GB2230346A - Optical instrument - Google Patents

Abstract

An optical instrument incorporates an optical limiter device which is arranged to attenuate very bright spots in an image, while allowing the rest of the image to pass through with low loss. The device combines a photoconductive layer 33 with an electro-optic element 36, e.g. a liquid crystal cell. The spatial intensity of incident light modulates the absorption of light in tHe device by changing the polarization locally in the electro-optic material. <IMAGE>

Description

OPTICAL INSTRUMENT This invention relates to an optical instrument,.

According to the invention, an optical instrument comprises first and second lenses, arranged such that an intermediate image plane lies in the optical path between said lenses, and an optical limiter device located at said intermediate image plane and arranged to reduce the proportion of incident light spatially transmitted or reflected thereby according to the spatial intensity of the incident light, the device comprising a light-transmissive layer in which the polarization of linear]y-polarized light passing therethrough is varied spatially according to the spatial intensity of light incident or, the device, and polarizing means for linearly polarizing light entering the device and for reducing the proportion of incident light emerging from the device whose polarization has been varied in the light-transniissive layer.

According to one embodiment of the invention, the optical limiter device comprises a layer of a photoconductive material in electrical contact with said light-transmissive layer, said layer comprising a material whose ability to change the polarization of linearly-polarized light passing therethrough varies according to the electric field imposed thereon, the layers being located between electrodes whereby an electric field may be applied to the layers.

Preferably, the device has a sensitivity threshold such that light below a predetermined intensity level passes through, or is reflected by, the device unchanged in intensity.

The optical limiter device may include means for applying an alternating electric field across the electrodes.

The photoconductive material is preferably amorphous silicon, and the light-transmissive material may be a liquid crystal or an electro-optic material: suitably an electro-optic polymer.

The optical limiter device may be arranged to operate by transmission of light therethrough, but in some applications it may be preferable to operate by reflection. For a device which operates by transmission of light, the liquid crystal is most suitably a 90~ twisted nematic liquid crystal, while a reflectance device will typically use a 450 twisted liquid crystal.

Although nematic liquid crystal will be 'preferred for most applications, it may also be possible to use liquid crystals displaying smectic or electroclinic behaviour.

In an alternative embodiment, the light-transmissive layer comprises a material exhibiting third-order electro-optic characteristics, the photoconductor then being inessential.

The optical instrument of the invention may be, for example, a TV camera providing headlamp spatial excision for traffic surveillance purposes, or flash gun protection, observation goggles, e.g. for welding and similar processes, or surveillance instruments providing laser dazzle protection or retention of dark adaptation in the presence of intermittent illumination. Sun image or glint spatial excision may also be afforded by an instrument according to the invention.

Reference is made to the drawings, in which: Figure 1 is a schematic diagram showing the arrangement of components in an optical instrument according to one embodiment. of the invention; Figure 2 is a schematic diagram showing the arrangement of components in an optical instrument according to an alternative embodiment of the invention; Figure 3 is a diagram of an optical limiter device for use in an instrument of the type illustrated in Figure 1; Figure 4 is a graph illustrating transmitted intensity as a percentage of input intensity for a device of the type illustrated in Figure 3; Figure 5 is a diagram of an alternative form of optical limiter device to that shown in Figure 3; Figures 6a and Eb are graphs of transmission against voltage for a modified form of the device illustrated in Figure 5; and Figure 7 is a diagram of an optical limiter device for use in an optical instrument of the type illustrated in Figure 2.

Figure 1 shows an optical instrument having a pair of lenses 1 and 2 which may be, as represented in the drawing, single lenses, or may be groups of lenses or lens elements. A transmissive optical limiter 3 of the type hereinafter described with reference to Figure 3 is located at the intermediate image plane of the two lenses, such that an image of the object 4 at the object plane is formed on the photosensitive layer in the limiter 3, the image is modified so as spatially to limit the intensity of that image to below a threshold value, and a final image of the modified intermediate image is formed at the final image plane, which may be the face plate of a TV camera tube, for example.

Figure 2 shows an alternative arrangement in which a reflective optical limiter 20, as described hereinafter with reference to Figure 7, is again located at the intermediate image plane of two lenses 21 and 22, but a beam splitter 23 is positioned in the optical path between the first lens 21 and the optical limiter 20, and the second lens 22 is positioned with its optical axis at right angles to that of the first lens 21. The light rays pass from the object 24 through the beam splitter 23 and a portion pass through a partial mirror on to the photoconductive surface of the limitel- 20.

Spatial modification of the reflected image occurs, as hereinafter explained, and after reflection it the beam splitter 23, the second lens forms the rays into a final image at the final image plane.

Referring to Figure 3, a transmissive optical limiter comprises a first glass support 31 on which is formed a transparent electrode 32, for example of indium tin oxide. On the electrode surface, a thin ( < 0.5. .m) layer of amorphous silicon photoconductor 33 is formed.

The photoconductor layer 33 has a high impedance, but absorbs only a small percentage of the incident light.

A second glass support 34 is also provided with a transparent electrode 35 and is arranged to sandwich a 900 twisted nematic liquid crystal 36 between itself and the photoconductive layer 33. A low voltage a.c. drive 37 is connected across the two electrodes 32 and 35.

The a.c. is suitably produced by a simple oscillator using a low voltage d.c. supply. A suitable frequency for the a.c. is about 1 to 10 kHz, with about lkHz being preferred.

Incident light passes through an input polarizer 38 before entering the photoconductive layer 33, and output light passes through an analyser or output polarizer 39 set at 900 to the input polarizer 38.

At low light levels nearly all the applied voltage falls across the high impedance photoconductor 33 and the liquid crystal layer 36 rotates the polarization of the transmitted light which passes through the output polarizer 39. Hence a signal passing through the device under these conditions will be minimally attenuated. As the light level increases, more of the applied voltage falls across the liquid crystal 36. The liquid crystal directors begin to tilt along the direction of the applied field and, as a result the induced polarization rotation decreases. The amount of light transmitted through the analyser 39 decreases. At the highest light levels no rotation occurs and transmission is at the limit of the polarizers' isolation. Hence a high average signal is highly attenuated by the action of the liquid crystal cell, as may be seen from Figure 4.

The intensity level at which the voltage threshold of the liquid crystal is crossed can be changes by varying the applied voltage. However the intensity level range over which the transmission changes from a maximum to a minimum is more fundamentally linked to the properties of the liquid crystal cell and photoconductor layer.

An alternative embodiment is illustrated in Figure 5. The device consists of thin layers of a photosensitive material 50 and an electro-optic material 51, sandwiched together. A voltage is applied across this bilayer using transparent electrodes 52 and 53.

Crossed polarizing layers 54 and 55 are provided between the photosensitive layer 50 and the electro-optic layer 51, and externally of the second electrode layer 53, respectively. The polarizers 54 and 55 are crossed relative to each other, i.e. set with their planes of polarization at 900 to each other. The photosensitive layer 50 must absorb only a small percentage of the incident light. With low levels of incident light the voltage across the electro-optic layer 51 should be such that all wavelengths are virtually fully transmitted.

If a bright beam is shone onto one section of the photosensitive area the impedance of this region will change. This will lead to a change in the voltage across the portion of the electro-optic element adjacent to where the light is incident. This voltage change cause the transmission of the electro-optic element to decrease.

It may be possible to use a number of different types of optical attenuators. However, any sultable one must have the following characteristics. Its transmission for low light levels must be as high as possible for all wavelengths over which the device operates. As the light intensity on a particular area increases, the impedance of the photo-sensitive area and hence the voltage across the electro-optic polymer will change. The device would have to be designed so that at a high enough light intensity the voltage across the electro-optic polymer reaches a saturated value. The transmission of the electro-optic element at this point must be as neai to zero as possible, for all wavelengths, in order to block the intense light beam.

Ideally, the transmission of the electro-optic element would be wavelength independent. Unfortunately, most designs of optical attenuators using the electro-optic effect have a highly wavelength dependent transmission. This preferred embodiment of the invention uses a variable birefringence Pockels cell, between crossed polarizers 54 and 55. The fractional transmission of a Pockels cell is give by: P=sin2 k. .nd 2 wherein: k is the wave vector of the light ..n is the birefringence of the electrooptic element d is the thickness of the electro-optic element The birefringence changes with the applied voltage.

The only condition where the transmission can be the same for all wavelengths is where the birefringence is zero and therefore P=0 for all k. Thus, ideally, the device is designed so that the saturated voltage across the electro-optic element gives zero birefringence.

At low light levels, there will be finite birefringence and so the transmission will depend on wavelength. The device will only be able to work over a limited range of wavelengths. The device could be designed so that with low light levels the birefringence gives 100% transmission in the middle of the wavelength range. The transmission at the extreme wavelengths of this range should still be quite high. For example, consider a device that is required to operate over the visible region (400-700 nm). If this device is constructed so that it theoretically has 100% transmission at 509 nm it will have a transmission of 83% at both 400 nm and 700 nm. The actual transmission figures will be reduced by losses in the polarizers and photosensitive element.

The device performance will be compromised if the material is dispersive, i.e. the birefringence varies with wavelength. Provided this variation is not too great, it will only slightly affect the device properties at low light levels. However a more significant limitation will occur if the birefringence, when the device was saturated with light, is not zero over the whole wavelength range. In this case there would be a limit to the amount that some wavelengths would be attenuated.

In the device described with reference to Figure 5, the voltage across the electro-optic element is determined in a straight-forward way by the light intensity on the photosensitive area. The problems encountered with this design can be solved by passing light through the electro-optic element first. in this arrangement there is feedback between the two halves of the device. This is because the intensity of the light reaching the photosensitive element will depend on the voltage across the electro-optic element. However, this voltage depends on the intensity of the light reaching the photosensitive element. We therefore need to consider how the following functions relate: T(V) Percentage transmissions of light as a function of voltage across electro-optic element V(T) Voltage across the electro-optic element as a function of the percentage of light it transmits.

Both of the functions can be plotted on a VT graph as shown in Figure 4. The point where the two functions intersect will be the operating point of the device.

The concept can probably best be understood by considering a simple example. Suppose the electro-optic element is made from a linear electro-optic material which is used to form a Pockels cell. If we let the transmission be a maximum at zero voltage then 2 T = cos (aV) where a is a constant which depends on the electro-optic coefficient, wavelength and refractive index.

If the photosensitive element is a photodonductor, its impedance will decrease with increasing light intensity. This will lead to an increase in voltage across the electro-optic element.

The intensity I incident on the photoconductor will equal I T where 0 I = intensity incident on the device T = 0 transmission of electro-optic element We can see that the voltage across the electro-optic polymer, V, will increase with I. The exact relationship between V and I will be quite complicated. However the principle of operation can be understood if we assume a linear relationship.

Therefore let V = b +CI = b + CIoT where b and c are constants Curves of V(T) and T(V) for different values of lo are plotted out in Figure 6. It can be seen that as 10 increases, the percentage transmission decreases. As 10 tends to infinity, it is obvious that the percentage transmission must tend to zero. At another wavelength the form of the T(V) and V(T) will be slightly different. This is because: The transmission of a Pockels cell inherently depends on wavelength The electro-optic coefficient and sensitivity of the photoconductor may vary with wavelength A new set of curves could be drawn and they would be slightly different. However, the same principle would still apply, namely increasing the intensity leads to a steady decrease in the proportion of incident light transmitted through the device.

For light incident at a different angle, the form of T(V) will be different. Again this will not affect the basic functioning of the device, but the percentage of light transmitted for a certain input intensity will change slightly.

Thus having the light incident on the electro-optic element first will probably increase the range of wavelength and incidence angle over which the device could operate. However, since the modulation of the light must occur before it reaches the photosensor, it will be necessary to place the analyser between the electro-optic and photosensing element.

Referring to Figure 7, an alternative construction of the device shown in Figure 1 includes a mirror layer 70, formed of multiple alternate layers of SiO2 and TiO2, or MgF and ZnS, or Cryolite and ZnS, and arranged to allow a small proportion of the incident light which, in this embodiment, falls first on the liquid crystal layer 36, to pass therethrough into the photoconductive layer 33. The light arriving in the photoconductive layer is sufficient. to change its impedance locally where the intensity exceeds a threshold level, thereby changing the local electric field in the liquid crystal and in turn changing the degree of polarization of the light passing therethrough. The liquid crystal in this embodiment is a 450 twisted liquid crystal, and a single polarizer 7 is used for both the incident and output light.

In Figure 7, corresponding parts are given the same reference numerals as in Figure 3. However, it should be noted that the support 31 and the electrode 32 need not, in this case, be transparent.

In use, incident light is plane polarized by the polarizer 71, passes through the liquid crystal 36, is reflected by the mirror 70, except for a small proportion which passes through the mirror into the photoconductive layer 33, passes again through the liquid crystal 36, and re-emerges through the polarizer 71. The liquid crystal is arranged so that the light below the threshold intensity is not changed in its polarization in passing therethrough. When light exceeding the threshold intensity reaches the photoconductive layer 33 through the mirror 70, as hereinbefore explained, the impedance of the layer is changed locally, causing a change in the electric field in the corresponding area of the liquid crystal 36.

This in turn causes the molecules in the liquid crystal to change in their orientation, thereby changing the polarization of the light where it locally exceeds the threshold intensity. Thus, light entering the liquid crystal in these areas is changed in its polarization twice, i.e. before and after reflection, such that the plane of polarization on again arriving at the polarizer 71 is no longer aligned wit that of the polarizer, and so the light is locally attenuated. The degree of attenuation depends upon the degree of rotation of the liquid crystal molecules, which in turn depends upon the intensity of the light (above the threshold), with the maximum attenuation effect being achieved when a double pass through the liquid crystal cell produces a rotation of the polarization of the light of 900.

It will be appreciated that, in the arrangement described with reference to Figure 2, the beam splitter may be a polarizing beam splitter, thus avoiding the necessity for the polarizer 71 in the device 20, described in more detail hereinbefore with reference to Figure 7.

Claims (10)

1. An optical instrument comprising first and second lenses arranged such that an intermediate image plane lies in the optical path between said lenses; and an optical limiter device located at said intermediate image plane and arranged to reduce the proportion of incident light spatially transmitted or reflected thereby according to the spatial intensity of the incident light, the device comprising a iight-transmissive layer in which the polarization of linearly-polarized light passing therethrough is varied spatially according to the spatial intensity of light incident on the device, and polarizing means for linearly polarizing light entering the device and for reducing the proportion of incident light emerging from the device whose polarization has been varied in the light-transmissive layer.

2. An instrument according to Claim 1, wherein the optical limiter device comprises a layer of a photoconductive material in electrical contact with said light-transmissive layer, said layer comprising a material whose ability to change the polarization of linearly-polarized light passing therethrough varies according to the electric field imposed thereon, t.hfo layers being located between electrodes whereby an electric field may he applied to the layers.

3. An optical instrument according to Claim 2, including means for applying an alternating electric field across the electrodes.

4. An instrument according to Claim 2 or 3, wherein the photoconductive material is amorphous silicon.

5. An instrument according to Claim 2, 3 or 4, wherein the light transmissive material is a liquid crystal.

6. An instrument according to Claim 2, 3 or 4, wherein the light-transmissive material is an electro-optic material.

7. An instrument according to any of Claims 2 to 6, wherein the optical limiter device comprises a mirror layer interposed between the photoconductive layer and the light-transmissive layer, the mirror transmitting therethrough to the photoconductive layer a minor portion of the light falling thereon sufficient for incident light above a predetermined intensity to change the conductivity of the photoconductor.

8. An instrument according to any of Claims 2 to 6, wherein the photoconductive layer of the optical limiter device is substantially transparent to incident light.

9. An instrument according to Claim 1, wherein said light-transmissive layer comprises a material exhibiting third-order electro-optic characteristics.

10. An optical instrument, substantially as described, or as illustrated in the drawings.