GB2147172A - Imaging or like apparatus - Google Patents

Abstract

Where detectors, for example in thermal imaging apparatus, have been necessarily periodically irradiated at a predetermined level to provide a grey level reference this has hitherto been effected by periodically moving an opaque member in the form of a blade onto the optical path to intercept the radiation normally incident upon the detector. Since it is a mechanical arrangement, the blade is difficult to operate in the precise time intervals; thus the present invention comprises the replacement of the blade and its drive mechanism with a liquid crystal assembly 1, 2, 3. The liquid crystal assembly becomes diffusive when a voltage is applied giving a relatively uniform level of radiation at the detector 5 which may be used to provide a grey level reference. <IMAGE>

Description

SPECIFICATION Imaging or like apparatus This invention relates to imaging or like apparatus (eg. thermal imaging apparatus) having detector means upon which electromagnetic radiation from an object or scene is directed and which detector means requires to be periodically irradiated art a predetermined low level, preferably related to the radiance of the object or scene, in order to provide a reference, for example a grey level reference.

Hitherto, this irradiation has been achieved by periodically moving an opaque member, for example in the form of a blade, into the optical path to interrupt the radiation normally incident upon the detector means. Such an arrangement has various shortcomings. Since it is a a mechanical arrangement, it is difficult to drive the blade at precise time intervals especially during conditions of vibration and acceleration and where there are limitations on space for the drive arrangement. Moreover, the opaque blade gives a radiance which is unrelated to the object or scene radiance.

It is therefore an object of the present invention to overcome these shortcomings in a relatively simple manner.

Accordingtothe present invention, imaging apparatus having detector means upon which electromagnetic radiation from a source is directed and which detector means requires to be periodically exposed to a predetermined radiance level, includes liquid crystal means located in between the detector means and the source, the liquid crystal means being changeable from a condition in which there is substantially no effect on the radiation directed onto the detector means, to a condition or conditions in which the radiation is diffused, and means to effect such change in condition.

Thus, any mechanically movable parts are eliminated; it is found that in its diffuse condition the liquid crystal means provides a radiance which is related to the object or scene radiance. The liquid crystal means also destroys the structure of the object or scene within the radiation giving a relatively uniform irradiance on the detector means.

One embodiment ofthe invention is illustrated by way of example with reference to the accompanying drawing.

In this embodiment, a liquid crystal assembly is incorporated in thermal imaging apparatus, the liquid crystal assembly comprising a liquid crystal layer 1.

sandwiched between two germanium discs 2 and 3 respectively. In the normal quiescent condition, the layer 1 is clear. When a voltage is applied across the discs, the layer 1 becomes diffuse to an extent depending upon the magnitude of the voltage.

The imaging apparatus itself comprises focussing means 4 by means of which incoming radiation from an object or scene is focussed onto detector means 5, for example an array of detector elements. The detector mears is mounted within a vacuum encapsulation 6 and cooled to its operating temperature by cooling means 7.

Awindowisformed in the encapsulation 6; this, when of germanium or similar material, thus can provide one layer2 of the liquid crystal sandwich. The liquid crystal layer 1 and its two sandwiching layers 2 and 3 lie external to the encapsulation 6 but between it and the focussing means 4. Leads 8,9 allow a voltage to be applied across the germanium discs 2 and 3 of the liquid crystal sandwich.

A suitable liquid crystal layer 1 is of a thickness of several wavelengths of the radiation to be detected. It has a high transmission to those radiation wavelengths forwhich the apparatus is to be used when in the quiescent condition.

When, periodically, avoltage is applied, the layer 1 scatters the radiation to an extent depending on the magnitude ofthe voltage. If desired, more than one voltage level may be used to effect different degrees of scattering sothatthe gains of the channels of any amplifier means receiving the outputs ofthe detector means may be matched as well as having the D.C.

output levels restored.

Although the sandwiching layers 2 and 3 have been specified as being formed of germanium and are therefore electrically conductive, other infra red transmissive materials can be used. Naturally, ifthese, like zinc sulphide and sapphire, are not electrically conductive, then some form of treatment is necessary to render them so.

1. Imaging apparatus, having detector means upon which electromagnetic radiation from a source is directed and which detector means requires to be periodically exposed to a predetermined radiance level, including liquid crystal means located in between the detector means and the source, the liquid crystal means being changeable from a condition in which there is substantially no effect on the radiation directed into the detector means, to a condition or conditions in which the radiation is diffused, and meansto effect such change in condition.

2. Imaging apparatus according to claim 1 wherein the detector means includes a radiation transparent window, the liquid crystal means comprises a liquid crystal layer sandwiched between two radiation transparent layers, and one of said layers comprises the radiation transparent window.

3. Imaging apparatus according to claim 2 wherein the transparent layers are formed of germanium.

4. Imaging apparatus substantially as described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Imaging or like apparatus This invention relates to imaging or like apparatus (eg. thermal imaging apparatus) having detector means upon which electromagnetic radiation from an object or scene is directed and which detector means requires to be periodically irradiated art a predetermined low level, preferably related to the radiance of the object or scene, in order to provide a reference, for example a grey level reference. Hitherto, this irradiation has been achieved by periodically moving an opaque member, for example in the form of a blade, into the optical path to interrupt the radiation normally incident upon the detector means. Such an arrangement has various shortcomings. Since it is a a mechanical arrangement, it is difficult to drive the blade at precise time intervals especially during conditions of vibration and acceleration and where there are limitations on space for the drive arrangement. Moreover, the opaque blade gives a radiance which is unrelated to the object or scene radiance. It is therefore an object of the present invention to overcome these shortcomings in a relatively simple manner. Accordingtothe present invention, imaging apparatus having detector means upon which electromagnetic radiation from a source is directed and which detector means requires to be periodically exposed to a predetermined radiance level, includes liquid crystal means located in between the detector means and the source, the liquid crystal means being changeable from a condition in which there is substantially no effect on the radiation directed onto the detector means, to a condition or conditions in which the radiation is diffused, and means to effect such change in condition. Thus, any mechanically movable parts are eliminated; it is found that in its diffuse condition the liquid crystal means provides a radiance which is related to the object or scene radiance. The liquid crystal means also destroys the structure of the object or scene within the radiation giving a relatively uniform irradiance on the detector means. One embodiment ofthe invention is illustrated by way of example with reference to the accompanying drawing. In this embodiment, a liquid crystal assembly is incorporated in thermal imaging apparatus, the liquid crystal assembly comprising a liquid crystal layer 1. sandwiched between two germanium discs 2 and 3 respectively. In the normal quiescent condition, the layer 1 is clear. When a voltage is applied across the discs, the layer 1 becomes diffuse to an extent depending upon the magnitude of the voltage. The imaging apparatus itself comprises focussing means 4 by means of which incoming radiation from an object or scene is focussed onto detector means 5, for example an array of detector elements. The detector mears is mounted within a vacuum encapsulation 6 and cooled to its operating temperature by cooling means 7. Awindowisformed in the encapsulation 6; this, when of germanium or similar material, thus can provide one layer2 of the liquid crystal sandwich. The liquid crystal layer 1 and its two sandwiching layers 2 and 3 lie external to the encapsulation 6 but between it and the focussing means 4. Leads 8,9 allow a voltage to be applied across the germanium discs 2 and 3 of the liquid crystal sandwich. A suitable liquid crystal layer 1 is of a thickness of several wavelengths of the radiation to be detected. It has a high transmission to those radiation wavelengths forwhich the apparatus is to be used when in the quiescent condition. When, periodically, avoltage is applied, the layer 1 scatters the radiation to an extent depending on the magnitude ofthe voltage. If desired, more than one voltage level may be used to effect different degrees of scattering sothatthe gains of the channels of any amplifier means receiving the outputs ofthe detector means may be matched as well as having the D.C. output levels restored. Although the sandwiching layers 2 and 3 have been specified as being formed of germanium and are therefore electrically conductive, other infra red transmissive materials can be used. Naturally, ifthese, like zinc sulphide and sapphire, are not electrically conductive, then some form of treatment is necessary to render them so. CLAIMS

1. Imaging apparatus, having detector means upon which electromagnetic radiation from a source is directed and which detector means requires to be periodically exposed to a predetermined radiance level, including liquid crystal means located in between the detector means and the source, the liquid crystal means being changeable from a condition in which there is substantially no effect on the radiation directed into the detector means, to a condition or conditions in which the radiation is diffused, and meansto effect such change in condition.

2. Imaging apparatus according to claim 1 wherein the detector means includes a radiation transparent window, the liquid crystal means comprises a liquid crystal layer sandwiched between two radiation transparent layers, and one of said layers comprises the radiation transparent window.

3. Imaging apparatus according to claim 2 wherein the transparent layers are formed of germanium.

4. Imaging apparatus substantially as described with reference to the accompanying drawing.