GB2047424A - Infra red imager having rotatable prism - Google Patents

Abstract

An infra red imager comprising a lens system 1 for focussing infra red radiation from a scene onto a rotatable refracting prism 3 which sweeps the scene across a parallel detector array 4. Output from the detectors is used to intensity modulate a display on a cathode ray tube 9 and hence provide a visible display of the thermal image. The prism has pairs of parallel faces angled to provide both a scanning of the scene in bands and an interlacing. An optional aperture stop 2 between the lens 1 and prism 3 allows the lens to be designed to correct for astigmatic error produced by the rotating prism. <IMAGE>

Description

SPECIFICATION Infra red imager This invention relates to an infra red (IR) imager.

In these imagers, infra red radiation is focussed onto a detector whose output is used to modulate a visible beam, e.g. a cathode ray tube, thereby forming a visual image of the infra red (thermal) scene. Such imagers find many uses e.g. providing a thermal picture of buildings to show heat losses.

One form of imager uses a rotating prism to sweep the thermal scene onto a detector. This detector may be a single detector in which case the prism sweeps out a line of a visual display at a time and an oscillating mirror causes a vertical scanning of the scene.

Another form of imager uses a plurality of detectors in a line each detector output forming a line of information on the CRT, i.e. a parallel scanned system. Usually, in this parallel system, a vertical scan e.g. by an oscillating mirror, is still used and the display is then observed as a series of horizontal bands.

In one system the rotating prism is a reflective prism and successive faces are at different angles to provide the required vertical scan, the number of bands being equal to the number of differently angled faces.

According to this invention an infra red imager includes a lens system for focussing infra red radiation from a scene onto a parallel array detector, whose output is used to intensity modulate a display on a cathode ray tube and hence provide a visible display, and further includes a rotatable refractive prism for sweeping parts of the scene across the detector array, the prism having a plurality of pairs of parallel spaced faces with each pair at a different angle to sweep out a different band of the scene, the different angles also providing an interlacing in each band on each complete revolution of the prism.

The display may be arranged in three bands in which case the prism has six angled faces.

The prism may be of germanium or any other suitable infra red transparent material e.g. a glass as described in U.K. Patent Application No. 27,359/76. Serial No. 1546828.

Preferably an aperture stop is provided intermediate the lens and the prism so that astigmatic errors in the rotating prism can be cancelled or reduced as the image moves across the appropriately designed lens.

The invention will now be described, by way of example only with reference to the accompanying drawings of which: Figure 1 shows in diagrammatic form an infra red imager; Figures 2a, b show a plan and cross section of the prism of Fig. 1; Figure 3 shows a detector array; Figure 4 shows a three band display; Figure 5 shows interlacing on part of the display of Fig. 4.

As seen in Fig. 1 germanium lens 1 focusses an infra red scene through an aperture stop 2, a rotatable prism 3, onto a detector array 4. The detector array, Fig. 3, may be a row of 32 elements 5 of cadmium mercury telluride alloy material, and cooled by liquid nitrogen cooler 6.

The detector 4 outputs, one from each element 5, are amplified 7, passed through a multiplexer 8 for subsequent display on a cathode ray tube 9. A timing circuit 10 receives an input from the prism 3 shaft to control the multiplexer 8 and CRT 9 timing circuits. The prism 3 is rotated by a motor not shown.

In operation infra red radiation is focussed onto the detector 4 and the rotating prism 3 sweep the image across the detector 4 in three bands, i.e. parallel scanning, indicated on Fig. 4 as an upper band U, a middle band M, and a lower band L. To provide such a banded sweep the six faces F, to F6 of the prism 3 are angled as follows. If the parallel opposing faces F1 and F4 were perpendicular to the axis of rotation then the middle band of the scene would be swept across the detector array 4 twice per revolution. This would result in a stripped appearance on the CRT due to the small 'dead space' between the detector elements 5, Fig. 3. To avoid this the two sweeps are interlaced, that is one sweep is of the scene a 41 element pitch up and the other sweep is of the scene a 4 element pitch 4 down.This is achieved by tilting the faces F1, F4 three minutes from the perpendicular so that in one revolution of the prism two partly overlapping sweeps of the scene centre band are made. The arrows on Fig. 2a indicate which direction the upper edge of the face is moved from its nominal position to provide interlace.

The parallel faces F2, F5 are angled nominally + 5 and - 5" respectively to provide a scan of the upper and lower band alternately once per revolution of the prism. To provide the interlace F2 is angled 3' more than 5" and F5 3' more than - 5'. Similarly for the parallel faces F3 and F5 face F3 is angled - 5" less 3' and F5 5" less 3'.

If the top sweep in each band is a and the bottom b then for clockwise revolution of the prism the scene is written on the CRT in the following order; Mb, Lb, Ub, Ma, Ua, La.

Thus in one complete revolution all three bands are written once plus a further three bands interlaced with the first three as shown in part in Fig. 5 where each detector 5 output on the CRT 9 is indicated by a' and b' shown side by side for ease of illustration.

As seen in Fig. 1 the aperture stop 2 is located between the lens and prism. This ensures that radiation is focussed onto the detector from different parts of the lens as the prism rotates and allows for aberrations in the lens to counter aberrations caused by refraction through the rotating prism.

Detailed dimensions vary with the task required to be performed by the imager. In one example the lens diameter was 68mm, the prism was 25.48mm between faces, the detector element pitch of 62.5ELm, the lens focal length was 114.6mum, lens aperture 50mm, prism rotation 1500rum.

In a modification additional detector elements are used to form a serial parallel scan system. Each detector 5 in the parallel array 3 is replaced by a line of detectors. As a result each line of an observed, on the CRT 9, image is formed from the integrated output of line of detectors.

Claims (5)

1. An infra red imager comprising a lens system for focussing infra red radiation from a scene onto a parallel array detector, whose output is used to intensity modulate a display on a cathode ray tube and hence provide a visible display, characterised by a rotatable refractive prism for sweeping parts of the scene across the detector array, the prism having a plurality of pairs of parallel spaced faces with each pair at a different angle to sweep out a different band of the scene, the different angles also providing an interlacing in each band on each complete revolution of the prism.

2. The imager claimed in claim 1 and further characterised by an aperture stop intermediate the lens and prism.

3. The imager claimed in claim 2 wherein the prism has six angled faces for sweeping the scene in the three bands.

4. The imager claimed in claim 2 wherein the prism is a germanium prism.

5. An infra red imager constructed, arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.