GB2189898A - Light valve - Google Patents

Abstract

A light valve for a thermal imager comprises a liquid-crystal cell containing a viscous liquid-crystal (18) material which is at least partly non-monomeric. The polymeric liquid-crystal material has a viscosity in excess of 5 poise, and may have a viscosity exceeding 100 poise, as compared to monomer liquid-crystal viscosities between 0.3 and 1.0 poise. The material may be a mixture of alkyl cyanobiphenyl material and a polysiloxane side-chain copolymer. This has been found to have the advantage of greatly reducing the undesirable degree of noise generated in prior-art light valves. The liquid-crystal cell includes two spaced- apart polyimide pellicles (12, 14) containing therebetween the liquid-crystal material. One pellicle (12) has a surface layer (26) which is transparent to visible light but absorbing to another wavelength interval such as infra-red. The cell may be arranged with a quarter-wave plate between a polariser and an analyser to form the light valve. The light valve may be arranged as part of a thermal imager to provide a visible-light image of a thermal scene. <IMAGE>

Description

SPECIFICATION Light valve This invention relates to a light valve, and more particularly to a light valve incorporating liquid crystal material.

Light valves are known devices, one such being described in published United Kingdom Patent Application No 2,150,387A (Reference 1). The light valve described therein is employed as part of a thermal imager. It comprises two collodion pellicles spaced apart by an annular spacer. Liquid crystal material fills the volume between the pellicles and within the spacer, an appropriate material being a mixture of nematic monomer material and chiral molecules (cholestaric-material 41, Merck Co). The light valve incorporates an analyser, and is operated at the cholesrericisotropic phase transition temperature of 28"C.

At this temperature the light valve has a rotary power of about 30 per degree C temperature change. Polarised light passing through the light valve experiences a rotation in its plane of polarisation to a degree depending on liquid crystal material temperature. The light valve incorporates a polarisation analyser arranged close to extinction of the polarised light employed. In Reference 1 the light valve is arranged to receive an infra red image of a scene. The liquid crystal material in the valve absorbs the infra red light, and accordingly the temperature of the material varies with position in accordance with the variation in image intensity over the valve. A source of polarised visible light of uniform intensity illuminates the valve, and is transmitted by the analyser to a degree varying over the valve according to local liquid crystal temperature.This produces a visible light image of the infra red scene, the modulation of the liquid crystal material temperature caused by infra red absorption being transformed into a spatial intensity modulation of visible light. The infra red radiation falling on the light valve is temporally modulated by a rotating chopper. The visible light is detected by a detector array connected to an image difference processing circuit. This circuit is arranged to subtract dark-field signals from light-field signals, these corresponding to the chopper in a closed or scene-obscuring position and an open or non-obscuring position.

Conventional liquid crystal light valves incorporating for example alkyl cyanobiphenyl material have been discovered to suffer from the disadvantage of producing an undesirable degree of noise. In thermal imaging devices in particular, the obtainable system performance is limited by the liquid crystal noise.

It is an object of the present invention to provide a liquid crystal light valve with low noise properties.

The present invention provides a light valve of the heat sensitive kind and incorporating a liquid crystal cell containing a viscous liquid crystal material which is at least partially nonmonomeric. For the purposes of this specification, a viscous liquid crystal material is one having a viscosity in excess of 5 poise, and the viscosity may be in the region of 200 poise. This compares with typical liquid crystal viscosities for monomer materials in the range 0.3 to 0.1 poise.

It has been found surprisingly that use of a viscous liquid crystal in a light valve in accordance with the invention greatly reduces thermally induced noise without appreciably affecting optical rotary power as a function of temperature. While for many purposes viscous liquid crystal materials are disadvantageously slow in response to electric fields, their thermal response remains adequate for light valve applications and they exhibit reduced noise.

In a preferred embodiment, the light valve of the invention is incorporated within a radiation sensing device such as a thermal imager or a microwave radiation pattern sensor.

The viscous liquid crystal material may comprise a mixture of one or more alkyl cyanobiphenyl materials and a viscous polymer or oligomer additive. The mixture may comprise the following three components: (1) 4-n-pentyl-4'-cyanobiphenyl, (2) 4-n-heptyl-4'-cyanobiphenyl, and (3) a polysiloxane side chain copolymer.

The light valve of the invention may include a liquid crystal cell comprising liquid crystal material retained between polyimide pellicles spaced apart by polyimide pillars. One pellicle may have a surface layer of material which is transparent to visible light but absorbing to radiation to be detected. The other pellicle may be provided with a small degree of heat sinking. The cell may comprise a two dimensional pixel area, each pixel being defined as a region between adjacent pillars. The liquid crystal cell may be arranged with a quarter wave plate between a polariser and an analyser to form the light valve.

In a preferred embodiment, the light valve of the invention is arranged as part of an imaging system which provides a visible light image from non-visible electromagnetic radiation, such as infra red or microwave radiation.

In order that the invention might be more fully understood, an embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a sectional view of a one-pixel element of a liquid crystal cell for a light valve of the invention; Figure 2 illustrates a schematic optical arrangement of a light valve of the invention; Figure 3 is a schematic block diagram of a thermal imager incorporating a light valve of the invention; Figure 4 is a graph of transmitted intensity I against polarisation analyser angle and temperature for a light valve of the invention; and Figure 5 shows graphs of rms noise against frequency for light valves of the invention and the prior art.

Referring to Figure 1, there is shown a sectional view of a onepixel element 10 of a liquid crystal cell suitable for use in a light valve of the invention. The liquid crystal cell is a 200 + 200 planar array of elements 10.

The cell element 10 comprises upper and lower polyimide pellicles 12 and 14 separated by two polyimide pillars 18 with centres 70 apart. This defines a space 18 filled with viscous liquid material to be described later. The polyimide pellicles 12 and 14 have inner surfaces which are rubbed to align the liquid crystal material in a manner well known in the liquid crystal field. A pair of subsidiary pillars 20 and 22 space the lower pellicle 14 from a transparent SrF heat sink 24 and define an intervening region 28 which may either be evacuated or gas filled. A small degree of thermal conduction takes place from the liquid crystal material in the space 18 to the heat sink 24.

The upper pellicle 12 has a layer 28 of Chromium a few hundred Angstroms thick.

The layer 28 is largely transparent to visible light but absorbing or black to infra red.

Referring now also to Figure 2, a schematic optical arrangement for a light valve is shown.

Unpolarised visible light from a source 30 passes along an optical axis 32, and is polarised by a polariser 34 in a direction indicated by an arrow 34a. The light then passes to a liquid crystal cell 38 having birefringent properties indicated by arrows 36a and 36b denoting maximum and minimum refractive index polarisation directions. The arrangement is such that the projection of arrow 34a is at 45" to arrows 38a and 38b. The cell 38 renders the light elliptically polarised, and a quarter wave plate 38 converts the light back to linear polarisation. An analyser 40 then detects the degree of polarisation rotation which has occurred and transmits a corresponding intensity of visible light.

Referring now also to Figure 3, there is shown a schematic block diagram of a light valve 50 employed in a thermal imaging device 52 of the kind described in Reference 1.

The device 52 comprises an optical stage 54 arranged to image infra red light 58 from a scene (not shown) on to the light valve 50 via reflection at a dichroic mirror 58. The mirror 58 is infra red reflective but transparent to viaible light. A chopper 80 is disposed to modulate the infra red light received by the light valve 50. The light valve 50 consists of the polariser-cell-quarter wave plate-analyser arrangement of Figure 2. Visible light from a source 82 is transmitted by the mirror 58 to the valve 50. Light transmitted by the valve 50 passes to a two dimensional array of detectors 84. Output signals from the detector array 84 pass to a signal processing circuit 88.

The arrangement described with reference to Figures 1, 2 and 3 operates as follows.

Infra red light from the scene imaged on to the light valve 50 is absorbed in the layer 28.

The absorption creates a variation in the temperature of the liquid crystal material in planes parallel to the layer 28. The variation corresponds to the variation in temperature over the imaged scene. The rotation in the plane of polarisation of light due to the liquid crystal material at 18 varies with temperature in the material. The visible light intensity transmitted by the analyser 40 is accordingly spatially modulated in planes perpendicular to the optical axis 32. This provides a visible light image of the original thermal scene for detection by the detector array 64.

Referring now to Figure 4, transmitted intensity I is shown plotted at 70 against analyser angle 0. As shown I varies as sin26 (arbitrary units). 1 varies with liquid crystal temperature T in the same manner, since polarisation rotation is approximately proportional to temperature. Arrows 72 and 74 indicate analyser angles for high contrast and high responsivity respectively. For high contrast, the analyser angle is set closer to extinction (I=0) than that required if high responsivity were to be more important.

Referring now to Figure 5, there are shown graphs 80 and 82 of the frequency variation of rms electrical noise (arbitrary units) typical of a prior art light valve and of a light valve of the invention respectively. Graph 80 was obtained using the material designated El and manufactured by BDH, a British Company. El is a eutectic mixture of BDH materials 5CB and 7CB, where: 5CB is 4-n-pentyl-4'-cyanobiphenyl, and 7CB is 4-n-heptyl-4'-cyanobiphenyl.

The prior art device in Reference 1 employed the nematic BDH material 6CB, this having the formula: 4-n-hexyl-4'-cyanobiphenyl. ;q; The electrical noise properties of materials 5Cb, 6CB and 7CB are similar.

Graph 82 was obtained using a nematic mixture of 58% of material El with 44% of a polysiloxane side chain copolymer. This material is polymer a in Table II of the article by Gemmell et al, Mol Cryst Liq Cryst 122, pp 205-218 (1985). The rms noise of graph 82 is approximately a factor of 10 less than that of graph 80 if simple difference signal processing is employed, ie subtraction of dark field from light field signals as previously mentioned. Better noise refection still can be achieved using more complex image processing or higher chopper speed.

The effect of the copolymer additive (polymer a) is to produce a mixture with a viscosity which is a factor of approximately 200 times that of the monomer mixture El. El has a viscosity in the region 0.3 to 1.0 poise near the nematic/isotropic phase transition. The monomer/copolymer mixture however has a viscosity in the region of 200 poise near this transition. Many applications of liquid crystal materials require realignment of the director in response to a change in applied elertric field.

For theae applications, an increase in viscosity would be disadvantageous since the speed of director realignment would be greatly reduced-by orders of magnitude in fact. However, birefringence is a function of the liquid crystal order parameter, and macroscopic director realignment is not required. Furthermore, it has been found that the thermal responsivity, response speed and operating temperature of the liquid crystal are very little affected by the copolymer additive. Accordingly, whereas a viscous liquid crystal material is disadvantageous for many purposes, it is advantageous for the purposes of a light valve for thermal detection.

Other types of viscous liquid crystal material may be employed in light valves of the invention. The material may be a liquid crystal monomer or a mixture of monomers with a viscous additive. The additive may be an oligomer or a polymer. Alternatively, a viscous oligomer or polymer liquid crystal may be employed. The viscosity should be at least 5 poise to obtain an appreciable noise reduction, and may be very much greater.

The light valve of the invention may be employed in applications other than thermal imaging. In particular, it may be employed to map out the electromagnetic radiation pattern of a microwave antenna. In this application, the liquid crystal material absorbs and is heated to varying degrees by the radiation, and the radiation pattern then gives rise to a visible light image using apparatus analogous to Figure 2.

Claims (8)

1. A light valve of the heat sensitive kind and incorporating a liquid crystal cell containing a viscous liquid material which is at least partially non-monomeric.

2. A light valve accordiang to Claim 1 wherein the liquid crystal material is a mixture of at least one liquid crystal monomer and a polymer or an oligomer.

3. A light valve according to Claim 2 wherein the liquid crystal material contains an alkyl cyanobiphenyl material and a polysiloxane side chain copolymer.

4. A light valve according to Claim 3 wherein the liquid crystal material is a mixture of: (1) 4-n-pentyl-4'-cyanobiphenyl, and (2) 4-n-heptyl-4'-cynanobiphenyl.

5. A light valve according to any preceding claim wherein the liquid crystal cell includes two pellicles spaced from one another and containing the liquid crystal material therebetween.

6. A light valve according to Claim 5 wherein one pellicle has a surface layer of material which is transparent to visible light but absorbing to radiation in a different wavelength interval.

7. A light valve according to Claim 8 wherein the other pellicle is adjacent to a heat sink.

8. A light valve according to any preceding claim arranged as part of an imaging system to absorb radiation in one wavelength interval and provide an image thereof by spatially modulated transmission of radiation in a second wavelength interval.