GB2307125A - Optical equipment - Google Patents

Abstract

This invention relates to optical equipment, (particularly but not exclusively to equipment) such as an infra-red line-scanner, which images infra-red radiation received from a viewed scene. Low-pass filter 5, converter 7 and inverter 8 function so that static aspects of the scanned scene are cancelled out. The equipment is aircraft mounted and change depends upon aircraft movement. Should the change be due to roll-exceeding 50, the cancelling signal is held fixed. In particular, parts of the image due to the equipment itself or the aircraft are cancelled out.

Description

optical EquiPment This invention relates to optical equipment, more particularly but not exclusively to equipment, such as an infra-red line-scanner, which images infra-red radiation received from a viewed scene.

In a line-scanner, a rotating polygonal scanner mirror receives infra-red radiation from say the ground beneath an aircraft and directs it, via suitable collecting mirrors and focussing optics, to an infra-red radiation detector. The result is a video signal indicative of a line by line scan of the ground. The nature of the scanning mechanism is such that, as well as the ground scene, the scanner 'sees' its own internal structure which imposes an unwanted temperature profile on the image. Attempts to shade the line scanner structure by the use of baffles and attempts to reduce temperature differences within the structure e.g. by the use of heat pipes, are not always successful.

An object of the invention is to electronically compensate for the effect noted.

According to the invention there is provided optical imaging equipment including an electrical circuit for forming a compensation signal representative of that part of a generated video signal which is due to the equipment viewing its own structure or other error including areas, and for combining the video signal and the compensation signal to nullify said part.

The single figure of the accompanying drawing is a block diagram of a video compensation circuit for a line-scanner, the diagram being given by way of example only.

The line-scanner (not shown) for which the illustrated compensation circuit is intended produces two channels of video, CHA and CHB. One of these channels is tapped to a low pass analogue filter, amplifier and limiter 1 from which the signal passes to an analogue to digital convertor 2.

The digitised signal is then passed to a digital filter arrangement 3, for example a microprocessor controlled by software stored in a programmable read only memory (PROM), which is functionally equivalent to a short time constant low pass filter 4 and a longer time constant low pass filter 5 coupled to one another by way of an edge to one another by way of an edge and asymmetry threshold detector 6.

The line scanner (not shown) scans the ground beneath the carrier aircraft along scan lines which are generally transverse to the along-track direction, i.e. the direction on the ground along which the aircraft is travelling. The result is a video signal comprising successive portions, separated by line flybackisynchronisation intervals, representing successive across track scan lines. Because the aircraft is travelling over the viewed scan, the video signal is not divided into frames or field periods as with a normal sort of T.v. picture signal - instead, if the video signal, were displayed on a T.V. monitor there would be seen a continuously rolling picture of the viewed ground scene.The effect of the scanning mechanism of the scanner 'seeing' its own internal structure and perhaps parts of the aircraft structure also, is to cause each line portion of the video signal to comprise a background pattern which is more or less the same for each portion so that, if displayed the picture might comprise a bright column or perhaps a dark column, usually at one or perhaps both sides of the picture.

If a blank scene were viewed, then on an oscilloscope the video signal would ideally comprise a series of flat-topped square pulses. The effect of the scanner seeing the scanner structure might be to cause the pulse tops to be humped or bell-shaped, the actual shape of course depends upon the structure of the scanner and its carrier, and the precise conditions, e.g. the aircraft speed.

The low pass filter 5 produces a running average of the video signal in the along-track direction. This average is stored by a pattern memory, (functionally included in block 5) so that the memory holds a pattern representing the long term mean level of the video signal at intervals across each scan line, i.e. so that from block 5, there is delivered a digital signal corresponding to the average shape of each line portion of the video signal. This is converted to an analogue signal by converter 7, low pass filtered and inverted by unit 8 then subtracted from each video channel by summing amplifiers 9 and 10. As the long term average changes, the pattern of signals held in block 5 changes to suit, i.e. so that the compensation signal remains in correspondence with the 'unwanted' pattern imposed on each line portion of the video.

The effectiveness of the method depends on the black shading pattern being fixed in position with respect to the airframe of the carrier aircraft and changing slowly with time compared with the video signal. Since the horizon will be present continuously, correction is not applied at the extremes of the video field-of-view. Also, to avoid correcting for the horizon or the pedestal offset signal, computation if halted while the roll angle exceeds 5 , so that the correction applied is held fixed.

Furthermore, along-track features such as coastlines and motorways, which can present for significant time, result in the generation of a false correction pattern. This problem has been minimised by using the second digital filter 4 of shorter time constant to control the operation of the main filter 5. If the pattern generated by the second filter 4 exhibits slew rate or asymmetry levels exceeding preset thresholds, operation of the main filter is inhibited and the correction pattern is not updated. This second filter operates continuously and allows normal operation to resume as soon as its generated pattern is within the threshold limits.

The digital filtering functions and pattern storage may be performed by a Transputer (Type T212). The characteristics of both filters are controlled by software stored in a PROM, thus allowing design flexibility.

For the purpose of trials an "inhibit" switch may be provided which allows the uncorrected video signal to be output instead of the corrected video, so that the effectiveness of the black shading correction can be judged; while the uncorrected video is being output, computation of the correction pattern continues normally.

Claims (1)

1. Optical imaging equipment including an electrical circuit for forming a compensation signal representative of that part of a generated video signal which is due to the equipment viewing its own structure or other error including areas, and for combining the video signal and the compensation signal to nullify said part.

Amendments to the claims have been filed as follows CLAIMS

1. Optical imaging equipment for generating a composite video signal, said signal comprising two parts, one part representing a viewed scene and a second part representing an unwanted error signal which is at least in part derived from the internal structure of said equipment, and in which the equipment is provided with an electrical circuit including first filter means for producing a running average of the composite video signal,compensating means for subtracting the running average from the composite video signal thereby compensating for the presence of an unwanted error signal and second filter means, having a shorter time constant than the first filter means, for producing a filtered version of the composite video signal and for inhibiting operation of the first filter means when said filtered composite signal exceeds a pre-determined threshold value.