GB2187301A

GB2187301A - Thermally compensated anamorphotic optical system

Info

Publication number: GB2187301A
Application number: GB08604893A
Authority: GB
Inventors: Eric Gordon Nightingale
Original assignee: Rank Taylor Hobson Ltd
Current assignee: Rank Taylor Hobson Ltd
Priority date: 1986-02-27
Filing date: 1986-02-27
Publication date: 1987-09-03
Also published as: GB2187301B; GB8604893D0

Abstract

An anamorphotic optical system comprises one or more optical energy transmissive elements of a material sensitive to temperature variations, but constructed and positioned to compensate at least partly for the effect of such variations. The optical system may comprises first and second germanium prisms 8 and 10 arranged between a collimating lens 7 and a focusing lens 12, for directing radiation onto a detector. The parameters of the prisms, in particular, the refractive indices and the angles of the input and exit faces to the direction of the incident radiation on the first of the prisms are selected so that over a predetermined temperature range the radiation leaving the second prism may be displaced laterally but without any substantial change in direction, thus to compensate for temperature changes. A conventional scanning system 2 is shown. <IMAGE>

Description

SPECIFICATION Optical systems This invention relates to optical systems.

Optical systems may have 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 which includes one or more prisms or other elements arranged to render the apparatus anamorphotic and arranged so that the effect of temperature variations is minimized or at least partly compensated for.

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 shows part of the apparatus in more detail.

With reference to Fig. 1, a conventionai scanning system 2, indicated only diagrammatically as a block, and an optical system 3 cause 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-to-noise-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 carriers along the body of the material should be as high as practicable. To provide for increased scanning speed, the optical system 3 is made anamorphotic, with the plane containing the larger focal length being parallel to the direction 5 in which the elongated body of thermally responsive material of the detector extends i.e. the direction in which the detector is scanned. The scanning system 2 forms an image at 0' from which collimated light 6 is directed by a lens 7 towards the detector 4 through first and second prisms 8 and 10 which are inverted relative to each other and positioned immovably at difference 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.

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 characteristic 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. Unless special steps are taken, the image would, as a consequence of this, be displaced laterally of the optical axis relative to the detector 4. In accordance with this embodiment of the present invention, the special steps involve arranging the parameters of the prisms 8 and 10 such that any change in temperature results in a lateral displacement of the rays emerging from prism 10 without any significant change in the direction thereof so that the displaced rays A, B, C are still focused by lens 12 to the same point 0".Thus, Fig. 1 shows in full lines the path of ray A when the apparatus is at 20"C and in broken lines the path which this ray follows if the temperature changes to 85"C or -46"C. The following is a numerical example whereby this may be achieved.

Example Each of the prisms 8 and 10 is made of germanium of a quality that has a refractive index of 4.03024 at 85"C, 4.0032 at 200C and 3.9792 at -46"C for a wavelength of 10 micrometres. The anamorphotic ratio at 20"C is 1.607:1.

The input face 8a of prism 8 is arranged with its normal at an angle of 4.5 to the direction D which is parallel to the rays A, B and C emerging from the collimating lens 7.

The exit face 8b of the prism 8 is arranged with its normal at 12.4" to the direction D.

The input face 10a of the prism 10 has its normal at 22.742 to the direction D and the output face 10b of the prism 10 has its normal at 32.742 to the direction D, these angles being illustrated in Fig. 2.

With these parameters, the deviation in the direction of the rays leaving face 10b of prism 10 when the temperature changes from 460C to 85"C is only 0.028 milliradians.

This deviation is acceptable but may be further reduced by further refining the prism angles.

Accordingly, the invention has the advantage that compensation for temperature variations is achieved without any moving parts.

Claims

1. An anamorphotic optical system including one or more optical energy transmissive elements of a material sensitive to temperature variations, but constructed and positioned to compensate at least partly for the effect of such temperature variations.

2. Apparatus according to claim 1, said one or more elements being arranged so that over a predetermined temperature range the energy output thereby is displaced laterally without any substantial change in direction.

3. Apparatus according to claim 2, wherein the or each element is a prism.

4. Apparatus according to claim 3, comprising first and second said prisms inverted relative to each other.

5. Apparatus according to claim 4, wherein each prism is of germanium of a quality that has a refractive index of 4.03024 at 85"C, 4.0032 at 20"C and 3.9792 at -46"C for a wavelength of 10 micrometres; and wherein the angle between a given direction and the normal to the input face of the first prism is 4.5 , the angle between said given direction and the normal to the exit face of the first prism is 12.4", the angle between said given direction and the normal to the input face of the second prism is 22.742 and the angle between said given direction and the normal to the exit face of the second prism is 32.742", said given direction being the direction in which radiation is incident upon the input face of the first prism.

6. Apparatus according to any preceding claim, said one or more elements being arranged between a collimating lens and a focusing lens.

7. An anamorphotic optical system for use with thermal energy, comprising a collimating lens for collimating energy from an image, a first prism of a material whose refractive index varies with temperature arranged for receiving energy from the collimating lens, a second prism of a material whose refractive index varies with temperature arranged for receiving energy from the first prism, and a focusing lens arranged for receiving energy from the second prism, said first and second prisms having their faces arranged relative to the direction in which energy from the collimating lens travels such that a change in temperature over a predetermined range results in lateral displacement of energy leaving the second prism without any substantial change in direction thereof.

8. Infrared scanning apparatus provided with an anamorphotic optical apparatus according to any preceding claim arranged for directing image radiation to a detector.

9. An anamorphotic optical system substantially as herein described with reference to the accompanying drawings.