Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
  • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
  • G02F1/135—Liquid crystal cells structurally associated with a photoconducting or a ferro-electric layer, the properties of which can be optically or electrically varied
  • G02F1/1354—Liquid crystal cells structurally associated with a photoconducting or a ferro-electric layer, the properties of which can be optically or electrically varied having a particular photoconducting structure or material

Definitions

• the present invention relates to the field of optical devices and more particularly to a light valve for amplifying light.
• the mirror layer Over the mirror layer is a layer of a substance having an electric Kerr effect in which this layer has an optimum index of refraction which is variable when subjected to a variable electric field. Over this layer is a glass plate provided with an electric layer. Thus the mirror layer is located between the photo- conductive layer to which the controlling light is applied and the layer with the electric Kerr effect through which the control light from the light source passes to the mirror and is reflected by the same through the layer with the electric Kerr effect.
• This arrangement described in the Baumann et al. '380 patent provides a basis for describing the essential features of current day light valves.
• U.S. patent no. 3,592,527 (Conners et al.) describes a display device utilizing a nematic liquid crystal layer for displaying a projected radiation pattern by means of reflected ambient light. Subsequent use of light valves is described in U.S. patent no. 3,824,002 issued to Beard, in which an improved photo-activated liquid crystal light valve is described. In the Beard et al. '002 patent the electrically conductive elements of the light valve are separated from the liquid crystal layer by various insulating layers.
• prior art light valves have switching characteristics which are severely limited. For example, in a video projection device which is to display motion, an image is typically changed 24 times per second in order for the human eye to satisfactorily perceive motion. Because of the limited switching characteristics of prior art light valves, resolution and performance of light valve amplification is severely limited and curtail the available grey scale rendition.
• the present invention describes a light valve which incorporates a bilayer photoconductor structure.
• the bilayer photoconductor structure has a charge separation region and a charge generation region.
• This bilayer structure allows for optimization of the switching of a liquid crystal on an area basis.
• the transparent charge separation layer allows for the accumulation of photo-generated charges.
• the charge generation layer concentrates the light absorption in a narrow physical region, where high electric fields are present due to the presence of a n-p-n junction.
• the light valve has switching characteristics dominated by the light and dark carrier densities in the narrow reverse bias region of the photoconductor structure.
• the four parameters are (1 ) a long time delay between the generation of positive and negative free carriers and their recombination; (2) the availability of an electric field to move the charges; (3) a long drift path; and (4) a high drift mobility.
• Figure 1 is a pictorial representation of a prior art light valve.
• Figure 2 is an equivalent circuit of the prior art light valve of Figure 1.
• Figure 3 shows a physical geometry of a single layer photoconductor of the prior art light valve of Figure 1.
• Figure 4 is a pictorial representation of a light valve of the present invention.
• Figure 5 is an equivalent circuit of the light valve of the present invention.
• Figure 6 is a physical geometry of a photoconductor structure of the light valve of the present invention.
• Figure 7 is an equivalent circuit showing the circuit components which develop voltage for the light valve of the present invention.
• Figure 8a is a pictorial diagram showing a charge separation at equilibrium.
• Figure 8b is a pictorial diagram showing a charge separation when an incoming light flux is presented to the photoconductor structure of the present invention.
• the present invention describes a light valve apparatus which allows for the optimization of the switching of a liquid crystal on an area basis.
• numerous specific details are set forth, such as the overall characteristics of materials, circuit parameters, etc., in order to provide a thorough understanding of the present invention. It will be obvious, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods and structures have not been set forth in order not to unnecessarily obscure the present invention.
• FIG. 1 a physical construction of a typical prior art light valve device is shown in which the basic features required for the operation of the device is shown. Although the diagram separates the various layers, it is understood that the layers are actually formed one atop another.
• the prior art light valve 10 of Figure 1 is essentially the light valve disclosed in the aforementioned Baumann et al. patent (U.S. 2,892,380).
• a transparent electrode 12 is applied onto a glass substrate 11.
• Upon electrode 12, a photoconductive layer 13 of suitable thickness is applied.
• photoconductor 13 Upon photoconductor 13, a light block layer 14 is disposed. Subsequently, on light block 14 a mirror layer 15 is disposed.
• a modulator layer 16 is disposed onto mirror 15 followed by a second transparent electrode layer 18 and a second glass substrate 17.
• a voltage source 19 applies a voltage between the two transparent electrodes 12 and 18 in order to activate the photo-sensitive surface of the photoconductor 13 and the modulator 16.
• input light signal 20 passes through the glass substrate 11 and the transparent electrode 12 and penetrates to the photoconductor 13.
• the amount of light 20 incident on photoconductor 13 controls the electric field potential through layers 13, 14, 15 and 16. This electric field causes a corresponding variation in the index of refraction of modulator layer 16.
• a light source projects light through layers 17, 18 and 16 and is reflected from mirror 15. This light is modulated by modulator 16, which modulation is controlled by the electric field.
• modulator 16 which modulation is controlled by the electric field.
• light 20 incident on the photoconductor 13 controls output light 21 which is reflected back to a viewing surface.
• the amount of input light 20 per area incident on the photoconductor layer 13 controls the modulation provided by the modulator layer 16 to provide a corresponding per area amplified light signal 21 to be reflected back to a viewing surface.
• a typical modulator for a light valve is comprised of a liquid crystal.
• FIG. 1 An equivalent circuit 30 for the prior art light valve 10 of Rgure 1 is shown.
• This equivalent circuit 30 is provided in order to analyze the circuit response of the prior art light valve 10 of Rgure 1.
• the liquid crystal modulator 16 is represented as a resistor 31 having a resistance value R1 in parallel with a capacitor 32 having a capacitance C1.
• the capacitor 32 is shown as a variable capacitor because the capacitance C1 of most liquid crystals varies with the detailed orientation of the molecules. Also, with most present day liquid crystals, the leakage resistance depicted by resistance R1 is generally extremely high and, therefore, this leakage is negligible.
• Mirror 15 Adjacent to the liquid crystal13 is the mirror layer 15.
• Mirror 15 is represented by a parallel configuration of resistor 33 having a resistance R2 and a capacitor 34 having a capacitance of C2. Again, the leakage resistance R2 of mirror 15 is sufficiently high so that the actual leakage current in the mirror 15 is negligible.
• Adjacent to the mirror 15 and light block 14 is the photoconductor layer 13.
• Photoconductor 13 is represented by a parallel configuration of resistor 36 having a resistance R3, capacitor 37 having a capacitance of C3 and a diode 38 (D1).
• resistors 35 and 36 represent the combination of the light block 14 and the reverse bias resistance of the diode 38.
• the forward resistance of the combination is represented by resistor 35.
• diode 38 conducts effectively removing resistor 36.
• the total back resistance formed by the photoconductor 13 and light block 14 is the combined resistance of resistors 35 and 36 (resistance values of R3 + R4) but in the forward direction the combined resistance is approximately R4.
• the parameters represented by resistors 35 and 36, as well as the capacitance C3 of capacitor 37, are light sensitive and therefore are shown to have a variable value.
• the actual difference in potential between the input to the photoconductor 13 and the output from the liquid crystal 16 is shown as voltage V in Rgure 2. This voltage V is an AC voltage.
• the overall switching of the device 10 as described is due to optically stimulated differences in the current i that flows in circuit 30.
• the described circuit 30 is not dissimilar to the description provided in reference article by W. P. Bleha et al., Optical Engineering, vol. 7, no. 4, pages 371 et Seq. It is to be noted that one particular feature of this type of device is the build up of a positive field across the photoconductor 13 and light block 14 by the rectifying action of diode 38.
• the input light signal 20 When the input light signal 20 is coupled to the photoconductor 13, it is absorbed at some internal distance d from surface 40 of photoconductor 13. The absorbed light generates an electron-hole pair within the photoconductor layer 13. It is to be noted that the amount of light available to be turned into electron-hole pairs decreases exponentially with distance d.
• FIG. 3 Three hypothetical situations are shown in Figure 3.
• the absorption occurs at surface 40 so that distance d equals 0.
• example B light is absorbed toward the center of the photoconductor layer 13 and in example C, light is absorbed at the junction between the photoconductor 13 and light block 14. It is to be noted that light can be absorbed at any distance d.
• the voltage gradient between surfaces 40 and 41 is shown by curve 39.
• Figures 1 , 2 and 3 an analysis of the functioning of the prior art light valve 10 can be made.
• the average current ⁇ L is less than the average current JD flowing in the dark.
• the corresponding light and dark voltages, VL and VQ developed across the modulator 16 are above and below the switching threshold for the modulator 16.
• a liquid crystal modulator responds to the root mean square (rms) voltage and diode 38 action is not necessary to operate this type of a light valve.
• the diode action thought to enhance the switching ratio, is observed in practical devices and is expected as a consequence of the internal electronic structures that have been developed. Due to the rectifying action of the diode 38 junction, there is an average positive potential bias developed across the photoconductor 13 and light block 14.
• the forward dark current can be considered a parameter of a material and thus largely independent of junction parameters.
• the reverse dark current is typically dependant upon the rectification ratio and will be small for a good rectifying junction and the bias across the photoconductor layer 13 and light block 14 will be enhanced. It is comparable to the forward current with a poor rectifying junction with a corresponding small bias developed across the photoconductor 13 and light block 14. The question then is how to maximize . for a given light flux F.
• parameters (1) and (4) are parameters of materials wherein these parameters are selectable within narrow limits. A consequence of these parameters is the rapid fixation of one type of carrier and relative freedom of the other with a generally increased mobility.
• the following analysis will consider the positive charges to have the lower mobility and to be quickly fixed close to their point of generation. The higher mobility negative charges move in the applied field to contribute to current The remaining parameters restrict the performance of the typical light valve. Since the action of the diode 38 junction is complex, parameters (2) and (3) will be considered in the analysis without a rectifying junction first and then subsequently considered with the presence of a rectifying junction.
• Light valves that have injecting contacts on the photoconductor 13 can be particularly troublesome with persistent images and ghosts during the operation of the amplifier. These effects are associated with significant excess electron injection. Good charge separation and a high ratio of JL to io are in conflict with the desirability of having minimal persistence of amplified images. Thus, in summary, there are significant unresolved, fundamental problems associated with the conventional prior art light valve devices.
• a light valve 42 of the present invention is shown.
• the light valve 42 provides an improved performance over the prior art light valve in that light valve 42 optimizes the absorption of the incident light flux F and couples the effect of this absorption to the switching of the modulator.
• the outward basic structure of the light valve 42 of the present invention is equivalent to the prior art light valve 10 shown in Figure 1.
• glass substrates 11a and 17a, the two electrodes 12a and 18a, light block 14a, mirror 15a and modulator 16a are equivalent to those same elements 11 , 12, 14-18 of Figure 1 (A letter "a” has been appended to those elements of Figure 1 to differentiate the present invention).
• the modulator 16a of the preferred embodiment is comprised of a liquid crystal which is well known in the prior art.
• An AC voltage source 19a provides an AC voltage V to the electrodes 12a and 18a.
• photoconductor layer structure 43 of the present invention is unlike the photoconductor layer 13 of Figure 1.
• the present invention employs two distinct and separate layers 46 and 47 for the photoconductor structure 43. That is, photoconductor structure 43 is comprised of a charge separation layer 46 and a charge generation layer 47 to provide a bilayer photoconductor 43 instead of the single layer photoconductor 13 of the prior art device 10 of Rgure 1.
• the charge separation layer 46 is adjacent to the first electrode 12 while the charge generation layer 47 is adjacent to the light block 14a.
• the purpose of the bilayer photoconductor 43 is to provide a n-p-n junction on both sides of the portion of the photoconductor structure 43 which generates electron-hole pairs.
• An equivalent circuit 49 for the light valve 42 is shown in Figure 5 and a physical geometry diagram of the photoconductor structure 43 is shown in Figure 6.
• the liquid crystal and the mirror portion of circuit 49 are equivalent to those elements of the prior art light valve equivalent circuit 30. Namely, liquid crystal 16a is represented by a parallel combination of resistor 31a having a resistance R1 and a variable capacitor 32a having a capacitance C1 ; and the mirror 15a is represented by a parallel combination of resistor 33a having a resistance R2 and a capacitor 34a having a capacitance C2.
• the equivalent circuit representing the photoconductor structure 43 of the present invention is considerably different from the equivalent circuit portion of the photoconductor 13 of the prior art device 10.
• the photoconductor structure 43 is represented by a combination of two back-to-back diodes 50 and 51 (D2 and D3, respectively).
• diode 50 represents a p-n junction which occurs at surface 62 at the junction of the charge generation layer 47 and light block 14a.
• the second diode 51 represents an n-p junction at surface 61 , which is the junction of the charge generation layer 47 and charge separation layer 46.
• the two p-n junctions are formed by making the charge generation layer 47 more "p-type" than the light block 14a, as well as the charge separation layer 46.
• the equilibrium charge distribution across the photoconductor structure 43 is such that a positive layer is formed on either surface 61 or 62 of the charge generation layer 47.
• the charge generation layer 47 becomes negative in potential by the diffusion of holes into the outer layers 46 and 14a and by the diffusion of electrons into the central layer 47 until an equilibrium is established.
• a potential gradient curve 64 depicts the potential well formed by the n-p-n junction formed by the three regions 46, 47 and 14a.
• Incoming light flux 20a (F) incident on surface 60 of the charge separation layer 46 drift rapidly and with little spatial spread and minimal recombination into the central region 47, because the charge generation layer 47 is thin and highly absorbing for the light flux F.
• the positive holes of the hole-electron pair generated by the light are trapped within the potential well of region 47.
• the equivalent circuit 49 components for the photoconductor structure 43 is shown in Figure 5.
• a parallel combination of diode 50, resistor 52 having a resistance of R5 and capacitor 55 having a capacitance of C5 represent the p-n junction formed at surface 62.
• R5 and C5 represent the leakage resistance and capacitance of the diode 50.
• the n-p junction formed at surface 61 is represented by the parallel combination of diode 51 , resistor 53 having a resistance of R6 and capacitor 56 having a capacitance of C6.
• R6 and C6 depict the leakage resistance and capacitance of the diode 51.
• Resistor 54 coupled in series between the two diodes 50 and 51 represents the forward resistance of the diode 50 or 51 , whichever is conducting, plus the resistance of the light block 14a.
• Resistance R7 typically has a small value since it is always in series with a reverse bias diode 50 or 51.
• the charge separation layer 46 is represented by a capacitor 57 having a capacitance of C7. Voltage V applied to the serial loop formed by the charge separation layer 46, charge generation layer 47, the mirror 15a, light block 14a and the liquid crystal 16a provides for a loop current i which flows through circuit 49.
• the rms potential in the liquid crystal 16a is determined by the values of capacitors 57, 56, 55, 34a and 32a and the reverse leakage resistors 52 and 53. Because the impedance of capacitor 34a in a typical device is negligible at the operating frequency, capacitor 34a of the mirror can be ignored. Further, because the resistive component R2 associated with the mirror and the resistive component R1 of the liquid crystal are significantly high, their leakage can also be ignored.
• the key remaining circuit parameters are the leakage capacitances C5 and C6 of the diodes 50 and 51 , the capacitance of the liquid crystal C 1 and the capacitance C7 of the charge separation layer 46 and the reverse leakage resistances R5 and R6.
• the capacitance C1 of the liquid crystal 16a is governed by the materials parameters and cavity geometry for well established systems of practical interest and these parameters are well known in the prior art. For these well established devices, it can be considered as fixed within moderate limits.
• the capacitance C7 of the charge separation layer 46 is controlled by its thickness and dielectric properties. During operation either diode 50 or 51 is forward biased on a given voltage cycle.
• the capacitance value of the reverse bias diode of a given cycle need only be considered in the analysis since the other leakage capacitance is shorted due to the conduction of its diode.
• the charge generation region 46 is to be made sufficiently thin.
• a sufficiently thin charge generation region 47 allows junction fields to efficiently separate the photo-generated charge, retain high resolution and minimize recombination losses.
• the primary function of charge generation region 47 is to absorb the light flux F.
• the thinness of the charge generation region 47 of the preferred embodiment is approximately one micron in depth and has a fully depleted capacitance associated with this thickness and, further, the capacitance is large in comparison to the capacitance C1 of the liquid crystal.
• the expected capacitance of a reverse bias junction can be estimated by applying general semiconductor theory (see Grove. "Physics and Technology of Semiconductor Devices", Wiley, 1967, page 159).
• the depletion width in the simple case of a step junction is given by:
• W is the width (W 2 being the square of the width)
• K is the dielectric constant
• Ko permittivity of free space
• ⁇ the junction height
• q the electronic charge
• Na the dominant impurity concentration.
• W represents the charge generation region 47
• W equals 1 micron.
• a dielectric constant equals 5
• ⁇ is equal to 0.5 volts
• the impurity concentration Na approximately equal to a nominal 2.7 * 10 14 ions/cm 3 .
• the charge separation layer 46 has a nominal dielectric constant of five and can be designed with a thickness in the range of 5-20 microns.
• the minimum capacitance of the diode junction 61 and 62 is determined primarily by the thickness of the charge separation layer 46.
• the parallel capacitance C6 of diode 51 serves the function of separating the photo-excited carriers with its large internal field of approximately 5,000 volts/cm. Thus, most of the externally applied potential is dropped across capacitor 57 of the charge separation layer 46.
• an n-p-n device is a common circuit element and that the principle source of leakage is due to two components.
• the two components are referred to as the generation current within the depletion region and the diffusion term outside of the depletion region.
• the diffusion component is small in comparison to the drift component.
• Equation (2) is believed to be appropriate for practical light valves and the values of R6 and R5 are determined by this current generation mechanism.
• the incoming light flux F will generate carriers in addition to the above thermal rate which will also decay with a similar characteristic lifetime.
• the relative values of IGEN and ILIGHT determine the effective value of R6 or R5 and the voltage across the liquid crystal, namely capacitor 32a.
• the number of carriers thermally generated in the thickness L of region 47 competes with the external flux absorbed in the same region.
• the signal to noise ratio is improved when the thickness of the charge generation layer 47 relatively thin and preferrably this layer is approximately 1 micron or less.
• the band diagrams illustrate the development of the electronic charges in the charge generation region 47 of the n-p-n structure, which charges are free to flow to regions 46 and 14a at either side of the charge generation region 47. It is to be noted that carriers can be swept across the thin charge generation region 47 with little loss to recombination. Thus, electrons generated by the incoming light flux F can make many transits between the two capacitors 57 and 32a.
• the gain is limited by the ratio of the depletion capacitance (i.e., the capacitance associated with W of Equation 1 ) to the charge separation capacitance (i.e. the capacitance associated with the width of layer 46). This ratio is that of the width of the charge separation layer 46 to the width of the charge generation layer 47, corrected for the small differences in their dielectric properties. Similar consideration applies for both the positive and negative cycles of the applied potential V. On the negative half cycle, the limit is determined by the ratio of the depletion capacitance to that of the dominant capacitance, which is the liquid crystal/mirror combination of capacitors 32a and 34a.
• the physical origin of the enhanced sensitivity lies in the n-p-n transistor action and the effective increase in the lifetime of a photo-excited carrier by having the electron charges spend most of their time away from the positive holes trapped in the potential well.
• Making the value of the capacitance C7 similar to that of the liquid crystal/mirror capacitance combination allows symmetrical operation and most efficient use of the incident flux F. If the value of the capacitance C7 is much larger than the liquid crystal/mirror capacitance combination, then it can store little charge on the positive half cycle. On the other hand, if C7 is much smaller than the liquid crystal/mirror capacitance combination then the voltage drop across capacitor 57 dominates the operation of the device. It is to be noted that the current gain associated with the charge separation layer 46 is similar to the advantages obtained from injecting contacts in the conventional light valve of Figure 1 but without the debilitating persistence of images.
• Figure 8a illustrates the generation of the charges at equilibrium.
• the n- p-n structure is represented by having the p-type charge generation layer 47 bounded by the two n-type regions 46 and 14a.
• the source of the reverse leakage current is due to the generation of the electron-hole pairs.
• the dark current D is determined by the thermal generation at equilibrium.
• the diagram exemplifies a condition when an incoming flux F penetrates to the charge generation region during a positive voltage half cycle across the back-to-back diodes. Flux F causes the generation of additional electron-hole pairs wherein the holes are trapped in the p-type well while the electrons generated by flux F are free to be mobile.
• the results to be obtained from the operation of the light valve of the present invention provides for a significant advantage over the prior art light valve.
• an alternating voltage is divided between the two capacitors 57 and 32a and the reverse resistance of the back-to-back diodes are such that there is an alternating potential across the liquid crystal 16a whose rms value aligns the molecules of the liquid crystal.
• the alignment will cause a half-wavelength phase delay between orthogonal components of polarized light traversing the device.
• the reduction in the value of the leakage resistance by the light causes an increase in the rms voltage across the liquid crystal 16a which reduces the phase delay to zero.
• the light valve of the present invention can be evaluated with respect to the four switching parameters discussed previously.
• the four parameters are (1) a long time delay between the generation of positive and negative free carriers and their recombination; (2) the availability of an electric field to move the charges; (3) a long drift path; and (4) a high drift mobility.
• the light valve of the present invention strongly enhances the first parameter by removing the free carriers from the vicinity of the positive ground state. Though this does not change the dynamics of the relaxation of carriers in the depletion region, it allows for much better utilization of the input signal. Electrons are swept from the excitation region 47 to the charge storage element where the electrons can be used to contribute to the charging of the liquid crystal 16a.
• the electric field that quickly and efficiently separates the photo-excited carriers is provided by the n-p-n junctions. This condition also optimizes the second parameter in that the fields are stronger and closer to where the light is absorbed.
• parameter 3 an effective long drift path is provided in that electrons contribute to the operation of the light valve by responding to both half cycles of the alternating field. The drift path is limited by the integrated losses, principally the recombination of the free carrier as it traverses the charge generation region 47. Great flexibility is enjoyed to select materials which exhibit high mobility since layer 46 and layer 47 do not combine the photoconductor function. It is to be noted that this last parameter is not of significance to the charge separation structure which is only required to store a charge and not transport free carriers.
• Another particular advantage of the light valve of the present invention is the retention of very high quality imaging by minimizing the contribution of diffusion currents to the operation of the light valve, ft is to be noted that all significant movement of the image bearing carriers is under the influence of strong electric fields. The image is not degraded by lateral diffusion of carriers associated with steep carrier concentration gradients. In addition, the light is absorbed close to the modulating medium rather than at the input face of the device. High sensitivity is associated with the physical displacement of the charge carriers. On removal of the incoming light flux F and/or the alternating potential V, the photo-excited carriers can rapidly recombine in the narrow charge generation region 47.
• the light valve design of the present invention overcomes the basic sensitivity and speed limitations inherent in the conventional prior art light valve structures.
• a typical liquid crystal of 10 microns thickness can be constructed of materials well known in the prior art. Such materials have a dielectric constant of 10 and switch with a nominal 10 volts.
• the capacitance of such a structure is approximately 885 picofarads/cm 2 .
• the charge on such a structure to switch from 5 volts to 10 volts typically is in the range of 4.4 to 8.8 nanocoulombs per square centimeter. This requires the switching of 2.7x10 10 electrons per square centimeter. This number of electrons, at unit quantum efficiency with two electron volt (eV) photons, can be provided by 100th of a microjoule of incident flux F.
• present day light valve devices use from 10 to 100 microjoules to switch in time periods that are typically 100 milliseconds. Further, present day light valve quantum efficiencies are no more than 1.0 percent and more typically are significantly less. The light valve of the present invention offers significantly improved quantum efficiencies.
• the light valve of the present invention uses the well established advantages present in an n- p-n photo-transistor structure. There is a direct linear relationship between the light flux F and the electrons available to switch the liquid crystal. This fact additionally provides for a good gray scale when the light valve is operated with a suitable liquid crystal geometry.
• the optimization of the light valve of the present invention focuses on the signal to noise ratio (or detectivity) of a basic charge generation layer.
• the charge separation layer 46 is used to optimize the switching of charges to the liquid crystal modulator 16a under the control of photo-excited carriers in the charge generation layer 47. The switching speed of the modulator 16a can therefore be adjusted.
• the modulator speed depends on the square of the voltage which is controlled by the capacitance C7 of the charge separation layer.
• the fundamental limits are ones of signal to noise within a given bandwidth.

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• Physics & Mathematics (AREA)
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• Chemical & Material Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Liquid Crystal (AREA)
• Testing Of Short-Circuits, Discontinuities, Leakage, Or Incorrect Line Connections (AREA)

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• 1991
  • 1991-03-04 WO PCT/US1991/001467 patent/WO1991014962A1/en active Application Filing
  • 1991-03-04 AU AU74967/91A patent/AU7496791A/en not_active Abandoned
  • 1991-03-04 JP JP91506142A patent/JPH05507363A/ja active Pending
  • 1991-03-04 DE DE19914190569 patent/DE4190569T/de not_active Withdrawn

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