GB1596766A - Thermal imaging screen - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THERMAL IMAGING SCREEN (71) I, THE SECRETARY OF.

STATE FOR DEFENCE, London, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention relates generally to the detection of infra-red radiation and in particular to the reduction of thermal noise in such detection.

Infra-red imaging devices commonly employ an optical system to focus radiation from a field of view on to a radiation sensitive detector. In known arrangements for reducing background noise in infra-red detectors a cold shield is placed around the cooled detector to limit the detector view to the radiation beam transmitted from the field. This arrangement is a compromise since the aperture of the detector shield is made large enough to allow detection of a wide cone of radiation while being made narrow enough to limit detection of unwanted background radiation.

When an array of detectors formed as a matrix on a substrate is employed the effective aperture varies if the incident beam wanders across the focussing lens aperture. The viewing aperture for each detector differs and therefore the noise seen varies with the position of the incident beam.

The object of this invention is to provide an improved shield.

According to the present invention a thermal imaging screen for an infra-red radiation detector to prevent detection of unwanted radiation comprises an openended generally cylindrical body adapted to surround the detector, having an internal surface of high thermal reflectivity and low thermal emissivity formed with a plurality of indentations such that radiation incident on an indentation is reflected along a path parallel to the path of the incident radiation.

Preferably the indentations form quasicontiguous comer-cube reflectors.

The shield can conveniently be made of aluminium or alternatively can be of plastics material with a silvered inner surface. In a preferred embodiment the cylindrical shield is co-axial with the optical axis of the detector and a further plane annular screen is provided co-axial with the optical axis such that the opening in the annular screen defines the acceptance aperture for the detector, the screen being formed on the surface nearer to the detector of material of high thermal reflectivity and low thermal emissivity and having a plurality of indentations such that radiation reflected from an indentation follows a path parallel to the incident radiation.

An embodiment of the invention will be described by way of example only with reference to the drawings accompanying the Provisional Specification of which: Figure 1 is a schematic view of a raster scan imaging system; Figure 2 is a ray diagram of a known detector shield; Figure 3 is a ray diagram of a detector shield according to the invention.

Figure 1 shows thermal radiation raster scan detector wherein thermal radiation is incident on an afocal telescope comprising an objective lens 1 and eyepiece lenses 2 and 3. Light from the field of view transmitted by the telescope is incident on a mirrored surface 4 of a mirror structure or rotor 5. Rotor 5 which can have any number of facets, is as shown, a regular hexagon which is spun about a spin axis 6 at high speed by means of a motor (not shown). The spinning rotor produces the line sweeps of the raster scan. Light reflected from the rotor surfaces 4 is incident on a frame mirror 7. The frame mirror 7 is arranged to oscillate about an axis in the plane of the mirror and perpendicular to the spin axis 6 of the rotor and is synchronised with the rotor so as to provide the framing deflection for the raster scan. Light reflected from the frame mirror 7 is focussed by lens 8 on to a radiation detector 9.

In known arrangements the radiation detector 9, shown as an extended array in Figure 2, is cooled and is surrounded by a cold shield 10 having an aperture 11 symmetrical about the optical axis 12 of lens 8 and dimensioned such that all elements of the detector array 9 can "see" the whole of the lens 8. In this arrangement of the cold shield 10 elements of the detector array 9 are able to see hot electronics and lens mounts in the space surrounding lens 8 within the solid angle 13 and similar sources of unwanted background radiation through the lens 8 by means of optical paths which are within the solid angle 14 of the optical axis 12.

In the arrangement according to the invention shown in Figure 3 a cooled detector array 9 is surrounded by a thermally reflecting shield 15 which does not need to be cooled. The shield 15 is arranged to screen the detector array 9 from substantially all radiation except that coming from the viewing direction. Shield 15 is formed by a sheet multiply indented with quasi-contiguous corner cube reflectors, and has a polished silver-coloured surface to provide high reflectivity and low emissivity. The shield 15 thus made has the property of reflecting incident radiation in a path parallel to the incident path. The shield 15 is made from aluminium sheet by using a hardened steel punch to form the indentations. By this means complex indented curves may be formed. The shield is generally cylindrically shaped, being placed symetrically about the optical axis 12 of the lens 8 and of diameter approximately equal to that of lens 8.

Employing such a shield the detector 9 is only able to "see" itself, a cold body, any radiation such as 16 being reflected back towards the source along a parallel path. A second shield in the form of a plane annular disc 17 is placed intermediate between the cylindrical screen 15 and the lens 8 and adjacent to the lens 8. The surface of the annular disc 17 facing the detector 9 is reflective and indented in similar manner to shield 15. The aperture of the disc 17 being such as to completely screen the mount of lens 8 from detector 9. The disc 17 defines the solid acceptance angle to radiation of detector 9. In a raster scan detector the read-out beam transmitted through the lens 8 is found to wander. Using a screening arrangement comprising a cylindrical shield 15 and an annular disc 17, the aperture in the annular disc can be cut to precisely the required shape to fit the wander of the leadout-beam of thermal radiation so that the signal to noise ratio can be optimised.

By changing the aperture size in the annular disc 17 the f/No of the detector array can readily be altered.

Using a shield according to the invention any stray thermal radiation whether it is internally or externally generated other than the small amount from the detector itself will be reflected back towards the point of origin and will not be detected.

WHAT I CLAIM IS: 1. A thermal imaging screen for an infrared radiation detector to prevent detection of unwanted radiation comprising an openended generally cylindrical body adapted to surround the detector, having an internal surface of high thermal reflectivity and low thermal emissivity formed with a plurality of indentations such that radiation incident on an indentation is reflected along a path parallel to the path of the incident radiation.

2. A thermal imaging screen as claimed in claim 1 wherein the indentations form quasi-contiguous comer-cube reflectors.

3. A thermal imaging screen as claimed in claim 1 or claim 2 wherein the cylindrical shield is made of aluminium.

4. A thermal imaging screen as claimed in claim 1 or claim 2 wherein the cylindrical shield is made of a plastics material formed with a reflecting coating on its inner surface.

5. A thermal imaging detector system comprising a thermal imaging screen according to any one of claims 1 to 4 wherein the cylindrical shield is co-axial with the optical axis of the detector and a further plane annular screen is provided coaxial with the optical axis such that the opening in the annular screen defines the acceptance aperture for the detector, the screen being formed on the surface nearer to the detector of material of high thermal reflectivity and low thermal emissivity and having a plurality of indentations such that radiation reflected from an indentation follows a path parallel to the incident radiation.

6. A thermal imaging detector system substantially as described with reference to Figure 3 of the Drawings accompanying the Provisional Specification.

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

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. In known arrangements the radiation detector 9, shown as an extended array in Figure 2, is cooled and is surrounded by a cold shield 10 having an aperture 11 symmetrical about the optical axis 12 of lens 8 and dimensioned such that all elements of the detector array 9 can "see" the whole of the lens 8. In this arrangement of the cold shield 10 elements of the detector array 9 are able to see hot electronics and lens mounts in the space surrounding lens 8 within the solid angle 13 and similar sources of unwanted background radiation through the lens 8 by means of optical paths which are within the solid angle 14 of the optical axis 12. In the arrangement according to the invention shown in Figure 3 a cooled detector array 9 is surrounded by a thermally reflecting shield 15 which does not need to be cooled. The shield 15 is arranged to screen the detector array 9 from substantially all radiation except that coming from the viewing direction. Shield 15 is formed by a sheet multiply indented with quasi-contiguous corner cube reflectors, and has a polished silver-coloured surface to provide high reflectivity and low emissivity. The shield 15 thus made has the property of reflecting incident radiation in a path parallel to the incident path. The shield 15 is made from aluminium sheet by using a hardened steel punch to form the indentations. By this means complex indented curves may be formed. The shield is generally cylindrically shaped, being placed symetrically about the optical axis 12 of the lens 8 and of diameter approximately equal to that of lens 8. Employing such a shield the detector 9 is only able to "see" itself, a cold body, any radiation such as 16 being reflected back towards the source along a parallel path. A second shield in the form of a plane annular disc 17 is placed intermediate between the cylindrical screen 15 and the lens 8 and adjacent to the lens 8. The surface of the annular disc 17 facing the detector 9 is reflective and indented in similar manner to shield 15. The aperture of the disc 17 being such as to completely screen the mount of lens 8 from detector 9. The disc 17 defines the solid acceptance angle to radiation of detector 9. In a raster scan detector the read-out beam transmitted through the lens 8 is found to wander. Using a screening arrangement comprising a cylindrical shield 15 and an annular disc 17, the aperture in the annular disc can be cut to precisely the required shape to fit the wander of the leadout-beam of thermal radiation so that the signal to noise ratio can be optimised. By changing the aperture size in the annular disc 17 the f/No of the detector array can readily be altered. Using a shield according to the invention any stray thermal radiation whether it is internally or externally generated other than the small amount from the detector itself will be reflected back towards the point of origin and will not be detected. WHAT I CLAIM IS:

1. A thermal imaging screen for an infrared radiation detector to prevent detection of unwanted radiation comprising an openended generally cylindrical body adapted to surround the detector, having an internal surface of high thermal reflectivity and low thermal emissivity formed with a plurality of indentations such that radiation incident on an indentation is reflected along a path parallel to the path of the incident radiation.

2. A thermal imaging screen as claimed in claim 1 wherein the indentations form quasi-contiguous comer-cube reflectors.

3. A thermal imaging screen as claimed in claim 1 or claim 2 wherein the cylindrical shield is made of aluminium.

4. A thermal imaging screen as claimed in claim 1 or claim 2 wherein the cylindrical shield is made of a plastics material formed with a reflecting coating on its inner surface.

5. A thermal imaging detector system comprising a thermal imaging screen according to any one of claims 1 to 4 wherein the cylindrical shield is co-axial with the optical axis of the detector and a further plane annular screen is provided coaxial with the optical axis such that the opening in the annular screen defines the acceptance aperture for the detector, the screen being formed on the surface nearer to the detector of material of high thermal reflectivity and low thermal emissivity and having a plurality of indentations such that radiation reflected from an indentation follows a path parallel to the incident radiation.

6. A thermal imaging detector system substantially as described with reference to Figure 3 of the Drawings accompanying the Provisional Specification.