US2987624A - Solid-state image-intensifier for the reproduction of images produced by radiation pulses - Google Patents

Description

June 6, 1961 G. DIEMER 2,987,624

SOLID-STATE IMAGE-INTENSIFIER FOR-THE REPRODUCTION OF IMAGES PRODUCED BY RADIATION PULSES Filed April 26, 1956 IG.7 GESINJS X'FQIEZ United States Patent O ware Filed Apr. 26, 1956, Ser. No. 580,954 Claims priority, application Netherlands May 9, 1955 Claims. (Cl. 250-213) This invention relates to solid-state image-intensifiers,

in which impedance variations produced in a photo-sensitive material by incident radiation govern the emission of light of a juxtaposed electro-luminescent material. As is shown diagrammatically in Fig. 1 of the drawing, such a device may for example consist of a plurality of layers directly adjacent one another, comprising outer transparent electrodes 1 and 5 sandwiching between them a photo-conductive layer 2, an intermediate layer 3 and an electro-luminescent layer 4. An alternating voltage V is applied to the electrodes 1 and 5, and when a beam of input rays S from the left-hand side produces an image on the photo-sensitive layer 2, so that the electric con- 'ductivity of the material and hence the electrical impedance of this layer are varied locally in accordance with the intensity of the incident radiation, the distribution of the voltage V among the various layers will also be varied locally to a greater or smaller extent.

The electro-luminescent layer 4 thus emits output light (L in FIG. 1) in the pattern of the image produced on the photo-sensitive layer 2. Thus an image produced by the radiation S can be amplified in intensity, while, if the beam S is not formed by visible rays (for example X- rays) the image is not only intensified, but also rendered visible.

'If the photo-sensitive layer 2 is sensitive to the electroluminescence light emitted by the layer 4 and this light is capable of interacting with the photo-sensitive layer, optical feed-back occurs, which may give rise to regeneration and instability of the device. Such feed-back may be "avoided by choosing an intermediate layer 3 such that it impedes the electro-luminescence light radiated towards the photo-sensitive layer. In order not to disturb the pattern of the image, this intermediate layer must have a high resistance in directions lying in its plane.

The sensitivity of a solid-state image intensifier, similar to-that of an alternating-voltage amplifier, may be increased-by means of feed-back. If the feed-back is so small that no instability occurs, no particular dificulties arise.

-It has been suggested to apply such a strong feed-back that the image intensifier is very unstable. To this end the intermediate layer 3 shown in Fig. 1 is omitted and the alternating voltage V is not supplied continuously but suppressed periodically. The latter measure prevents the electro-luminescent layer from becoming and remaining, as a whole, light-emissive at a maximum value.

The invention has for its object to provide a device, in which, as in the use referred to above, the radiation image produced on the photo-sensitive material is formed dotwise in order of time, or else, as with stroboscopy and with X-rays produced by alternating voltage, a device in which the radiation image is produced as a whole periodically; in general, a device in which the image to be reproduced is formed by radiation pulses and in which feedback of the electro-luminescent light on the photosensitive material giving rise to instability is utilized in a different and simpler manner than before.

" The image-intensifier of the invention is characterized by feedback of the electro-luminescent light to the photosensitivematerial to such an extent, and with a certain 1 voltage applied to the electrodes, which is continuously operative across these electrodes during operation, 'that the static characteristic curve of the image intensifier has an unstable range between a lower and an upper branch. This range is, however, restricted such that the image intensifier adjusts itself, in the absence of the radiation image to be reproduced, to a state which is indicated by a point on the lower branch of this characteristic curve outside the unstable range.

The term static characteristic curve of the image intensifier is to be understood to mean the curve indicating, for the states of equilibrium, the relationship between the intensity of a radiation emanating from an external source and incident on the photo-sensitive material, i.e., the light or radiation input, and the intensity of the electro-luminescence light radiated in consequence thereof by the electro-luminescent material, i.e., the light output. In the image intensifier shown in FIG. 1 this is the relationship between the intensity of S and the associated intensity of L.

The invention will now be described with reference to the accompanying drawing, in which:

FIG. 1, as indicated earlier, shows a conventional image intensifier,

'intensifier as shown in FIG. 1,

FIG. 3 illustrates the efiect of the value of the voltage at the electrodes, other conditions being the same,

'FIG. 4 is a diagrammatical view of one embodiment of a device according to the invention,

FIG. 5 shows a detail of this device,

FIGS. 6 and 7 show time diagrams indicating the relationship between various radiation pulses of an image to be reproduced by a device as shown in FIG. 4 and the electro-luminescence light thus emitted.

In a solid-state image intensifier the extent of feed-back is determined by:

(l) The fraction of the electro-luminescence light produced capable of acting upon the photo-sensitive material. In the image intensifier shown in FIG. 1 this fraction is determined by the permeability of the intermediate layer 3 to the electro-luminescence light. It should be noted that the permeability for all wavelengths of the electro-luminescence light need not be the same. The intermediate layer may be coloured and thus exhibit a selective absorption.

(2) The sensitivity of the photo-sensitive material to light of the same spectral composition as the fraction of the electro-luminescence light produced striking this material, and the extent to which such light is capable of penetrating wholly or only partly the section of the photo-sensitive material. In the image intensifier shown in Fig. l the thickness of the photo-sensitive layer together with the nature thereof may therefore be an important factor.

The static characteristic curve of a solid-state image intensifier is determined not only by the nature and the quality of the photo-sensitive and the electro-luminescent material, but also by the value of the voltage at the electrodes, the frequency of this voltage and the extent of feed-back of the electro-luminescence light on the photosensitive material.

With respect to the latter factor FIG. 2. shows various characteristic curves which apply to image intensifiers operating in the same conditions and difiering only in the extent of feed-back.

In FIG. 2, as well as in FIG. 3 to be described hereinafter, the intensity S of an input radiation striking the photo-sensitive material and emanating from an external source of radiation, this radiation causing the initially very poor conductivity of the photo-sensitive material to continually increase with an increase in its intensity, is plotted on the horizontal axis, and the intensity L of the output electro-luminescence light is plotted on the vertical axis. The curve found by indicating the value of 'L associated with each constant value of S is termed herein the static characteristic curve. The expression static is emploeyd herein to indicate that we are concerned here with states of equilibrium-which may otherwise be instable-so that inertia or after-effects inherent in the substances used are left out of consideration.

In general, the characteristic curve of a solid-state image intensifier has a lower and an upper branch, connected by an ascending part, which may be more or less strongly curved in accordance with various factors and which may even bend backwards, as will be explained hereinafter.

In FIG. 2 the curve 21 indicates the characteristic curve of an image intensifier as shown in FIG. 1, in which the intermediate layer '3 is completely impervious to the 'elec'tro-lumine scent light, and in which, consequently, no optical feed-back occurs. The voltage at the electrodes is designated by V With the same voltage at the electrodes the characteristic curve of an image intensifier having a certain degree of feed-back, since the intermediate layer permits the electro-luminescent light to pass to some extent, may be indicated by curve 22. In contradistinct'ion to the characteristic curve 21, this curve has two points, i.e. A (the associated 5:5 and D (the associated S=S at which it bends back, i.e. the tangent at this point is vertical. In such a case the image intensifier is not stable under all circumstances. With an increase in S beyond S L increases more or less abruptly up to point B over A, after which the upper branch BC of the curve is followed. This part of the curve, which is actually a region of saturation, may be termed a lit operating condition, in the sense that a relatively large amount of output light is generated. If S is then caused to decrease, L follows the upper branch back to point D. At this point L also decreases more or less abruptly and returns to point E, vertically beneath D on the lower branch FA of the curve 22. The .area outlined by EABDE indicated in FIG. 2 by cross-hatching, is, consequently, an unstable region or an area of unstable conditions: the theoretical course of the curve 22 in this area, which can be calculated, is indicated by the broken line therein.

Since the line S=0 falls out of this unstable area, an image intensifier having a characteristic curve corresponding to curve 22 of FIG. 2 will adjust itself, in the absence of external radiation incident on the photo-sensitive material, always in a manner such that the electroluminescence light is at a minimum (lower branch of the characteristic curve). This may be termed the dark or substantially dark operating condition. Under conditions otherwise the same, this is different with an image intensifier in which the characteristic curve has the shape of curve 23. Such a characteristic curve may be obtained by employing a higher degree of optical feedback than in the image intensifier to which applies the characteristic curve 22. This higher degree of feedback may be realized by means of a higher permeability of the intermediate layer 3. (FIG. 1). Since the curve 23 has more than one point of intersection with the line S=0, so that this line passes through the unstable area, a reduction of the external radiation S to zero is no longer capable of bringing an image intensifier having such a characteristic curve from a state indicated by a point on the upper branch (C-H) into astateindicated by a point on the lower branch F--G (G is the point having a vertical tangent). This can be realized only by suppressing or reducing substantially the voltage at the electrodes. Such is the case in the aforesaid known method used with an image intensifier having such a high degree of feed-back (complete omission of the intermediate layer 3).

Whereas FIG. 2 shows various characteristic curves applying to the same electrode voltage, but to difierent degrees of feed-back, FIG. 3 shows characteristic curves which are found with a variation in electrode voltage, the degree of feed-back remaining the same. We start from an image intensifier which, at an electrode voltage V has a characteristic curve equal to curve 22 of FIG. 2. This characteristic curve is also indicated in FIG. 3 and designated by 30. At a lower electrode voltage V the same image intensifier has a curve 31, which has no points with a vertical tangent and hence no unstable area. With a still lower voltage (V the characteristic curve has the shape of the curve 32, which has a still flatter course. If, however, the electrode voltage is raised to a value V exceeding V;,, the instability will increase and the characteristic curve can again intersect the line S=O (curve 33).

The frequency of the voltage applied to the electrodes of the image intensifier affects practically only the positions of the lower and upper branches of the characteristic curve. The values of S with which the characteristic curve has a vertical tangent vary little with the frequency of the alternating voltage.

From the foregoing it is evident that with a solidstate image intensifier the shape of the characteristic curve may be governed by the choice of the degree of optical feed-back, or by the value of the electrode voltage. This is utilized with the device according to the invention.

FIG. 4 shows diagrammatically one embodiment of a device according to the invention.

The image on the screen of a cathode-ray tube 40 is reproduced by means of an optical system 41 on the photo-sensitive layer 45 of a solid-state image intensifier, designated as a whole by 42. This intensifier is constituted by a transparent, flat electrode 44, a photo-sensitive layer 45, an intermediate layer 46, an electroluminescent layer 47, a second transparent, flat electrode 48 and finally, as a base for the whole structure, a glass plate 43. The electrode 44 may be constituted by a transparent layer of metal, for example gold, and the electrode 48 may be constituted by a very thin layer of conductive tin oxide on the glass plate 43.

The photo-sensitive layer 45 is made mainly of cadmium sulphide, which is activated with copper and gallium, its thickness being about several hundred ,u..

The layer 47 is mainly made of an electro-luminescent powder, consisting of copperand aluminum-activated zinc sulphide, and ureaformaldehyde, the thickness of this layer being about 50,41

The intermediate layer 46 is of a nature such that the transmission of the electro-luminescence light from the layer 47 is about 0.01, i.e. only about 1% of the electroq luminescence light radiated in the direction of the photosensitive'layer 45 can interact with this layer. The intermediate layer 46 may be made of an organic colour substance suspended in a synthetic substance, for example aniline black, which absorbs the electro-luminescence light.

Instead of using an absorbing intermediate layer, use may be made of a reflecting layer, if desired together with an absorbing layer between the photo-sensitive layer and the electro-luminescent layer, if only care is taken that the desired fraction of the electro-luminescence light (in this case about 1%) is capable of striking the photosensitive layer. The use of a reflecting layer directly adjacent the electro-luminescent layer, this reflecting layer being made for example of titanium oxide in ureaformaldehyde, has the advantage that the quantity of light emitted to the right hand, i.e. observable light, is increased.

In order to prevent the electro-luminescence light emitted by an image point of the electro-luminescent layer and passed by the intermediate layer 46 from reacting on a larger portion of the photo-sensitive layer than that corresponding to the associated image point,

a frame of opaque black lines is printed between these layers, preferably directly on the electro-luminescent layer. These lines, which are designated by 50 in FIG. 5, which is a front view of the image intensifier 42 with layers partly cut away, have a width approximately equal to or larger than the thickness of the layer 47, while their intermediate space is a multiple thereof. This intermediate space determines the definition of the electroluminescence image. From FIG. it is evident that the lines form a rectangular grating: this however, is not necessary; the lines may for example be parallel wavelines, for example sinusoidal lines, as is known with X- ray intensifying screens to suppress stray radiation.

By means of a source of alternating voltage 49 the electrodes 44 and 48 of the solid-state image intensifier 42 have applied to them continuously an adjustable, otherwise constant alternating voltage V which is about 350 v. in the present case. The frequency of this voltage is about c./s.

In accordance with the invention the degree of optical feedback and the value of the electrode voltage V are such that the characteristic curve of the image intensifier 42 has a shape corresponding more or less to curves 22 and 30 in FIGS. 2 and 3. This means that in the absence of an image on the screen of the cathode-ray tube 40 the intensifier adjusts itself to a state corresponding to the lower branch of the characteristic curve and that no or little electro-luminescence light is produced in the layer 47, which has been referred to as the dark operating condition.

When an image is written on the screen of the cathoderay tube 40, for example a television image or a radar image, each image point of the photo-sensitive layer 45 receives, once in one image period, a more or less strong luminous pulse. Distinction may be made between the case in which the screen of the cathode-ray tube 40 has only a short persistence and the case in which the screen has a long persistence, the duration of which exceeds the inertia or response time of the solid-state image intensifier 42.

This inertia is determined by the time constants of the photosensitive material of the layer 45 and any aftereffect of the electro-luminescent layer.

For the case of a short persistence of the screen of the cathode-ray tube, FIG. 6 shows a time diagram, from which is evident the eifect of the intensity of the input luminous pulse on the consequent emission of output electro-luminescence light. It is assumed that an image point'of the photo-sensitive layer 45 is first struck by a luminous pulse 61, having an intensity exceeding only little 8; (vide FIGS. 2 and 3), and one image period later by a luminous pulse 62, having a materially higher intensity. The two pulses will bring the image point of the electro-luminescent layer associated with the corresponding image point of the photo-sensitive layer into a lit state indicated by a point on the upper branch of the characteristic curve. The intensity of the electroluminescence light is at this instant in both cases L associated with the larger part of the upper branch. Since the intensity of the luminous pulse '61 exceeds the value S to a much smaller extent than that of the luminous pulse 62, the point indicating the state in which the image point of the electro-luminescent layer lies is much nearer point B for the first than for the latter pulse. Thus, when the two luminous pulses terminate, owing to which the state of the image point of the electroluminescent layer goes back to point D along the upper branch of the characteristic curve, the electro-luminescence of the image point of the electro-luminescent layer will persist shorter for the luminous pulse 61 than for the luminous pulse 62. The return along the upper branch is performed with a velocity which is determined in the first instance by the inertia of the photo-sensitive material, but also by the distance between the line 8:0 and the unstable area, i.e. the distance F-E. After the state indicated by point D has been reached, the electroluminescence intensity decreases rapidly, as is described hereinbefore with respect to curve 22. In FIG. 6 the dot-and-dash lines 63 and 64 indicate the luminous pulses emanating from the electro-luminescent layer associated with the luminous pulses 61 and 62 respectively.

The pulses 63 and 64 consist each, properly speaking, of a large number of successive luminous flashes, since the electro-luminescent material does not continuously electro-luminesce, but luminesces once in half a period of the alternating voltage at the electrodes. The pulses 63 and 64 shown are, in fact, the envelopes of these series of luminous flashes. It should be noted that the vertical scale for the luminous pulses on the photosensitive layer (S-scale) differs from that for the luminous pulses emanating from the electro-luminescent layer (L- scale), so that the ratio between the heights of the two pulses in FIG. 6 is not a measure for their intensity ratio.

Since, as stated above, the time (At and At: respectively in FIG. 6) during which the electro-luminescence of an image point persists is the larger, the higher is the intensity of the input luminous pulse on the photo-sensitive layer, the total light output of an electro-luminescence pulse is an indication of this intensity. Consequently, it is evident that intensification occurs, on the one hand because the intensity of the electro-luminescence light exceeds the intensity of the luminous pulse on the photo-sensitive layer, and on the other hand because the luminous pulse emanating from the electro-luminescent layer has a longer duration than the luminous pulse on the photo-sensitive layer, this duration being the longer, the higher is the intensity of the incident luminous pulse. In FIG. 6 the light content of the luminous pulses on the photo-sensitive layer is indicated by crosshatching ascending to the right hand, and that of the electro-luminescence pulses by cross-hatching descending to the right hand.

In order to ensure that upon the occurrence of a luminous pulse on the photo-sensitive layer the associated image point of the electro-luminescent layer is brought into a state of maximum emission (L the frequency of the alternating voltage at the electrodes of the solidstate image intensifier is chosen to be so high that during the luminous pulse on the photo-sensitive layer a plurality of periods of this alternating voltage are performed. With a screen of the cathode-ray tube 40 having a short persistence, the duration of a luminous pulse is of the order of 10" sec. Therefore, in the example shown, the frequency of the alternating voltage V is chosen to be 10 c./s.

If the screen of the cathode-ray tube 40 has a long persistence, the input luminous pulses forming an image point on the photo-sensitive layer have a shape which is indicated in the time diagram of FIG. 7 by 71 for low intensity and by 72 for high intensity. Owing to the persistence of the screen of the cathode-ray tube, the two pulses have a decay tail which is longer according as the maximum intensity of the pulse is higher. As a startingpoint it is now assumed that the persistence of the screen of the cathode-ray tube is longer than the inertia or response time of the solid-state image intensifier. In this case an image point of the electro-luminescent layer which is brought by a luminous pulse on the associated image point of the photo-sensitive layer into the lit state of maximum electro-luminescence, continues to emit light until the intensity on the photo-sensitive layer drops below the value S (vide curve 22). At that instant the electroluminescence decreases strongly. For the luminous pulses 71 and 72 of FIG. 7 the associated electro-luminescence pulses are indicated by the dot-and- dash lines 73 and 74 respectively. The termination of these pulses is determined practically by the points of intersection of the decay characteristic of the pulses 71 and 72 with the line S=S these points being indicated in FIG. 7 by P and Q respectively. The duration Ar and A1 of the electro- luminescence pulses 73 and 74, respectively, are therefore determined by the initial intensity of the associated input luminous pulse on the photo-sensitive layer. Since, as in the case to which applies the time diagram of FIG. 6, the intensity of the electro-luminescence pulse is materially higher than that of the pulse on the photosensitive layer, an intensification is obtained, while the contrast is maintained, since the duration of the electroluminescence pulses varies with the light content of the luminous pulses on the photo-sensitive layer.

If the screen of the cathode-ray tube 40 has a persistence of a shorter duration than the inertia of the solidstate image intensifier, the inertia of the latter determines the duration of the electro-luminescence pulse. The device operates, in this case, in accordance with the time diagram of FIG. 6.

It will be obvious that instead of using'a cathode-ray tube 40 with an optical system 41, use may be made of an X-ray tube, which is fed by a pulsatory voltage, for example an alternating voltage of line frequency, as a source of radiation. Also in this case the photo-sensitive layer is irradiated by pulses. It is also possible to house the image intensifier in the cathode-ray tube, the photosensitive layer being made of a material such as cadmium sulphide, the conductivity of which can be acted upon directly by the electron beam, i.e. without the intermedial-y of a luminescent screen.

It will be obvious that difficulties may arise if the inertia of the image intensifier is so high that a maximum electro-luminescence produced by a radiation pulse on the photo-sensitive layer has not yet decreased when the associated image point of the photo-sensitive layer is struck by a subsequent radiation pulse. The response time of the image intensifier must therefore .be smaller than an image period.

What is claimed is: V

1. An electroluminescent device comprising an impedance-varying, radiation-responsive layer and an electroluminescent layer in juxtaposed relationship, electrode means connected to said radiation-responsive and electroluminescent layers in such manner that a potential may be applied across them in series arrangement, means mounted between the radiation-responsive and electroluminescent layers and providing a given optical feedback between them, whereby a predetermined proportion of the light generated by each elemental area of the electroluminescent layer impinges upon a corresponding elemental area of the radiation-responsive layer, means connected to said electrode means for applying continuously thereto a periodically-varying voltage of constant amplitude, said optical feedback and said voltage of constant amplitude having values at which the static light output-light input characteristic of the device possesses a first stable portion corresponding to a substantially dark operating condition, a second stable portion corresponding to a lit operating condition, and a third unstable portion between the first and second portions, whereby, during operation but in the absence of impinging radiation, the device always remains in its dark operating condition, and means including a pulsing radiation image source for impinging on said radiation-responsive layer a periodically-interrupted radiation image, whereby each elemental area of the device when struck by radiation of the radiation image is excited from its dark to its lit condition, said device having a response time shorter than an image period.

2. A device as set forth in claim 1 wherein the duration of the impinging radiation pulses exceeds the period of the applied voltage.

3. A device as set forth claim 1 wherein the optical feedback means comprises means for confining the optical feedback from a given point of the electroluminescent layer to a point of the radiation-responsive layer corresponding thereto and initially responsible for the light output from said given point.

4. A device as set forth in claim 1 wherein the source of radiation includes a cathode-ray tube and beamscanning means.

5. A device as set forth in claim 3 wherein the feedback means comprises an opaque grating composed of opaque grating composed of opaque lines whose thickness is not less than that of the electroluminescent layer.

References Cited in the file of this patent UNITED STATES PATENTS 2,120,916 Bitner June 14, 1938 2,525,156 Tink Oct. 10, 1950 2,805,360 McNaney Sept. 3, 1957 2,858,363 Kazan Oct. 28, 1958 FOREIGN PATENTS 157,101 Australia June 16, 1954 OTHER REFERENCES A Solid-State Image Intensifier, by Orthuber et al. Journal of the Optical Society of America, volume 44, No. 4, pp- 297-299, April 1954.

Kazan: Proceedings of the IRE, vol. 43, No. 12 'December 1955, pp. 1888-1897.

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