GB2182457A - Compensated optical mounting - Google Patents

Abstract

In an optical system producing an image, lateral displacement of the image relative to the means for detecting it is compensated for by positioning a movable optical element, for example a mirror, in the system and arranging for it to move so as to provide the necessary compensation. This may be achieved by mounting the movable element on, or coupling it to, a thermally responsive material which changes shape or size in response to temperature variation. Fig. 2 shows an anamorphatic system where radiation incident on skewed prisms (8) and (10) is focussed by lens (12) and reflected by mirror (14) on to detector (4) to achieve thermal compensation, mirror (14) forms part of a flanged mounting (18) which is positioned by PTFE collar (24) secured to both flange and casing. The expansion of the collar is such as to achieve the necessary compensation. <IMAGE>

Description

SPECIFICATION Optical systems This invention relates to optical systems.

Optical systems may heve to be used in environments in which temperature variations may be encountered that are sufficient to affect the optical system. For example, a particular problem arises in an optical system for a thermal imager in which various elements such as lenses or prisms are made of material such as germanium which has a high thermal coefficient of refractive index so that significant deterioration in the performance of the optical system arises as a consequence of temperature variations. The invention is especially applicable to a thermal imager having an optical system which is anamorphotic, this being achieved preferably by the provision of prisms in the optical path.

In one aspect, the invention provides optical apparatus provided with means for compensating, at least partly, for the effect of temperature variations on the apparatus.

The invention is described further by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of optical apparatus according to an embodiment of the invention; and Figure 2 is section through part of an apparatus constructed in accordance with an embodiment of the invention.

With reference to Fig. 1, a conventional scanning system 2, indicated only diagrammatically as a block, causes an image to be scanned across a detector array 4 which comprises one or more sprite detectors. As is well known, a sprite detector comprises a body of electrothermally responsive material which is elongated in the direction in which the image scans across the detector and an electric field is applied to this body of material so as to cause charge carriers created therein to move, along the direction of scan, in synchronism with the scanning image. This provides integration thereby increasing signal-tonoise-ratio. As the generated charge carriers drift along the body of material of the sprite detector, they tend to diffuse and this diffusion impairs resolution.In order to minimize the diffusion, the scanning speed and the speed of movement of the charge carrier along the body of the material should be as high as practicable. To provide for increased scanning speed, the optical system forming the images is made anamorphotic, with the plane containing the larger focal length being parallel to the direction in which the elongated body of thermally responsive material of the detector extends i.e. the direction in which the detector is scanned. In the embodiment of Fig. 1, the scanning system 2 directs coilimated light 6 towards the detector 4 through first and second prisms 8 and 10 which are inverted relative to each other and postioned at different angles to the optical path in order to render the system anamorphotic as described.A lens 12 focuses the image onto a plane in which the detector 4 is located and a mirror 14 is positioned between the lens 12 and detector 4 for reflecting the light from the lens onto the detector.

In thermal imaging systems, the optical elements are generally made of material, such as germanium, which has a high coefficient of thermal change of refractive index but a low characteriatic of dispersion. Thus, the prisms 8 and 10 may be made of such a material.

As a consequence of temperature changes, therefore, the angle through which the thermal energy is refracted by the prisms 8 and 10 may change as indicated by broken lines 16.

As a consequence of this, the image would be displaced laterally of the optical axis relative to the detector 4. However, to compensate for this, and retain the image in its correct position, the mirror 14 is movably mounted and its position is controlled by a temperature responsive means, not shown in Fig. 1. The mirror may be at any point within the optical sub-system.

A preferred structure for achieving this compensation is illustrated in Fig. 2, in which the elements 4, 8, 10, 12 and 14 are shown mounted in a housing 16. The mirror 14 is constituted by a specularly reflective 45Q surface on an aluminium cylindrical member 18 which is tightly but slideably fitted within a circular aperture 20 in the housing 16. The outside end of the member 18 is provided with a flange 22. A spacer 24, in the form of a collar, is located between the flange 22 and the housing 16. The spacer 24 is secured to both the housing 1 6 and flange 22, for example by studs. The housing 16 would normally contain a gas such as nitrogen. To prevent escape of such gas through the aperture 20, a sealing ring 26 is provided around the member 18.

The spacer 24 is made from a material, such as PTFE, whose thickness and coefficient of thermal expansion is related to the thermal refractive index change of the prisms 8 and 10 so that as temperature varies, thereby affecting the prisms 8 and 10, the member 18 is moved to the right or the left as seen in Fig. 2 by expansion or contraction of the spacer 24 to adjust the position of the mirror 14 to compensate for the effect of temperature changes on the prisms 8 and 10.

Various modifications are possible within the scope of the invention. For example, instead of moving the mirror, the prisms themselves could be moved or other elements in the optical system could be moved to compensate for the temperature variation.

Further, instead of utilizing the thermal expansion and contraction of a material for mov ing the mirror or other element, it would be possible to provide temperature sensing means from which an electrical signal is derived and the required compensation achieved electro-mechanically.

Although the invention is particularly applicable and particularly valuable in correcting for image position variations arising from thermal effects on anamorphotic thermal imaging optical systems, it may also have other uses. For example, it may be used to compensate for shifting of the image laterally of the optical axis resulting from thermal expansion and contraction of the mountings of elements such as scanning devices, for example polygonal or oscillating mirrors, or servo control mechanisms for such scanning devices.

Claims (12)

1. Optical apparatus comprising an optical system for producing an image and means for compensating at least partly for displacement of the image laterally of the optical axis due to thermal effects.

2. Apparatus according to claim 1, wherein said compensating means is thermally responsive.

3. Apparatus according to claim 2, wherein said means comprises a member made of a material whose shape or size changes in response to temperature variations, said member being coupled to a movable element in said optical system.

4. Apparatus according to claim 3, wherein said member is mechanically coupled to said element.

5. Apparatus according to claim 3 or 4, wherein said element is a mirror.

6. Apparatus according to any preceding claim, wherein said optical system is anamorphotic.

7. Apparatus according to any of claims 1 to 6, wherein said optical system is operable to scan a thermal image across a thermal detector for producing a signal representing said image.

8. Apparatus according to claim 7 as dependent upon claim 6, wherein said optical system comprises at least one prism rendering said system anamorphotic.

9. Apparatus according to claim 8, wherein said detector is a sprite detector and the longer focal length of said anamorphotic optical system is in a plane parallel to the direction of scanning across said detector.

10. Optical apparatus including an optical system and means for compensating at least partly for an effect of temperature variations on said optical system, said compensating means comprising a movable element and a member whose shape or size changes in response to temperature variations coupled to said movable member for effecting movement thereof.

11. Optical apparatus substantially as herein described with reference to Fig. 1 of the accompanying drawings.

12. Optical apparatus substantially as herein described with reference to Fig. 2 of the accompanying drawings.