GB2110897A

GB2110897A - Imaging systems

Info

Publication number: GB2110897A
Application number: GB07835096A
Authority: GB
Inventors: Aurther Harold Lettington
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1977-09-13
Filing date: 1978-08-31
Publication date: 1983-06-22
Also published as: GB2110897B

Abstract

An imaging system includes first and second scanning rotors 1, 2 having sets of (n1+n2) and n1 plane mirrors, respectively, (where n1=5 and n2=1, for example), the sets of mirrors extending around the rotor axes W1, W2, the adjacent mirrors in each set being inclined at different angles to the rotor axis and the rotors being arranged such that radiation from a scene may be reflected from one set to the other and thence to a detector, and means for driving the first rotor 1 at <IMAGE> x the speed of the second rotor 2. Where the system is used with infrared radiation, the detector is composed of cadmium mercury telluride and its output is connected to a cathode ray tube to form a display of the scene being scanned. <IMAGE>

Description

SPECIFICATION Improvements in or relating to imaging systems The present invention relates to imaging systems and particularly, though not exclusively, to imaging systems which collect infra-red radiation from a scene and produce an image of the scene by means of a rotating scanner.

A known imaging system employs a rotating prism which has reflecting facets and a flapping mirrorfor sweeping a scanned image of a scene onto a detector which is responsive to the radiation from the scene. Such a system requires accurate synchronisation between the rotating prism and the flapping mirror.

Another known system employs a rotor with facets which are inclined so as to produce a banded scan. In this system it is necessary to have a large number of plane surfaces and consequently a large rotor to produce a satisfactory image.

According to the present invention an imaging system includes first and second scanning rotors having sets of n, and (n, + n2) plane mirrors, respectively, (where n, and n2 are integers) the sets of mirrors extending around the rotor axes, wherein the adjacent mirrors in each set are inclined at different angles to the rotor axis and wherein the rotors are arranged such that radiation from a scene may be reflected from one set to the other and thence to a detector, and means for driving the first rotor at (n, + n2) -----x the speed of the second rotor.

n, In a preferred form of the invention, the rotors are of truncated pyramidal configuration, the one facing the other.

The rotors may be driven at constant speeds or one or both rotors driven intermittently to achieve a particular scanning pattern.

The system may include means for producing an afocal image of the scene at one of the rotors.

The mirrors of each set may be attached to a rotor bodyorthe mirrors may be integrally formed with the rotor body by, for example, polishing plane faces of the rotor.

The invention will now be described by way of example only, with reference to the accompanying drawings of which: Figure lisa schematic plan view of an imaging system Figure 2 is a perspective view of a pyramidal rotor pair of the system shown in Figure 1.

The imaging system shown in Figure 1 includes an afocal lens system L, which focuses infra red radiation from a source Son a six sided scanning rotor 1 which reflects the radition from its sides which con stitute plane mirrors onto a scanning rotor, 2, having five sides which also constitute plane mirrors and which then reflects the radiation from its sides onto a single detector, D, composed of cadmium mercury telluride whose output is connected to a cathode ray tube C to from a display of the source in a conventional manner.

The scanning rotors 1,2 are driven by an-electric motor via a gearbox (not shown) and are rotated in the same direction at constant speeds W1 and W2, respectively, where the speed ratio W, equals the ratio N2 = 5 (where N, and N2 are 6 W2 N1 the number of sides of the rotors 1 and 2 respectively). The rotors are machined from aluminium bar and the sides are highly polish planar reflectors.

Successive sides of each rotor are inclined at progressively increasing angles to the rotor axis as shown in Table 1 below. The total angle that the incident radiation is reflected through is 2#( 1 + 1 )whereN,and N,arethenumberof N1 N2 sides on the rotors 1 and 2.

(i) Rotor 1 2.5 5 7.5 10 12.5 15 2.5 5 7.5 10 12.5 15 2.5 5 7.5 (ii) Rotor 2 0.5 1.0 1.5 2.0 2.5 0.5 1.0 1.5 2.0 2.5 0.5 1.0 1.5 2.0 2.5 (iii) Angular Difference 2.0 4.0 6.0 8.0 10.0 14.5 1.5 3.5 5.5 7.5 12.0 14.0 1.0 3.0 5.0 (degree) (iv) Coinoiding-pair 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 number (i) Rotor 1 10 12.5 15 2.5 5 7.5 10 12.5 15 2.5 5.0 7.5 10.0 12.5 15 (ii) Rotor 2 0.5 1.0 1.5 2.0 2.5 0.5 1.0 1.5 2.0 2.5 0.5 1.0 1.5 2.0 2.5 (iii) Angular Difference 9.5 11.5 13.5 0.5 2.5 7.0 9.0 11.0 13.0 0 4.5 6.5 8.5 10.5 12.5 (degree) (iv) Coinoiding-pair 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 number In Table I lines (i) and (ii) give the angles of each mirror with respect to a fixed datum for the rotors 1 and 2, each column containing the angles for an aligned pair of mirrors.Line (iii) gives the difference between the angles in (i) and (ii) for the aligned pairs and line (iv) gives reference numbers fo the aligned pairs in order of coincidence as rotors 1 and 2 rotate.

As the rotors rotate, radiation is reflected from a first pair of sides which are aligned and then by subsequent aligned pairs until the fifth pair has come into alignment. The next reflection in the sequence is from the sixth side of rotor 1 to the first side on rotor 2. The sequence continues for alignment of 30 pairs of sides, which constitutes one cycle (and therefore a single scan of the source, S) which is then repeated.

Table 1 above shows alignments of rotor sides over one cycle. The display produced by the system shown in Figures 1 and 2 has 30 bands, each band corresponding to an aligned pair of sides. The relative position of each band within a final display of the source S, created by the collection of bands over a cycle is indicated by the value of the angular difference given in line (iii) of Table 1. The scanning pattern (ie the manner in which the bands go together to form a complete display) of the imaging system is therefore illustrated by Table 2 below which correlates the coinciding pair numbers of line (iv) in Table 1 with the angular differences of line (iii).

2 ANGULAR DlmRENcE (DEGREES) COINCIDING PAIR NUMBER

The sides of the rotor are only exactly aligned at the centre of each line scan and they are in error by #[(1/N1) - (1/N2)] at the extremities of the scan, where ss is the scan efficiency. The result of this alignment error is antisymmetric distortion in the display. Hence it is important that [(11N1) - (11N2)] is minimised.

Claims

1. An imaging system including first and second scanning rotors having sets of n1 and (nt + n2) plane mirrors, respectively, (where n1 and n2 are integers), the sets of mirrors extending around the rotor axes, wherein the adjacent mirrors in each set are inclined at different angles to the rotor axis and wherein the rotors are arranged such that radiation from a scene many be reflected from one set to the other and thence to a detector, and means for driving the first rotor at (n, + n2) - x the speed ofthe second rotor.

n,

2. An imaging system as claimed in claim 1 wherein the rotors have a common rotational axis.

3. An imgaging system as claimed in claim 1 or claim 2 wherein each set of rotors is of truncated pyrimidal configuration.

4. An imaging system as claimed any previous claim including means for producing an afocal image of a scene on one of said rotors.

5. An imaging system as described herein with reference to the accompanying drawings.