GB2152328A - Scanning an imaged scene - Google Patents

Abstract

A plurality of infra-red detector elements D1-DNx are, in a simplest case, arranged in two parallel lines I and II with the elements of one line shifted with respect to the elements of the other line by a distance DELTA xs in the direction of the lines. Each line has DNx, individual elements. Signals from the detector lines are processed in delay and interlacing devices. The length of each line corresponds to a complete horizontal field of vision spanning 2Nx columns of the imaged scene. This field of vision is scanned vertically by a mirror sweeping over the fixed detector array during a TV field duration, an object line raster being produced by each line after optoelectronic conversion of the incident radiation. These object line rasters are interlaced by means of the delay and interlacing devices to produce, on a TV monitor, a video signal which can be represented without interposition of a standard converter and which is free from aliasing effects. <IMAGE>

Description

SPECIFICATION Scanning Method and Apparatus This invention relates to a method and apparatus for scanning radiant energy by means of two or more parallel detector lines each containing several detector elements sensitive to the radiant energy (typicaliy infra-red radiation).

It is known from DE-AS 2332245 to scan such an array of detectors using a scanning mirror. The video signals obtained by scanning in parallel require a comparatively complex standard converter if they are to be processed for display on a television monitor. Scanning methods for topographical profiles using a detector row are also disclosed in DE-PS 2250251 and DE-OS 2224275.

In the former case, the detector row itself takes over the function of the scanning mirror by being moved over the terrain in question. The information obtained is transferred to delay elements, partly by way of ring switches. These documents fail to give any indication of TV-compatible reproduction of the information obtained.

It is an object of the present invention to improve on the known scanning methods by generating a video signal which can be reproduced on a monitor in a TV compatible form without the interposition of a standard converter. According to one aspect of the present invention, a method of scanning an imaged scene using a radiation sensitive detector arrangement comprises the steps of: providing a detector array including at least two parallel detector lines, each line comprising a plurality of regularly spaced detector elements with the elements of each line staggered relative to the elements of the or each other line; irradiating the detector array with an image of the scene and causing relative movement of the image and the array in a transverse direction with respect to the detector lines thereby to scan the image; generating, at predetermined time instants during a scan, a line raster from electrical signals produced by the detector elements of one of the detector lines and, after a delay, interlacing the raster with a raster or rasters produced from a subsequently irradiated detector line or lines. A TV-compatibie video signal can be obtained by these means and moreover aliasing effects can be avoided in the direction of the lines.The scanning theorem can be fulfilled by means of the number of detector lines used and their staggering in relation to each other. These unwanted aliasing effects occur when sampling or interrogation is not carried out continuously but only at certain points in time, because multiple spectra are then produced and the unwanted sidebands of the basic spectrum overiap to a greater or less extent if the scanning theorem is not fulfilled, with the result that false frequencies not present in the original spectrum occur and manifest themselves in Moirée effects (aliasing).

Preferred features of the invention are disclosed in the dependent claims following this description.

The features of claim 7 yield the advantage that the contents of the shifting register can be represented as monitor line or input immediately after the interlacing process. The features of claims 9 and 10 prevent interference of the predetermined local frequencies bysidebands of the multiple spectrum.

Regardless of whether the detector lines make use of, on the one hand, the so called CCD-FPA technique (charge coupled devic--focal plane array), that is to say, with serial read-out of the line raster signals, or, on the other hand, the conventional technique of bringing out electrical connections in parallel, it is advantageous to employ intermediate storage of the line rasters as set out in claim 11. "CCD" refers to known semiconductor charge transfer devices.

The other sub-claims contain further preferred features.

The invention will now be described by way of example with reference to the drawings, in which corresponding parts in the various figures carry the same reference numerals. In the drawings, Figure lisa diagram of a two line detector arrangement with a delay line and summing device, in which the individual detector elements are spaced apart by the width of one element, the two lines being parallel to each other and staggered by the width of one element; Figure 2 is a diagram of a four line detector arrangement similar to the arrangement of Figure 1 but with each line shifted by only half the width of an element with respect to the adjacent line or lines; Figure 3a is a simplified representation of a two line detector; Figure 3b is a schematic diagram of the two line detector of Figure 3a, each line having a parallel buffer and CCD lines;; Figure 3c is a pulse diagram corresponding to the detector arrangement of Figures 1,3a and 3b; Figure 3d shows the forward and return sweep signals of a monitor electron beam line written by the detector of Figures 1 and 3a to 3c; Figure 4a is a propagation diagram of the individual line rasters at various points in time in a four line detector arranged as shown in Figure 2 using CCD-FPAtechnology; Figure 4b is a propagation diagram of the individual line rasters of a four line detector constructed as shown in Figure 2 and using conventional technology; Figure 5 is a schematic diagram of the four line detector of Figure 2 with parallel shift register and buffer similar to Figure 3b; and Figure 6 is a diagram illustrating the use of the four line detector for TDJ operation in the scanning direction of the detectors.

Figure 1 shows a detector arrangement consisting of two parallel detector lines I and II each having DNX individual elements of width XD arranged with their centres spaced apart by a distance nx,. The two detector lines are staggered with respect to each other by a distance Vex5. The length of the line corresponds to the total horizontal field of vision of the scene to be imaged, which is obtained from the column numbering 1 to 2Nx of the object lines. The vertical field of vision of the scene is received by the scanning mirror (not shown, for the sake of clarity) continuously guiding the scene in question over the rigid detector arrangement from above downwards (in the direction of vision) during aTVfield duration of, say, 20 ms.

The two detector lines I and II sweep over an object line one after the other, that is to say in a time interval equal to one TV line duration. At each point in time in the sweep, an object line raster corresponding to the particular detector line is formed. The two object line rasters formed in this manner, which are shifted in relation to each other spatially (in the x-direction) by Ex5 and in time by theTV line duration, are interlaced both spatially and in time by means of an electronic device and produce the video signal for the electron beam of the TV system.

If, for example, local frequencies up to the detector local or position frequency of 1 fx,D= XD are to be transmitted and aliasing is to be avoided, then, as shown in Figure 2, for example, the values no=4 and Axis=2 XD should be chosen. If one makes do with the half local frequency 1 2xD, then the two line detector arrangement shown in Figure 1, where EXS=XD is sufficient.

The manner of the operation will first be described with reference to the simpler embodiment of Figure 1, where ny=2 and Ax=x0:- At the point in time the detector line I is situated in the object line y1 and forms the following object line raster, where "object" means the scene image in the detector plane.

y": 1 3 5...

The vertical scanning velocity is so adjusted that the complete vertical deflection corresponds to the TV field time, and the detector line II reaches the object line y1 64 us later than detector line I. At that moment, detector line I lies in object line y3. The raster produced is therefore adapted to the intermediate line process of the TV system. Detector line I then forms the raster y1111: 2 4 6...

If the raster Y1, is delayed by Tz=64 us and clock signals I and II are interlaced in time as indicated in the lower part of Figure 1, then the following sequence is formed at the output of the adder E Y1 1 2 3 4 5 6... 2Nx.

The rastering obtained is therefore finer by the factor 2 than that which corresponds solely to the distance between the individual detectors of the detector lines I and 11. The scanning distance is thus x,, so that local frequencies of up to 1 2 XD can be transmitted free from aliasing by suitable pre- and post-filtration, whereas without these measures the aliasing limit would be 1 4 ' XD This detector arrangement may also be constructed using CCD-FPA devices. In this case, it is of interest to know whether it is possible to realise the necessary clock frequency with which the charge is shifted within a COD line.This clock frequency fHH, i.e. the reciprocal of the charge shifting time from one main store H to the next main store H via a temporary store Z must be chosen, as indicated in Figure 3b,so that the whole detector line is read out in the line forward run sweep time TH=52 Cis of the monitor electron beam.

The terms "main store" and "temporary store" are used in this example in connection with the bucket chain charge transport mechanism by which CCD's operate. Each main store is associated with one detector and receives from it the detector signals whereas the temporary stores only serve to transport the charge. It should be emphasized, however, that this is purely an imaginary concept used as an aid and is not an exact description of the physical process.

1/Nx of the sweep time tH is thus available for the clock period tHH, so that 1 1 fHH = tHH THINS A numerical example will now be given for assessing the clock frequency when the detector number N=3OO:- 1 fHH= 300=5.769 MHz.

52 us Even with this relatively high detector number, one obtains a clock frequency which is within the range of what can be realised for future IR-CCD detector arrangements.

As already mentioned, as shown in Figure 2, the values ny=4 and EXS=2 XD must be chosen if aliasing-free resolution is to be achieved up to the detector frequency x,D XD As shown in Figure 2, detector line I is again situated in the object line y, at the point in time t and forms the following object line raster: y11=1 3 5...

The vertical scanning interval again corresponds to the TV field time, and detector line II reaches object line y, by to=64 pus later than detector line I.

The latter is at that moment situated in object line y3 (intermediate line process). Detector line I then forms the following raster: y",: 1.5 3.5 5.5....

64 us later, the following raster is formed: y"": 2 4 6..., and a further 64 us later, the following raster: y11: 2.5 4.5 6.5....

If raster y,, is delayed by t=3 . tz (tz=64,us), raster y" is delayed bye=2 . tand raster y"" is delayed by l=1 . tz compared with rastery,,vand clock signals I to IV are interlaced in time as shown in the lower part of Figure 2, then the following sequence is obtained at the output of the summation device E: y,: 1 1.5 2 2.5 3 3.5....

The rastering obtained is then four times finer than that which would correspond to the distance between the individual detectors, and the frequency limit for aliasing-free transmission has been multiplied by the factor 4, in other words improved towards 1 XD The value obtained for the clock frequency HH is the same as that obtained with a two line arrangement if the same number is chosen for the detector lines Nx. Thus, for example, when No=300, the clock frequency fHH=5.769 MHz.It should be noted here that all the detector lines pick up new object line rasters every tz=64 us whereas the rasters produced before the time 3tz, 2t, and ltz in detector line I or before the time 2tz and ltz in detector line II or before time ltz in detector line Ill are still in the corresponding delay line (or shift register or store) to be subsequently worked off in stages.

The procedures described up to now are applicable to the first field or half image of the TV standard. These procedures are repeated in the second field or half image duration but the line rasters are now derived from the lines having the even numbered index, namely Y2, Y4, Y6, etc.

It is not necessary for the detector arrangement to carry out an opto-mechanical line shift jump for this purpose; only the transfer points for the object line signal must be suitably shifted, electrically and in time, in relation to the vertical scanning movement.

By means of the delay- and signal carry over times, the process described can be used to produce not only a double line jump but also a multiple line jump. With a 4:1 line jump, the frequency limit for aliasing-free transmission can also be increased in the vertical direction to the local frequency 1 yD determined by the geometry of the detector, y, being the size of the detector in the vertical direction.

The detector arrangement for IR detectors described above may be conventional, that is to say the individual detector leads extend from an associated Dewar vessel which surrounds the detectors as shown in Figure 4b. This entails certain difficulties in view of the large number of detectors and their leads. Alternatively, the detector arrangement may use COD technology with so called FPA's. In that case, the delay elements may be realised internally on the focal plane by means of COD shift registers, or externally. In the conventional detector technique, only the external solution is possible.

If one considers first the internal solution, in the arrangement of Figure 1, in the case of an FPA each detector line has a COD line (not shown) assigned to it. It is sufficient to read out the CCD I, with an appropriate delay, after CCD li. Figure 3c shows the corresponding pulse sequence.

In the arrangement of Figure 2, it is necessary to provide additional 3-stage registers la, Ib and Ic for detector lines I, II and III, or 2-stage registers Ila and llb, or a single stage register Illa, as shown in Figure 4, with serial transfer of the line rasters in order to produce the delay times. Alternatively, as illustrated in Figure 5, multiple buffer stores B with transfer between the buffers parallel but shifted in time, and subsequent serial read-out from the given lines COD must be provided.

With the external solution, on the other hand, the usual arrangement of one COD line per detector line must be provided in the first place. The necessary delays may then be realised either with conventional analogue delay lines or with external shift registers or analogue RAM's (random access memory).

Solution according to Figure 3 for the two line detector of Figure 1.

Figure 3 shows a delay arrangement for a two line detector, including a pulse diagram. As already mentioned, it is sufficient in this simple case to read out the CCD I with the appropriate delay after receiving line raster I whereas CCD II can be read out virtually without delay. The two line rasters are fed alternately to the circuitry controlling the monitor electron beam. In this solution, referring to Figure 3b, each of the two detector lines I and II has assigned to it a buffer line B for temporary storage and a COD line for charge transport. In the drawing only the three individual detectors D1, D2 and D3 are shown in each detector line for clarity. The drawing therefore also shows only 3 buffers B and 3 COD building blocks, each with a main store H and a temporary store Z.The individual detectors are connected to their respective buffers B through electronic switches SW1, and SW1 " and these buffers are connected to the corresponding main stores H through the electronic switches SW2, and SW211.

Placed above the pulse diagram of Figure 3c, Figure 3a shows the detector positions at times tyi, ty3, t andt yS y7 and ty7. The reception of the signal from object line y1 by way of detector line I takes place at time tyi, and the reception of the signal by way of detector line Il takes place tatty3, etc. Between tyi and ty3 may be seen the relatively long temporary storage of the line signal in buffer B,. This signal is fed to the main store H, only shortly before ty3, so that the buffer B, is available for accepting a new signal at ty3.The complete object line I, in this case comprising 1 3 5, is then read out between (ty1l)start and (ty1,)end. A similar process takes place in the elements of detector line 11. The only difference between this process and the events in I is that in this case B" transfers its charge immediately after receiving the signal while B, has completed the relatively long temporary storage. Object line II, comprising 2 4 6, is read out between (t, lI)start (which differs from (ty, )sta" only by tHH) and (t"|)end, and is automatically interlaced with object line I in the sequence 23456.

Figure 3d shows the sweep and flyback signals of the monitor electron beam line, from which it may be seen that the pick up of the signal and temporary storage take place during the return orflybacktime.

The ratio of forward trace to return trace in this line (52 to 12 us) has merely been indicated in this example for reasons of clarity of the drawing.

The external and internal solution of Figure 4 for the four line detector of Figure 2.

If the necessary delays of 3tz, 2tz and ltz indicated in Figure 2 are generated by serial shifting registers, then the principle illustrated in Figure 4, which shows how the line rasters Y, y111, Y11v propagate during tyi to ty7 and are available for interlacing at the given output of the detector lines I, 11, III, IV at ty7, is applicable both to the external and the internal solution.

For the external solution, the boundary between the COD detector arrangement and outside world lies on the line marked "external", and for the internal solution it lies on the line marked "internal".

In the latter case, the additional shift registers Ia, 1b and le, 11a and llb, and 111a are constructed as CCD's in exactly the same manner as the shift registers I, Il, Ill and IV, and for each arrangement part I, II, Ill they are not arranged linearly one behind the other but, for example, in parallel beside 1, II and Ill, although of course electrically they are arranged in series.

The two concepts, "externai" and "internal" refer to the arrangement of the delay elements outside or inside the focal plane, and that means outside or inside the Dewar. If, in particular, the detector arrangement shown in Figure 4b is constructed conventionally, then the arrangement parts I, II, lil and IV are to be regarded as situated to the left of the boundary line.

The internal solution of Figure 5 for the four line detector of Figure 2.

Parallel buffers B or shift registers may be provided instead of the serial shift registers between the detector lines I to ny. The basic layout may be seen from Figure 5 which shows the position of the individual line rasters in relation to tyi to ty7.

TDJ operation (time delay integration) The process described also enables time delay integration known per se to be carried out in the direction of scanning of the detector arrangement.

The arrangement required for this purpose is shown in the four line detector of Figure 6. Again, only three detectors (viewed vertically) have been shown per line for the sake of clarity. Scanning velocity must in this case be adjusted so that the distance ay,, (i.e. not only ay, as without TDJ) is traversed during the line duration tz. m transfers of signal must be carried out from one line trigger point to the next, where m is the number of TDJ detectors.

Claims (24)

1. A method of scanning an imaged scene comprising the steps of: providing a detector array including at least two parallel detector lines, each line comprising a plurality of regularly spaced detector elements with the elements of each line staggered relative to the elements of the or each other line; irradiating the detector array with an image of the scene and causing relative movement of the image and the array in a transverse direction with respect to the detector lines thereby to scan the image; generating, at predetermined time instants during a scan, a line raster from electrical signals produced by the detector elements of one of the detector lines and, after a delay, interlacing the raster with a raster or rasters produced from a subsequently irradiated detector line or lines.

2. A method according to claim 1, wherein the detector elements are sensitive to infra-red radiation and the said relative movement is produced by reflecting radiation from the scene onto the detector array from a scanning mirror.

3. A method according to claim 1 or claim 2, wherein the distances between individual detector elements of each detector line are equal to the width of elements.

4. A method according to any preceding claim, wherein each detector line is irradiated continuously and receives radiation energy from the whole field during each scan.

5. A scanning method with detector lines and electronic interrogation of the lines, using an infrared (IR) scanning mirror, in which: a) the radiant energy encounters an IR detector composed of several individual detector elements, which detector gives off corresponding electric signals after optoelectronic conversion, b) the scanning mirror directs the radiant energy in a vertical direction over the IR detector, and c) the distances between the individual detector elements of a detector line are equal to the dimension of the individual detector elements in the scanning direction, wherein::- d) the scanning mirror directs the radiant energy of the whole vertical field of vision continuously to the individual elements (D, to DNx) which are extended to horizontal, parallel detector lines (I to ny) and are shifted from line to line with a constant shift (nix,), and e) the line raster or rasters resulting from interrogation at certain points in time is or are interlaced with the raster of the following line or lines in an electronic interpretation after suitable delay.

6. A method according to claim 5, wherein the radiant energy and/or corresponding signals for predetermined local frequencies are filtered before and after rastering is carried out.

7. A method according to claim 5 or claim 6, wherein: a) for a two line detector, the first line raster is temporarily stored in buffers until shortly before the second line raster is received, and is then received in a shift register (CCD I), so that a delay is produced, b) the second line raster is temporarily stored in other buffers (B II) shortly after its reception by the detector arrangement and is then transferred to another shift register (CCD II), and c) the two rasters are interlaced by alternating, serial read-out of the corresponding shift registers and used as monitor line, the transfer and reception pulses occurring during the return run of the monitor electron beam (Figures 1 and 3c).

8. A method according to any of claims 5 to 7, wherein a time delay and integration step is carried out in the scanning direction, the scanning speed being adjusted so that the vertical path ny,, from the detector line of one block (I to IV) to the corresponding line of the next block is traversed in the line duration tz and m signal transmissions are carried out from one line trigger point the next (Figure 6).

9. A detector arrangement for carrying out a method according to any preceding claim, wherein the individual detector elements (D, to DNx) extend in horizontal, parallel detector lines (I to ny), and these lines are all shifted from one another by a constant distance.

10. An arrangement according to claim 9, wherein the shift (vexs) of the detector lines (I to ny) is less than or equal to half the reciprocal value of the predetermined local frequency.

11. An arrangement according to claim 9 or claim 10, wherein the line rasters are temporarily stored in an internal COD (charge coupled device) shift register or in an external shift register.

12. An arrangement according to claim 11, wherein, in the electronic interpretation circuitry, each individual detector element (D, to DNx) has assigned to it a respective main store (H) for receiving the corresponding detector signal, a temporary store (Z) serving for charge transport is provided between the individual main stores of a line in each case, and at least one buffer (B), also serving for intermediate storage, is provided between the detectors and the main stores assigned to them, all the stores feeding a common adder (t) (Figures3b,4,5).

13. An arrangement according to claim 12, wherein, between the detectors (D, to DNx) and associated buffers (B) on the one hand and between these buffers (B) and associated main stores (H) on the other hand, an electronic switch (SW) is provided in each case (Figure 3b).

14. An arrangement according to any of claims 10 to 13, characterised by CCD-FPA (charge coupled device-focal plane array) detector lines containing delay elements (la to Illa) provided (a) internally on the focal plane by means of COD shift registers or (b) externally (Figure 4).

15. An arrangement according to any of claims 10 to 13, wherein the individual detector elements (D, to DNx) have electrical output leads which extend out of a Dewarvessel surrounding the detectors and are electrically connected in parallel to first delay elements (Ic; 11,; Ills) associated with each detector line (Figure 4b).

16. An arrangement according to claim 14, wherein, in the case of the internal delay elements, for the serial transport of the line rasters when there are ny detect or lines, line I has (ny-1)times, line II has (ny-2) times, etc. the number of additional registers (Figure 4).

17. An arrangement according to claim 14, wherein buffers (B) which, as regards space, continuously increase in number from line to line and, as regards time, transport with a time shift, are connected in series between the detectors (D, to DNx) and the corresponding main stores (H) (Figure 5).

18. An arrangement according to claims 14 or 15, wherein, for the external solution, the delay from detector line to detector line is achieved by means of CCD's, analogue delay lines, shift registers or RAM (random access memory) devices.

19. An arrangement according to any of claims 9 to 18, wherein the distance between the centre of one detector element and the centre of the next is, as regards space, constant between the individual detector lines, being equal either to twice the detector height or to an integral multiple n and, as regards time, corresponds to the TV line duration or to an integral multiple thereof.

20. An arrangement according to claim 19 wherein the delay of the line rasters in the individual lines amounts to (n- 1), (n-2), (ny-3) times the monitor line duration when n=1 and ny=lV, or it amounts to n(ny 1), n(n,-2), n(ny-3) times the monitor line duration when ni1.

21. An arrangement according to any of claims 9 to 20, rotated through 90" when the monitor does not operate in the TV standard.

22. Scanning apparatus including a detector arrangement as claimed in any of claims 9 to 21.

23. A method of scanning an imaged scene, substantially as herein described with reference to the drawings.

24. A detector arrangement constructed and arranged substantially as herein described and shown in the drawings.