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Performance and Reliability of Third-Generation Image Intensifies
ADVANCES IN ELECTHONICS A N I ) fil.l
Performance and Reliability of Third-Generation Image Intensifiers H . K . POLLEHN U.S. Army Night Vision and Electro-Optics Laboratory, Fort Belvoir, Virginia, U . S . A .
Over the past years third-generation image intensifiers have been developed to a point where first production can be initiated and most of the performance characteristics initially envisioned can now be obtained routinely. For practical applications it is most important that these performance characteristics do not degrade under various and often extreme environmental and operational conditions. This article will describe the performance currently obtainable and the reliability characteristics of third-generation image intensifiers together with some of the remaining problem areas and possible improvements. It will also discuss the trade-offs that had to be made in respect of performance in order to achieve satisfactory reliability.
INTRODUCTION In all image intensifiers the basic detection mechanism is the photoelectric effect. Electrons are released from the photocathode by incoming photons. These electrons are accelerated andlor multiplied by the gain mechanism and focused onto the output phosphor screen, where they generate photons, generally in the visible region. Image intensifiers are classified as zero, first, second, and third generation. The distinguishing factor for the four generations are the type of photocathode and the gain mechanism. S . 1 photocathodes were used in what have been called zero-generation tubes. First- and second-generation intensifiers incorporate the multialkalide S 20 photocathode and thirdgeneration tubes the negative affinity single-crystal GaAs cathode. To obtain a usable luminous gain, zero- and first-generation tubes rely solely on the acceleration of electrons and the conversion of its kinetic energy in the generation of output photons. Second- and third-generation intensifiers utilize the electron multiplication capabilities of a microchannel plate (MCP). Focusing of the electrons onto the MCP or the phosphor screen is 61 Ci3pyright
L’, IYX? hy Auiidemic Prer\, Inc. (London) Lid. All right\ of reproduction in any form re\erved. ISHN U-I?-O14hM-Y
62
H. K. POLLEHN
achieved by three distinctive mechanisms: magnetic, electrostatic, and proximity focus. Since all third-generation intensifiers currently in production or development are proximity-focused wafer tubes, only this type of intensifier will be discussed further. PROXIMITY-FOCUSED IMAGEINTENSIFIERS Second-generation proximity-focused image intensifiers have been in production for almost 10 years and are utilized in night vision goggles (AN/PVS-5). First production contracts for third-generation intensifiers were awarded in 1982. They will be utilized in the aviators night vision imaging system (ANVIS). Figure I shows a cross-section of a second- and third-generation intensifier. The similarities of construction are obvious. Going from input to output, we have the input faceplate with the photocathode, the MCP and the phosphor screen deposited onto a fiber optic inverter. In both cases these components are sealed vacuum tight into a metal ceramic housing, but except for the output phosphor screen the components utilized and the method of construction are quite different. Second-generation tubes generally use a fiber optic input window and third-generation tubes a plain glass faceplate. Although both types of intensifiers have been fabricated with both types of input windows, higher quality, and, especially, better yields have been experienced with glass windows. The glass windows are constructed so as to prevent incoming photons hitting and being reflected from the metallized parts of the window. This is done either by insertion of black glass outside the image plane or by special coatings underneath the metal. Second generation
Phosphor screen
MCP
I n seal
Ill v itocathode Fiber optic face plate
Bulls eve for veiling glare reduction
F ~ GI.. Structure of second- and third-generation proximity focused image intensifiers. Shaded areas, metal; light areas, ceramic.
THIRD-GENERATION IMAGE INTENSIFIERS
63
Fabrication techniques for third-generation photocathodes have been described in the literature. Both liquid phase and organometallic vapor phase epitaxial growth techniques are currently used. The vapor phase technique has an advantage in respect of reproducible cosmetic quality and cost but, as will be discussed in some more detail below, achieving a good balance between cathode sensitivity and dark current seems to be more critical. Microchannel plates are fabricated identically for second- and thirdgeneration intensifiers, but before insertion into a third-generation tube an important additional fabrication step has to be performed: the input of the MCP is coated with a thin film (about 40 A) of Alz03 or Si02.This film is the main factor allowing the fabrication of third-generation image intensifiers of great stability and reliability; its presence requires that the front and the back end of the tube have to be open during processing and cleaning operations. After completion of these procedures the tube has to be sealed at both ends (cold Indium compression seal) as compared to a single hot Indium seal at the input only for second-generation tubes. The difficulties originally encountered with the double seal, compounded by the requirement for very high vacuum (lO-'O Torr), have largely been overcome. The double sealing operation has become a standard procedure with high yield. In Table I the basic differences between second- and third-generation image intensifier tubes are summarized.
TABLEI Characteristics of second- and third-generation intensifiers
Cathode material Cathode fabrication Cathode thickness ( p m ) Faceplate material MCP: Center-to-center spacing ( p m )
Second generation
Third generation
Na-K-Sb-Cs (S 2.0 Evaporation 0. I Fiber optics (glass)
GaAs-CsO Epitaxial growth I .s Glass (fiber optics) 10 for ANVlS tube 15 for PVS-7 tube 40 A thick A12Q or SiOt ion barrier film I .8 for ANVlS tube; 2.0 for PVS-7 tube
15
Input
Open
Noise figure
I .6
Seal Type Vacuum level (Tom)
Single hot indium 10-7
Double cold indium 10-1"
64
H . K . POLLEHN
PERFORMANCE OF IMAGEINTENSIFIERS Before comparing the performance of third- and second-generation intensifiers and discussing the trade-offs made between different performance parameters and between performance parameters and reliability characteristics, a short description of the static intensifier performance model will be given. The basic assumption of this model is that the perception of brightness differences between two areas (i.e., target and background) is proportional to the average brightness difference divided by the fluctuations in the difference. Based on this theory a formula can be derived3that relates a 50% probability of detection or recognition (K5O%) of a target at distance R to image intensifier and environmental parameters:
where H,(A) = Scene incident irradiance as function of wavelength B(A) = Cathode sensitivity SI(A)= Reflection coefficient of target &(A) = Reflection coefficient of surrounding background u(A) = Atmospheric attenuation coefficient Nf = Noise figure, defined as the signal-to-noise ratio at the output to the signal-to-noise ratio at the input of the intensifier tube ZD = Tube dark current H ( u ) = Not well-defined function of the modulation transfer function (MTF), dependent on target shape, complexity of target, and background, etc. K = Contains various variables as T number of optics, integration times, etc., not generally related to the “generation” of the intensifier. Figure 2 shows the scene incident irradiance H,(A) for a moonless night sky, the cathode sensitivity O(A) for typical second- and third-generation image intensifier tubes, and reflection coefficients SI(A) for some “natural” materials. From this figure and Eq. (1) the advantages obtainable through use of the third-generation GaAs photocathode are obvious. The factors &(A) and [Sl(A)- &(A)] are increasing above 700 nm where B(A) stays significantly higher for the GaAs photocathode as compared to the second-generation S .20 cathode.
THIRD-GENERATION IMAGE INTENSIFIERS
65
Wavelength (nm)
FIG.2. Spectral characteristics of photocathodes, night sky, and materials.
Performance-Reliability Trade-08s
The performance parameters specified for ANVIS or the third-generation night vision goggles (AN/PVS-7) are well below the maximum values obtained so far (see Table 11). Using these maximum values in Eq. ( I ) , it can be shown that detection and recognition ranges will increase by about a factor of two,2 or the same detection or recognition ranges will be obtained at light levels lower by more than a factor of 5 . Obviously maximum obtained values for the performance parameters cannot be specified in a production contract. The yield would be extremely low and the cost therefore extremely high. This is the only reason for the lower specified values of the limiting resolution. For the other parameters additional, more fundamental limitations exist, as will be discussed below. GaAs photocathodes are activated by depositing a CsO layer onto its surface. As shown in Fig. 3, the cathode sensitivity and the dark current, generally expressed as EBI (equivalent background input) in lumens per square centimeter rise sharply at the beginning of the activation, but after
66
H . K . POLLEHN
TABLE I1 Specified intensifier parameters and maximum values achieved
Cathode sensitivity (PA 1m-I) Noise figure Resolution (Ip mm-l) Reliability (hr) Standard? Accelerated'
Maximum obtained
ANVIS specification
ANIPVS-7 specification
I000 1 .8"
800 2.0"
36
26
48
2000 400
7500 1500
>6000 >3000
2060
I .35 MgO coated MCP 1.56
Noise figure not specified. Value of 1.8 or 2.0 corresponds to specified cathode sensitivity and signal-to-noise ratio. Routinely obtained in second-generation wafer tubes. Test still continuing, for test conditions see Table 111.
some CsO is deposited the slope of the cathode sensitivity decreases; a maximum is obtained after which a further increase in CsO reduces the sensitivity. The dark current on the other hand continues to rise sharply when the quantity of CsO is further increased. The absolute values for dark current and cathode sensitivity as a function of deposited CsO are dependent on various processing and material parameters most of which are not well defined. There is only one parameter for which a relationship between dark current, cathode sensitivity and amount of CsO is reasonably well known: the cleanness of the GaAs surface, especially in respect to carbon. For a given cathode activation curve, the dark current may increase as indicated by curve a of Fig. 3 for a cathode with a contaminated surface, whereas a relatively smaller increase (curve b) is experienced with a clean surface. Another important factor that influences the relationship between dark current and cathode sensitivity is the growth technique used to fabricate the photocathode. It seems to be easier to satisfy the specified sensitivity (1000 pA Im-I) and the specified EBI (2.5 X lo-" Im cm-2) with photocathodes grown by the liquid phase epitaxial techniques, but for other reasons stated before, better cosmetic quality and lower cost, the vapor phase method is the preferred technique. The high performance achieved with third-generation image intensifiers would be of no value if at the same time satisfactory stability and reliability could not have been obtained. The key factor for achieving excellent stability, operational reliability, and meeting bright source protection requirements (Table 111) has been the thin film deposited at the input of the MCP. This film serves two purposes; it prevents ions generated in the
TH IRD-GENERATION I M A C E I NTENSl FI ERS
67
Amount of CsO on cathode surface (relative units)
FIG. 3. Dependence of photocathode sensitivity and dark current on quantity of CsO deposited.
MCP or at the phosphor screen from hitting the photocathode and it preserves the delicate CsO balance at the photocathode. This balance seems to be disturbed at elevated temperatures since a decrease in the cathode sensitivity is experienced when measured at the operational test point (52°C) of the environmental/temperature test (Fig. 4), but the original sensitivity is again obtained when measured at room temperature after the test. The application of the ion barrier film has caused an increase in the noise figure. Attempts have been made to decrease the noise figure of filmed MCP by funneling the input and by depositing a material of high secondary emission (MgO) into the MCP. Funneling is now a standard procedure for MCPs for ANVIS. A gain in noise figure of between 5 and 10% is obtained. A reliable manufacturing process for depositing MgO into the MCP has not been found yet. Many tubes incorporating MgO coated MCPs have experienced persistence problems.
SUMMARY The performance improvements, originally expected for third-generation image intensifiers, have been realized in currently fabricated tubes.
68
H. K. POLLEHN
TABLEI11 Reliability test conditions Cycle time (min)
On time (min)
Standard reliability
60
55
1 x
Accelerated reliability
12
11
5 x
On time
Area
Intensity
1
1 mm2
50 mlm
Bright source protection
Light level
Flashes during cycle
Temperature
5 x FC for 5 sec 5 FC for 3 sec 5 x FC for 5 sec 5 FC for 3 sec
40
2
5°C
40
2
5°C
Test criteria
No discernible damage after 24 hr
95%RH 15%RH
+65"C
+23"C
+65"C
t t l
---.A
I
I
Reduce humidity from 95% to less than 15%
Operational test
I
iermal shock
!3" c
-
t
Operational test Thermal shock
Operational test
+23"c
Opkrational posttest
35°C 1-
Reference only
FIG.4. Environmental test applied to intensifiers.
THIRD-GENERATION IMAGE INTENSIFIERS
69
These improvements have been verified in numerous field experiments (not reported in this article) by comparing second- with third-generation image intensifiers. Initial problems in respect of stability and reliability have been overcome. Current third-generation intensifiers are more stable and have higher reliability than second-generation tubes. To achieve this reliability and stability some sacrifices in performance had to be made. Current efforts are directed to minimize these sacrifices.
REFERENCES 1. Escher, Y. S. and Anlypas, G . A . , Appl. Phys. Lett. 30, 314 (1977). 2. Csorba, I. P., Appl. Opt. 18, 2440 (1979). 3. Pollehn, H . , Appl. Opt. Opr. En#. 6 (1980).
Performance and Reliability of Third-Generation Image Intensifiers H . K . POLLEHN U.S. Army Night Vision and Electro-Optics Laboratory, Fort Belvoir, Virginia, U . S . A .
Over the past years third-generation image intensifiers have been developed to a point where first production can be initiated and most of the performance characteristics initially envisioned can now be obtained routinely. For practical applications it is most important that these performance characteristics do not degrade under various and often extreme environmental and operational conditions. This article will describe the performance currently obtainable and the reliability characteristics of third-generation image intensifiers together with some of the remaining problem areas and possible improvements. It will also discuss the trade-offs that had to be made in respect of performance in order to achieve satisfactory reliability.
INTRODUCTION In all image intensifiers the basic detection mechanism is the photoelectric effect. Electrons are released from the photocathode by incoming photons. These electrons are accelerated andlor multiplied by the gain mechanism and focused onto the output phosphor screen, where they generate photons, generally in the visible region. Image intensifiers are classified as zero, first, second, and third generation. The distinguishing factor for the four generations are the type of photocathode and the gain mechanism. S . 1 photocathodes were used in what have been called zero-generation tubes. First- and second-generation intensifiers incorporate the multialkalide S 20 photocathode and thirdgeneration tubes the negative affinity single-crystal GaAs cathode. To obtain a usable luminous gain, zero- and first-generation tubes rely solely on the acceleration of electrons and the conversion of its kinetic energy in the generation of output photons. Second- and third-generation intensifiers utilize the electron multiplication capabilities of a microchannel plate (MCP). Focusing of the electrons onto the MCP or the phosphor screen is 61 Ci3pyright
L’, IYX? hy Auiidemic Prer\, Inc. (London) Lid. All right\ of reproduction in any form re\erved. ISHN U-I?-O14hM-Y
62
H. K. POLLEHN
achieved by three distinctive mechanisms: magnetic, electrostatic, and proximity focus. Since all third-generation intensifiers currently in production or development are proximity-focused wafer tubes, only this type of intensifier will be discussed further. PROXIMITY-FOCUSED IMAGEINTENSIFIERS Second-generation proximity-focused image intensifiers have been in production for almost 10 years and are utilized in night vision goggles (AN/PVS-5). First production contracts for third-generation intensifiers were awarded in 1982. They will be utilized in the aviators night vision imaging system (ANVIS). Figure I shows a cross-section of a second- and third-generation intensifier. The similarities of construction are obvious. Going from input to output, we have the input faceplate with the photocathode, the MCP and the phosphor screen deposited onto a fiber optic inverter. In both cases these components are sealed vacuum tight into a metal ceramic housing, but except for the output phosphor screen the components utilized and the method of construction are quite different. Second-generation tubes generally use a fiber optic input window and third-generation tubes a plain glass faceplate. Although both types of intensifiers have been fabricated with both types of input windows, higher quality, and, especially, better yields have been experienced with glass windows. The glass windows are constructed so as to prevent incoming photons hitting and being reflected from the metallized parts of the window. This is done either by insertion of black glass outside the image plane or by special coatings underneath the metal. Second generation
Phosphor screen
MCP
I n seal
Ill v itocathode Fiber optic face plate
Bulls eve for veiling glare reduction
F ~ GI.. Structure of second- and third-generation proximity focused image intensifiers. Shaded areas, metal; light areas, ceramic.
THIRD-GENERATION IMAGE INTENSIFIERS
63
Fabrication techniques for third-generation photocathodes have been described in the literature. Both liquid phase and organometallic vapor phase epitaxial growth techniques are currently used. The vapor phase technique has an advantage in respect of reproducible cosmetic quality and cost but, as will be discussed in some more detail below, achieving a good balance between cathode sensitivity and dark current seems to be more critical. Microchannel plates are fabricated identically for second- and thirdgeneration intensifiers, but before insertion into a third-generation tube an important additional fabrication step has to be performed: the input of the MCP is coated with a thin film (about 40 A) of Alz03 or Si02.This film is the main factor allowing the fabrication of third-generation image intensifiers of great stability and reliability; its presence requires that the front and the back end of the tube have to be open during processing and cleaning operations. After completion of these procedures the tube has to be sealed at both ends (cold Indium compression seal) as compared to a single hot Indium seal at the input only for second-generation tubes. The difficulties originally encountered with the double seal, compounded by the requirement for very high vacuum (lO-'O Torr), have largely been overcome. The double sealing operation has become a standard procedure with high yield. In Table I the basic differences between second- and third-generation image intensifier tubes are summarized.
TABLEI Characteristics of second- and third-generation intensifiers
Cathode material Cathode fabrication Cathode thickness ( p m ) Faceplate material MCP: Center-to-center spacing ( p m )
Second generation
Third generation
Na-K-Sb-Cs (S 2.0 Evaporation 0. I Fiber optics (glass)
GaAs-CsO Epitaxial growth I .s Glass (fiber optics) 10 for ANVlS tube 15 for PVS-7 tube 40 A thick A12Q or SiOt ion barrier film I .8 for ANVlS tube; 2.0 for PVS-7 tube
15
Input
Open
Noise figure
I .6
Seal Type Vacuum level (Tom)
Single hot indium 10-7
Double cold indium 10-1"
64
H . K . POLLEHN
PERFORMANCE OF IMAGEINTENSIFIERS Before comparing the performance of third- and second-generation intensifiers and discussing the trade-offs made between different performance parameters and between performance parameters and reliability characteristics, a short description of the static intensifier performance model will be given. The basic assumption of this model is that the perception of brightness differences between two areas (i.e., target and background) is proportional to the average brightness difference divided by the fluctuations in the difference. Based on this theory a formula can be derived3that relates a 50% probability of detection or recognition (K5O%) of a target at distance R to image intensifier and environmental parameters:
where H,(A) = Scene incident irradiance as function of wavelength B(A) = Cathode sensitivity SI(A)= Reflection coefficient of target &(A) = Reflection coefficient of surrounding background u(A) = Atmospheric attenuation coefficient Nf = Noise figure, defined as the signal-to-noise ratio at the output to the signal-to-noise ratio at the input of the intensifier tube ZD = Tube dark current H ( u ) = Not well-defined function of the modulation transfer function (MTF), dependent on target shape, complexity of target, and background, etc. K = Contains various variables as T number of optics, integration times, etc., not generally related to the “generation” of the intensifier. Figure 2 shows the scene incident irradiance H,(A) for a moonless night sky, the cathode sensitivity O(A) for typical second- and third-generation image intensifier tubes, and reflection coefficients SI(A) for some “natural” materials. From this figure and Eq. (1) the advantages obtainable through use of the third-generation GaAs photocathode are obvious. The factors &(A) and [Sl(A)- &(A)] are increasing above 700 nm where B(A) stays significantly higher for the GaAs photocathode as compared to the second-generation S .20 cathode.
THIRD-GENERATION IMAGE INTENSIFIERS
65
Wavelength (nm)
FIG.2. Spectral characteristics of photocathodes, night sky, and materials.
Performance-Reliability Trade-08s
The performance parameters specified for ANVIS or the third-generation night vision goggles (AN/PVS-7) are well below the maximum values obtained so far (see Table 11). Using these maximum values in Eq. ( I ) , it can be shown that detection and recognition ranges will increase by about a factor of two,2 or the same detection or recognition ranges will be obtained at light levels lower by more than a factor of 5 . Obviously maximum obtained values for the performance parameters cannot be specified in a production contract. The yield would be extremely low and the cost therefore extremely high. This is the only reason for the lower specified values of the limiting resolution. For the other parameters additional, more fundamental limitations exist, as will be discussed below. GaAs photocathodes are activated by depositing a CsO layer onto its surface. As shown in Fig. 3, the cathode sensitivity and the dark current, generally expressed as EBI (equivalent background input) in lumens per square centimeter rise sharply at the beginning of the activation, but after
66
H . K . POLLEHN
TABLE I1 Specified intensifier parameters and maximum values achieved
Cathode sensitivity (PA 1m-I) Noise figure Resolution (Ip mm-l) Reliability (hr) Standard? Accelerated'
Maximum obtained
ANVIS specification
ANIPVS-7 specification
I000 1 .8"
800 2.0"
36
26
48
2000 400
7500 1500
>6000 >3000
2060
I .35 MgO coated MCP 1.56
Noise figure not specified. Value of 1.8 or 2.0 corresponds to specified cathode sensitivity and signal-to-noise ratio. Routinely obtained in second-generation wafer tubes. Test still continuing, for test conditions see Table 111.
some CsO is deposited the slope of the cathode sensitivity decreases; a maximum is obtained after which a further increase in CsO reduces the sensitivity. The dark current on the other hand continues to rise sharply when the quantity of CsO is further increased. The absolute values for dark current and cathode sensitivity as a function of deposited CsO are dependent on various processing and material parameters most of which are not well defined. There is only one parameter for which a relationship between dark current, cathode sensitivity and amount of CsO is reasonably well known: the cleanness of the GaAs surface, especially in respect to carbon. For a given cathode activation curve, the dark current may increase as indicated by curve a of Fig. 3 for a cathode with a contaminated surface, whereas a relatively smaller increase (curve b) is experienced with a clean surface. Another important factor that influences the relationship between dark current and cathode sensitivity is the growth technique used to fabricate the photocathode. It seems to be easier to satisfy the specified sensitivity (1000 pA Im-I) and the specified EBI (2.5 X lo-" Im cm-2) with photocathodes grown by the liquid phase epitaxial techniques, but for other reasons stated before, better cosmetic quality and lower cost, the vapor phase method is the preferred technique. The high performance achieved with third-generation image intensifiers would be of no value if at the same time satisfactory stability and reliability could not have been obtained. The key factor for achieving excellent stability, operational reliability, and meeting bright source protection requirements (Table 111) has been the thin film deposited at the input of the MCP. This film serves two purposes; it prevents ions generated in the
TH IRD-GENERATION I M A C E I NTENSl FI ERS
67
Amount of CsO on cathode surface (relative units)
FIG. 3. Dependence of photocathode sensitivity and dark current on quantity of CsO deposited.
MCP or at the phosphor screen from hitting the photocathode and it preserves the delicate CsO balance at the photocathode. This balance seems to be disturbed at elevated temperatures since a decrease in the cathode sensitivity is experienced when measured at the operational test point (52°C) of the environmental/temperature test (Fig. 4), but the original sensitivity is again obtained when measured at room temperature after the test. The application of the ion barrier film has caused an increase in the noise figure. Attempts have been made to decrease the noise figure of filmed MCP by funneling the input and by depositing a material of high secondary emission (MgO) into the MCP. Funneling is now a standard procedure for MCPs for ANVIS. A gain in noise figure of between 5 and 10% is obtained. A reliable manufacturing process for depositing MgO into the MCP has not been found yet. Many tubes incorporating MgO coated MCPs have experienced persistence problems.
SUMMARY The performance improvements, originally expected for third-generation image intensifiers, have been realized in currently fabricated tubes.
68
H. K. POLLEHN
TABLEI11 Reliability test conditions Cycle time (min)
On time (min)
Standard reliability
60
55
1 x
Accelerated reliability
12
11
5 x
On time
Area
Intensity
1
1 mm2
50 mlm
Bright source protection
Light level
Flashes during cycle
Temperature
5 x FC for 5 sec 5 FC for 3 sec 5 x FC for 5 sec 5 FC for 3 sec
40
2
5°C
40
2
5°C
Test criteria
No discernible damage after 24 hr
95%RH 15%RH
+65"C
+23"C
+65"C
t t l
---.A
I
I
Reduce humidity from 95% to less than 15%
Operational test
I
iermal shock
!3" c
-
t
Operational test Thermal shock
Operational test
+23"c
Opkrational posttest
35°C 1-
Reference only
FIG.4. Environmental test applied to intensifiers.
THIRD-GENERATION IMAGE INTENSIFIERS
69
These improvements have been verified in numerous field experiments (not reported in this article) by comparing second- with third-generation image intensifiers. Initial problems in respect of stability and reliability have been overcome. Current third-generation intensifiers are more stable and have higher reliability than second-generation tubes. To achieve this reliability and stability some sacrifices in performance had to be made. Current efforts are directed to minimize these sacrifices.
REFERENCES 1. Escher, Y. S. and Anlypas, G . A . , Appl. Phys. Lett. 30, 314 (1977). 2. Csorba, I. P., Appl. Opt. 18, 2440 (1979). 3. Pollehn, H . , Appl. Opt. Opr. En#. 6 (1980).