GB2363205A

GB2363205A - Thermal imaging apparatus

Info

Publication number: GB2363205A
Application number: GB8804925A
Authority: GB
Inventors: Kenneth Rodney Pollett
Original assignee: Rank Taylor Hobson Ltd
Current assignee: Rank Taylor Hobson Ltd
Priority date: 1988-03-02
Filing date: 1988-03-02
Publication date: 2001-12-12
Anticipated expiration: 2008-03-02
Also published as: GB2363205A8; GB2363205B; GB8804925D0

Abstract

Thermal imaging apparatus includes a polygonal mirror which is rotated to effect the line scanning operation and an infra-red laser rangefinder operable to emit energy which, upon return, is directed to a rangefinder detector after reflection from first and second facets of the polygonal mirror. A reflective device comprising a collimating lens, a pair of opposed mirrors arranged parallel to and on opposite sides of the axis of the collimating lens and a third mirror transverse to the opposed mirrors reflects the return pulses from the first to the second facet. The arrangement is such that the angle between the input and output beams of the reflective device remains substantially constant. This provides a compact arrangement.

Description

SOZO9CE THERMAL IMAGING APPARATUS This invention relates to thermal

imaging apparatus and particularly concerns such apparatus provided with 5 a rangefinder, preferably an infra-red laser rangefinder.

It has already been proposed to incorporate a laser rangefinder in a thermal imaging apparatus having a 10 rotatable polygonal mirror for effecting line scanning, in which the return laser pulses of the rangefinder are directed to a rangefinder detector element after reflection from first and second facets of the polygonal mirror, the laser return pulse being 15 directed from the first facet to the second facet by reflection at a concave mirror. The reflection at the second facet compensates for the change in the beam direction in the rangefinder system which would otherwise arise due to the fact that the angular 20 position of the first facet when the return pulse is incident upon it varies with distance of the object whose range is to be detected.

The invention aims to provide a more compact 25 arrangement than that which has been proposed.

2 The invention provides thermal imaging apparatus including a polygonal mirror which is rotated to effect a scanning operation and including a rangefinder operable to emit energy which, upon 5 return, is directed to a rangefinder detector after reflection from first and second facets of the polygonal mirror, the apparatus comprising a reflective device which receives said return energy from said first facet and directs it to said second 10 facet, said reflective device comprising first and second opposed reflective surfaces, a third reflective surface arranged transversely to said first and second reflective surfaces, and optical means with power arranged to focus energy from said first 15 face at or near to said transverse reflective surface after reflection at one of said opposed surfaces and to direct energy to said second facet after reflection at said other opposed reflective surface, the arrangement being such that the angle between the 20 output from and input to said reflective device remains substantially constant at least over a limited range of angles of the energy input to said device.

The invention is described further by way of example 25 with reference to the accompanying drawings in which:

-c 3 Fig. 1 illustrates diagrammatically part of a combined thermal imaging apparatus and laser rangefinder in accordance with a preferred embodiment of the invention; and Figs. 2a and 2b are diagrams to illustrate the operation of part of the apparatus of Fig. 1.

The apparatus shown in the drawings comprises a 10 conventional afocal telescope (not shown) attached to the front of a housing 2 for supplying thermal energy from a thermal scene to a line scanning polygonal mirror 4 which is rotated about an axis 6. The energy reflected from the facets of the mirror 4 is supplied 15 via a suitable optical system (not shown) and an oscillating mirror (not shown) for effecting frame scan, to a thermal detector 8, via a filter 10, so called "cold" reflector 12, detector lens 14 and a further "cold" reflector 16. This arrangement causes 20 a thermal image of the scene to be scanned horizontally and vertically across thermal detector elements in the detector 8. Since this is performed in a conventional manner, further description of this and the elements whereby it is achieved is not 25 necessary.

The apparatus also includes a laser rangefinder which comprises an infra-red laser (not shown) for generating pulses of infra-red energy, a range finder detector (not shown) incorporated in the detector 8 5 and circuitry for measuring the time between transmission of an infra-red pulse by the laser and reception of the return pulse by the rangefinder detector after reflection from the object. The return pulse is directed by the above mentioned telescope on 10 to the polygonal mirror 4, the timing of transmission of the laser rangefinder pulse being chosen so that the return pulse travelling along path 18 is reflected by a facet 4a of the mirror 4 into a reflective device 20 from which the return pulse is directed to a second 15 facet 4b of the mirror 4 from which it is passed, via further mirrors 22 and 24 and the elements 10, 12, 14 and 16 to the detector 8 to be received by the laser rangefinder detector element therein.

20 It will be appreciated that since the delay between transmission of the laser rangefinder pulse and arrival of the return pulse at the polygonal mirror 4 is of necessity dependent upon the distance of the object whose range is to be detected, the angular 25 position of the polygonal mirror 4 at the time when the return pulse arrives thereat is dependent upon the range of the object. As a consequence, unless special steps are taken, the direction in which the return pulse is reflected within the apparatus would vary with the range of the object, which is undesirable. In 5 the apparatus illustrated in the drawings, such variation is substantially eliminated by directing the return pulse from facet 4a on to facet 4b, via the device 20.

10 The device 20 comprises a thin focussing lens 26, a plane mirror 28 parallel to the axis of the lens 26, a further plane mirror 30 normal to the mirror 28 and a third plane mirror 32 parallel to the plane mirror 28.

The lens 26 is arranged such that the energy in the 15 return pulse is focussed thereby substantially at the surface of mirror 30 after reflection at mirror 28 and the energy reflected from mirror 30 is directed to facet 4b via the lens 26 after reflection at mirror 32. The arrangement is such that the angle between 20 ray 18a entering the device 20 and ray 18b leaving device 20 remains substantially constant despite variations (over a limited range) in the angle between the ray 18a and the axis of lens 26, which variation takes place dependent upon the angular position of the 25 polygonal mirror 4 at the instant when the return pulse is incident thereon. Since the angle between 6 rays 18a and 18b remains substantially constant, the direction of ray 18c leaving facet 4b remains substantially constant so that the return pulse is always directed to the rangefinder detector element 5 within detector 8. Further, the device 20 relays the pupil of the optical system from facet 4a to facet 4b.

The way in which the angle between rays 18a and 18b is maintained substantially constant will be understood 10 from consideration of Figures 2a and 2b.

In Figure 2a, the return pulse is shown as an input collimated beam 18a at an angle A to axis 34 of lens 26 such that, after reflection from the mirror 28, it 15 is focussed to a point 36 on mirror 30 which is coincident with the axis 34 and as a consequence the angle B which the output beam 18b makes with the axis 34 Is equal to the angle A. Reference 36a in Figure 2a indicates the virtual image of point 36 associated 20 with the reflection of the beam from the mirror 32 and thus point 36a is spaced behind (to the left as seen in Fig. 2a) mirror 32 by a distance equal to the distance by which the point 36 is spaced in front of the mirror 32. In considering Figure 2a, it should be 25 noted that a ray 18c of the output beam which has passed through the centre of lens 26 does not change 7 its direction upon passing through the lens 26. Thus, the direction of the output beam 18b may be determined by drawing a straight line between the centre of the lens 26 and the virtual image 36a.

In Figure 2b, the angle between the input beam 18a and the axis 34 is shown as having a value C larger than the angle A and as a consequence the point of focus 38 on mirror 30 is to the left of the axis 34 and 10 therefore to the left of point 36 of Figure 2A. In other words, point 38 is closer to mirror 32 than point 36. It follows that the virtual image 38a of point 38 is closer to mirror 32 than virtual image 36a and determination of the direction of the output beam 15 18b by drawing a straight line between the virtual image 38a and the centre of the lens 26 will show that as the angle between the axis 10 and the input beam 18a is increased, the angle between the output beam 18b and the axis 34 has correspondingly decreased.

20 This latter angle is shown as having a value D in Figure 2b. Thus, within limits, the angle of the incident beam 18a may change whilst maintaining a substantially constant angle between the input beam 18a and the output beam 18b.

The mirrors 28, 30, 32 may be constituted by the surfaces of a solid block of thermally transparent material, with which the lens 26 may also if desired be integral. Reflection at these surfaces maybe achieved by total internal reflection and/or by the 5 provision of silvering or the like on the surfaces of the block.

The device 20 as shown in the drawing provides a more compact apparatus than that which has already been 10 proposed in which energy is reflected from the first to the second facet via a concave mirror.

9

Claims

CLAIMS:

1. Thermal imaging apparatus including a polygonal mirror which is rotated to effect a scanning operation 5 and including a rangefinder operable to emit energy which, upon return, is directed to a rangefinder detector after reflection from first and second facets of the polygonal mirror, the apparatus comprising a reflective device which receives said return energy 10 from said first facet and directs it to said second facet, said reflective device comprising first and second opposed reflective surfaces, a third reflective surface arranged transversely to said first and second reflective surfaces, and optical means with power 15 arranged to focus energy from said first face at or near to said transverse reflective surface after reflection at one of said opposed surfaces and to direct energy to said second facet after reflection at said other opposed reflective surface, the arrangement 20 being such that the angle between the output from and input to said reflective device remains substantially constant at least over a limited range of angles of the energy input to said device.

2. A device according to claim 1, wherein said reflective surfaces are plane.

3. A device according to claim 1 or 2, wherein 5 said transverse reflective surface is plane.

4. A device according to any preceding claim, wherein said optical means comprises a single optical element.

5. Apparatus according to claim 4, wherein the optical element is a collimating lens.

15

6. A device according to claim 5, wherein said opposed surfaces are positioned at equal distances on opposite sides of the axis of said lens, said lens and said transverse reflective surfaces being at opposite ends of said opposed surfaces.

7. Thermal imaging apparatus substantially as herein - described with reference to the accompanying drawings.

7. A reflective device substantially as herein described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows Thermal imaging apparatus including a polygonal mirror which is rotated to effect a scanning operation 5 and including a rangefinder operable to emit energy which, upon return, is directed to a rangefinder detector after reflection from first and second facets of the polygonal mirror, the apparatus comprising a reflective device which receives said return energy 10 from said first facet and directs it to said second facet, said reflective device comprising first and second opposed reflective surfaces, a third reflective surface arranged transversely to said first and second reflective surfaces, and. optical means with po wer is arranged to focus energy from said first facet at or near to said transverse reflective surface after reflection at one of said opposed surfaces and to direct energy to said second facet after reflection at said other opposed reflective surface, the arrangement 20 being such that the angle between the output from and input to said reflective device remains substantially constant at least over a limited range of angles of the energy input to said device.

I I j Z-' 2. Apparatus according to claim 1, wherein said reflective surfaces are plane.

3. Apparatus according to claim 1 or 2, wherein 5 said transverse reflective surface is plane.

4. Apparatus according to any preceding claim, wherein said optical means comprises a single optical element.

15 6. Apparatus according to claim 5, wherein said opposed surfaces are positioned at equal distances on opposite sides of the axis of said lens, said lens and said transverse reflective surfaces being at opposite ends of said opposed surfaces.