Adaptive attenuating spatial light modulator

• US 5,917,568 A
• Filed: 07/08/1994
• Issued: 06/29/1999
• Est. Priority Date: 07/08/1994
• Status: Expired due to Fees

• Current Assignee: The Board of Regents of the University of Colorado (University of Colorado System)
• Original Assignee: The Board of Regents of the University of Colorado (University of Colorado System)
• Inventors: McKnight, Douglas John, Johnson, Kristina Mary, Bar-Tana, Ido
• Primary Examiner(s): Sikes, William L.
• Assistant Examiner(s): NGUYEN, TIEP HUU
• Application Number:
  US08/272,726
• Time in Patent Office: 1,817 Days
• Field of Search: 359/72, 359/85, 359/55, 359/56, 359/63, 345/48, 345/99, 345/147
• US Class Current: 349/116
• CPC Class Codes: G02F 1/135 Liquid crystal cells structurally associated with a photoconducting or a ferro-electric layer, the properties of which can be optically or electrically varied G02F1/133348 takes precedence
• CPC Class Codes: H04N 23/75 by influencing optical camera components

Abstract

An optically addressed, optical relaying device placed between crossed polarizers utilizes an array of pixels made up of photodetectors, circuitry, and modulating pads, and an analog liquid crystal to modulate the intensity of an optical signal on a pixel by pixel basis. The photodetector generates a current proportional to the light detected. Once the current exceeds a certain level, the circuitry stores a charge on the modulating pad proportional to the photo-induced current. This stored charge rotates the molecules of the liquid crystal thereby rotating the polarization of the light signal. The output polarizer then attenuates the light beam according to the change in polarization.

Claims

1. Apparatus responsive to a variable intensity input optical beam for modulating the intensity of that beam in a variable manner, based on the magnitude of its input intensity, comprising:

means detecting the intensity of the input beam for generating a variable magnitude modulating signal that is related to the input intensity detected,

liquid crystal means for changing polarization of the input beam in response to the magnitude of said modulating signal, and

polarization sensitive attenuating means responsive to said changed polarization of the input beam, said attenuating means generating an output beam whose intensity varies as a function of the magnitude of said modulating signal,

whereby attenuation of the input beam is a monotonically increasing function of the intensity of the intensity of the input beam.

2. Apparatus in accordance with claim 1 wherein said attenuation is substantially constant for all input beam intensities that are below a predetermined threshold intensity level, and wherein said attenuation is a direct function of input beam intensity above the predetermined threshold intensity level.

3. Apparatus in accordance with claim 1 wherein said liquid crystal means includes a liquid crystal device selected from the group including

analog liquid crystals using nematic and chiral smectic liquid crystals, including electroclinic materials and DHF,

ferroelectric and anti-ferroelectric liquid crystals, guest-host liquid crystals, and

polymer dispersed liquid crystal light scattering materials.

4. Apparatus for relaying polarized light from a source with a variable and controlled intensity, comprising;

photodetector means responsive to light received from the source for producing an electrical output signal having a magnitude that is directly indicative of the intensity of the light thus received,

a liquid crystal for passing light received from the source, and responsive to said magnitude of said photodetector output signal for shifting the polarization of light passing therethrough in direct relation to said magnitude of said photodetector output signals, and

polarization sensitive light filtering means for receiving light which has passed through said liquid crystal, and for passing it to an output light path with an intensity that is proportioned to the amount of polarization shifting effected by said liquid crystal.

5. Apparatus in accordance with claim 4 wherein said liquid crystal further includes first and second electrical conductors positioned relative to said liquid crystal for coupling said photodetector output signal thereto.

6. Apparatus in accordance with claim 5 wherein said conductors are positioned on opposite sides of said liquid crystal.

7. Apparatus in accordance with claim 6 wherein said conductors are positioned transverse to the path of light passing through said liquid crystal, and at least one of said conductors is constructed of transparent material.

8. Apparatus in accordance with claim 7 wherein said first conductor is of transparent material and is positioned between the source of light and said liquid crystal, and

said second conductor is located on the side of said liquid crystal opposite said first conductor and is constructed so as to include light reflective material.

9. Apparatus in accordance with claim 7 wherein said conductors are both of transparent material, and

said light filtering means is positioned on the side of said liquid crystal opposite from the side on which light from the source enters said liquid crystal.

10. Apparatus in accordance with claim 4 including means establishing a threshold level relative to said photodetector means'"'"' output signal for causing said liquid crystal to pass light without polarization shift in response to a said photodetector means'"'"' output signal that is below said threshold level, and for causing said liquid crystal to shift light polarization in response to a said photodetector means'"'"' output signal that is above said threshold level, said polarization shift being in proportion to an amount of the output signal that exceeds said threshold level.

11. The method of relaying light from a variable intensity source to an output light path with a controlled intensity that is a variable function of the variable intensity of the source, comprising the steps of;

receiving light from said source,

determining the variable intensity of said received light,

shifting polarization of said received light by a variable amount that is representative of the determined variable intensity of said received light,

providing polarization responsive light attenuating means, and

delivering said polarization shifted light to said attenuating means and then to the output light path with an intensity proportioned to the variable amount of polarization shifting introduced by said shifting step.

12. The method in accordance with claim 11 which includes the step of preventing the operation of said shifting step until the intensity of said received light exceeds a predetermined threshold level.

13. Apparatus for relaying light representing a variable intensity image emanating from a source with an intensity that varies as a function of the variable intensity of the images, comprising;

a plurality of modules each positioned for receiving a variable intensity polarized respective segment of the light from the image from the source, wherein each of said modules operates to effect an additional variable polarization shift of the received polarized segment, the variable polarization shift being a function of the variable intensity of the received polarized segment, and

means for producing output light with a variable intensity magnitude that is proportional to the variable polarization shift of light introduced thereto,

each said module including,

photodetector means responsive to the light segment received from the source for producing an electrical output signal having a variable magnitude correlated to the variable intensity of the light thus received,

a ferroelectric liquid crystal for shifting polarization of light passing therethrough in proportion to a variable biasing voltage applied thereto,

modulator means responsive to said photodetector means output signal for applying the variable biasing voltage to said liquid crystal in proportion to said variable magnitude photodetector output signal, and

means coupling light passing through said liquid crystal to an input of said output light producing means.

14. Apparatus for relaying polarized light from a source with controlled intensity, comprising;

photodetector means responsive to light received from the source for producing an electrical output signal having a magnitude indicative of the intensity of the light thus received, means responsive to said photodetector output signal for producing light at an output light path with an intensity that is reduced for light from the source which has an intensity greater than a preselected threshold,

a liquid crystal responsive to said photodetector output signal for shifting the polarization of light passing therethrough in proportion to the magnitude of said photodetector output signal,

light filtering means for receiving light which has passed through said liquid crystal and for passing it to said output light path with an intensity proportioned to the amount of polarization shifting present on the light introduced thereto,

first and second electrical conductors positioned relative to said liquid crystal for coupling said photodetector output signal thereto, said conductors being positioned on opposite sides of said liquid crystal transverse to the path of light through said liquid crystal, at least one of said conductors being constructed of transparent material, and

means cyclically coupling said conductors to said photodetector means output signal during a first time period, electrically isolating said conductors for a second time period, and electrically discharging said conductors for a third time period.

15. Apparatus in accordance with claim 14 which includes means controlling said preselected threshold for adjusting said preselected threshold.

16. The method of relaying light from a source with controlled intensity, comprising the steps of;

receiving light from said source,

determining the intensity of said received light,

shifting the polarity of said received light by an amount representative of the intensity of said received light by applying a voltage to a modulating pad for a liquid crystal during a first predetermined time period, and retaining said modulating pad applied voltage for a second predetermined time period,

delivering said polarity shifted light to an output light path with an intensity proportioned to the amount of polarity shifting introduced by said shifting step,

preventing the operation of said shifting step until the intensity of said received light exceeds a predetermined threshold level, and

discharging said modulating pad during a third predetermined time period.

17. Apparatus for relaying light representing an image emanating from a source with controlled intensity, comprising;

a plurality of modules each positioned for receiving a polarized respective segment of the light from the image from the source,

means for producing output light with a magnitude proportioned to a polarity phase shift of light introduced thereto,

each said module including

photodetector means responsive to a light segment received from the source for producing an electrical output signal having a magnitude correlated to the intensity of the light segment thus received,

a ferroelectric liquid crystal for shifting the polarity of light passing therethrough in proportion to a biasing voltage applied thereto,

modulator means responsive to said photodetector means output signal for applying said biasing voltage to said liquid crystal in proportion thereto,

means coupling light passing through said liquid crystal to the input of said output light producing means,

means for producing a threshold signal, and

each said module further including means responsive to said threshold signal for shifting the polarity of light received by that said module after said photodetector output signal exceeds said threshold signal.

18. Apparatus in accordance with claim 17 which further includes means generating a duty cycle controlling signal for determining a slope of the polarity shift magnitude as a function of the incident light intensity, each said module further including means responsive to said duty cycle controlling signal for correlating the amount of polarity shifting introduced by said liquid crystal in accordance therewith.

19. Apparatus in accordance with claim 18 wherein said modulator means of each said module includes a light reflecting and electrically conductive substrate on a side of said liquid crystal opposite a side receiving light from the source,

a transparent electrically conductive layer on the side of said liquid crystal, and

means applying said photodetector output signal across said substrate and said conductive layer.

20. Apparatus in accordance with claim 18 wherein said modulator means of each said module includes transparent electrically conductive layers on opposite sides of said liquid crystal, and

said output light producing means including a polarized light filter for receiving light from said crystal and for reducing the intensity of said received light as a function of the polarity phase shift applied to said light by said liquid crystal.

21. Apparatus in accordance with claim 17 which includes control means coupled to each said module for sequentially discharging said modulating means during a first time period, charging said modulating means to said photodetector output signal during a second time period, and retaining the charge on said modulating means from said second time period throughout a third time period.