GB2279762A

GB2279762A - Optical scanning system

Info

Publication number: GB2279762A
Application number: GB8204356A
Authority: GB
Inventors: Hillary Gil Sillitto
Original assignee: GEC Ferranti Defence Systems Ltd; Ferranti PLC
Current assignee: Ferranti International PLC; Leonardo UK Ltd
Priority date: 1982-02-15
Filing date: 1982-02-15
Publication date: 1995-01-11
Anticipated expiration: 2002-02-15
Also published as: GB8204356D0; GB2279762B; FR2704325A1

Abstract

An optical scanning system includes scanning means 10, 15 operable to scan an object field and including a rotatable drum 10 having a plurality of plane reflecting faces 11 each perpendicular to the axis of rotation. A detector 16 is provided which is sensitive to radiation directed on to it by the scanning means. Stabilising means 20, 21 are provided which are operable to direct a line of sight through the scanning means towards any selected point in the object field. Preferably the stabilising means comprise a second rotatable drum 20 having the same number of plane reflecting faces as a first drum. The two drums are arranged to rotate at the same speed through the phase angle between them may be varied at will. The stabilising means are used in association with a laser rangefinder 22. <IMAGE>

Description

OPTICAL SCANNING SYSTEM This invention relates to optical scanning systems of the type wherein optical radiation from a field of view is directed on to a detector by means of a rotating drum having a plurality of plane reflecting surfaces. Scanning systems of this type are well known, and may be used, for example, for inspection of moving surfaces or for scanning stationary fields. One disadvantage with such a system is that it is not possible to use the existing scanning optical system to view continuously one particular part of the field. There are instances, for example, where it would be useful to be able to examine one particular part of an area being scanned whilst scanning of the whole area continues.

It is an object of the present invention to provide an optical scanning system with which simultaneous scanning of an object field and continuous viewing of part of that field is possible.

According to the present invention there is provided an optical scanning system which includes scanning means operable to scan an object field and including a first plurality of plane reflecting members arranged to rotate continuously about an axis, each member being disposed substantially parallel to said axis, detector means sensitive to optical radiation directed on to it by the scanning means, and stabilising means operable to direct a line of sight through said scanning means towards any selected point in said object field.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a schematic diagram of a known optical arrangement for scanning a field of view; Figure 2 shows a first embodiment of the invention; Figure 3 shows a second embodiment of the invention; and Figure 4 shows a schematic diagram of control means.

Referring now to Figure 1, the known scanning arrangement comprises a rotatable polygonal drum 10 comprising a number of plane reflecting surfaces 11, each parallel to the axis of rotation of the drum. Radiation from the object field is collimated by an optical system (not shown) and is reflected from a surface 11 of the drum 10 on to a plane mirror 12 which in turn reflects the radiation on to a toroidal mirror 13.

This latter mirror 13 focusses the radiation on to a further plane mirror 14 and it then passes to a scanning mirror 15.

The scanning mirror 15 is a plane mirror rotatable about an axis optically perpendicular to the axis of rotation of the drum 10, and it directs the radiation on to a detector 16. The collimated radiation is focussed on to the detector 16 by a lens 17.

In operation, the drum is rotated continuously so that radiation from the object field falls on successive faces of the drum. The angular movement of the drum has the effect of scanning the object field in a direction perpendicular to the axis of rotation of the drum 10. If the scanning mirror 15 is moved slightly each time radiation from the object field passes from one face of the drum to the next, then the field is also scanned in a perpendicular direction. The result is to produce a gradual scanning of the object field in two perpendicular directions, enabling a television-type raster scan picture to be built up.

Scanning arrangements of the type described above are frequently used for infra-red scanning systems using thermal imaging techniques. In such systems it is frequently desirable to use laser range-finding techniques to determine the range of an object in the object field. Obviously if the laser beam was projected through the scanning system described above, it would scan the entire field. A range-finder could not function in this way since the receiver would not be "looking" at the target when a reflected pulse reached the scanning system. It is, of course, possible to provide a completely separate optical system for the laser, but this leads to problems of alignment.

The optical system shown schematically in Figure 2 allows a laser to be used through part at least of the scanning system, by modification of the scanning system described above. The arrangement of Figure 1 is retained, but in addition a second rotatable drum 20 is provided, having the same number of plane reflecting surfaces as the drum 10, and rotatable about an axis parallel to that of the drum 10.

Radiation is reflected from a surface of the drum 10 on to a surface of the second drum 20, and thence to a scanning mirror 21, rotatable about an axis optically perpendicular to the axis of rotation of drum 20. From the scanning mirror 21 radiation passes to the laser receiver, or from the laser transmitter 22. The transmit and receive paths of the laser are separated by known techniques such as the use of a beam-splitting polariser.

In operation, the two drums rotated at the same speed, so that the angle between the corresponding faces of each remains constant. This angle also determines the direction of the laser sightline in a direction perpendicular to the axis of rotation of the drums. This direction may be varied by changing the angles between corresponding faces of the drums, that is the phase angle between the drums. The speed of rotation of the drums, and the phase angle between them, is readily controlled by a servo system.

The direction of the sigbtline in the perpendicular direction is determined by the scanning mirror 21, which may be rotated about its axis as required.

The simple optical system described above suffers from a restriction in the angular movement of the sightline, and the arrangement of Figure 3 may be used to give greater freedom of movement. This arrangement differs only by the inclusion of a pair of toroidal mirrors, one mirror 30 being located between the two drums 10 and 20, whilst the other mirror 31 is located between the drum 20 and the scanning mirror 21. This enables the greatest possible field of view to be covered by the laser sightline.

Figure 4 shows a schematic diagram of a control system for controlling the movements of the drums 10 and 20 and the scanning mirrors 15 and 21. For each of these elements the control system has pick-off such as a shaft encoder, and drive means. The drive means for the drums 10 and 20 are motors of a type such that the relative angular positions between them may be changed readily and quickly. For the scanning mirrors 15 and 21 stepping motors are the most suitable. Details of the control system are not given, since the techniques are well-known in the servo control field. Demand inputs DM are necessary to determine the required direction of the line of sight controlled by drum 20 and scanning mirror 21.

Although the scanning system described above is for use with an infra-red thermal imaging system, both the application and the wavelength of the radiation are only by way of example. Similar scanning systems may be used, with either infra-red or other optical radiation for other purposes, such as the inspection of a surface to detect faults. It may be required to inspect a possible fault whilst scanning continues, and in such a case the stable optical sightline will be used for inspection purposes.

Claims

What we claim is:

1. An optical scanning system which includes scanning means operable to scan an object field and including a first plurality of plane reflecting members arranged to rotate continuously about an axis, each member being disposed substantially parallel to said axis, detector means sensitive to optical radiation directed on to it by the scanning means, and stabilising means operable to direct a line of sight through said scanning means towards any selected point in said object field.

2. A scanning system as claimed in Claim 1 in which the stabilising means includes a second plurality of plane reflecting members arranged to rotate continuosuly about a second axis, each member being disposed substantially parallel to said second axis, and control means operable to cause said second plurality of members to rotate at the same speed as the said first plurality of members, and to vary the relative positions of said first and second pluralities of members as required.

3. A scanning system as claimed in Claim 2 in which the scanning means includes a first pivotable reflecting surface movable to deflect radiation incident upon it in a direction parrallel to the axis of rotation of the first plurality of plane reflecting members, the control means being operable to move the same pivotable reflecting surface in synchronism with the rotation of the said first plurality of members.

4. A scanning system as claimed in either of Claims 2 or 3 in which the stabilising means includes a second pivotable reflecting surface movable to deflect radiation incident upon it in a direction parallel to the axis of rotation of the second plurality plane reflecting members, the control means being operable to move the said second pivotable reflecting surface as required.

5. A scanning system as claimed in any one of Claims 1 to 4 which includes a laser system operable to direct a beam of radiation along said line f sight towards the object field.

6. An optical scanning system substantially as herein described with reference to Figures 2 to 4 of the accompanying drawings.

6. A scanning system as claimed in Claim 5 in which the laser system includes a laser transmitter and radiation-sensitive means responsive to radiation from the transmitter which is reflected back along the line of sight by an object in the object field.

7. An optical scanning system substantially as herein described with reference to Figures 2 to 4 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. An optical scanning system which includes scanning means operable to scan an object field and including a first plurality of plane reflecting members arranged substantially parallel to a first axis to form a first polygonal surface rotatable about said first axis, and a like plurality of plane reflecting members arranged substantially parallel to a second axis to form a second polygonal surface rotatable about said second axis, detector means sensitive to optical radiation directed on to it by the scanning means, and control means operable to cause said second plurality of members to rotate at the same speed as the said first plurality of members, and to vary the relative positions of said first and second pluralities of members as required so as to direct a line of sight through said scanning means towards any selected point in said object field by reflection from the first and second polygonal surfaces in succession.

2. A scanning system as claimed in Claim 1 in which the scanning means includes a first pivotable reflecting surface movable to deflect radiation incident upon it in a direction parallel to the said first axis the control means being operable to move the same pivotable reflecting surface in synchronism with the rotation of the said first plurality of members.

3. A scanning system as claimed in either of Claims 1 or 2 in which the scanning means includes a second pivotable reflecting surface movable to deflect radiation incident upon it in a direction parallel to the said second axis, the control means being operable to move the said second pivotable reflecting surface as required.

4. A scanning system as claimed in any one of Claims 1 to 3 which includes a laser system operable to direct a beam of radiation along said line of sight towards the object field.

5. A scanning system as claimed in Claim 4 in which the laser system includes a laser transmitter and radiation-sensitive means responsive to radiation from the transmitter which is reflected back along the line of sight by an object in the object field.