GB2042304A - Optical scanning device - Google Patents

Abstract

An optical scanning device, in which optical elements 17, 20 direct an incident light beam to scan a detector array 11 by rotating and/or oscillating, is provided with means for changing the field of view parallel to the line scan, and therefore the effective magnification in this direction of the image produced, without requiring changes in the optical system, by varying the relationship between the rate at which output information signals from the detector array 11 are fed to a converter 23 which converts them into serial format for television display and the rate at which this serial output is generated; at the same time the field of view perpendicular to the line scan direction is adjusted by changing the range of movement of the oscillating optical component 20 which causes scanning in this direction, preferably, but not necessarily, proportional to the variation of the field of view parallel to the line scan. <IMAGE>

Description

SPECIFICATION Optical scanning device The present invention relates to an optical scanning device for producing images, particularly, but not exclusively, images from light in the infra-red region of the electromagnetic spectrum. When used in this specification the term "light" will be understood to refer to all electro-magnetic radiation, extending beyond the visible spectrum and including, inter alia, infra-red radiation.

Known image forming optical scanning devices of the type to which the present invention relates generally include a telescope operating to direct incident light towards an optical system comprising a polygonal reflector which is driven to rotate at a certain speed so that the light beam reflected thereby is sequentially scanned as a plurality of unidirectionally swept reflected beams over a plane reflector (in practice after one or more beam folding reflections at plane or curved reflectors) which is driven to oscillate about an axis which lies in or parallel to the plane defined by the sweep ofthe reflected beam incident thereon.The oscillation of the plane reflec torthus causes the reflected beam to scan in a direction orthogonal to the scan introduced by the rotating polygon and the orthogonally scanning beam is then directed onto an array of light sensitive detector elements, arranged in a matrix of orthogonal rows and columns, which produce electrical output information signals in response to the light incident on them. The scan of the beam introduced by the rotating polygonal reflector is termed the azimuth scan and that introduced by the oscillating plane reflector is termed the elevation scan. Although reflectors are used in the known devices other optical components capable of introducing varying beam deflection could be employed to achieve the scanning of the beam.

It is sometimes necessary, or at least desirable, when using optical scanners of this general type, to be able to change the field of view, thereby varying the effective magnification of the image produced by the device, either by reducing the field of view to concentrate attention on a particular region of the image, thereby effectively magnifying that part of the image, or by increasing the field of view to obtain an overall picture of events of which the original image formed only part.Now although this could be achieved by any known optical means, such as by altering the telescope through which the incident light arrives, or by introducing additional lenses into the optical path, this is not always a practical solution to the problem of providing for field of view changes due to the very restricted space available within the scanner, however, and the present invention is aimed at providing an electrical solution to this problem by operating on the electrical signals in order to modify the image eventually displayed, without requiring any adjustment to the optical components of the system.

The electrical signals generated by the detector array are fed to a converter in which they are put in serial format suitable for television display, and although changes in the effective field of view could in theory be achieved by modifying the scan rates of the television picture tube whilst maintaining the signals from the detector array unmodified this would lead to certain technical difficulties, in particular the difficulty of varying waveform which controls the horizontal or line scan.

According to the present invention there is provided an optical scanning device of the type having optical elements which in use of the device cause incident light to be scanned across a detector array comprising a plurality of rows of detector elements which produce electrical output information signals in response to the light incident thereon, the detector elements being sampled and the output information signals therefrom being fed to a converter which converts the information signals into a serial format suitable for CRT display, in which there are provided means for varying the rate at which the information signals are fed into the converter and/or the rate of production of serial output signals from the converter whereby to vary the effective field of view of the device in a direction corresponding to the rows of the detector array.

The change in the field of view, and therefore effectively in the magnification of the image, (at least in the direction parallel to the detector rows) is thus effected simply by varying the number of detectors of each row the signals from which contribute to the eventual picture information fed to the CRT. In order to select the required area there must, of course, be provided some means for selecting and adjusting the initial address position along the rows at which the signals are fed to the converter. Otherwise the mere alteration of either the rate at which the information signals are fed to the converter, or the output rate of the converter, will adjust the picture by omitting or including information toward one lateral edge only.That is, if the scan proceeds from left to right the picture will be enlarged or reduced by omitting or including information towards the right hand edge of the picture. The variation of the initial address position may be automatic, in order to obtain automatic centring of the enlarged image, or may be selectable in order to permit the precise area of the image for enlargement to be selected in dependence on different requirements from time to time.

For most practical purposes it will be desirable to vary not only the lateral magnification but also the vertical magnification, and this latter by an amount which corresponds to the lateral magnification so that the same dimensional relationships are maintained. In some circumstances independence of adjustment could be required, but in either case the adjustment can be made in accordance with the invention in one of two ways. Either the vertical scanning wave form can be modified to vary the number of scan lines accommodated in each frame, or alternatively, and preferably, the range of movement of the oscillating optical component can be varied, this having the same effect of modifying the number of scan lines included in a frame, reducing the number of scan lines if the range of movement is reduced and increasing it if the range of movement is increased.

When the range of movement of the oscillating optical component is reduced (this being achieved either by reducing the amplitude of the drive motor control signals or by means of a variable transmis sion mechanism between the drive motor and the element) and at the same time the rate at which the information signals are fed to the converter from the detector array is increased, both being in order to increase the magnification of the image, and being therefore of corresponding amounts, for example the former being halved and the latter doubled to achieve a two-fold magnification, the number of scan bands per frame remains the same, or at least substantially the same. The vertical field scan is reduced, however, (by half in the example given above) and therefore successive scan bands must contain an overlap of the scene.This would cause confusion in the image produced unless steps were taken to avoid superimposition of the picture information. This can be achieved by means of a system in which the overlap of scan bands is deliberately introduced to reduce the signal-to-noise ratio and also to permit an increase in the speed of rotation of the rotating polygonal reflectors, which speed would otherwise be limited by various determining factors.

In this system the signals from each row of detectors are separately stored in digital stores and each time a detector row is scanned the newly produced signals are added to the signals generated as a result of the or each previous scan of that row. When a row has been scanned as many times as it is going to be during a given frame then the summed signals of the detectors are passed for further processing in the converter. This system requires that the converter have one or more different modes of operation because such summing is not required if there is no scan band overlap.It is probable that this would lead to the device being provided with means for switching to discrete magnification changes rather than being continuously variable, because this would allow programming to be effected to accommodate the various changes required in the converter to deal with the situation of non-overlapping scan bands as well as situations where the overlap is two fold, threefold or more.

One embodiment of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure lisa partly diagrammatic, partly block schematic representation of an embodiment of the invention, and Figure 2 is a block schematic diagram of a part of the embodiment of Figure 1.

Referring first to Figure 1 the embodiment shown comprises a telescope 16 which receives incoming light from the scene being viewed and directs it towards a rotary polygonal reflector 17 which in operation is caused to rotate in a sinqle directional sense at a constant speed causing the reflected beam to scan repeatedly over an arc. The direction of rotation is indicated by the arrow A. From the polygonal reflector 17, the light beam is directed, via two beam-folding reflectors 18, 19 to a plane reflector 20 the orientation of which is such that the lateral scanning introduced by the movement of the rotary polygonal reflector 17 causes the spot of light incident on the reflector 20 to scan parallel to an axis B-B about which the reflector 20 is caused to oscillate by a drive motor 21.The axis B-B may lie in the plane of the reflector 20 or be spaced from this plane either to the front or the rear of the reflector 20. The oscillation of the reflector 20 about the axis B-B causes the beam reflected thereby additionally to scan in a direction which is orthogonal with respect to the scanning introduced by the rotary polygonal reflector 17. Light reflected by the reflector 20 falls onto a detector array 11 comprising a plurality of rows 1 lea, 11b....11n of photo-sensitive detectors, in this embodiment sensitive to light in the infra-red region of the spectrum. Although in the embodiment illustrated there are shown only a relatively limited number of rows of detector elements there could in practice be very many more than this.The number of detectors in each row may vary from one to several hundred depending on the purpose to which the scanner is to be put.

Each detector in the array produces an electrical output signal in response to the incident light falling on it as the light beam is scanned over it, and in general more than one detector will be illuminated at any one instant in time. The signals from the detector array 11 are fed in parallel as they occur to electronic logic circuitry for processing and converting them into serial form and a format compatible with television picture standards, the converted signals then being fed to a cathode ray tube CRT as the information signals, separate horizontal and vertical scanning control signals being generated separately and in a conventional manner, these being indicated X and Yin the drawing.

The processing and conversion of the detector signals takes place in a processor circuit 22 and a converter circuit 23. The processor circuit 22 is controlled from a control logic circuit 24 which includes a control panel by means of which a required magnification of the image can be selected in a manner which will be described in more detail in relation to Figure 2. The control logic circuit 24 also feeds control signals to a drive control logic circuit 25 the output of which is fed to the motor 21 which drives the oscillation of the reflector 20.

Referring now to Figure 2 there is shown in block schematic form some of the components of the processor 22 and the converter 23. As can be seen from Figure 2 the processor 22 includes a plurality of summing circuits 12, one for each row of detectors, and the output of each summing circuit is fed to a respective analogue-to-digital converter 13. From the A/D converters 13 the digital signals are fed to a processor logic circuit 14 which, in dependence on the control signals received from the control logic circuit 24, controls the further processing of the signals.

For the purposes of the following description it will be assumed that the system has a "normal" operating mode and one or more modes of operation with a narrower field of view giving an enlarged image. In the "normal" mode of operation each row of detec tors 1 la, 1 1b etc. is scanned only once during each field in the generation of the picture and the summing circuits 12 in this case merely pass the signals as they arrive to the A/D converters 13 which feed corresponding digital signals into the processor logic circuit 14; this latter serialises the signals and feeds them out on an output line 29 to converter logic circuit 30 at a rate R1 determined by clock signals on a control line from the control logic circuit 24.

The converter logic circuit feeds out the converted signals at a rates2, also determined by the value of different clock pulses fed on another control line from the control logic circuit 24.

When a narrower field of view is selected, for example to double the image size this is achieved by doubling the clock rate R1 at which information signals are fed to the converter logic circuit 30 from the processor logic circuit 14. This means that now only signals from half the detectors in each row will pass in the time taken for each horizontal scan of the television picture tube CRT to which the information signals, modified into suitable format by the circuit 31 are eventually fed. The same effect may, of course, be obtained by halving the output clocking rate R2 from the converter logic circuit 30 or by an intermediate rate change of both R1 and R2. In general the doubling of R1 is preferred because this maintains a high level of horizontal sampling of the picture.Coupled with this horizontal field of view change there is a corresponding vertical field of view change achieved by halving the amplitude of the oscillations of the frame drive motor 21 by appropriately changing the drive signal (the drive control logic circuit 32 will do this automatically on receipt of an appropriate control signal from the control logic circuit 24) which causes the signals from only half of the rows of detector elements to appear in the eventually produced television signal. Because of this the number of scan bands per frame is not changed and therefore since the width of the scanning beam and of each row of detectors remains the same, it follows that each row of detectors will now be scanned twice per frame.In order to avoid confusion the control logic circuit 24 feeds a control signal to the converter logic circuit which now operates in a different mode, feeding control signals to stores 15" 152 and 153 of the store array 15. Likewise the processor circuit 22 also now operates differently, the summing circuits 12 operating to sum the two output signals from each detector of a row before passing them onto the processor logic circuit 14, which latter passes the signals from the row being scanned for the first time during the current frame to the store 15, and the signals from the row being scanned for the second time alternately to the second or third store 152, 153. At the same time the previous contents of store 15, are fed either into store 152 or 153 as appropriate, to be added to the signals just arriving, from the detector row being scanned for the second time, whilst the third store, 153 or 152 alternately, feeds out the signals accumulated therein to the format changer 31. All four stores are used for a tripling of the image size when there is a triple overlap of scanned rows each row being scanned three times per frame.

A discussion of this operation, which also has the advantage of improving the signal-to-noise ratio (as does the double overlap operation) will be found in our copending British Patent Application No.

By providing more stores 15 the system can accommodate a greater degree of enlargement of the image, although it will be realised that such enlargement also carries with it a consequent loss of resolution since it is achieved by selecting only a part of the available scanned information and presenting it on the full area of the display screen, so that each picture element occupies a successively larger area of the display screen at each enlargement.

Over-enlarged pictures can thus appear "grainy".

The arrangement operates entirely satisfactorily within certain limits however.

When changing the clock rates R1 and/or R2 an initial address position must also be chosen to obtain the desired part of the horizontal scan on the screen.

Likewise the part of its angular range of movement must be selected for the oscillating mirror by suitable control of the motor drive signal in order to obtain the desired part of the vertical scan.

Claims (12)

1. An optical scanning device of the type having optical elements which in use of the device cause incident light to be scanned across a detector array comprising a plurality of rows of detector elements which produce electrical output information signals in response to the light incident thereon, the detector elements being sampled and the output information signals therefrom being fed to a converter which converts the information signals into a serial format suitable for CRT display, in which there are provided means for varying the rate at which the information signals are fed into the converter and/or the rate of production of serial output signals from the converter whereby to vary the effective field of view of the device in a direction corresponding to the rows of the detector array.

2. An optical scanning device as claimed in Claim 1, in which one of said optical elements oscillates to cause scanning of the light beam in one of the scanning directions and there are further provided means for varying the range of movement of the oscillating optical element whereby effectively to adjust the field of view of the device in a direction corresponding to the said one scanning direction.

3. An optical scanning device as claimed in Claim 2, in which the said one scanning direction is substantially perpendicular to the rows of the detector array.

4. An optical scanning device as claimed in Claim 2 or Claim 3, in which the range of movement of the oscillating optical element is varied by varying the amplitude of the drive signal fed to a motor which causes the movement of the oscillating optical element.

5. An optical scanning device as claimed in Claim 4, in which the said oscillating optical element is a reflector.

6. An optical scanning device as claimed in Claim 1, in which the said direction corresponding to the rows of the detector array also corresponds to the direction of the line scan of the CRT and there are further provided means for varying the vertical frame scan waveform whereby to adjust the effective field of view of the device in a direction perpen dicularto the direction of the rows of the detector array.

7. An optical scanning device as claimed in any of Claims 2 to 5, in which the said converter includes means for separating information from overlapping scan bands on the detector array caused upon reduction of the range of movement of the oscillating opt cal element when there is also an increase in the rate at which information signals are fed to the converter and/or a decrease in the rate of production of serial output signals from the converter.

8. An optical scanning device as claimed in Claim 7, in which the said converter includes a plurality of stores for storing information signals generated by respective detectors of each row of detectors upon each scan, and means for summing the values stored in each store upon successive scans of the same row of detectors, whereby at any one time each store contains signals representing the sums of the signals generated by respective detectors during all the scans over the row containing these detectors during the current frame.

9. An optical scanning device as claimed in Claim 8, in which the number of stores is at least one greater than the number of times any one row of detectors is scanned, each of the stores being fed with signals generated by a row of detectors during a scan and the sum of the signals generated by that row of detectors during all the preceding scans for which that row of detectors was illuminated.

10. An optical scanning device as claimed in Claim 9, in which the stores are digital stores and each row of detectors has an associated analogueto-digital converter.

11. An optical scanning device as claimed in Claim 9 or Claim 10, in which the output or outputs from the store or stores containing all the signals generated by a row of detectors, after the last scan in which that row is illuminated in the production of a picture field, is or are passed on to the next stage of the converter to be converted into a format suitable for television display.

12. An optical scanning device substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.