties of high reflectivity and high dielectric constant. There is evidence that the titanate penetrates some way into the phosphor layer during preparation. Such a panel is shown on Fig. 611.1b, in this case with a protective layer of wax which is applied to keep out moisture. Micanized shellac has been used as an alternative (1955:115). It can be flame-sprayed by the "Schori" technique. Atmospheric moisture is one of the main factors which control the long term stability. On the other hand, there have been suggestions that the most thorough drying of the phosphor leads to a suppression of electroluminescent emission. This matter is in need of further investigation. t MASH, D. H., then at Thorn Electrical Industries Ltd., personal communication.

262

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A modification as regards general shape has been suggested by Jenkins (1955:9) who applied a layer structure of the kind already described to the inside of an evacuated or gas-filled glass bulb. This is a convenient way of obtaining a light source of relatively large surface area and the electroluminescent layer would be protected from atmospheric influences. Moreover, the use of glass bulbs would enable many well known techniques of lamp manufacture to be adapted for the production of electroluminescent light sources. O n the other hand, this construction disposes of the two most important features which are otherwise characteristic of electroluminescent light sources : their robustness and their flatness. Rulon (1955:1) has given a description of a different structure which has many potential advantages. This has been designated as a "Panelescent L a m p " and is shown in Fig. 611.1c. In this case the phosphor is embedded in a vitrified ceramic material supported by a metal plate. I n some cases the edges of the plate are bevelled to provide greater stiffness. During preparation, the ceramic dielectric must be fused in the shortest possible time to avoid chemical damage to the phosphor. The chemical composition of the ceramic is evidently important, but details of the materials actually used are not available. The materials must be free from impurities which react with phosphors, particularly lead. Ceramics which dissolve the phosphor during fusion are, of course, unsuitable. The vitrified dielectric may be expected to be more resistant to moisture and to effects arising from temporary exposure to high temperatures than the materials otherwise used. The conducting layer is applied by a spraying technique, using a tin salt solution, and firing. By using the metal plate as the main structural member, the panel becomes a light source of extraordinary robustness.! Panels based on ceramic dielectrics do not at present exhibit quite as high brightness levels as those based on organic resins, but they are superior in certain other respects (Sub-section 6.1.2). If extreme robustness were not essential, the main structural member could alternatively be made of glass. I n this way and by using two transparent conducting layers, it would be possible, in principle, to produce a panel which emits light on both sides, i.e. over almost the entire surface of the device (Fig. t This claim has been verified by convincing, if unorthodox, extensions of normal testing practice. Thus, an electrically energized panel can be cut with shears and each segment produced in this way will continue to emit light until finally severed. The emission also survives when the panels are used as targets for small-bore gunnery. The commercial literature provides picturesque illustrations of this kind.

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611.Id). A similar effect could, of course, be achieved by the structurally simpler (but electrically less economical) device of mounting two normal panels back-to-back. A more complex electroluminescent light source has been described by Michlin (1956:5). In principle it would be possible to sandwich thin layers of transparent (or, at any rate, translucent) electrodes, phosphors and insulators in such a way as to obtain several electroluminescent systems superimposed on one another within a single plate. Light

emission

(a) Plasfic film with dispersed — phosphor

Transparent //plastic films

\{/f{us//tfw/tff/(\Uit/(Mv; '//// '//·

'/////M/WWW/W/A

7

/ Transparent Υ/Λ.Υ/////ΛΥΛΥ.Υ/Λ', ν,',·,Υ///Λ'/////22Ζ3 Λ χ m e t a 11 i z e d electrodes

Light emission (b) Phosphor suspended" in wax

W////////////////////^^^^^ l'////////////77Z

>//YY///W////;//77\ /λψΥ///,ν///Α '//M'A Y, V/WM/M/M

Light

emission

Main structural " m e m b e r : Brass Transparent -plastic film ~~ T r a n s l u c e n t metal I ized electrode

FIG. 611.2. Electroluminescent light sources—II. Types of construction. (Thicknesses not to scale.) After Gillson (1956:6).

Each of the phosphors (three have been suggested) could be selected so as to cover different parts of the visible spectrum. By electrically energizing the various layers in turn it would be possible to provide a choice of colours. Alternatively, by energizing the layers simultaneously to different degrees, it would be possible to vary the emission spectrum of the panel in a continuous manner. The success of such a device would, of course, depend on the extent to which efficient luminescent layers can be made which are at least partially translucent to the light emitted by associated sources. This involves conditions which are difficult to satisfy and the practical importance of this particular design must thus remain in doubt.

264

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An interesting development arises from the possibility of dispersing an electroluminescent phosphor in a thermosetting plastic film. Semitransparent electrodes can be applied to such a film and the resulting light source is extremely thin and flexible. Gillson (1956:6) has described such a design, as shown on Fig. 611.2a. Only one of the metallized layers has to be semi-transparent, the other could be thick enough to form a reflecting surface. The top and bottom layers are protective films, bonded together with an adhesive cement. Figure 611.2b shows a variant in which the plastic is used not as the phosphor carrier but as a dielectric which protects an otherwise weak phosphor layer from electrical breakdown. Polyethylene terephthalate has been suggested as a suitable medium, used preferably in the form of stretched film, thermally set after reaching full extension, f Films of 0-001 or 0-002 in. thickness can easily be made in this way. The phosphor is dispersed in the molten wax at about 300°C and extruded in sheet form. Aluminium, zinc or silver electrodes can be applied by wellknown vacuum evaporation techniques. Detailed information on the performance of these devices as light sources is not yet available.

6.1.2

PERFORMANCE AND STABILITY

A typical relation between emitted light intensity and voltage applied to an electroluminescent panel has already been shown on Fig. 131.1a. Although operating voltages are normally much higher and fields of the order of 20,000 to 50,000 V/cm are common, panels of conventional design can be made which emit light with only 5 V (r.m.s.) applied. This corresponds to an average field of only 1000 V/cm (1955:115). By using cells which consisted of single layers of crystals of mean diameter 2-3 /z, Thornton obtained electroluminescence at less than 2 V (r.m.s.). The actual fields were reasonably high in this case. The efficiency in terms of light output per unit power input decreases somewhat with increasing voltage (and power), as shown on Fig. 612.1a. The power supplied by the external source is dissipated partly in the dielectric and partly in the phosphor. It is possible to estimate these two components of the total wattage (1953:4). One way in which this can be done is to construct an auxiliary cell of equal dimensions in which the dielectric does not contain any phosphor and then to measure the f For preparational methods, see U.S. Patent No. 2,465,319.

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265

power factor of this unit. This involves difficulties because the power factor of the electroluminescent lamp is voltage dependent, a property which must be ascribed to the phosphor rather than the dielectric. Conventional instruments for the measurement of capacitance and loss angle are therefore unreliable. Two other methods remain, namely calorimetry and the simultaneous observation of voltage and current.

Total

power,

W

Applied

voltage,

V

FIG. 612.1. Power consumption of electroluminescent light sources. ZnS phosphor dispersed in organic resin dielectric. After Jerome and Gungle (1953:4).

The second procedure is usually preferred. Results on the voltage dependence of the power factor are shown on Fig. 612.1b. If the phosphor is regarded as a conductor, these relations can be extrapolated to zero voltage and the intercept thus obtained (which also corresponds to zero current, of course) can be identified with the power factor of the pure dielectric. A comparison of results obtained by different methods shows fair agreement. In one or other of these ways, the power losses in the phosphor itself can be assessed and plotted against various parameters. Figure 612.2 gives such results, as obtained by Jerome and Gungle. It is found that the power factor of electroluminescent panels first decreases with increasing frequency and then rises again after reaching a minimum value. The position of this minimum depends on the nature of the phosphor. It has also been found that the power factor

266

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decreases with increasing area-to-circumference ratio, which suggests that the regions near the edge of the plate are responsible for a relatively large fraction of the total power loss. This is, of course, an incidental outcome of the methods of preparation. Quantitative results obtained on plates without special tests for homogeneity must be interpreted with caution. Typical values for the current consumption of a panel are of the order of 0-05 mA/cm2 at 600 V. The practical efficiencies now obtainable are in the range of 10-15 lu/W and are thus quite comparable with those of conventional gas-filled tungsten lamps.

(a)

2-0 6 0 c/s Room

temperature

I-Ç

5 o

Q.

0

200

400

Applied

600

800

voltage,

1000 V

1-0 Power

in

phosphor,

2-0 W

FIG. 612.2. Power dissipation in electroluminescent phosphor particles. ZnS phosphor dispersed in organic resin dielectric. After Jerome and Gungle (1953:4).

These values apply to complete panels in which, of course, the performance is necessarily diminished by the presence of the dielectric. The corresponding efficiencies of the phosphor itself (if they could be realized) are about 18-19 lu/W. If we assume that the apparent brightness of an electroluminescent surface is independent of the angle at which it is viewed, then the lumen output per unit area is numerically equal to the surface brightness measured in ft-L. The surface brightness can exceed 50 ft-L, but if adequate life is to be

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maintained present day plates should be operated at lower brightness levels. (1956:25 and 1955:115).f When an electroluminescent cell is subjected to pulse excitation, the power losses depend on pulse duration, but not nearly as sharply as one would perhaps expect. For a 40 jLcsec rectangular pulse, losses are about twice those for a 2/xsec pulse of the same amplitude (1958:38). The light output of the longer pulse has been reported as being about 8 times that of the shorter. The efficiency therefore increases with pulse length, at any rate within this range of durations. As under sinusoidal conditions, the losses under pulse excitation are voltage dependent, the efficiency being greater at high than at low voltages. The losses in the dielectric are small in comparison with those in the phosphor. This ratio is much greater under pulse conditions than that shown on Fig. 612.2a (see Sub-section 6.4.3). As regards stability and ageing, electroluminescent light sources differ considerably from conventional filament lamps. Following the first application of an external voltage, the brightness usually increases. This is the build-up phenomenon already described in Sub-section 5.3.8. It is, of course, the long-term build-up which is of principal interest in the context of practical applications, and this property is known to vary a good deal from cell to cell (1949:1, 1949:2, 1947:3 and 1953:21). There are recorded instances in which the build-up extended over a hundred hours or more and involved a brightness increase by factors of two or three, e.g. as shown on Fig. 538.1. The build-up period is followed by ageing. This process is continuous and there are no sudden total failures of the kind associated with conventional light sources. Some commercially available panels show no detectable loss of brightness after an initial ageing period of 2000 hr or so. Figure 612.3 shows this and records a correlation between ageing properties and the hardness of the dielectric. Cells based on ceramic dielectrics are claimed to have generally superior ageing characteristics. The life expectation of the best devices has been quoted as being between 25,000 and 40,000 hr under continuous operation. The assessment of "working life" is, of course, critically dependent on one's definition of "death". In order to protect the panels during operation it is desirable to use them in conjunction with a current-limiting device. Deterioration is more rapid in the presence of moisture than in dry air t Surface brightnesses up to 1000 ft-L have been claimed for experimental versions of the Panelescent Lamp though, presumably, such units have only a short operational life at the present time.

268

ELECTROLUMINESCENT

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and also more rapid during operation than during storage. Figures 612.1a and 612.2b give results before and after ageing.

0

I

i

IOOO

i

2000

i

3000

Hours

FIG. 612.3. Ageing of electroluminescent phosphors. (1) and (2) After Waymouth, Jerome and Gungle (1952:14). (3) and (4) After Rulon (1955:1). 6.1.3

BRIGHTNESS AND GRAIN SIZE

The performance of an electroluminescent panel based on microcrystalline phosphor depends not only on the thickness of the phosphor layer but also on the grain size. Three systematic investigations have been reported, all concerned with copper-activated zinc sulphide powder suspended in castor oil. The choice of this medium was probably governed by convenience but the presence of a liquid may in fact make the interpretation of results more difficult. The powders used by Lehmann (1958:47) had mean diameters of 6, 8, 10, 15 and 20/x, with a certain amount of overlap between the groups. The observed relationships are complicated. At low voltages a lower brightness was obtained with smaller grains; at higher voltages the change was in the opposite direction or else was zero. As one would expect, the power absorption increases with particle size for a given amount of phosphor. The efficiency itself, expressed as the ratio of brightness to power absorption,

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269

increased with decreasing particle size. The results obtained by Goldberg (1959:5) are essentially similar. In this case the experiments were designed to test in a semi-quantitative way how the potential barriers, across which most of the applied voltage is believed to exist, may be distributed within the thickness of an electroluminescent layer. The results agree best with the hypothesis that each grain contributes a fixed number of barriers. This, in turn, implies the existence of an optimum grain size. For a given volume fraction of phosphor and a given applied voltage, smaller grains would produce a lower brightness because the voltage across each grain would be small. Larger grains would have greater voltages across them, but fewer such grains could be accommodated in one thickness and the number of light sources would thus be reduced. In Kremheller's case (1960:8) the small grains ( < 20 μ diameter) were always less efficient than large grains, contrary to previous results. It may be that these "large" grains were larger than those in the powders used by Lehmann and Goldberg. Kremheller associated the diminution of brightness at successive stages of etching with the possibility that the grains may become roughened and thus increasingly opaque as a result of internal light reflections. Every phosphor is, of course, characterized by a certain distribution of grain sizes. Lehmann (1960:41) found that the nature of this distribution has an influence on the brightness-voltage relationship, and in principle it is reasonable that it should be so. In practice, wide distributions of particle size favoured eqn. (131.2), narrow distributions eqn. (131.1). The correlation was only approximate and its origins are not yet clear. 6.1.4

SUMMARY OF SPECIAL CHARACTERISTICS

Considered as practical light sources, electroluminescent plates suffer from a low surface brightness as compared with conventional lamps. To remedy this shortcoming is evidently an important aim of future development and research. There are, however, existing and potential applications for which high brightness is not essential and sometimes not even desirable. In these cases electroluminescent panels have attractive features to offer which can be summarized as follows (1953:3): (a) The panels can be made to luminesce in many different colours, depending on the phosphor used. Colour control by adjustment of the frequency is possible but normally impracticable, partly

270

(b)

(c)

(d)

(e)

ELECTROLUMINESCENT

DEVICES

because the frequency is rarely variable in power systems, and partly because frequency and intensity are not independent. Electroluminescent panels can be made in highly reliable forms which are mechanically more robust than existing light sources. The self-luminous area can easily be arranged in the form of non-overlapping letters or symbols which, by a suitable switching process, can be energized in turn. Multiple-electrode indicator panels can be designed in this way. The audiofrequency power supply systems used in aircraft offer especially favourable conditions for the application of such light sources. (See also Sub-section 6.2.1.) Substantial advantages are associated with the fact that electroluminescent panels are almost two-dimensional. They can be produced in a great variety of sizes and shapes and, if necessary, in flexible form. Panels from J i n 2 to 18ft2 have been made, though as the area increases, it becomes more difficult to ensure uniformity of the phosphor layer. The fact that the plates are virtually two-dimensional can, in some circumstances, lead to an increase in practical efficiency over conventional devices. This arises from the fact that nearly all the light emitted from an electroluminescent panel can be directly utilized, whereas light from conventional sources is often absorbed or scattered over an appreciable portion of the solid (emission) angle. Light sources more complicated than those discussed above but based on the same principles can be used as picture display panels as described in Sections 6.3 and 6.4.

6.2 APPLICATIONS OF ELECTROLUMINESCENT PANELS 6.2.1

ILLUMINATION AND INFORMATION DISPLAY

There are obvious applications in the general field of illumination which arise directly out of the special characteristics summarized above. A number of commercial units are being manufactured,! including f These are being marketed under various trade names. The "Rayescent" lamp is a Westinghouse product, based on a glass plate as the main structural member. The "Panelescent" lamp is made by General Telephone and Electronics, and its structure is as shown in Fig. 611.1c. "Thorn Panelume" devices made by Thorn Electrical Industries can be based on metal or glass, and Ericsson Telephones market panels and alphanumerical displays under the names "Phospholites" and "Phosphotrons".

APPLICATIONS

OF E L E C T R O L U M I N E S C E N T

PANELS

271

low intensity panels (0.02 W) for direct use on power mains. An even more promising development is the use of electroluminescent devices as self-luminous signs and, to some extent, as decorative and architectural features. For these applications a stable brightness of 0-5 ft-L (after ageing) has been found acceptable.! This is the level which can now be generally achieved in the course of operation at power frequencies. Much greater brightness (e.g. exceeding 25 ft-L) can be obtained when direct mains operation is not essential. In some cases, it is practicable to energize an electroluminescent panel by means of a transistor oscillator supplied from a low voltage (9 V) battery. The power factor of approximately 20 per cent must be taken into account when designing such sources. Panels can be made in a variety of regular and irregular shapes. For special applications, holes can be provided within the illuminated area. The fact that the lamps are cool in operation and two-dimensional suggests a number of applications in photography and dark-room processes, e.g. as dark-room safelights, transparency illuminators and light sources in contact printers. Safelights must, of course, be free from blue emission. An interesting application has been devised which depends not on any new principle but merely on the arrangement and selective use of properly designed counter-electrodes on a single panel. Figure 621.1 shows two such electrode designs. There is also a simpler version of (a) in which the diagonal strips are omitted. With the 14-segment panel it is possible to form numerals and letters by selecting the appropriate electrodes and leaving the remainder un-energized (alpha-numerical display). Sixteen-segment panels have also been described and, in return for the additional complication, they provide a set of symbols which are more easily legible. The (effectively) ten-segment version on Fig. 621.1 b is intended for the display of numerals only. Mash ( 1960:10) has devised a "coding matrix" which makes use of non-linear resistance elements of silicon carbide (see also Sub-section 6.4.1) and greatly simplifies the switching procedure. The coding matrix can be made in the form of a flat plate in contact with the electrodes of the electroluminescent panel. Isolating components must be incorporated, since no two electrodes (except X and Y on Fig. 621.1b) can be allowed to remain permanently connected. The voltage-dependent silicon carbide f For the sake of comparison, this would be equivalent to 30 times the brightness of a sheet of white paper in full moonlight (1955:129). On the other hand, the brightness of a cool white 40 W fluorescent lamp is about 1900 ft-L.

272

ELECTROLUMINESCENT

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elements fulfil this function. Such a coding matrix enables a panel designed for numerical information to be operated by means of a single pole ten-way switch. In effect, the switch selects one of ten different circuit patterns and thereby one of ten numerals. The low level of surface brightness so far realized makes it difficult at present to construct displays of small size. (a)

(b)

FIG. 621.1. Electrode arrangements of read-out panels, (a) Westinghouse alpha-type RA. 14 segments, (b) After Mash (1960:10). Numerical type. 11 segments, of which X and Y are permanently connected, so that they could be considered as a single electrode.

A method of using electroluminescence for the production of selfluminous half-tone pictures has been described by Schwertz and co-workers (1954:54). A picture is produced on an amorphous selenium layer, using normal xerographic techniques. However, contrary to normal practice, the developing powder in this case is a mixed zinc sulphide-selenide phosphor of very small grain size. After application to the selenium, the distribution of this powder corresponds to the light distribution on the original picture. This powder layer can then be transferred to a thermoplastic resin and fixed in position by heating to the softening point. The resin layer (25 μ thick) is mounted on a sheet of conducting glass. An aluminium electrode is applied to it by

APPLICATIONS

OF ELECTROLUMINESCENT

PANELS

273

vacuum evaporation, thus providing all the elements of an electroluminescent cell. On application of a field, a self-luminous picture is obtained which reproduces the original half-tones in a reasonably satisfactory manner. 6.2.2

CONTROLLED ELECTROLUMINESCENCE

The descriptive accounts given in Section 1.1 and Sub-section 1.3.6 have already indicated the existence of interaction between optical and electrical stimuli. Such interaction leads to the possibility of control on which a number of interesting devices can be based. The most important control process involves irradiation of an electroluminescent specimen by means of some external source. Williams (1955:37) has pointed out that such a system has the inherent possibility of giving light amplification. The arrangement is shown on Fig. 622.1a. External radiation can be envisaged to act upon an electroluminescent phosphor in a number of ways. If it is of sufficiently short wavelength, it could supply free charge carriers which are necessary for the electroluminescent process or which, at any rate, are able to take part in it. Alternatively, or in addition, it could produce space charges which alter the prevailing field conditions in a beneficial way. The observations recorded by Ballentyne (1961:8) could be interpreted in this way. The radiation could also affect the degree of ionization of luminescence centres. This conclusion agrees with experimental results obtained by Pâtek (1958:70). Similar control effects should be obtainable by the use of cathode rays instead of external light. The opposite case, namely the control of cathodo-luminescence by the application of electric fields to the phosphor has already been explored (1957:67). If additional charge carriers are able to affect an electroluminescent system, then it should alternatively be possible to achieve control by the extraction of existing free charge carriers. Daniel and co-workers (1958:48) have demonstrated this in the case of photoluminescence. As far as is known, the practical demonstration for electroluminescence has not yet been provided. It may also prove possible to effect the electroluminescent process directly by other means, e.g. by mechanical pressure. It is known from the work of Leistner (1957:69) that photoluminescence can be modulated in this way. Experiments by Greguss and Weiszburg (1959:43 and 1960:34) have achieved such control for electroluminescence, though possibly in a different sense. The normal emission colour was blue-green and the impact of ultrasonic radiation

274

ELECTROLUMINESCENT

DEVICES

produced a shift towards yellow. In this case barium titanate was used as a matrix for the phosphor suspension, and the piezo-effect may have arisen from this rather than from the phosphor itself. An entirely different form of controlled electroluminescence which is likewise capable of giving power gain has already been described in Subsection 4.1.4. (a)

Light

emission i

Transparent electrode

External control source

1>t

TV

WmMMW^ ν

ν'^^: -'''''^{::;'^}\-

Glass

Ph osphor

Metal electrode

L i g ht emission

L i g h t emission

Translucent /electrode -

r

)"■'·" v.:'."..']W'/wAK l

>>»/>>->;.

"Piezolayer Metal electrode'

Short circuit Pressure (b)

m.

3—-Phosphor -

W0^M^^Mi Piezoresistive

pressure (C)

FIG. 622.1. Controlled electroluminescence. [See also (1957:73).]

The optical control method discussed above is "direct" in the sense that other (non-electroluminescent) components are not required. It is also "positive" in as much as increased radiation produces increased brightness. The opposite process, resulting in "negative" control, is also possible. Heckscher (1957:42) and Narita (1960:2) have shown that at any rate certain electroluminescent phosphors can be

APPLICATIONS OF ELECTROLUMINESCENT PANELS

275

quenched by infra-red light. Indeed, Narita's crystals did not recover their electroluminescence unless subsequently irradiated by violet or ultra-violet light. Various devices could be based on this principle [e.g. seeHenisch (1958:59)]. Electroluminescent cells can also be controlled by using them in conjunction with other sensitors. Figure 622.1b shows how this could be done by means of a piezo-electric material. The two conducting electrodes would be short circuited. The electroluminescent film would have to be of a kind which operates at very low voltages and it would also have to be sufficiently robust not to be damaged by the pressure. If these requirements could be satisfied then the brightness distribution over the area of the cell should correspond to the pressure distribution. The device could not operate under static conditions, but may have possibilities as an energy transducer. Similar structures making use of piezo-resistive instead of piezo-electric elements could also be envisaged (Fig. 622.1c). In this case, the application of external pressure would bring about a redistribution of the applied voltage between the phosphor and the piezo-resistive element in series. Alternatively, it is possible to use magnetic, photoconductive and ferroelectric sensitors as control elements. Examples of such applications are discussed in the following Sections.

6.2.3

RADIATION AMPLIFICATION THROUGH PHOTO-ELECTROLUMINESCENCE

Radiation amplification through direct control has been demonstrated by Cusano (1955:21 and 1955:38), who used a ZnS—Mn, Cl phosphor, deposited in the form of a continuous film by a vapour reaction. It had a negligible light output in the presence of the electric field alone. A large increase of brightness was observed when the phosphor was irradiated with ultra-violet light or X-rays. Figure 623.1 shows these results. As expected, the effect was critically dependent on the polarity of the applied field. Under favourable conditions, 10 photons of visible light were produced for each photon of 3650 Â. The amplification ratio diminished with increasing irradiation intensity. This is in agreement with results obtained by Halsted (1957:73). Long time constants have been observed in the response of these systems to changes of irradiation intensity. For intensities as low as 1 fiW/cm 2 they were of the order of 2-3 sec. 19

276

ELECTROLUMINESCENT

DEVICES

Using a different kind of phosphor, Destriau had found earlier that the output could actually be lowered by ultra-violet irradiation but increased by X-rays, first by a factor of only two and later (as quoted in 1957:59) by a factor of eight. However, overall amplification was not achieved in these cases. Transparent conducting layer AL electrode Phosphor -

\0μ

J.^^^^■^.^■^^■^■^^■"■:^.'^'^ ■^^^^'^■^^^ Λ^^^ ■:^>'^^^^^ "

Irradiation UV or X-ray

Constant irradiation level

6 Applied

field,

V/cm

8

ΙΟχΙΟ4

d.c.

FIG. 623.1. Radiation amplification through photo-electroluminescence. Brightness level negligible in the absence of external radiation. After Gusano (1955:21).

The photo-electroluminescence phenomena discussed in Sub-section 1.3.6 differ from those above only by degree. We are here dealing with instances of photo-electroluminescence in which the initial light output is near to or below the detectable threshold. A quantitative analysis has been attempted (1955:28), taking into account trap densities and transition probabilities, but it would seem that the systems so far examined have not been known with sufficient precision to make detailed calculations reliable.

PHOTOGONDUGTIVE

CIRCUIT

ELEMENTS

277

Dealing with problems of radiation amplification in general, Loebner (1955:89) and (1956:35) has drawn attention to the distinction between luminous power gain and radiant power gain. The former is significant in cases in which the human eye is the ultimate radiation detector. It is defined by reference to the spectral sensitivity of the eye, whereas radiant power gain represents an uncorrected integration over the whole of the wavelength spectrum. Even within the concept of luminous gain, there is the possibility of colour conversion. Radiation amplification is called homochromatic when the output and input spectra are the same, heterochromatic when they are not. Because of their structure, radiation amplifiers which make use of photo-electroluminescence are sometimes called single-layer cells. The control radiation falling on to a Cusano cell could obviously take the form of a half-tone picture. The outcome would be an intensified (and visible) image. Kazan and Nicoll (1957:59) have given a description of Cusano's display panel. The resolving power was about 10 μ, and was governed by the thickness of the phosphor film. The response time was of the order of seconds. Half-tones were well reproduced on a 4 in. screen. The principal shortcoming of the device as a picture amplifier is, of course, the need for an ultra-violet input picture. In practice, an input is rarely available in this form. As far as is known, it has not yet been possible to construct a similar display panel which responds to a visible or infra-red input. It is for this reason that the more complex (multi-layer) display panels described in Sub-section 6.3.4 have been devised.

6.3 ASSOCIATION OF ELECTROLUMINESCENT AND P H O T O C O N D U C T I V E C I R C U I T ELEMENTS 6.3.1

MONOSTABLE AND BISTABLE OPTRONS

The term i c optron" has been suggested by Loebner (1955:89) for solid state devices which utilize the interaction of optical and electronic processes. The term is more specifically applied to electroluminescent cells which are associated with photoconductive elements. As far as is known, photovoltaic control elements have not yet been made, though the possibility exists in principle, by analogy with Fig. 622.1b. The electroluminescent and photoconductive components of an optron can be optically or electrically coupled, or both. The simplest

278

ELECTROLUMINESCENT

DEVICES

form, providing only optical coupling, is shown on Fig. 631.1, together with a family of operating characteristics. As a rule, cadmium sulphide photoconductors are used for this purpose. Their chief characteristics are high sensitivity and slow response. This conforms with the general expectation according to which both factors are controlled by the effective carrier life-time. Precise analysis of the device is complicated because the principal elements are both non-linear. Power amplification factors of 70,000 have been reported (1957:73). With further development it may eventually become possible to use single crystal phosphors which respond to constant voltage excitation and thus to achieve useful d.c. amplification. (a)

EL

Electroluminescent

PH

Photoconductor

cell

FIG. 631.1. Optron operation with simple optical coupling. After Halsted (1957:73).

Different situations arise when the two active elements are electrically coupled (Fig. 631.2a). We are now concerned specifically with a device in which control is exercised by the light input, the electrical power supply being kept constant. In this version, the light output increases with increasing light input. The opposite result could also be achieved, at any rate, in principle (Fig. 631.2b). The photoconductor which receives the input radiation now controls the bias current through a transformer. In this way, the transformer could be gradually saturated and the light output diminished.

PHOTOGONDUGTIVE

CIRCUIT

279

ELEMENTS

On Fig. 631.3a the device is shown with positive optical feedback. The corresponding operating characteristics under typical conditions (b)

(a)

Hl·Saturable reactor

Light input

Θ

Θ

UH

ill!

Light input

Light

Light

output

output

FIG. 631.2. Optron operation with simple electrical coupling, (a) Light intensifier, (b) Light inverter. (b)

(a) 400

300 Variable input and

Ô

constant bias illumin-

200

(c)

y

yB

f\

J\

--V

'Bright emission

/ Slight 1 emission 1 Constant bias 1 Illuminance of / 3 lm/m2

100

50

r

-I200V

^80 )V ^

Γ

0

100 Light output and optical feedbock

[^Dark i

-50 1

i

1

Current,

0-1

/«A

Total

600 V

1-0

\

400 V

10

I luminance,

100

lm/m 2

FIG. 631.3. Operation of an electrically coupled optron with optical feedback. After Loebner (1955:89).

are also given. Figure 631.3b illustrates the bi-stable character of the regenerative optron. For applied voltages which increase from zero to (in this case) 380 V (point A)9 the electroluminescent cell will be either dark or only very slightly emitting. If the voltage is then

280

ELECTROLUMINESCENT

DEVICES

increased and maintained at a higher level, the optron stabilize at a different point (B or above) in the light emitting condition. The manner in which this characteristic functions is not straightforward, since the optron requires time to adjust itself to a new external stimulus. The time-dependence arises from the slow response of the photoconductor and from the build-up process within the electroluminescent cell. On these grounds, the use of a simple load line on this diagram would not be meaningful, especially also since the series resistance, represented by the photoconductor, is not constant. The switching characteristics therefore differ considerably from those of (say) bi-stable transistor devices. If the voltage is reduced slowly from the point B, the optron follows the full line. It can switch back to the non-emitting state if the applied voltage is kept below the point C for a sufficiently long time. Loebner's curve on Fig. 631.3b refers to an optron operated with a constant bias illumination. In the absence of such a light bias, the critical voltage for bi-stable operation (in most cases so far examined) exceeded the dielectric breakdown voltage of the phosphor layer. This may be regarded as an accidental shortcoming and not as a fundamental limitation. It may well be that optrons will be constructed which are bi-stable without external light bias. Excessive bias illumination destroys the bi-stable character altogether. At 60 lu/m 2 the characteristic is a simple straight line through the origin. It is possible to use these properties for triggering an optron in an alternative way: by temporarily increasing the light bias. A dark optron can thus be made to emit by allowing light to fall on it for a short period. It can be returned to the dark state by immobilizing the photoconductor. This can be done by means of infra-red radiation, if intensity and duration are suitably chosen. An approximate analysis and representation of the system will be found in Sub-section 6.3.5. It is possible to devize many variants on the present theme. Figure 631.4, for instance, shows an arrangement proposed by Halsted which depends on the fact that the impedance characteristics of electroluminescent cells make them suitable for inclusion in resonance circuits. When the circuit is at resonance a high voltage would exist across the electroluminescent cell. The resulting emission would reduce the impedance of the photoconductor which, in turn, would lead to a higher output voltage. The device is bi-stable and can be triggered by changes in voltage, frequency, input radiation intensity or by a combination of these parameters. In principle, devices of this kind could be made which include electro-photoluminescent rather than

PHOTOGONDUGTIVE

CIRCUIT

ELEMENTS

281

electroluminescent elements. Applied fields would then be used to quench a prevailing photoluminescence and, correspondingly, a new and complicated set of operating characteristics would be obtained. (See also Sub-section 6.3.6.) (a) -ΛΛ/VWPC

Vin

S=î

X XJ

FIG. 631.4. Operation of optrons in conjunction with resonance circuits. After Halsted (1957:73).

6.3.2

RADIATION AMPLIFICATION BY MEANS OF OPTRONS

Devices of the kind described above can give homo- and heterochromatic light amplification. It would be largely homochromatic, for instance, if the arrangement of Fig. 631.3a were used and the input radiation derived from an identical electroluminescent cell. However, since the spectral characteristics of cells depend somewhat on the loading conditions, it must be expected that completely homochromatic amplification is difficult to achieve. Systems can be devised which have a wide separation between input and output spectra. The sensitivity region of the photoconductor must, of course, extend over both spectra to permit positive feedback. In this way, it is possible to use optrons for converting a stimulus outside the visual range (e.g. X-rays, ultra-violet or infra-red radiation) into a visible signal. Figure 631.3c illustrates the operation of an optronic light amplifier. In this case, it was necessary to exceed a certain amount of illuminance before gain appeared. Similarly, it was necessary to apply a certain minimum energizing voltage. However, electroluminescent cells are now known (Sub-section 5.4.2) which can be operated at very low voltages and which, by themselves, show no sign of a threshold. Light amplifying optrons can easily be made using commercially

282

ELECTROLUMINESCENT

DEVICES

available electroluminescent cells and cadmium sulphide photoconductors. The results in Fig. 631.3c were obtained in this way. It was found possible to match the components very critically so that as little as 10~5 of the emitted flux was capable of triggering the optron. In principle, at any rate, the emitter and detector could be single crystals.

ΟΌΟΙ

0-01

Incident illumination,

0-1

1-0

ft candles

FIG. 632.1. Gain as a function of incident illumination and frequency for an amplifying optron without optical feedback. Photoconductive layer of sintered cadmium selenide. After Nicoll (1959:21).

Operation under fluctuating light conditions is again determined by the response time of the photoconductor and, by the reactive and non-linear behaviour of the two components. The last two factors in particular make analysis of optronic circuits very complicated. Nicoll (1959:21) has given detailed examples of the relationship between photocurrent, applied voltage and intensity of illumination for a sintered cadmium selenide photoconductor. He has also shown that the response time which governs optical activation diminishes with increasing intensity and that these relations are different for direct and alternating current. In contrast, the decay time after optical activation appeared to be independent of activation intensity.

PHOTOGONDUGTIVE

CIRCUIT

ELEMENTS

283

An optronic amplifying system has many parameters which need to be optimized if the operation is to be efficient. One of these is the applied voltage. At very low voltages, the gain is low, and at very high voltages the cell may emit light even when the photoconductor is in darkness. There is, therefore, an optimum voltage. For NicolPs light amplifiers which were based on sintered cadmium selenide and employed no optical feedback, this was found to be 600 V. The gain was a complicated function of frequency and incident intensity. It was always smaller at higher frequencies owing to the diminished impedance of the electroluminescent cell. Figure 632.1 illustrates this behaviour. The optimum operating conditions thus depend on the intended input level and the maximum response time which any particular application may permit. 6.3.3

OPTO-ELECTRONIC NETWORKS

Networks consisting of coupled and modified optrons can perform a variety of electrical operations. The bi-stable circuits given above are merely the simplest networks of this kind. Others can be devised, making use of the fact that a photoconductive element connected in parallel

FIG. 633.1. Simple optron storage unit. After Loebner (1955:89).

with an emitter can serve to turn the emission off. I n darkness, such a parallel element has no effect since its impedance is high. Under illumination, its impedance collapses and with it the voltage across

284

ELECTROLUMINESCENT

DEVICES

the emitting cell. Figure 633.1 gives an example. ELb is the bi-stable cell, in conjunction with photoconductors PCV in parallel and PCS in series. In one of the switch positions, the cell EL\ is energized. This acts upon PCS and thus brings EL\> into the emitting condition. When

(a)

PC,

■PC,

PC,

0

(b)

EL

Read out trigger

(cl· EL a Feedback

PCa Read in

trigger^ pulse

PC h EL b

Θ

Read out pulse

r

FIG. 633.2. Opto-electronic logic networks

I. After Loebner (1959:18).

the switch position is changed, EL2 becomes operative. It acts upon PCP and causes the voltage across ELb to drop sharply. As a result, ELb reverts to the dark condition. The device thus has the making of a storage element, though it is slow in operation with the

PHOTOCONDUCTIVE

CIRCUIT ELEMENTS

285

components now available. The switching stimuli could be transient and could, of course, be derived from non-electroluminescent sources. Other logic networks have been constructed by Loebner, including various kinds of shift registers. Figures 633.2a and 633.2b show optron circuits corresponding to a triple "or-gate" and a triple "and-gate". Arrangements could be made for each photoconductor to react to light of a particular colour with a sensitivity which could be adjusted optically or electrically. In this way the discriminating functions of the network can be made more versatile. Figure 633.2c gives the circuit of a device which can store information, can provide a stimulated "read-out" signal and thereby reset itself. ELa is optically and electrically coupled to PCa and is thus a bi-stable element which can be activated by a transient optical signal. Once this has happened, ELa will be emitting, but ELb will not. A second light impulse on PCb will lead to the application of a voltage to ELb which will then become emitting. This emission could be permanent, but the components can alternatively be designed so that the activation of PCb is sufficient to cause ELa (and hence, ultimately, ELb) to extinguish. The light pulse emitted from ELb can be accepted by the sensitor of another network which could be of the same kind. We thus have the basis of an optically sensing shift register. Such devices have also been described by Tomlinson (1957:88).

6.3.4

STATIC DISPLAY SCREENS WITH PHOTOCONDUCTIVE CONTROL

The shortcomings of direct optical control for picture amplification have already been mentioned in Sub-section 6.2.3. In order to obtain amplification in the visible region of the spectrum it is thus necessary to employ some form of photoconductive layer which covers the entire picture area. The principle of operation would, at any point, be the same as that of an optron. This could lead to a homochromatic device or else to an image converter, depending on the sensitivity range of the photoconductor. (1953:33). The simplest form of multi-layer image intensifier is that described by Orthuber and Ullery (1954:8) and illustrated on Fig. 634.1a. The photoconductive cadmium sulphide layer was prepared by vapour deposition. Strongly illuminated regions of this layer will obviously coincide with phosphor regions which are strongly emitting. In the version shown there is optical feedback and although this increases

286

ELECTROLUMINESCENT

DEVICES

the amplification, it is undesirable in other ways. Thus, if the system is bi-stable, any particular region might become "locked" in the emitting state, so that the picture cannot be changed except by interruption of the supply voltage. Moreover, the emitting phosphor scatters Incident

picture

(a)

'

f

r

Glass

v/////;///mw////;///////m^^^

\ . /

Photoconductor

Phosphor—

^ Ä Ä ^ Ä ^

'

Glass

T

T

Amplified

Incident

Transparent electrodes

picture

picture

(b)

'' Conducting _ 1 i nes

'

-

T\/\/\/\A ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^

Phosphor--—. o

^ ^

^^^^^^^^^^^^s^^^^^^s WMM?M//////M/M/?M/M////M/??/ ^ Glass

r

r

A m pi if i ed

Photo conductor Current d i f f u s i n g layer Opaque l a y e r Transparent

f

picture

FIG. 634.1. Picture display screens with photoconductor control (thicknesses not to scale), (a) After Orthuber and Ullery (1954:8). (b) After Kazan andNicoll (1957:59).

its light and thus illuminates not only the photoconductor immediately adjoining, but a certain neighbouring region as well. This tends to make the bright regions of a picture spread at the expense of the dark regions and in due course the whole picture area becomes bright. It is therefore desirable to prevent optical feedback and this can be done

PHOTOGONDUGTIVE

CIRCUIT

ELEMENTS

287

by placing a thin opaque film (e.g. lamp black) between the photoconductor and the electroluminescent layer. There are, however, cases in which the bi-stable characteristics can be utilized. Such devices are discussed in Sub-section 6.3.6. The spreading effect can take place only if the photoconductor is sensitive to the radiation emitted by the phosphor. It does not, therefore, occur in picture amplifiers which are specifically designed for X-ray input, and this makes the design and construction of such amplifiers somewhat simpler. The optimum flux gain which has been obtained with such a device is 480, using an input illumination of 0-08 mlu/cm2 and a source giving power at 200 c/s. If simplifying assumptions are made and the empirical brightness-voltage relation of the phosphor is taken as a basis, then the gain of the device can be estimated as a function of operating frequency and input illuminance in a straightforward manner. Orthuber and Ullery obtained fair agreement with their experimental results in this way. Ideally, the photoconductor in such a system would be a single crystal. However, crystals of sufficient size are not available and the sensitive material (usually cadmium sulphide) must therefore be used in a different form. Vapour deposition is a convenient process, but the layers so prepared are generally thin, whereas the need to achieve proper impedance matching calls for a thick layer. Again, thick layers can be prepared by powder sintering techniques, but their sensitivity is then limited to X-rays. This is so because visible light is too quickly absorbed in a thin surface region. Considerations of this kind led Kazan and Nicoll (1957:59) to adopt a different method. They made use of a finely divided cadmium sulphide powder which could be bonded with transparent plastic without suffering any loss of sensitivity. The bonded phosphor itself has been described by Thomsen and Bube (1955:69). It has a non-linear voltage-current relation (J = AV4) arising out of multiple grain contacts. The same behaviour is found on unbonded powder specimens. The relationship between photocurrent and incident illumination is likewise non-linear, beginning as a square law at low intensities and becoming linear as the intensity increases. The light sensitivity of these bonded powders is reported as being comparable with that of the best single crystals. Their build-up time is approximately inversely proportional to the level of the input radiation. At the lowest intensities, it is of the order of seconds, and the decay times are very similar. The bonded powders are thus much slower in operation than single crystals.

288

ELECTROLUMINESCENT

DEVICES

The combined impedance matching and sensitivity problem was solved by the arrangement shown in Fig. 634.1b. The thickness is sufficient to provide impedance matching and its shape, with a large exposed surface area, leads to high sensitivity. The incident light is absorbed close to the regions in which most of the resistance is located. The current diffusing layer is needed so as to prevent the grooves which are 0-025 in. apart from appearing on the final image. It is made of a plastic-bonded conductive powder. The panel described by Kazan and Nicoll was of substantial size (12 x 12 in.). Screens have been made which give a homochromatic energy gain of 100 for the yellow band. The gain is always smaller at very low and very high input intensities. As Kazan and Nicoll have pointed out, homochromatic amplification is of special value since it permits the cascading of amplifiers with a view to achieving a higher total gain. It also makes it possible to use optical feedback from one panel to another. It was later shown (1957:87) that the gain of these devices can be improved by applying to alternate strips a superimposed d.c. bias of opposite polarity. The detailed performance characteristics of these light amplifiers depend critically on the nature of the photoconductor. Under certain conditions, its behaviour is controlled by space charges and Rose and Bube have given a detailed analysis of such a system (1959:23).

6.3.5

APPROXIMATE THEORY OF LIGHT AMPLIFIERS

As mentioned above, calculations which are concerned with the performance of light amplifiers are difficult or at any rate inconvenient because of the optical and electrical non-linearities involved. Moreover, the photoconductive and electroluminescent components are rather variable from case to case, and precise calculations would be applicable to individual optrons only. For a more general assessment of modes of behaviour, rough calculations are therefore appropriate. The approximations can take a variety of different forms. One such treatment has been given by Diemer, Klasens and van San ten (1955: 99), another by Hadley and Christensen (1959:22). The latter can be summarized as follows and the possibility of introducing variations will be clear. Optical feedback is assumed to be absent. The analysis is based on the equivalent circuit shown on Fig. 635.1 and on two basic equations :

PHOTOGONDUGTIVE

289

CIRCUIT ELEMENTS

GL = GQL»

(635.1)

which gives the conductance GL of the photoconductor as a function of incident light intensity L (Go and n being constants), and B = B0œaUELb

(635.2)

which gives the brightness B as a function of (angular) frequency ω and the voltage across the cell UEL- B^ a and b are again constants B.SB.

npc

log Θ,

|>Cx

<·

Γ /BS= Β

™\7ΰ\

B,= B,

/ /

log L

FIG. 635.1. Schematic representation of static amplifier response in the absence of optical feedback. After Hadley and Ghristensen (1959:22).

(see Sub-section 1.3.1). These equations are not accurately true, but they are obeyed well enough to make their use in the present context desirable. The voltage UEL can be simply calculated as a fraction of the external applied voltage U9 and when the result is substituted into (635.2) we obtain GLb B = Bs = Bm (635.3) where Bm = B0œaUb. The brightness has been called Bs because we

290

ELECTROLUMINESCENT

DEVICES

are dealing with a static characteristic. The cell capacitance C has been regarded as constant here though in practice, it is known to be a function of UEL (e.g. see Fig. 534.1a). A schematic plot of Bs versus Z, in accordance with eqn. (635.3), is shown on Fig. 635.1 One mayconsider this as being composed of three lines, each appropriate to a range of optical input levels. At low inputs GL <ζ Cœ so that Bs = Bd = Bm(GdICœ)*> = constant

(635.4)

where Ba is the dark background level of the electroluminescent screen and Ga is the leakage conductance of the photoconductor. I n the intermediate region, GL can still be neglected in the denominator, in comparison with Cœ. Thus Bs = Bm{G0LnlCœ)b

(635.5)

which, on a log-log plot, represents a straight line. Saturation must set in when GL > Cœ, in which case B = Bm. It is also possible to find the condition of maximum gain from d(B[L)ldL = 0 and this leads at once to GoL" = Cœ^(bn-\).

(635.6)

The illumination at which the gain is a maximum thus increases with frequency ω, and this in qualitative agreement with the results on Fig. 632.1. The maximum value of the gain, as calculated in accordance with the present equations, should diminish with increasing frequency, though not as rapidly as the results on Fig. 632.1 show. Hadley and Christensen found that the curve on Fig. 635.1 is reasonably well obeyed by practical systems. The various parameters could thus be evaluated from experimental results plotted in this way. For a typical zinc sulpho-selenide phosphor, b was about 3-5 and a cadmium sulphide photoconductor was characterized by n = 0-7. A parallel combination of photoconductor and electroluminescent cell can, of course, be analysed in a very similar way. It is a simple matter to introduce optical feedback qualitatively into this picture and thus to clarify the working of the bi-stable device. To do this, we consider eqn. (635.5), according to which Bs is proportional to Lbn, where bn is about 2-5. When optical feedback is present, we can put and

^TOTAL = ^ E X T + ^ E L

£EL =

ßBs

(635.7)

PHOTOCONDUGTIVE

CIRCUIT ELEMENTS

291

where ß is a constant, determined by geometry and by the degree of spectral overlap. The linear relationship between ZEL and Bs is shown by the broken lines on Fig. 635.2a for three different values of ß, increasing from βι to £3. The brightness Bs as a function of Ζ,ΕΧΤ can be (a)

/L EL

/

//

L

/*

EL

*\ / ^r—// / à

/ 1 1 1 1

/

/

/

/

//

is J

' 's sSS'7 '-/ \&> 1

1

/ /

L· Total s s

j ^

/L

y

*'fi*

y

Fixed frequency

/

Illumination (c) A

>2

High frequency

FIG. 635.2. Schematic representation of the behaviour of a light amplifier with optical feedback, (a) Graphical method of evaluating B s as a function of text 03 > /?2 > j8i. (b) Switching by means of light impulses, (c) Switching by means of a temporary frequency change.

obtained graphically by subtracting the abscissae, and results of this kind are shown on Fig. 635.2b for different amounts of feedback. A negative slope can now appear and it characterizes the bi-stable nature of the arrangement. The turnover points correspond to those brightness levels Bs on Fig. 635.2a for which the gradients equal 1/jS. It is clear that switching can be achieved by means of a light impulse. On Fig. 635.2c, essentially the same relationships are given for several different

292

ELECTROLUMINESCENT DEVICES

frequencies of excitation. A given operating point Αχ would be stable for a fixed external light input, but a temporary increase of frequency could switch the device to A%. When the original frequency is restored, the operating point will be A3. A similar picture governs switching by voltage impulses. In other contexts, the dynamic response characteristics may be of greater interest, as, for instance, when the input radiation acts for a short time only and when the output is to be photographically recorded. The quantity which is then significant is u

\Βά> o

where t% is the time during which the photographic film is exposed. It is possible to calculate this from the above equations and the known time-dependent behaviour of the photoconductor. These considerations lead to a comparison between film speed and the speed of response of an electroluminescent light amplifier. Using a cadmium selenide photoconductor and the best preparational techniques now available, these speeds can be made comparable. A detailed analysis of time-dependent behaviour has been given by Kazan (1959:68). The earlier calculations by Diemer and co-workers made G^ a linear function of incident light intensity, assumed that B depends on U in accordance with eqn. (131.2) and took account of the capacitance of the photoconductive layer. In spite of these variations of detail, the conclusions derived from the two sets of approximations are very similar. It has been shown (1955:99) that by a suitable combination of operating frequencies, the (more or less) linear part of the curve on Fig. 635.1 can be considerably extended. 6.3.6

STORAGE LIGHT AMPLIFIERS

As mentioned above (Sub-section 6.3.4) it is possible to exploit optical feedback with a view to achieving the permanent or, at any rate, long term storage of a picture. This cannot be done simply by means of the type of screen on Fig. 634.1, since the bright regions would spread out and would eventually cover the whole area. In order to obtain stable storage, optical feedback must be permitted within discrete picture elements, but discouraged from element to element·

PHOTOGONDUGTIVE

CIRCUIT

ELEMENTS

293

Kazan and Nicoll (1957:59) have proposed two ways of achieving this in principle. They involve deposition of the photoconductor in the form of "pedestals" as shown on Fig. 636.1, of which version (b) evidently permits a great deal more feedback than.(a). Such devices Incident

Wm

picture

YMM

YMM\

Transparent electrodes

Photoconductive "pedestals"

Phospn

f}//M///M/M?/BmmMMm>M#^ Glass

Amplified

picture

Incident picture

mm\ Transparent electrodes

V/>////////////\

Wm

„Photoconductive "pedestals'*

V///////0/A \//////////ψ/ί

wMmmmmmMMmm//m Glass

Amplified

picture

FIG. 636.1. Principle of the light amplifying storage matrix, (thicknesses not to scale). After Kazan and Nicoll (1957:59).

are somewhat more difficult to construct, but not prohibitively so. Indeed, the individual cells can be very small so that several hundred can be accommodated in a square centimetre. A 12 in 2 panel with 250,000 storage cells has been made in this way. Some constructional details have been given by Loebner (1959:18) and Hook (1959:20).

294

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There are, essentially, three types of practical design, based on very similar principles. The structure of single picture elements is shown on Fig. 636.2a and b. (a) Corresponds closely to the schematic representations on Fig. 636.1. The glass element helps to ensure that the photoconductor is efficiently illuminated. The alternative of making the pedestals transparent and surrounding them with a photoconductive (b)

Phosphor

Continuous transparent electrode

Transparent plastic

Conducting mesh

Grooved photoconductor

^^0MMï$i)?Mi$^

Opaque mesh

L-VGlass

Transparent elec trode

mm

XPhosphor

support

dots (c)

FIG. 636.2. Light amplifying storage matrices (thicknesses not to scale). After Hook (1959:20). (a) and (b) Single pedestals, (c) Section through matrix.

layer has also been tried (b), but is less successful from the manufacturing point of view. More successful was a layer structure of the type illustrated on Fig. 636.2c. This is a modification of the continuous display screen shown on Fig. 634.1b. A current diffusing layer is not required in this case. An opaque mesh divides the screen area into discrete picture elements and greatly diminishes the optical interaction between neighbouring regions. The outcome is a high degree of

PHOTOCONDUGTIVE CIRCUIT ELEMENTS

295

stability. However, some interaction remains and the stored images deteriorate in the course of time. The mesh itself can be "photoformed" on glass. It can alternatively consist of regularly spaced glass spheres (1960:17). The limits of intelligibility depend, of course, on the nature of the picture. Hook has shown that alphanumerical information can be successfully stored for periods of the order of 30 min. A performance analysis of the pedestal system in general has been given by Diemer and co-workers (1955:99). Quite generally, the loss of picture definition with time is due to residual amounts of cross-coupling between picture elements, both optical and electrical. Difficulties arise from the back reflection of emitted light by the front surface of the glass plate and from any ambient light on the viewing side of the panel. Such light can reach the photoconductor through the phosphor layer which is slightly translucent. To overcome these problems, Kazan (1959:68) has proposed a composite panel consisting of the usual photoconductive elements and two phosphor layers, separated by a completely opaque screen. One of the phosphor layers supplies a feedback signal, the other (on the viewing side) provides the emitted picture. Such an arrangement has the additional advantage of providing for the possibility of colour conversion. The response of the feedback phosphor could be properly matched to the photoconductor and that of the output phosphor could be designed for optimum viewing conditions. The devices considered above employ optical feedback to achieve bistable conditions. Such conditions can alternatively prevail (without optical feedback) when cadmium selenide powder is used as the photoconductor. Such a powder, when mixed with a very small amount of binder, exhibits a form of discontinuous hysteresis in its current-voltage relationship. This hysteresis is most prominent in the dark but it still occurs in the presence of illumination. The photo-element itself is then bi-stable. The phenomenon is not yet satisfactorily understood, but Nicoll (1958:55) has been able to make use of it for the construction of a feedbackless storage light amplifier. A further interesting storage panel has been made by Ranby, Hobbs and Turner, f It consists of a single layer in which phosphor and photoconductor have somehow been combined. The device operates from a direct voltage source. Half-tones are satisfactorily reproduced immediately after exposure, but deteriorate subsequently over a period of several minutes. Details of the design are not yet available. f Brit. Patent Application 26957/59, Thorn Electrical Industries, Enfield, Middx.

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6.4 SCANNED PICTURE DISPLAY SCREENS 6.4.1

GENERAL PROBLEMS

The image intensifies so far discussed all suffer from a common limitation, namely that they need a half-tone picture as input. Many new applications would be open, if it were possible to achieve a satisfactory picture display, using an electrical signal as starting point. For this purpose, an electroluminescent panel has many potential advantages over a conventional television tube, in as much as it is virtually two-dimensional and not subject to the same limitations of size. The practical difficulties are, however, severe. In the last resort, and as far as domestic television applications are concerned, they have not yet been overcome. Nevertheless, a great deal of progress has been made in the design and construction of such display devices and the present Section is concerned with a review of this work.

FIG. 641.1. Matrix display screen with direct intensity control, as proposed by Piper (1955:42).

It is necessary to provide for some form of scanning over the picture area and, simultaneously, for intensity control. I n the simplest display screen which could be envisaged, the control would be achieved by direct variation of the power applied to each picture element. In the more sophisticated screens discussed below the control is indirect. Figure 641.1 shows the essential parts of a direct system. By selecting any two strip electrodes and applying a potential difference between them, one picture element (at the crossover point) can be made luminous. Scanning is obviously complicated but not impossible. Piper (1955:43) proposed, in the first instance, that it should be done mechanically by the use of rotating switches. For semi-static displays

SCANNED PICTURE DISPLAY SCREENS

297

the switching could be done manually. The intensity modulation would have to be superimposed on the switching process. Such devices may have limited applications, but there are inherent difficulties which preclude their use for the display of moving half-tone pictures. In order to make any particular picture element luminescent, we shall assume that a balanced alternating voltage is applied between the two appropriate strips and that the remaining strips are all earthed. This would mean, however, that there are numerous intersections, in the form of a cross, at which one half of the external voltage would be applied across the phosphor. The observed pattern would thus appear as a bright spot at the intersection of a less bright cross in a dark field. If the spot were moving during scanning, the cross would also move and would result in a substantial reduction of contrast. The actual degree of reduction would, of course, depend on the shape of the brightness-voltage characteristics and this, in turn, would depend on the frequency used. In particular, the effect is much diminished at high frequencies. Under static conditions the spot could be made about thirty times brighter than the cross without too much difficulty. Greater contrast ratios have been achieved, but only by the inclusion of non-linear resistance elements in series with the phosphor layer. O'Connell and Narken (1960:13) have described such a system. The non-linear resistive element was a layer of Carborundum (600 grit), suspended in a binder. Layer thickness and composition could be varied. Under optimum (static) conditions, brightness ratios greater than 104:1 could be obtained at useful brightness levels and operating voltages. Residual consequences of the cross-effect are always intensified by the scanning process itself. Thus, each wanted bright spot would be energized with the frame frequency and its average brightness would depend on the time-integral of the individual light pulses, bearing in mind that the kind of phosphor which is otherwise most suitable is likely to have a very short afterglow. On the other hand, the unwanted picture elements situated on a cross would be energized much more often, i.e. with the line frequency of any one electrode array. Although each light pulse would be be less intense, the time-integral of all the pulses must be expected to give an impression of high brightness. As mentioned above, the damaging effect would be diminished by using high frequency signals, but this would introduce difficulties of its own. A further grave problem arises from the fact that each spot could be energized for a very short time only. If the frame frequency were 25/sec and if there were as few as 100 spots in any one line on a screen

298

ELECTROLUMINESCENT

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of 100 lines, each spot would have an applied voltage for only 4/xsec, that is, in all likelihood, for only a small fraction of a cycle. In the course of such a short time, the brightness developed would be minimal. The outcome of these considerations is that such a simple system is not suitable as the basis of a television display. The systems described in the following Sub-sections have been designed to overcome at least some of the difficulties. 6.4.2

DISPLAY SCREENS WITH FERROELECTRIC CONTROL

The above considerations lead to the conclusion that each picture element of a display screen should remain energized throughout the period required for persistance of vision, i.e. 1/25 to 1/20 sec. Since this cannot be done by direct scanning, it must be done by means of some control element which is itself very rapid in action and which is capable of storing the video information. Two such systems have been developed, with limited though, considering the difficulties involved, gratifying success. The first makes use of ferroelectric control elements and has been described by Sack (1959:16). The principle of the "Elf Screen", as it has been called, is quite simple and is illustrated on Fig. 642.1a. A ferroelectric capacitor made of ceramic barium strontium titanate is placed in series with an electroluminescent cell. The voltage appearing across the cell depends on the series capacitance and this, in turn, depends on the unidirectional field applied across the ferroelectric. With increasing field, the effective dielectric constant diminishes sharply (Fig. 642.1b). The alternating voltage across the electroluminescent cell thus diminishes with increasing control voltage. In principle (given perfect insulation) the control would be maintained even if the external video source were removed, since electric charge is not consumed. I n practice there is, of course, a slow decay. The circuit on Fig. 642.1c is an improvement on the simplest arrangement since it avoids application of the control voltage to the electroluminescent cell itself. In order to achieve a picture of reasonably high resolution, this configuration would have to be reproduced for each of a large number of picture elements. Figure 642.Id shows how this is done. Special techniques have been developed to achieve intimate bonding between the wafers. The various layers are first applied and bonded in continuous form and then machine-slotted as shown. The first system described by Sack had ten elements per inch and a more recent screen had sixteen. It is believed that this does not

SCANNED PICTURE DISPLAY SCREENS

299

represent the ultimate limit of resolution, and that perhaps a hundred elements per inch might eventually be obtainable. Contrast ratios of 100:1 have been achieved with control voltages of 200 V. The highlight brightness was 25 ft-L. No significant electrical or optical interaction (o)

(c)

Electroluminescent /cell

Θ

ËL~T ■ I

'

Barium - strontiun titanate Isolating -^vV\A

1

j y Con trol

Ä

signnal

2

40

Ferroe lectric capacitor 80

160 Control

(d) eta I

24 0

320

volts Ferroelectric material

\

Phosphor

FIG. 642.1. Display screen with ferroelectric control. After Sack (1959:16). (a) and (b) Control circuits, (c) Ferroelectric characteristics, (d) Two elements of a display panel.

between neighbouring elements was observed. Sack has also given a brief discussion of the practical ways in which the video signal corresponding to a half-tone picture might be distributed to the individual picture elements. 6.4.3

DISPLAY SCREENS WITH MAGNETIC TRANSFLUXOR CONTROL

A different mode of approach has been used by Rajchman, Briggs and Lo (1958:38). It exploits the interesting properties of magnetic transfluxors, previously devised by the authors in a different context.

300

ELECTROLUMINESCENT

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Only a brief description of these devices can be given here.f A transfluxor is made of ferrite in the shape illustrated on Fig. 643.1. The saturation on (a) can be established by passing a temporary current pulse through the setting winding. The resulting flux divides itself between the legs 2 and 3 as shown. It is permanent in view of the fact that the remnant and saturation magnetizations are almost equal for this type of material. The smaller aperture of the ferrite disk carries a primary and a secondary winding. In the situation envisaged by Fig. 643.1a, there would be no secondary output, since this would involve an increased flux either in leg 2 or leg 3 beyond saturation. The transfluxor is thus "blocked". A secondary output can be obtained only if either leg 2 or leg 3 are not completely saturated. In order to alter the flux through leg 2, a small and reversed current pulse is passed through the setting winding. The flux which such a pulse will locally produce depends on the magnetic reluctance of the system. This is proportional to the circumference of the magnetic path and thus to its radius. A critical flux value is required before the magnetic saturation in any part of the core is reversed. A small pulse can thus achieve a flux reversal up to a certain radius but no farther. This condition is shown on Fig. 643.1b. The total flux through legs 2 and 3 is now unequal. Accordingly, power can be passed from the primary to the secondary, its amount depending on the magnitude of the original pulse. The maximum power condition is shown on Fig. 643.1c. For higher setting pulses the output must diminish because of the presence of the aperture (Fig. 643.3). Each picture element of a display panel is connected to the secondary of such a transfluxor. Its brightness level can thus be permanently controlled by a single setting pulse. In order to obtain reproducible results, each re-setting to a new level should begin with complete saturation, as represented by Fig. 643.1a. A powerful blocking pulse is therefore applied before each setting pulse. A special "blocking winding" can be provided for this purpose. A pulse length of 2 /xsec has been found successful. There are reasons for believing that even this short duration could be reduced. Simple considerations show that it is advisable for the driving voltage in the primary to be asymmetrical, since this prevents spurious unblocking. It is now necessary to consider the conditions under which the electroluminescent picture elements are energized. There are technical t For technical details see RAJCHMAN, J. A. and Lo, A. W., The transfluxor—a magnetic gate with stored variable setting, R.C.A. Review 16, 303 (1955), and The transfluxor, Proc. I.R.E. 44, 321 (1956).

SCANNED PICTURE DISPLAY SCREENS

301

difficulties in providing a large number of secondary windings as would be required to match the transfluxor to an electroluminescent cell. In practice the number of turns must therefore be kept small (e.g. 10-20). The cell needs a high voltage and, in the ordinary way using sinusoidal inputs, this could be achieved only at high signal frequencies. Indeed, the drive frequency would have to be several megacycles per second which is entirely impracticable. An alternative way of obtaining a high secondary voltage is to use an input of lower frequency but of angular wave-shape, so as to make the flux changes as rapid as possible.

Set to maximum

FIG. 643.1. Operation of a magnetic transfluxor. After Rajchman, Briggs a n d L o (1958:38).

In a display panel, each element is thus pulse-excited, using 40/x-sec pulses of* 183 V, repeated at the rate of 12 kc/s. The behaviour of electroluminescent cells stimulated by square wave pulses of this duration has already been discussed in Sub-sections 5.2.4 and 6.1.2. In the present case, we are not, of course, dealing with a square pulse. On the other hand, the observed behaviour is not very different. Figure 643.2 shows these relationships. A maximum (average) brightness of 4 ft-L has been recorded.

302

ELECTROLUMINESCENT

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In a display screen the picture elements (1200 in the experimental model described) are arranged in rows and columns. Through each large transfluxor aperture p asses a row setting and one column setting

Drive current through p r i m a r y

1

2

3

Time,

4

5 \

6

7

μ sec

-4

200

Transfluxor f u l l y set

IOO 0 -100 -200

Cell 0-15 Cell

Time ,

area in2

capacitance : 3 0 0

μ

μμΐ

sec

FIG. 643.2. Waveforms of transfluxor drive current, secondary voltage and electroluminescent cell light output. After Rajchman, Briggs and Lo (1958:38).

conductor, and the scanning pulses are applied to these conductors from two magnetic switch systems. Actual scanning is achieved by a coincidence method, which operates as follows. The relation between

SCANNED PICTURE DISPLAY SCREENS

303

1-5 Duration of setting impulses 12/asec

1-0

>

0-5

1

1 / ' —H

·

!

'

Setting current,

_i

2-0 A

FIG. 643.3. Setting characteristic of a typical transfluxor. After Rajchman, Briggs and Lo (1958:38).

Blocking winding

Setting winding

Driving volts 9 ~ 9

Picture element

7^P Transfluxor

FIG. 643.4. Electroluminescent cell controlled by a transfluxor-transformer combination with a resonant secondary output. After Rajchman, Briggs and Lo (1958:38).

304

ELECTROLUMINESCENT

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setting current and output flux is shown on Fig. 643.3. A pulse, whether row or column, of magnitude /o or less, does not achieve any significant amount of setting. However, the simultaneous presence of two pulses can achieve variable setting up to a certain maximum. The coincidences can be arranged to scan the picture area and thereby to set and store the video information. Technical details of screen construction and switching circuits can be found in the original paper. The screen actually built and tested had a low resolution because of the small number of picture elements used (a much larger number being very cumbersome). O n the other hand, it had good half-tone characteristics and adequate brightness. By the use of higher pulse repetition rates for driving, it was estimated that the brightness could be increased to 50 ft-L. Even the low frame rate of 15/sec gave an adequate illusion of continuous action, since the cells were emitting for 29/30th of each frame time. The storage properties were excellent and pictures could be viewed several months after being set. O n the other hand, the construction of such a panel is very complex and its power consumption is high. An interesting modification of the above system has been described by the same authors. It makes use of sinusoidal drive currents and involves coupling of the electroluminescent cell to the transfluxor by means of a resonant transformer. The circuit is shown on Fig. 643.4. The transformer is an additional component, but its presence actually simplifies matters, since it eliminates the need for one of the transfluxor windings and permits the use of a smaller transfluxor. The transfluxor reflects an impedance into the transformer secondary which depends on the magnitude of the setting pulse. The secondary is arranged to be at resonance when the transfluxor is blocked. Setting impulses then have the effect of de-tuning the output and thus of reducing the excitation of the electroluminescent cell. A control unit has been described measuring 0-1 x 0-1 x 0-5 in. and designed to drive a picture element of area 0-1 in 2 in this manner. The outside diameter of the transfluxor itself was only 0-08 in. It is believed that arrangements of this kind could be made about three times more efficient than those discussed above. 6.4.4

DISPLAY SCREENS WITH PIEZOVOLTAIC CONTROL

Any display screen which consists of a dot matrix and individual control elements is necessarily costly. Moreover, as the screen size

SCANNED PICTURE DISPLAY SCREENS

305

increases, uniform structure becomes difficult to achieve. In an attempt to overcome these shortcomings and to simplify the electronic drive mechanism, an entirely different form of approach has been proposed Strip

electrode

Acoustic terminations

Piezoelectric Base

slab

electrode

(a)

Acoustic terminations

S t r i p electrode

(b) , Phosphor

>///////////////{ψ/////;^ Pulsed video signal

-Si

Transparent conducting coating - Non-linear resistance layer

?

m/y////////////////////////////////////////m^ Base e l e c t r o d e

- Piezoelectric

slab

FIG. 644.1. Scanned picture screen with piezovoltaic control. After Yando. (a) Operating principle; generation of the elastic wave pattern, (b) Screen structure and method of superimposing video signal.

by Yando. t It depends on a combination of electroluminescent and piezoelectric layers. The principle and design are illustrated on Fig. 644.1. Figure 644.1a shows a thin piezoelectric panel made of lead zirconate-titanate which is a ceramic material. One of its surfaces is f YANDO, S., General Telephone & Electronics Inc. Lecture delivered at the International I.R.E. Convention, New York, March 1961, and private communication.

306

ELECTROLUMINESCENT

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entirely covered with a metallic base electrode, the other carries two narrow strip electrodes as shown. The application of a voltage pulse between base and a strip leads to the formation of an elastic wave which is propagated through the panel almost without loss and is eventually absorbed (without reflection) in the acoustic termination. The elastic wave is accompanied by a localized piezoelectric voltage, developed across the thickness of the panel. Voltage pulses in straight line form can thus be made to travel across the panel in two directions, emanating from each of the strip electrodes. A velocity of 3600 m/sec has been quoted as being typical. At their cross-over point, the two waves would reinforce each other and the voltage developed would therefore be twice as high. If simultaneous elastic waves were to be produced by each of the strip electrodes, then it is clear that the crossover point would travel across the sheet along a diagonal line. By suitable phasing of the two pulses, this line can be displaced and a complete scanning pattern can thus be achieved. Figure 644.1b shows the construction of the panel as a whole. A nonlinear resistance layer is included for the purpose already outlined in Sub-section 6.4.1. The layers are matched so that the double voltage generated at a cross-over is only just sufficient to produce an electroluminescent signal, but the single voltage generated elsewhere is not. The scanning process thus produces a uniform field of luminescence, presumably near the threshold of visibility, on which positive video modulation can then be superimposed. The piezoelectric voltage and the (smaller and pulsed) video signal are effectively connected in series and the diagram shows how this is done. Yando has made and demonstrated experimental panels of this kind up to 5 in. x 5 in. in size. With the appropriate electronic controls, they are capable of displaying normal oscilloscope patterns, with a brightness of 0-1 ft-L and an effective spot size of about 1 mm. The device is still in the development stage. Its relative simplicity of construction, the continuous character of the screen and the ease with which the video information can be distributed make it a promising line of development towards a practical solid state display for television purposes.