Title:
METHOD OF USING AN OPTIC RELAY COMPRISING AN ELEMENT OF AN ELECTRO-OPTICAL MATERIAL AND A PHOTOSENSITIVE ELEMENT
United States Patent 3855579
Abstract:
In order to erase an image recorded on an optic relay comprising an element of electro-optical material and a photosensitive element, the photosensitive layer is uniformly exposed and simultaneously is applied to the relay a voltage of a sign opposite to that of the voltage previously applied during the recording and having such a value that during the re-recording the voltage at the terminals of the electro-optical element is zero. Application: Television.
US Patent References:
COARSE GRAIN POLYCRYSTALLINE FERROELECTRIC CERAMIC OPTICAL MEMORY SYSTEM
Maldonado - June 1973 - 3740734
COARSE GRAIN POLYCRYSTALLINE FERROELECTRIC CERAMIC OPTICAL MEMORY SYSTEM
Maldonado - June 1973 - 3740734
Inventors:
Grenot, Michel (Brunoy, FR)
Pergrale, Jean (Paris, FR)
Pergrale, Jean (Paris, FR)
Application Number:
05/323848
Publication Date:
12/17/1974
Filing Date:
01/15/1973
View Patent Images:
Export Citation:
Assignee:
U.S. Philips Corporation (New York, NY)
Primary Class:
Other Classes:
365/166, 365/121
International Classes:
G02F1/05; G02F1/01; G11C13/04
Field of Search:
340/173LM,173LS
Primary Examiner:
Fears, Terrell W.
Attorney, Agent or Firm:
Trifari, Frank Cohen Simon R. L.
Claims:
What is claimed is
1. A method of using an optic relay in conjunction with at least a first source of light radiation having a first frequency, means for projecting the said radiation, at least a second source of light radiation having a second frequency, polarization and projection means from said second light radiation on an optic image relay constituted by an evacuated space having at least one window which is transparent to the said light radiation, a layer which is photosensitive to the said first radiation, a plate having an electro-optic effect maintained at a temperature near its Curie point and presenting a variable double refraction as a function of a potential difference existing between its faces, a first electrode which is electrically conductive and optically transparent to the said second light radiation being disposed against the said plate and a second electrode being placed on the opposite side in the proximity of the said plate, the method comprising displaying an image of electric charges while projecting an image on the photosensitive layer and in the presence of a voltage V between the electrodes, erasing said charges image while illuminating uniformly the photosensitive layer, and simultaneously applying between the two electrodes a voltage -V'.
2. A method as claimed in claim 1, characterized in that the applied voltage -V' has a value -V' = V C'/C, in which C is the capacity of the assembly electro-optical plate-dielectric mirror and C' is the capacity existing between said assembly and the electrode situated on the side opposite to the plate, in such manner that the voltage at the terminals of the assembly electro-optical plate-dielectric mirror is removed when the voltage V is again applied between the electrodes in the absence of light on the photosensitive layer.
1. A method of using an optic relay in conjunction with at least a first source of light radiation having a first frequency, means for projecting the said radiation, at least a second source of light radiation having a second frequency, polarization and projection means from said second light radiation on an optic image relay constituted by an evacuated space having at least one window which is transparent to the said light radiation, a layer which is photosensitive to the said first radiation, a plate having an electro-optic effect maintained at a temperature near its Curie point and presenting a variable double refraction as a function of a potential difference existing between its faces, a first electrode which is electrically conductive and optically transparent to the said second light radiation being disposed against the said plate and a second electrode being placed on the opposite side in the proximity of the said plate, the method comprising displaying an image of electric charges while projecting an image on the photosensitive layer and in the presence of a voltage V between the electrodes, erasing said charges image while illuminating uniformly the photosensitive layer, and simultaneously applying between the two electrodes a voltage -V'.
2. A method as claimed in claim 1, characterized in that the applied voltage -V' has a value -V' = V C'/C, in which C is the capacity of the assembly electro-optical plate-dielectric mirror and C' is the capacity existing between said assembly and the electrode situated on the side opposite to the plate, in such manner that the voltage at the terminals of the assembly electro-optical plate-dielectric mirror is removed when the voltage V is again applied between the electrodes in the absence of light on the photosensitive layer.
Description:
The present invention is related to the invention which forms the object of the U.S. Pat. 3,792,259 in the name of applicant. It relates more in particular to a method of using an optic relay described in the above patent application when in the manufacture of said relay a photosensitive element is included.
In the said patent application is stated in particular a method of erasing an image recorded in the relay, which erasing is necessary when one image is to be replaced by another.
For erasing a recorded image, the method described consists in exposing the photosensitive layer in a uniform manner, for example, by means of a flash of light, and simultaneously removing the direct voltage applied between the two control electrodes. In the case in which the photosensitive layer is a photoconductor, said control electrodes are constituted by two transparent conductive layers deposited on the faces of the stack consisting of the plate of electro-optical material, the dielectric mirror and the photoconductive layer. Under the influence of the light, numerous pairs of electrons-holes are produced in the photoconductor the resistivity of which decreses considerable.
The phenomenon or erasing charges deposited on the electro-optical material may be made equal to a capacitor discharge via a resistor which may be non-linear, the capacitor being constituted by the plate of electro-optical material and the resistance being that of the photoconductive element. The "erased" state is obtained while the applied external voltage is zero. When with a view to a new erasing said voltage is brought to the recording value, the capacitive divider comprising in series on the one hand the capacity C constituted by the assembly plate of electro-optical material - dielectric mirror and on the other hand the capacity C' constituted by the photoconductive element causes a voltage equal to V C'/C + C' to appear at the terminals of the said plate, which voltage is sufficient to produce a uniform modulation to which a uniform background corresponds. So the contrast of any new image is influenced by the existence of said uniform background. This phenomenon appears both in the case in which the photosensitive layer is a photocathode and in the case in which it is a photoconductor. Hence, it is generally more important in said latter case.
The method of using the optic relay according to the present invention endeavours to mitigate said inconvenience. It is remarkable in that, in order to erase a recorded image, the photosensitive layer is uniformly exposed and simultaneously a voltage -V' having a sign opposite to that of the voltage V applied during the recording is applied between the control electrodes. The value of V' is such that when the voltage V is again applied for a new recording, the voltage which appears at the terminals of the useful capacity C of the assembly electro-optical material-dielectric mirror is zero. Said value of V' is equal to C'/C V.
The invention will be better understood with the aid of the following description of the development of a few particular phases of the method of using the optic relay the constitution of which is repeated, for memory, on the sole FIGURE accompanying the said description in the case in which the photosensitive layer is a photoconductor.
It will be obvious that any conception of the method of using the relay comprising the same stages of changing the state of change of the plate of electro-optical material obtained with the same means but in a different sequence form part of the invention.
In the sole FIGURE, reference numeral 1 denotes a sheet of material which is transparent to the reading light and which serves to support the assembly of the relay, 2 is a conductive layer which is transparent to the reading light and permits the connection of the contact 3 deposited on the plate 4 of electro-optical material, for example, deuterated potassium biphosphate (KDP), 5 is the multi-layer dielectric mirror permitting the reflection of the reading light emitted by the source 6.
The photoconductive layer is shown at 7, the conductive layer at 8 which is transparent to the erasing light 9 and permits the connection to the contact 10.
During the recording of an image, a direct voltage +V is applied between the electrodes 3 and 10.
By means of a cooling device not shown in the FIGURE, the KDP is brought at a temperature near its Curie point (approximately -50°C).
In a first stage of the step of using the relay, the assumed initial charging state of the plate corresponds to a non-uniform charge density which may originate, for example, from a first image erasing by projection with the aid of the light 9 in the presence of the applied potential +V.
In a second stage, in order to erase the image the voltage -V' (C'/C V) is applied between the electrodes 3 and 10 and the photosensitive layer is exposed with the aid of a beam of uniform light 9 during a time which is sufficient for the assembly KDP-dielectric mirror to be charged to the voltage -V'.
In a third stage, the recording conditions of an image are while applying the voltage +V between the electrodes 3 and 10 and projecting an image on the photosensitive layer. Due to the capacity existing between the faces of the photosensitive layer, the various points of the assembly KDP-dielectric mirror are brought at potentials which vary between 0 for the non-exposed points and +V for the most exposed points.
In the said patent application is stated in particular a method of erasing an image recorded in the relay, which erasing is necessary when one image is to be replaced by another.
For erasing a recorded image, the method described consists in exposing the photosensitive layer in a uniform manner, for example, by means of a flash of light, and simultaneously removing the direct voltage applied between the two control electrodes. In the case in which the photosensitive layer is a photoconductor, said control electrodes are constituted by two transparent conductive layers deposited on the faces of the stack consisting of the plate of electro-optical material, the dielectric mirror and the photoconductive layer. Under the influence of the light, numerous pairs of electrons-holes are produced in the photoconductor the resistivity of which decreses considerable.
The phenomenon or erasing charges deposited on the electro-optical material may be made equal to a capacitor discharge via a resistor which may be non-linear, the capacitor being constituted by the plate of electro-optical material and the resistance being that of the photoconductive element. The "erased" state is obtained while the applied external voltage is zero. When with a view to a new erasing said voltage is brought to the recording value, the capacitive divider comprising in series on the one hand the capacity C constituted by the assembly plate of electro-optical material - dielectric mirror and on the other hand the capacity C' constituted by the photoconductive element causes a voltage equal to V C'/C + C' to appear at the terminals of the said plate, which voltage is sufficient to produce a uniform modulation to which a uniform background corresponds. So the contrast of any new image is influenced by the existence of said uniform background. This phenomenon appears both in the case in which the photosensitive layer is a photocathode and in the case in which it is a photoconductor. Hence, it is generally more important in said latter case.
The method of using the optic relay according to the present invention endeavours to mitigate said inconvenience. It is remarkable in that, in order to erase a recorded image, the photosensitive layer is uniformly exposed and simultaneously a voltage -V' having a sign opposite to that of the voltage V applied during the recording is applied between the control electrodes. The value of V' is such that when the voltage V is again applied for a new recording, the voltage which appears at the terminals of the useful capacity C of the assembly electro-optical material-dielectric mirror is zero. Said value of V' is equal to C'/C V.
The invention will be better understood with the aid of the following description of the development of a few particular phases of the method of using the optic relay the constitution of which is repeated, for memory, on the sole FIGURE accompanying the said description in the case in which the photosensitive layer is a photoconductor.
It will be obvious that any conception of the method of using the relay comprising the same stages of changing the state of change of the plate of electro-optical material obtained with the same means but in a different sequence form part of the invention.
In the sole FIGURE, reference numeral 1 denotes a sheet of material which is transparent to the reading light and which serves to support the assembly of the relay, 2 is a conductive layer which is transparent to the reading light and permits the connection of the contact 3 deposited on the plate 4 of electro-optical material, for example, deuterated potassium biphosphate (KDP), 5 is the multi-layer dielectric mirror permitting the reflection of the reading light emitted by the source 6.
The photoconductive layer is shown at 7, the conductive layer at 8 which is transparent to the erasing light 9 and permits the connection to the contact 10.
During the recording of an image, a direct voltage +V is applied between the electrodes 3 and 10.
By means of a cooling device not shown in the FIGURE, the KDP is brought at a temperature near its Curie point (approximately -50°C).
In a first stage of the step of using the relay, the assumed initial charging state of the plate corresponds to a non-uniform charge density which may originate, for example, from a first image erasing by projection with the aid of the light 9 in the presence of the applied potential +V.
In a second stage, in order to erase the image the voltage -V' (C'/C V) is applied between the electrodes 3 and 10 and the photosensitive layer is exposed with the aid of a beam of uniform light 9 during a time which is sufficient for the assembly KDP-dielectric mirror to be charged to the voltage -V'.
In a third stage, the recording conditions of an image are while applying the voltage +V between the electrodes 3 and 10 and projecting an image on the photosensitive layer. Due to the capacity existing between the faces of the photosensitive layer, the various points of the assembly KDP-dielectric mirror are brought at potentials which vary between 0 for the non-exposed points and +V for the most exposed points.