GB1587098A

GB1587098A - Image forming apparatus

Info

Publication number: GB1587098A
Application number: GB2229078A
Authority: GB
Original assignee: EMI Ltd
Current assignee: EMI Ltd
Priority date: 1978-05-25
Filing date: 1978-05-25
Publication date: 1981-04-01
Also published as: DE2921251A1; FR2427018A1; DE2921251C2; FR2427018B1

Description

(54) IMPROVEMENTS IN OR RELATING TO IMAGE FORMING APPARATUS (71) We, EMI LIMITED, a British company of Blyth Road, Hayes, Middlesex, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an image forming apparatus. In particular, the invention relates to such apparatus for forming a visible image representing the differential thermal radiance of a scene.

It is common practice, required by the technical difficulties of fabricating a large matrix of infra-red sensitive detectors, to dissect the image of a scene by a relatively small number of detectors which are scanned through the image, the electrical signals from the detectors being used to produce a visible representation of the scene, for example on a cathode-ray-tube display or a display coinprising an array of light-emissive diodes. In practice, because of the differences in responsivity of the detectors, the representation does not truly represent the differential radiance of the scene. The differences produce a "streaking" effect in the display, which is most troublesome in the so-called "parallel scanner" system where a scene is scanned in horizontal or vertical lines, there being as many detectors as there are lines.

The streaking effect can be eliminated by using only one detector to scan the whole scene point-by-point. However, the speed of scanning required may be quite impractical and the signal/noise ratio will probably be inadequate to produce a good representation of the temperature differentials in a typical scene.

An imaging system in which the effects of differences in responsivity of the detectors is reduced and in which an improved signal/noise ratio (relative to the single detector system) is obtained is disclosed in British Patent 1361144 (Hughes Aircraft Company). In that system a scene is scanned, in a raster scan, point-by-point by a single row of detectors. This is known as a "serial" scanner. The signals produced by the detec tors as they pass each point are delayed and summed to give the effect of only one detec tor of increased signal/noise ratio. This pro cess is known as "time delay and integrate".

In the system, the signals so produced are displayed on a television type cathode ray tube display or on a display comprising a two-dimensional array of light-emitting diodes with time multiplexing circuits, both types of display being provided with horizon tal and vertical synchronization signals derived from a scanner which is described in more detail in British Patent 1,361,145. The system is disadvantageous in that because in each field of the raster scan, all the detectors pass every scanned point of the scene, a high rate of scanning is required. Furthermore, the display is complex. The scanning rate could be reduced by using a plurality of paral lel rows of detectors, (giving what is known as the "serial/parallel scanner") but this would increase the complexity of the system because a line store would be required to store the signals from the detectors to display them using TV display standards.

British Patent 1482641 (EMI Limited) discloses a scanning arrangement for use with a CCIR standard 625 line TV monitor.

The arrangement uses 10 detectors which are scanned through the image of a scene in 629 sweeps per frame to give 629 lines per frame of which four are rejected. The scanning produces 2-fold interlacing of lines, and cor responding lines in successive frames are derived from different detectors, over ten frames. This allows the eye to integrate the overall picture on the monitor, which picture then appears less striated than would be the case without such scanning. Thus the effect of unequal detector sensitivities and/or detector channel gains, or of "dead" detec tors is reduced because the lines are derived from different detectors in successive frames and the eye integrates successive frames.

Such a scanning arrangement is, in practice, complex, line stores being provided for storing signals derived from the detectors before they are applied to the TV monitor.

It is an object of one aspect of the present invention to provide an image forming apparatus which avoids the complexity of TV displays and in which the operations of scanning of a scene and producing an image of the scene are performed using means common to both operations.

According to the said one aspect of the invention, there is provided a scanning means for use in an image forming apparatus for forming a visible representation of the variation of thermal radiance over a viewed scene, which apparatus comprises an array of detector means, responsive to said radiance for generating electrical output signals indicative of the variation of said radiance over respective areas of said scene, and an array of light emissive means, corresponding ones of said detector means and said emissive means being electrically coupled together via respective signal channels which operate upon said output signals to render them suitable for actuation of said emissive means, the scanning means comprising input scanning means for causing said detector to scan said scene, in synchronism, in elevation and azimuth, and output scanning means for causing the light emitted by said emissive means to provide said representation, said input and output scanning means sharing at least a component used for effecting the azimuthal scanning, wherein the input and output scanning means are arranged to scan in elevation at a first frequency and at the same rate in both senses of the elevational direction and in azimuth at a second frequency, the frequencies being such that, in operation, a scanning pattern is periodically repeated with a repetition period in the range of the integration time of the human eye viewing the said visible representation, during which period an integer number of cycles of scanning in elevation, and an integer number of cycles of azimuthal scanning takes place, the number of azimuthal cycles being not wholly divisible by the number of elevational cycles.

According to another aspect of the invention, there is provided an image forming apparatus for forming a visible representation of the variation of thermal radiance over a viewed scene, the apparatus comprising scanning means according to said one aspect, an array of detector means responsive to said radiance, the input scanning means being arranged to cause said devices to scan said scene, in synchronism, in elevation and azimuth, said detector means generating electrical output signals indicative of the variation of said radiance over respective areas of said scene, and an array of light emissive means, corresponding ones of said detector means and said emissive means being electrically coupled together via respective signal channels which operate upon said output signals to render them suitable for actuation of said emissive means, the said output scanning means being arranged to cause the light emitted by said emissive means to provide said representation.

For a better understanding of the present invention, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a schematic representation of an image forming apparatus according to the said one aspect of the invention; Figure 2(a) is a diagram illustrating the form of scanning performed by the apparatus of Figure 1, Figure 2(b) shows how detectors of the apparatus are arranged; Figure 3(a) is a side-elevational view, partly in section, of a practical embodiment of the scanning mechanism of the apparatus of Figure 1; Figure 3(b) is a detail of the mechanism viewed in section, perpendicular to the plane of the drawing, along line b-b of Figure 3a; Figure 3(c) shows in plan view the frame scanner and the line scanner of the mechanism; and Figure 4 shows a modification of the apparatus of Figure 1.

Referring to Figures 1 and 2, infra-red radiation from a scene 1 is directed via a germanium lens 2 to a reflective facet R of a line scanner 3. The line scanner comprises a regular polygonal member, in this example, a hexagon which is rotatable about an axis 4.

The line scanner is rotated by a motor M to scan the scene in a line scan direction 5 at a line scan frequency defined by the number of faces of the polygonal member and the rate of rotation of the member. The radiation reflects from the line scanner 3 onto a first reflector 6 of a frame scanner 7. The frame scanner 7 is oscillated at a frame scan frequency in a frame scan direction 8 perpendicular to the line scan direction 5 about an axis 9 perpendicular to, and extending along a line through, the axis 4. The frame scanner is so disposed in position and axis of rotation relative to the reflecting faces of the line scanner that the displacement of the image of an aperture stop As is minimised over the whole of the excursion of the frame scan. The axis 9 is positioned within the line scanner so that pure rotation (and no translation) is imparted to the image 7. The frame scanner is oscillated by a cam 10 rotated by the motor M, the cam 10 engaging with a cam follower 11 on the frame scanner 7. A spring 12 urges the cam follower 11 into engagement with the cam 10. The cam is shaped to impart an equiangular triangular waveform to the movement of the reflector 6.

The radiation reflected from the reflector 6 is directed by another germanium lens 13 onto an array of e.g. ten infra-red detectors 14 arranged side-by-side as shown in Figure 2b. The effect of the scanning is that the ten detectors simultaneously scan through a block 15 of ten lines 16 of the scene on each line scan at a constant rate in one sense of the line scan direction with instant fly back between scans. The detectors scan succesive blocks in the frame scan direction, at the same rate in both senses of the frame scan direction (i.e. without instant flyback).

The signals produced by the detectors 14 are supplied to respective signal processing channels 17 which process the signals and supply them to respective light-emissive diodes of an array 18 corresponding to the array of detectors 14. An example of the channels 17 is disclosed in our copending patent application 7903400 - publication number 2014395. The diodes 18 produce visible light which is directed by a lens 19 to a second reflector 20 of the frame scanner 7 and thence onto the line scanner 3. The frame and line scanners form the light into an image representing the differential radiance of the scene, which image is viewed via another lens 21, by the observer 22.

The eye of the observer integrates the light image, with an apparent integration time of about 0.2 seconds at the light levels which are likely to be experienced from the LEDs (when the apparatus is designed for night time use) and the pattern of the scanning performed by the line and frame scanners is arranged to take advantage of the integration by the eye thereby to reduce "streaking" in the image due to unequal detector sensitivities or unequal signal processing chan nelRains.

In this example, the line scan and frame scan frequencies are chosen so that the line scan and frame scan together complete an integral number of cycles only in the period of 3 frame scan cycles. The frame scan frequency is chosen to be e.g. 20Hz, and the line scan frequency is chosen to be eg. 713.33Hz.

Thus the period of 3 frame scan cycles is 0.15 seconds, which is less than the integration time of the eye, and in that time 107 line scan cycles takes place (bearing in mind that the frame scanning rate is the same in both senses of the frame scan direction). The numbers are chosen so that there is a noninteger number of line scans in each cycle of the frame scan; in this example the number (107) of line scans to be performed in 3 frame scan cycles is chosen to be not wholly divisible by 3. There are thus 17 5/6 line scans per half cycle of the frame scan. Consequentially, the scanning pattern by which the scene 10 is scanned and the corresponding image is built up has the form shown in Figure 2(a). (It is to be appreciated that Figure 2(a) is not an accurate representation of the scanning pattern but merely roughly indicates the form of the pattern).

The scene is scanned in successive blocks 15 of 10 lines 16, each block being the result of one line scan cycle. The lines 23 in Figure 2a indicate the centre line of blocks scanned in a first half cycle of 3 frame scan full cycles.

Line 23' indicates that 5/6 of a line scan takes place at the end of the first half cycle of the frame scan and the remaining 1/6 of the line scan is completed during the second half cycle of the frame scan as indicated by line 24'. Other line scans performed during the second half cycle are indicated by lines 24.

The second half cycle of frame scan ends after a further 17 5/6 line scans, with 4/6 of a line scan being performed as indicated by line 24", the remaining 2/6 thus being completed in the third half cycle of frame scan as indicated by line 25.

The effect of this is to produce interlacing of the line scans on successive half cycles of the frame scanning, and any one portion of the scene is scanned by different detectors on successive half cycles of the frame scanning.

The scanning pattern is arranged to repeat itself every 3 full cycles of frame scanning.

The image of the scene viewed by the eye is built up in exactly the same way. As the eye integrates the image over the 3 full cycles of the frame scanning unequal detector sensitivities or unequal signal processing channel gains are averaged out as any one portion of the image is derived from different detectors on successive half cycles of the frame scanning.

The ratio of the line scan and frame scan frequencies is defined by a gear train G coupling the motor M to the axis 4 and to the cam 10. The train includes a line scan gear Land a frame scan gear F.

An example of the calculation of the number of teeth on the gears L and F the speed of the line scanner and other parameters will now be given.

The frame scan mirror is driven with an equiangular triangle waveform at a frequency NF Hz, through an angle 00. The line scan mirror has R facets, and rotates at NL revs/sec, giving a line-scan frequency of RNL Hz. A particular point in the scanned field will reappear in its original position when both scans have simultaneously completed an integral number of scans; let this occur after 3 frame scans. We must first calculate a value for NL.

The frame scan velocity VF = 2ONF deg/sec. Let there be Q detector widths in the active line scan width, and the display aspect ratio, width height h be m. Then, there are Q/m detector widths/picture height. During a line scan period, let the array move downwards in the frame scan direction byr resolution elements.

The angular resolution element at the mirroris 8 = mB/Q So,0 = O8/m as VF2QNF VF=2 NF/m = 2UNF/m resolution elements/second.

Thus, to move down a distance r elements in a line time tL, as VF = r/tL tL = rm/2QNF so the line scan frequency RNL = 1/ tL = 2QNF/mr After the repeat period of 3 cycles of the frame scan, the line scan must also complete an integral number of cycles. The time available is 0.15 seconds (for 3 cycles of frame scan). Therefore 0.15 RNL must be integer, not wholly divisible by 3.

To allow integer values for the gear teeth on the drive, say F on the frame gear and L on the line gear, NL = NF X F/L. Therefore 0.15 RNF F/L must be integer. For a hexagon, R = 6. Put Q = 300,Therefore 0.15x 20x 6x L must be integer, not wholly divisible by 3 = 18 F/L = 3n+ 1, where n is an integer. Therefore F/L = (3n+ 1)/18 The number of sweeps in a top to bottom scan, Ns = RF = R(3ni1)/36 = (3n1)/6 2L where = 6 let (3n-+1)= 107 Therefore Ns = 17.833, and the number of teeth on the line gear L = 36. Therefore the number of frame gear teeth F = 2x 36x 17.833/6 = 214.

So mar = 2QL = 2x 300x 36 = 16.822 RF 6x214 resolution elements.

Referring to Figure 2b, the detector ele ments are pitched at 1.25 element widths, so this dictates the value of the width to height ratio m, because mr = 16.822 elements.

= 16.822/1.25 pitches = 13.458 pitches.

Because r = 11x1.25 elements, m = 13.458/11 = 1.223.

In practice, unity efficiency will not be achieved for the frame scan, because of the mirror inertia. For a likely value of frame scan efficiency of 0.85, the displayed aspect ratio will be 1,223/0.85 = 1.44.

The speed of the line scan drum will be NL = NF X F Revs/sec L = 20x214 x 60 Rev/min 36 = 7133.33 R.P.M.

an acceptable speed.

It is to be appreciated that the numbers given in the foregoing are exemplary only.

Other numbers of facets 12, frequencies of line and frame scanning, of detectors and diodes, of resolution elements Q per picture width and of other parameters could be cho sen.

Each detector of the array 14 could be replaced by a row 140 of two or more detec tors extending in the line scan direction. The detectors of each row would be connected to a a delay and summation circuit 141 as shown in Figure 4. The delay and summation circuit would delay the signals from the detectors of the row such that signals produced in response to the same portion of the scene are summed to give the effect of a single detector of increased signal to noise ratio. The summed signals produced by the circuits 141 would be applied to the respective channels 17.

Figures 3a to c show an example of a prac tical embodiment of the frame and line scan ners of the apparatus of Figure 1. In the figures the same reference numerals as are used in figure 1 have been used to indicate equivalent items of the two figures.

Referring to Figure 3a, a frame assembly 31 comprises columns 311 which support end plates 312, 313 and a central platform 314 in spaced relation. An axis 4 is rotatable in bearings 32 in the end plates and extends through an annular projection 36 of the cen tral platform 314. A hexagonal line scanner 3 having facets R is supported by the axis 4.

The scanner 3 has a recess 33 into which the projection 36 projects. Referring to Figure 3b, the projection 36 supports in the recess 33 bearings 34, defining an axis 9 perpen dicular to axis 4 about which a frame scanner 7 is pivotable.

The frame scanner comprises a rectangu lar frame 37 as shown in Figure 3c surround ing the axis 4 and projection 36. Legs 38 extend from the frame parallel to the axis 4 and projection 36, supporting the frame on the bearings 34. By pivoting the frame scan ner about a point within the line scanner rotation without translation is imported thereto. Reflectors 6 and 20 of the frame scanner are supported on arms 39 extending from opposite corners of the frame 37.

A motor M is supported by the central platform 314. The motor is coupled by gears G to the axis 4 to rotate the line scanner and by the gears G to a cam 10 which engages a cam follower supported on a finger 40 upstanding from the frame 7 to oscillate the frame scanner. The finger 40 projects through an opening 41 in the central plat form. A spring 12 is connected at one end to the end plate 312 and at the other end to a retainer 42 on the frame 7 to urge the cam follower 11 into abutment with the cam 10.

A chopper 43 is connected to the line scanner 3 to chop radiation reflected from the frame scanner to the detectors via an exit pupil 44 in the end plate 313.

Instead of using e.g. ten detectors extending in the frame scan direction, twelve, for example, or more detectors slightly inclined to the frame scan direction could be used to scan a block of the same width as is scanned by ten detectors.

The exemplary apparatus produces an image with reduced striations with a practical scanning speed whilst avoiding the complexities of TV type displays.

WHAT WE CLAIM IS: 1. A scanning means for use in an image forming apparatus for forming a visible representation of the variation of thermal radiance over a viewed scene, which apparatus comprises an array of detector means, responsive to said radiance for generating electrical output signals indicative of the variation of said radiance over respective areas of said scene, and an array of light emissive means, corresponding ones of said detector means and said emissive means being electrically couplied together via respective signal channels which operate upon said output signals to render them suitable for actuation of said emissive means, the scanning means comprising input scanning means for causing said detector means to scan said scene, in synchronism, in elevation and azimuth, and output scanning means for causing the light emitted by said emissive means to provide said representation, said input and output scanning means sharing at least a component used for effecting the azimuthal scanning, wherein the input and output scanning means are arranged to scan in elevation at a first frequency and at the same rate in both senses of the elevational direction and in azimuth at a second frequency, the frequencies being such that, in operation, a scanning pattern is periodically repeated with a repetition period in the range of the integration time of the human eye viewing the said visible representation, during which period an integer number of cycles of scanning in elevation, and an integer number of cycles of azimuthal scanning takes place, the number of azimuthal cycles being not wholly divisible by the number of elevational cycles.

2. Scanning means according to Claim 1, wherein a prime number of azimuthal cycles of scanning takes place in said repetition period.

3. Scanning means according to Claim 1 or 2 for use in an apparatus wherein the detector means, light-emissive means, and the signal channels are mounted to be stationary relative to each other, the scanning means being arranged to direct radiation from successive portions of the scene to the detector means and to direct light from the light-emissive means to form the said representation.

4. Scanning means according to Claim 3 comprising a regular polygonal reflector member mounted to rotate in the azimuthal direction about a first axis extending in the elevational direction, the member having reflective faces facing radially outward of the axis, a further reflector member mounted to pivot about a second axis perpendicular to said first axis, the further member having two reflective faces spaced from and on respective opposite sides of, the second axis, the reflective faces of the further member being respectively co-operable with the reflective faces of the polygonal member to direct radiation from the scene to the detector means, and to direct light from the light-emissive means to form the said image, and drive means for oscillating the further member at the said first frequency and for rotating the polygonal member at a rate dependent on the number of reflective faces thereon and the said second frequency.

5. Scanning means according to Claim 4, wherein the said second axis extends along a line which passes perpendicularly through the first axis at a point within the polygonal member.

6. Scanning means according to Claim 4 or 5, wherein the drive means includes an electric motor coupled via a gear to the polygonal member for rotating the polygonal member and to a cam arrangement for oscillating the further member.

7. Scanning means according to any preceding claim, wherein the said repetition period is in the range 0.1 to 0.3 seconds.

8. Scanning means according to Claim 7, wherein the said repetition period is in the range of 0.15 to 0.25 seconds.

9. Apparatus according to Claims 4, 5 or 6 or Claim 7 or 8 when appended thereto wherein the polygonal member has six reflective faces.

10. Scanning means substantially as hereinbefore described with reference to Figures 3a, b and c of the accompanying drawings.

11. An image forming apparatus for forming a visible representation of the variation of thermal radiance over a viewed scene, the apparatus comprising scanning means according to any preceding claim, an array of detector means responsive to said radiance, the input scanning means being arranged to cause said devices to scan said scene, in synchronism, in elevation and azimuth, said detector means generating electrical output signals indicative of the variation of said radiance over respective areas of said scene, and an array of light-emissive means, corresponding ones of said detector means and said emissive means being electrically coupled together via respective signal channels which operate upon said output signals to render

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the frame scanner to the detectors via an exit pupil 44 in the end plate 313. Instead of using e.g. ten detectors extending in the frame scan direction, twelve, for example, or more detectors slightly inclined to the frame scan direction could be used to scan a block of the same width as is scanned by ten detectors. The exemplary apparatus produces an image with reduced striations with a practical scanning speed whilst avoiding the complexities of TV type displays. WHAT WE CLAIM IS:

1. A scanning means for use in an image forming apparatus for forming a visible representation of the variation of thermal radiance over a viewed scene, which apparatus comprises an array of detector means, responsive to said radiance for generating electrical output signals indicative of the variation of said radiance over respective areas of said scene, and an array of light emissive means, corresponding ones of said detector means and said emissive means being electrically couplied together via respective signal channels which operate upon said output signals to render them suitable for actuation of said emissive means, the scanning means comprising input scanning means for causing said detector means to scan said scene, in synchronism, in elevation and azimuth, and output scanning means for causing the light emitted by said emissive means to provide said representation, said input and output scanning means sharing at least a component used for effecting the azimuthal scanning, wherein the input and output scanning means are arranged to scan in elevation at a first frequency and at the same rate in both senses of the elevational direction and in azimuth at a second frequency, the frequencies being such that, in operation, a scanning pattern is periodically repeated with a repetition period in the range of the integration time of the human eye viewing the said visible representation, during which period an integer number of cycles of scanning in elevation, and an integer number of cycles of azimuthal scanning takes place, the number of azimuthal cycles being not wholly divisible by the number of elevational cycles.

them suitable for actuation of said emissive means, the said output scanning means being arranged to cause the light emitted by said emissive means to provide said representation.

12. Apparatus according to Claim 11, wherein the array of detector means comprises a column of detectors inclined to the elevation scan direction.

13. Apparatus according to Claim 12, wherein the detector means comprises two parallel columns of detectors inclined to the elevation scan direction, the detectors in one column being offset from those in the other column in the direction of greatest extent of the columns.

14. Apparatus according to any one of Claims 11 to 13, wherein the array of detector means comprises n detectors arranged to simultaneously scan n lines of the scene in azimuth and wherein twice the said integer number of cycles of scanning in elevation is lessthann.

15. Apparatus according to Claim 14, wherein n equals ten or twelve and twice the said integer number of cycles of scanning in elevation equals six.

16. Apparatus according to Claim 11, wherein the light emissive means comprises light emitting diodes.

17. Apparatus according to Claim 11 wherein each detector means comprises a plurality of detectors arranged in a row extending in the azimuthal direction, and the processing means comprises means for delaying and summing the signals produced by the detectors of each detector means.

18. An image forming apparatus substantially as hereinbefore described with reference to Figures 1 and 2 of Figures 1 to 3, optionally as modified by Figure 4 of the accompanying drawings.