Thin films as photographic information storage devices

Thin Solid Films-Elsevier Sequoia S.A., Lausanne-Printed in Switzerland

313

THIN FILMS AS PHOTOGRAPHIC INFORMATION STORAGE DEVICES*

J. MALINOWSKI

Central Laboratory of Photographic Processes, Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia (Bulgaria) (Received May 19, 1972)

Recently efforts have been made to develop a novel photographic material on the basis of thin films of silver halides, prepared by deposition in vacuum. Besides some technological advantages of this new approach, thin films have additional attractive features. The main possibilities for overcoming certain limitations in the information storage capacity of conventional photographic materials are discussed. The mechanism of latent image formation is reviewed, with the aim of revealing the reasons for the unique position of the silver halides in conventional photographic materials. It is argued that the specific scheme of trapping and neutralization of photoelectrons in silver halides is the main cause for their wide use in photography. The repeated failures of numerous attempts to find a substitute for silver halides prove the futility of any expectations to discover another photographic system with similar photographic properties. It is shown, however, that with many substances it is possible in principle to create a photographic system based on the motion of mobile photoexcited holes. Such a system would work just reciprocally to the conventional, negative, photographic recording and lead to the building of a direct positive image. The possibility of creating novel photographic materials on the basis of evaporated thin films of non-silver photosensitive substances is illustrated and discussed. INTRODUCTION

One of the most striking characteristics of contemporary scientific, technical and economical progress is the outburst of information quantity, which each entity of a modern society has to receive, to process and to store. This expansion of current data, however, induces by itself an additional information flux, ever growing with constantly increasing rate. An approximate estimation shows that during the last century the data storage doubled every 100 years, now it doubles every 10-15 years and by the end of the century it is expected to double every 3-5 years. Extrapolating the increase of The Physical Review, one comes to the Paper presented at the International Conference on Thin Films, "Application of Thin Films ", Venice, Italy, May 15-19, 1972; Paper A.5.2.

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absurd conclusion that after the end of the century the weight of this journal will be comparable with that of the globe itself. This paradoxical extrapolation, however, is a vivid demonstration of the problem which mankind has to face in order to command the information explosion. It is clearly seen that we are at present engaged in an autocatalytic chain reaction, the development of which is hardly to be foreseen. Generally speaking, each data storage operation is basically a signal or an image registration process. Although not the only means, conventional silver halide photography is one of the most widely used media for image recording. Nevertheless, the exploitation of its outstanding capacity for information storage is still only beginning. Commercially available photographic emulsions have been shown 1 to be capable of storing about l0 s bits/cm 1. This enormous capacity is at least three orders of magnitude larger than the capacity of the conventional magnetic discs and magnetic tapes used as storage memory media in contemporary computers, and about two orders of magnitude larger than laboratory results obtained on evaporated metal magnetic thin films. It is therefore easy to understand the reason for the large number of patents published recently, aimed at developing new photorecording memory storage systems. A survey of the published achievements in this field, although indicating definite capacity advantages of the new systems compared with the conventional magnetic one, shows that the silver halide photographic emulsion is still unsurpassed in this respect. It is the personal opinion of the author that most of these new optical data recording systems cannot be expected to find wide use as information storage media if they do not match the information capacity of the silver halide photographic emulsions. It is the aim of the present paper to review some of the potentials of a novel photographic system, based on vacuum-evaporated thin films. TECHNOLOGICAL ADVANTAGES OF THE SYSTEM

A survey of the patent literature shows that recently the photographic industry has been making serious attempts to develop a principally new technological approach for the manufacture of photographic materials. This new technology is based on the vacuum deposition on a suitable support of thin silver halide films. The approach has some very important technological advantages compared with conventional methods. It makes it possible for the coating rate of the photographic material to be raised by an order of magnitude. Conventional coating units in the photographic industry at present allow coating rates of about 30-50 m/min, but because of technical difficulties many manufacturers still use coating rates substantially lower. The new approach follows in principle the experience obtained with vacuum metallizing plants developed for the manufacture of capacitors. In this respect similar results have been obtained for the production of photographic materials by evaporation of silver bromide in vacuum. Recently, patents have been published z claiming coating rates of about 300 m/min by vacuum evaporation, Thin Solid Films, 13 (1972) 313-327

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which even at this early experimental stage exceeds the rate of modern emulsion coating units by almost an order of magnitude. The main advantage of the vacuum deposition technique is the complete elimination of the emulsion drying system. The removal of large quantities of water from the coated emulsion layer, which is a very slow, expensive and intricate operation in the manufacture of photographic materials, is now being automatically dropped out. For some special cases, as for example the production of photographic materials designed to be used at extreme resolution and without any defects caused by dust particles, air filtration might become the bottle-neck of the production process. For such materials the vacuum evaporation technique, eliminating by necessity the contact of the material with uncontrolled media, offers attractive possibilities. FUNCTIONAL ADVANTAGES

Besides the purely technological merits mentioned above, photographic materials based on thin evaporated films have some functional advantages too. The conventional photographic emulsion is an optically heterogeneous system, consisting of tiny silver halide grains dispersed in gelatine. The large difference in the refraction coefficients of the two constituents makes considerable Rayleigh scattering of the light beam in the emulsion layer unavoidable. As is well known, the intensity of the scattered light is stronger for short wave radiation, being inversely proportional to the fourth power of the wavelength. Because of this, photographic materials designed for the registration of images with extremely fine details are sensitized to longer wavelengths. In order to improve the resolution the blue light in the beam used to project the image on such materials is cut off. The loss of sharpness due to increased Rayleigh scattering at shorter wavelengths2° is illustrated in Fig. 1. The necessity to cut off the

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blue light of the spectrum, however, immediately has another negative effect. The resolving power of the optical system used is higher for light of shorter wavelength. The incompatibility of the two factors is indicated by the expressions written in the figure. Therefore these contradictory requirements automatically set a limit to the resolving capability of any optical system using the conventional silver halide emulsion as an image recording medium. The emulsion thickness of contemporary photographic materials presents another limitation. It has been found to be technically impractical to produce materials with uniform emulsion layers thinner than 6-8 ~tm. At such thicknesses the back reflection of the light beam from the glass substrate introduces considerable halation of the emulsion, although with difficulty this can be controlled to a reasonable extent. The depths of focus of the objectives used to project the image, however, are in severe conflict with the thickness of the emulsion layer. This is illustrated in the following figures. Figure 2 represents the distribution 7 I

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of intensity around a slit 1 gm wide, calculated for ideally corrected objectives with different numerical apertures (NA). It is seen that for a 1 gm linewidth a considerable spread is observed with objectives having apertures smaller than 0.5. Thin Solid Films, 13 (1972) 313-327

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This is more vividly indicated in Fig. 3, expressing the capability of an optical system to transfer fine image details. This capability is quantitatively measured by the modulation transfer function ( M T F ) which depends on the numerical

Fig. 3. Modulation transfer function of objectives of different NA. aperture of the optical system. It is readily seen that for a linewidth of 1 rtm an objective with N A = 0.5 already causes 30 ~o loss of image definition, which for m a n y purposes is considered as an acceptable limit. The theoretical resolving power o f the system, indicated by the zero value of the modulation transfer function, usually has no practical meaning. Therefore for the exact reproduction o f images in the micron range one has to use an ideally corrected objective having at least N A = 0.5. The depth of focus of an objective, however, is inversely proportional to the square of the numerical aperture, and for N A = 0.5 it is about 2 lam. In Fig. 4 this depth is compared to scale with the emulsion thickness

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Fig. 4. Depth of focus of objective NA = 0.5 in the emulsion layer. Thin Solid Films, 13 (1972) 313-327

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of one of the best photographic materials on the market--Kodak HRP. As can be seen, a sharp image can be obtained only in a limited region of the emulsion layer. Outside this region considerable distortion of the image would be produced, because here the light beam is out of focus. This is a very serious limit for the use of contemporary photographic materials for the registration of fine image details. If the recorded image is viewed optically by an enlarging objective, this may improve the picture to a certain extent because the small depth of focus of this second objective helps to cut off some of the unsharp portions of the image. However, if the original is used to duplicate the image by contact printing, which is the practice on many occasions, especially in the photolytographic manufacture of integrated circuits, the difficulties become even greater. The sharp image in the original plate is always separated from the acceptor plate by a distance larger than the dimensions of the image itself. Numerous attempts in our laboratory as well as in Carl-Zeil3 Jena 3 have met with practically insurmountable difficulties when trying to use Kodak HRP for obtaining master plates in the submicron range. These could not be reliably reproduced by contact printing if they contained details finer than 0.8-1 ~tm. Perhaps I have indulged too far in these elementary consequences of geometrical optics. My intention was to argue that conventional emulsions, although having for the moment the highest information storage capacity, have also reached their limit. Although the limit resolution of Kodak HRP is evaluated as more than 2000 lines/ram, practically it is useless for an exact and reliable reproduction of details with linewidth smaller than 1 Jam, which is equivalent to only 500 lines/mm. Now let us consider again the photographic materials prepared by evaporation in vacuum. Under well-controlled conditions this technique produces optically homogeneous films, the photographic sensitivity of which reaches a limit when the film is only 0.2-0.4 ~m thick. At least in principle, these extremely thin, optically non-scattering films are free from the limitations considered above. They do not feel the depth of focus even of objectives with NA = 1. Such an objective has a limiting resolution of about 3000 lines/mm and is reliable for an exact reproduction of lines thinner than 0.5 ~m. The contact transfer from such materials, provided the surface of their carrier substrate is of sufficiently high optical quality, should not present any additional difficulties. The absence of swelling gelatine or another binder in the evaporated photographic layers is an additional advantage, which guarantees complete absence of distortion due to deformation during the wet processing and drying of the plates. This also allows the processing of the silver halide evaporated thin films to be accomplished in several seconds, which for some special purposes might be of considerable importance. Recently Shepp e t al. 4 and Masters 5 have shown that, due to the favourable silver bromide coverage density, evaporated silver bromide thin films are very efficient for recording low energy electrons. In the energy range below 20 kV they appear to be the most efficient recording medium, and can be very useful for mass spectrography. Attempts are now being made to create memory storage or Thin Solid Films, 13 (1972) 313-327

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information recording media utilizing the direct writing action of an electromagnetically scanned electron beam. If this is achieved, evaporated films of silver halides might prove to be a most efficient and even indispensable part of this new information storage system. FUTURE EXPECTATIONS

It should be heavily stressed, however, that this new technical approach is still far from practical realization. Solutions to numerous more or less serious problems have to be found before the new photographic materials based on vacuum-evaporated silver halide layers can hope to compete successfully with conventional photographic emulsions. Nevertheless, evaporated thin films, with regard to their use as data storage media with exceptionally high recording density, do have practical advantages. They at least have the potential for surpassing the capacity of photographic emulsions, which nowadays is the highest one. On purely technical grounds one could not expect that evaporated thin films would become widely available commercially in the next 5 or 10 years. However, if the technical difficulties are finally overcome, it is very probable that another important limitation will supervene. In spite of definite and wide fluctuations, the world market shows a rather steady tendency towards an increase in the price of silver 6, predicting a possible rise by a factor of 10 in the next decade. Nowadays the photographic industry is the largest consumer of silver, and its price already hinders substantially the wider use of conventional photographic materials in many fields. It is my firm belief, which I shall try to defend later on, that the new photographic materials of vacuum-deposited thin films are very promising even on the basis of non-silver photosensitive substances. Therefore it is tempting to consider that if a substantial increase in the price of silver becomes inevitable, this may prove a very active promoter for the development of the new thin film technique. FUNDAMENTALS OF THE PHOTOGRAPHIC PROCESS

The problem of avoiding the use of silver halides in photography is not a new one. Many non-silver photographic systems have been successfully developed and are in widespread technical use nowadays. Nevertheless, in spite of much work, all attempts to substitute the silver halides in conventional photography have been without success. One reason for this failure may be sought in the present primitive state of solid state physics. Our possibilities of controlling the photoprocesses in light sensitive substances is much smaller than, for instance, our knowledge about the classical semiconductors. In this paper I shall attempt to review briefly a principally new way for solving the problem of substitution of the silver halides in photography. A detailed argument of this possibility has already been published elsewhere ~. Compared with contemporary electronic devices the development of the photographic latent image seems to be an extremely efficient amplifier. Although Thin Solid Films, 13 (1972) 313-327

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controlled by very simple and cheap techniques, its intensification can be as high as 108. Nevertheless, it is difficult to assume that this process of catalytic growth of tiny metal nuclei is exclusively embodied in silver halides. Physical development after fixation, a well-known variation of conventional processing, has been in use for many decades. Obviously it has nothing to do with silver halides, since prior to its application the emulsion grains are completely dissolved. Recently the process of physical development has been brought to a high state of perfection and successfully applied to non-silver photographic systems 8. Electroless plating baths have also become commercially available, allowing the selective deposition of metals on many plastic substrates. It can be argued, then, that efficient amplification by physicochemical means should not be considered a unique property of silver halides, responsible for their monopoly in photography. On the other hand, the photodecomposition observed with silver halides is not a unique property either. Many substances decompose when exposed to actinic radiation with a quantum yield of the order of 0.1. In 1938, Gurney and Mott 9 proposed a very general scheme outlining the fundamental principles of the formation of silver in exposed photographic grains. According to them, the mobile interstitial silver ions provide the vehicle which compensates the electric field of the temporarily trapped photoelectrons, the fundamental process being eAg + Ag + ~ latent image electron stage ion stage metal nucleus --The appropriate sequence of these electronic and ionic stages in fact makes possible the occurrence of a measurable photodecomposition. Nevertheless, this general principle alone is not sufficient to elucidate the very efficient mechanism of latent image formation in the silver halides. The lead halides, more or less typical representatives of light sensitive salts, have photoelectric properties which should provide a reasonable photodecomposition by a two-stage electronic and ionic process. The situation with some thallium and cadmium halides is similar. In spite of these facts, photographic materials based on non-silver halides have never been successful. NON-SILVER HALIDE SUBSTITUTES

In principle the photolysis of these salts can proceed efficiently by an electronic and ionic mechanism reciprocal to that operating in silver halides. Recent investigations indicate that this seems actually to be the case. Due to the motion of photoexcited holes 1°, eventually bound as excitons 11, halogen is liberated on exposure of lead halides. The electric field set up by its displacement is compensated by the motion of halogen ion vacancies. The salt decomposes and as a result a metallic component, in this case lead, is formed in excess in the illuminated crystals. Tubbs and F o r t y 12 w e r e the first to show that on the basis of this photodecomposition process one might realize a new photographic system. Their Thin Solid Films, 13 (1972) 313-327

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investigations have indicated that the rate of photodecomposition of lead iodide substantially increases at elevated temperatures. Obviously the higher temperature enhances the possibility of the photolytic halogen leaving the crystal lattice. In this way the recombination of the primary products of photolysis becomes negligible. Tubbs and Forty lz have shown that a thin film of lead or cadmium iodide, prepared by vacuum deposition on a glass carrier, becomes reasonably sensitive to an image-wise exposure to visible radiation if heated to about 180 °C. The change of the optical reflectivity and absorption of the exposed areas, mainly due to destruction of the film and the accumulation there of excess metal, leads to the formation of a readily visible image. It is shown that due to the fine crystallinity of evaporated thin films, the products of the photolytic decomposition do not spread sideways by diffusion, allowing one to record lines 1 gm wide with very good definition. By intelligent use of the difference in optical properties of the exposed and unexposed areas, Tubbs 13 managed to demonstrate a reasonable gradation in the contrast of the exposed regions and to obtain pictures with intermediate tones. Some reproductions of pictures published by Tubbs are shown in Fig. 5.

Fig. 5. Micrograph of images of PbI 2 layers, after Tubbs 13 (x230).

A further development of this system was introduced by Kostishin 1¢ and co-workers. Following Tubbs and Forty they found that the sensitivity of evaporated thin films increases substantially if the film is evaporated not directly onto glass plates but onto metal or metallized substrates. This observation made it possible to prepare pictures keeping the evaporated films at room temperature. Thus Kostishin succeeded in avoiding the annoying exposure of the samples at elevated temperature. His work showed that besides lead and cadmium halides many other metal halides work satisfactorily if deposited on metallized substrates. The sulphides of arsenic were found to be especially suitable for this system. Pictures published by Kostishin are reproduced in Fig. 6. Thin Solid Films, 13 (1972) 313-327

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The work of Tubbs and Forty, as well as that of Kostishin and co-workers, follows the general principle of the conventional photographic system. The image is built up by accumulation of the product of photolysis. Excess of metal in one form or another is left in the system after the removal of the electronegative component. In the work of Tubbs and Forty the removal of this component is facilitated by the elevated temperature. It was later shown by the author 7' 15 that in the system of Kostishin, the metal substrate, reacting chemically with the photolytic halogen or sulphur, hinders the recombination of the latter in the exposed layer and thus enhances the building of visible metal excess in the exposed areas of the film. LATENT IMAGE FORMATION IN SILVER HALIDES

The most important difference between the silver halides and all other photosensitive substances, however, is the operation in the former of the so-called concentration principle. Due to this principle, most silver atoms built up on illuminating a silver halide microcrystal tend to concentrate into a limited number of specks situated mainly on the surface of the crystal. Because of their superficial position these metal nuclei come into contact with the developer and promote the further deposition of metal onto themselves v. This concentration principle has not been found to operate efficiently in any other photosensitive substance. Therefore, even if excess metal is formed on exposure, the metal remains finely dispersed in the volume of the system. As a consequence the developer cannot feel the catalytic action of the metal nuclei and intensification by development is inefficient. For this reason all photographic systems based on the photodecomposition of non-silver halide photosensitive substances are bound to have low sensitivity. Typical examples of such systems, not liable to be efficiently amplified by development, are the systems proposed by Tubbs and Forty 11, 12 and by Kostishin 14 and co-workers. It is the opinion of the author that the main reason for the operation of the concentration principle is the unique scheme of the primary photographic process in the silver halides. Investigations carried out in the last 15 years, aimed at studying the properties of the photoexcited charge carriers in silver bromide, make it possible to describe this process quite explicitly. Thin Solid Films, 13 (1972) 313-327

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Experiments with synchronized light and voltage pulses have shown that, before their final neutralization by mobile silver ions, the photogenerated electrons are repeatedly trapped at very shallow levels. Because of this a photoelectron in a silver halide grain will hop randomly from one shallow trap to another until it finally falls into a deeper centre on the surface of the grain, produced there during the manufacture of the emulsion ~6. In this case, as shown schematically in Fig. 7, the electron remains in the surface trap for a relatively long time and eventually combines with an interstitial silver ion to form a neutral atom of silver. Recent investigations of the properties of photoexcited holes have indicated another favourable peculiarity of silver halides, Contrary to the case of photoelectrons, photoholes, as shown in Fig. 8, find many relatively deep traps in

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the volume of the grains. The formation of neutral complexes with the participation of silver ion vacancies neutralizes the charge of the trapped holes 16. Thus, for reasons not well understood at present, silver bromide has exactly the properties necessary to promote an efficient concentration of silver atoms at a single speck on the surface. At the same time, holes are almost permanently trapped in the immediate vicinity of their origin. THE COMPLEMENTARY PHOTOGRAPHIC PROCESS

Numerous attempts already carried out indicate that the probability of finding another photosensitive compound with similar properties is very low. However, in many photosensitive substances, e.g. the lead halides, holes are actually the mobile photoexcited carriers. In these cases, one would expect a multiple trapping scheme just reciprocal to the one established in silver bromide crystals, but now allowing photoholes to drift easily to the surface to build the corresponding halogen there. Thin Solid Films, 13 (1972) 313-327

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This scheme immediately offers a possibility, in principle, for a new photorecording process, illustrated in Fig. 9. The sample represents a thin layer of hV

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photosensitive substance, say PbI 2, deposited by evaporation in vacuum onto a film or glass substrate. The fog layer, e.g. a monolayer of silver atoms, is then deposited on the sensor, again by well-controlled vacuum evaporation. In the illuminated region the photoexcited electrons are almost immediately trapped and build photolytic metal which is finely dispersed in the body of the layer and is therefore unavailable to the action of the photographic developer. The halogen equivalent, however, is transferred to the surface, there destroying the fogging monolayer and thus building a direct positive image. Many photosensitive substances were checked with this model system and most of them showed a reasonable sensitivity as a result of the bleaching process. Lead, thallium and cadmium halides were found to be especially suitable, as were some metal sulphides, selenides and some oxideG' 1:5

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The advantages of this reversal process, i.e. the bleaching photosensitive system, is demonstrated in Fig. 10. Curve A represents the characteristic curve of a thin layer of PbIz, fogged with a monolayer of Ag. On increase of the exposure the developed density is diminished--a direct positive image is obtained. Curve B represents the density developed on an identical PbIz layer which had Thin Solid Films, 13 (1972) 313-327

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not been previously fogged. The exposure time in this case was about 100 times larger, but in spite of that the developed negative image is negligible. Obviously, curve B illustrates the intrinsic inefficiency of lead iodide to build up developable metal nuclei on exposure. The inefficiency of the concentration principle in non-silver halide photosensitive materials is the main reason for the inherent low sensitivity of all photographic systems based on the conventional negative working process. As demonstrated, this chief difficulty may be surmounted by using the complementary direct positive process. Pictures made with this system are reproduced in Figs. 11-13. Films of AszS 3 are very promising as information storage devices. This substance is not

Fig. 11. Contact printing on PbI2 layer.

V Fig. 13. Silver deposit etched on exposure of As2S 3 layer. Thin Solid Films, 13 (1972) 313-327

Fig. 12. Contact printing on TII layer.

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crystalline but forms glassy films, and because of this its acuity can be very high. This is demonstrated in Fig. 13, the upper part of which represents an electron microscope picture of the edge of the silver deposit etched by photoholes generated on illumination of an As2S 3 layer. The extremely high resolution of these films is essential for the holographic recording of information. Illuminating similar materials with laser beams, Kostishin 17 claims to have registered more than 10 000 lines/min. Tubbs 18 has also obtained very encouraging holographs using evaporated thin films of lead iodide. CONCLUSION

This exceptional resolution has been obtained without any intensification of the photographic image by development. Published results for the standard development of evaporated thin films of silver bromide, however, are rather disappointing 4. Obviously this apparent failure of thin films as high capacity storage media should be attributed to our present poor ability to handle their development. Actually, Jonker et a1.19 have recently demonstrated the potential ability of physical development to intensify fine image details without serious loss. They have shown that the modulation transfer function of thin film photographic materials, combined with skilfully mastered development, is superior to that of Kodak HRP, the information storage capacity of which seems to be the highest one obtainable nowadays. Intensive experiments on this problem are also being carried out at present in the author's laboratory. It seems probable that the resolution obtained with developed thin films should at least exceed the modulation transfer function of modern objectives, designed to reproduce details finer than 0.6-0.8 ~tm reliably. If actually achieved, this may prove decisive for the introduction of thin films as photographic information storage devices with exceptionally high recording density. Furthermore, the technical and economic realization of the principal advantages of thin films as novel photographic materials would seriously challenge the monopoly of the expensive silver halides in photography. REFERENCES 1 2 3 4 5 6

J . H . Altman and H. J. Zweig, Phot. Sci. Engng, 7 (1963) 173. A.A. Rasch and H. B. Cowden, B.P. 1,150,626, 1969. H . G . Woschni, Personal communication, 1971. A. Shepp, R. E. Whitney and J. I. Masters, Phot. Sci. Engng, 11 (1967) 322. J.I. Masters, Nature (London/, 223 (1969) 611. L . A . Mannheim, Worm Report of the Photographic Industries, Technologies and Science, The Focal Press, London and New York, 1968, p. 125. 7 J. Malinowski, Phot. Sci. Engng, 15 (1971) 175. 8 H. Jonker, A. Molnaar and C. J. Dippel. Phot. Sci. Engng, 13 (1969) 38; 19 (1971) 96; U.S.P. 157502, 1964. 9 R . W . Gurney and N. F. Mott, Proc. Roy. Soc. A, 164 (1938) 151. 10 J . F . Verwey, J. Phys. Chem. Solids, 31 (1970) 163. Thin Solid Films, 13 (1972) 313-327

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11 R.I. Dawood, A. J. Forty and M. R. Tubbs, Proc. Roy. Soc. A, 284 (1964) 272. 12 N. R. Tubbs and A. J. Forty, Brit. J. Appl. Phot., 15 (1964) 1553; B.P. 1,045,487, 1966; B.P. 1,141,021, 1969. 13 M . R . Tubbs, J. Phot. Sci., 17 (1969) 162. 14 M.T. Kostishin, Fiz. Tverd. Tela, 8 (1966) 571. 15 B.P. 1,151,310, 1969. 16 J. Malinowski, Phot. Sci. Engng, 14 (1970) 112. 17 M.T. Kostishin, Personal communication, 1971. 18 H . R . Tubbs, M. J. Beesley and H. F. Foster, Brit. J. Phot., Ser. 2, 2 (1969) 197. 19 H. Jonker, L. K. H. yon Beck, A. J. Houtman, F. P. Klostermann and E. J. Spiertz, J. Phot. Sci., 19 (1971) 187, 20 G . W . W . Stevens, Microphotography, Chapman and Hall, London, 1968, p. 27.

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