GB2248310A

GB2248310A - Thermal imaging apparatus

Info

Publication number: GB2248310A
Application number: GB8728795A
Authority: GB
Inventors: William Thomas Moore
Original assignee: Rank Pullin Controls Ltd; Rank Xerox Ltd
Current assignee: Xerox Ltd; Rank Pullin Controls Ltd
Priority date: 1987-12-09
Filing date: 1987-12-09
Publication date: 1992-04-01
Also published as: GB8728795D0

Abstract

In a thermal imaging apparatus comprising an afocal telescope 2, an oscillating mirror 4 for effecting horizontal scan and a linear detector array 10 extending across the image in the vertical direction, a relay optical system 8 is positioned between the oscillating mirror and a detector lens so that pupils are formed at the oscillating mirror and the detector lens, thereby avoiding problems of pupil scan. <IMAGE>

Description

THERMAL IMAGING APPARATUS This invention relates to thermal imaging apparatus and is particular concerned with an optical system for such apparatus.

One conventional form of such apparatus comprises a scanning optical system which causes a thermal image of a scene to be scanned across a detector or detector array. Electrical signals derived from the detector or detector array are then processed and applied to a display device, such as a cathode ray tube, to produce a visible image of the scene. The scanning optical system may include one or more movable mirrors, such as oscillating and/or rotating polygonal mirrors, whereby horizontal and/or vertical scanning of the image across the detector or detector array is achieved. Conventionally, energy from the moving mirror or mirrors is imaged onto the detector or detector array by a detector lens. A problem which arises in such apparatus is that of "pupil scan".

That is to say, the pupil of the optical system changes its position during scanning and/or is seen to be in different positions by different detectors where an array of detectors is used. Various proposals for dealing with the problem of pupil scan have been used in prior art systems.

The present invention is especially (but not exclusively) concerned with a system in which a detector array is used which extends substantially across the image in one direction, scanning in said one direction being achieved electronically, and scanning in the other direction is achieved by a movable mirror. Pupil scan in such a system may be particularly severe in view of the fact that the detector array extends across the entire image since the pupil associated with the detector at one end of the array is significantly displaced relative to the position of the pupil associated with the detector at the opposite end of the array.

In a preferred form, the invention solves the problem by providing a relay optical system between a scanning optical element, such as a moving mirror, and detector means preferably comprising an array of detector elements.

In an alternative aspect, the invention provides a thermal optical system including a scanning optical element, such as a movable mirror, and thermal detector means including a detector lens, means being provided to image the detector lens onto the scanning optical element.

In a further aspect, the invention provides an optical system for a thermal imager in which a relay optical system forms a pupil at a position after the scanning element and in front of the detector. Preferably, the system comprises a single scanning element, such as an oscillating mirror for performing scan in one direction, a relay optical system being x between the scanning element and the detector so that pupils are formed both at the scanning element and in front of the detector, the detector preferably comprising an array which extends across the image in the second direction.

The invention is described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates diagrammatically thermal imaging apparatus according to a preferred embodiment of the invention; Figure 2 is a section through a thermal optical system, in a first plane, included in the apparatus of Figure 1; and Figure 3 is a section through the optical system taken at right angles to the section of Figure 2.

The thermal imager shown in Fig. 1 comprises an afocal telescope 2, a mirror 4 which oscillates about an axis 6 extending perpendicular to the plane of the paper, a detector relay optical system 8 and a linear thermal detector array 10. The array 10 extends in the vertical direction of the scene (perpendicular to the plane of the paper in Fig. 1) and may comprise, for example, 384 individual detectors, namely one for each horizontal line of the image to be reproduced.

Thermal energy is directed to the detector array 10 via the telescope 2, the oscillating mirror 4, the relay optics 8 and a detector lens (not shown in Fig.

1) whereby an image of the scene is scanned, by the oscillation of the mirror 4, horizontally across the vertically extending detector array 10. Thus, horizontal scanning is achieved by the movement of the mirror 4. Vertical scanning -is achieved electronically by reading signals from the detectors in the array into a signal processing circuit 12 which may carry out various signal processing operations, and supplies an analogue signal in TV format to a display device 14 in the form of a cathode ray tube. A timing circuit 16 controls the operation of the signal processing circuit 12, display device 14 and a mirror drive device 18.

The operation of the optical system will be understood by reference to Figs. 2 and 3 which, for simplicity in showing the ray paths, are drawn with the detector relay optics 8 and telescope 2 in alignment with each other. Fig. 2 is a "vertical" section and thus, as shown in that figure, the axis 6 about which the mirror 4 oscillates is parallel to the plane of the paper as is the direction in which the linear array of detectors 10 extends. The section of Fig. 3 is horizontal, and thus the axis 6 and the direction in which the array of detectors 10 extend are normal to the plane of the paper.

As seen in Figs. 2 and 3, the telescope 2 comprises a first lens group 20 and a second lens group 22 between which an image plane 24 is formed. The detector relay optics 8 comprises a group of four lenses, as illustrated, at the centre of which a further image plane 26 is formed. The detector lens, indicated at 28 in Figs. 2 and 3, is mounted in an aperture 29 in a cold shield 34 and receives energy from the relay optical system 8 and focuses the energy, via a slit shaped stop aperture 30, onto the detector array 10.

The arrangement is such that a first pupil plane is provided at the front lens of the lens group 20 (the left hand lens as shown in Figs. 2 and 3), a second pupil plane is provided at the mirror 4, and a third pupil plane is provided substantially at the aperture 29. Thus, the detector relay optics 8 effectively relays the pupil at mirror 6 into the aperture 29. As a consequence, pupil scan is substantially eliminated. In other words, the pupils associated with each of the detectors in the array 10 are all coincident thus avoiding the problems of image deterioration arising from pupil scan.

A further cold shielding element 36, is arranged in a conventional manner, adjacent the detector array 30.

Various modifications are possible within the scope of the invention. For example, although the portion of the optical axis extending from the mirror 4 to the detector 10 is straight in the embodiment of the drawings, it would be possible to include one or more folding mirrors within that portion of the optical axis if desired.

Further, although in the preferred embodiment the detector lens is mounted in a stop aperture, namely in the drawings the aperture 29 provided by the cold shield mirror 34, this is not essential and the detector lens could be mounted elsewhere in certain circumstances. Thus, it is a preferred feature of the invention that a pupil should be formed in or near to a stop aperture (which may be in a cold shield or other element) positioned in front of the detector.

Claims

1. Thermal imaging apparatus comprising optical means for producing a thermal image and including a movable element for scanning said thermal image in a first direction, and an array of detectors extending substantially across said image in a second direction transverse to said first direction, said optical means including a detector lens adjacent said array for focusing energy on to said detectors, and optical relay means positioned between said movable optical element and said detector lens, the arrangement being such that pupils are formed both substantially at the movable optical element and at a position between the optical relay means and the array.

2. Apparatus according to claim 1, wherein the pupil between the optical relay means and the array is located substantially at said detector lens.

3. Apparatus according to claim 1 or 2, wherein said detector lens is located in a stop aperture.

4. Apparatus according to claim 1, 2 or 3, wherein the optical relay means comprises a group of lenses.

5. Apparatus according to claim 4, wherein an image plane is formed within said lens group.

6. Apparatus according to any preceding claim, wherein said movable optical element is operable for effecting scanning in the horizontal direction of the image.

7. Apparatus according to any preceding claim, wherein said movable optical element is an oscillating mirror.

8. Apparatus according to any preceding claim, wherein said optical means includes a telescope for supplying thermal energy to said movable optical element.

9. Apparatus according to claim 8 wherein said telescope is an afocal telescope.

10. Thermal imaging apparatus, substantially as herein described with reference to the accompanying drawings.