GB2303756A

GB2303756A - Reduction of narcissus

Info

Publication number: GB2303756A
Application number: GB8800975A
Authority: GB
Inventors: Jonathan Paul Kettlewell
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1988-01-16
Filing date: 1988-01-16
Publication date: 1997-02-26
Anticipated expiration: 2008-01-16
Also published as: GB2303756B; GB8800975D0

Abstract

Narcissus in a thermal imager occurs when a cooled detector 1 is forced to "look at" itself by partial reflection from a component of the imager. Apparatus for reducing narcissus includes a hot IR source 8 projecting radiation towards any partially reflective component within the body of the imager, and a beam splitter 7 for passing a portion of the source radiation reflected at the component to the detector or detectors 1 to compensate for any narcissus radiation. Open or closed loop feedback control may be included to allow for changes in operating conditions, the control operating to control the output of the IR source according to the radiation intensity of the imager body.

Description

Reduction of Narcissus This invention relates to the reduction of narcissus in a thermal imaging system.

Thermal imagers generally comprise an optical system, one or more scanning elements and one or more detectors of infra-red radiation. It is important that noise, either generated within the detector or background radiation is not allowed to interfere with the readings of the detector or detectors, and hence the detectors are often cryogenically cooled to reduce the effects of such noise.

If one of the detectors does happen to look at an image of itself or of its cooled surroundings by, for example partial reflection from transmitting elements in the imagers telescope, then the effect known as narcissus is observed.

Narcissus generally occurs when rays projected within the imaging beam from the cold detector have a near normal incidence angle on a partially reflective element of the telescope. The rays which are imaged back into the detector are characteristic of a very cold source, i.e.

the detector itself. For large field angles, the imaging beam is less likely to be normally incident on the elements, and the rays reflected back to the detector will indicate a higher source temperature, that of the body of the imager. The narcissus effect is therefore seen on the observed image as a comparatively cold region, usually close to the centre of the imagers field of view.

The present invention seeks to reduce and at best to eliminate this degradation in the image caused by narcissus.

According to the present invention there is provided a thermal imager including an infra-red detector, means for cryogenically cooling the detector, means for providing an imaging beam at the detector, an infra-red source for providing radiation characteristic of a higher temperature than that of the detector and means for passing at least a portion of the radiation from the source into the imaging beam to compensate at least partially for narcissus.

Preferably a beam splitter is used to pass radiation to the detector.

By providing an open or closed loop feedback system then the amount of radiation from the hot source injected into the imaging beam can be adaptively altered in order to match the apparent temperature of the sum of the injected radiation and the radiation from the detector with the apparent temperature of the imager body to continually compensate for narcissus.

An open loop feedback loop may be arranged to respond to the output of the I.R. source and to the radiance of the body of the imager. Alternatively a closed feedback loop may be arranged to sense the summed radiation from the detector and I.R. source, and to match this with the radiance of the imager body.

The radiance of the imager body may be monitored by means of a thermocouple.

The invention also provides a method of reducing narcissus in a thermal imager of the type including at least one cryogenically cooled detector, which narcissus arises from reflection of the or each detector at a partially reflective element or elements; comprising; injecting radiation into the imaging beam on the detector side of, and directed towards, the partially reflective element; and directing at least a portion of the radiation, after reflection at the element, to the detector.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows schematically a conventional thermal imager; Figure 2 shows schematically a thermal imager according to the present invention; Figure 3 shows a closed loop feed-back system, and Figure 4 shows an open loop feedback system.

Referring to Figure 1 there is shown schematically a thermal imager comprising a detector 1 mounted behind a highly reflective radiation shield, or "cold shield" 2.

Imaging radiation from an optical system such as a telescope 3 and a scanning unit 4 passes through an aperture stop 6 and is focussed by a lens system 5 onto the detector. An output is taken from the detector to apparatus which processes the image and ultimately displays or stores it. In practice a plurality of detectors would probably be used in a swathe-scanned or staring array configuration. Although telescope 3 consists of transmissive elements it will inevitably have some reflection characteristics and a small proportion of radiation emitted from the cooled detector 1 at near normal incidence will be reflected back onto the detector.

Detector element 1 is cryogenically cooled to a very low temperature and thus, in seeing "itself" records a very low temperature at that point, rather than the actual temperature of the image it is supposed to be viewing.

Thus inaccuracies result in such previous systems.

Figure 2 shows a system according to the present invention which essentially injects radiation into the imaging beam on the detector side of, and directed towards, the partially reflective element or elements.

The apparatus is modified from that of Figure 1 in that it includes a hot and efficient IR radiating source 8 and a beam splitter 7. The I.R. source is typically characteristic of a source at around 10000C and is spatially matched to the cooled area of the detector as viewed fromt he scanning unit 4. In one application, the beam splitter may allow only about 3% of impinging radiation to pass.

The apparatus is arranged such that when the imaging beam has a near normal incidence to the partially reflective elements, the injected radiation is passed through beam splitter 7 towards the telescope together with the narcissus radiation from the detector 1. A small amount of the injected radiation is reflected by the telescope together with the detector radiation and is imaged back by reflection at beam splitter 7 onto the detector. Thus, by arranging that the sum of the amount of injected radiation reflected back to the detector and the detector radiation is equal to the radiance of the imager body, as seen by the detector, then an image free of narcissus is obtained.

The level of reflected detector and injected radiation must be equivalent to the radiation reflected from the body of the imager in order that perfect compensation is provided. Since the level of reflected radiation alters as the radiation source, i.e. the imager body, changes temperature then some form of feedback loop is useful to alter the amount of injected radiation.

Figures 3 and 4 respectively show alternative closed and open feedback loops for achieving this purpose.

In Figure 3 the I.R. source 8 is driven by a current source 9. Radiation from both source 8 and the detector 1 is passed, by means of beam splitter 7, to a first infrared sensor 10 which measures the sum of the detector radiation and the radiation from source 8. A second infra-red sensor 11 is arranged to measure the radiance of the imager body. A comparator 12 takes inputs from respective sensors 10 and 11 and generates an output signal if there is a difference between the two respective sensor outputs in order to increase or decrease the output from I.R. source 8 to balance or match the radiation from the imager body with the sum of the radiation from the I.R. source and the radiation for the detector.

Figure 4 shows an alternative open loop arrangement in which radiation emitted only by the hot source 8 is detected by sensor 10a. The radiation from the imager body is detected by a sensor lla which may conveniently be a thermocouple as shown. After comparison with a reference voltage, Vref the output from a thermocouple and I.R. sensor 10a are compared at comparator 12a and used to adjust a current source 9a as before. The circuit of Figure 4 does assume that the amount of radiation from the hot element 8 which is reflected back to a detector 1 is dependent upon, and bears a fixed relationship to the heat output from element 8. In practice this will generally be true and hence the circuit of Figure 4 will generally be used in preference to the more complex and difficult to perform circuit of Figure 3.

Claims

CLAIMS 1. A thermal imager including an infra-red detector, means for cryogenically cooling the detector, means for providing an imaging beam at the detector, an infra-red source for providing radiation characteristic of a higher temperature than that of the detector and means for passing at least a portion of the radiation from the source into the imaging beam to compensate at least partially for narcissus. 2. A thermal imager as claimed in claim 1 including means for injecting radiation from the source into the imaging beam on the detector side of, and directed towards, any partially reflective element or elements within the imager and means for directing at least a portion of the thus reflected radiation to the detector. 3. A thermal imager as claimed in claim 2 wherein the means for injecting radiation and directing a portion of the radiation to the detector includes a beam splitter. 4. A thermal imager as claimed in any of the preceding claims including a feedback loop for equalising the sum of the detector radiation and injected radiation with the radiance of the imager body. 5. A thermal imager as claimed in any of the preceding claims including a feedback loop responsive to the temperature of the imager body to alter the output of the radiation source. 6. A thermal imager as claimed in any of the preceding claims including means for limiting the spectral passband of at least the portion of the radiation from the source which reaches the detector. 7. A method of reducing narcissus in a thermal imager of the type including at least one cryogenically cooled detector, which narcissus arises from reflection of the or each detector at a partially reflective element or elements; comprising; injecting radiation into the imaging beam on the detector side of, and directed towards, the partially reflective element; and directing at least a portion of the radiation, after reflection at the element, to the detector. 8. A thermal imager substantially as hereinbefore described with reference to, and as illustrated by Figure 2 of the accompanying drawings. 9. A thermal imager including a feedback system, for reducing narcissus, substantially as hereinbefore described with reference to, and as illustrated by Figure 3 or Figure 4 of the accompanying drawings. 10. A method of reducing narcissus in a thermal imager substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings. Amendments to the claims have been filed as follows

1. A thermal imager including; a cryogenically cooled infra-red detector; means, including an imager body, for providing an imaging beam at said detector; an infra-red source for providing radiation characteristic of a higher temperature than that of said detector; means for injecting radiation from said source into the imaging beam on the detector side of, and directed towards, any partially reflective element or elements within the imager body and for directing at least a portion of any thus reflected radiation to said detector; and circuit means responsive to the radiation produced by said source and the radiation from said imager body for adjusting the output of said source to provide at least partial compensation for narcissus.

2. A thermal imager as claimed in claim 1 wherein the means for injecting radiation and directing a portion of the radiation to the detector includes a beam splitter.

3. A thermal imager as claimed in any of the preceding claims wherein said circuit means includes a feedback loop for equalising the sum of the detector radiation and injected radiation from said source with the radiation from the imager body.

4. A thermal imager as claimed in any of the preceding claims wherein said circuit means includes a feedback loop responsive to the radiation from the imager body to alter the output of said source.

5. A thermal imager as claimed in claim 3 wherein said feedback loop includes: first infra-red sensor means for sensing the sum of the output radiation from said source and radiation from said detector, and for providing a first output signal corresponding to said sum; second infra-red sensor means for sensing the radiation of said imager body and for providing a second output signal corresponding thereto; means for comparing said first and second output signals and for producing a third output signal corresponding to any difference between said first and second output signals; and means responsive to said third output signal for adjusting the output radiation of said source to cause said first and second output signals to be substantially equal.

6. A thermal imager as claimed in claim 4 wherein said feedback loop includes: a first infra-red sensor for sensing the output radiation from said source and for producing a first output signal corresponding to said output radiation; second infra-red sensor means for sensing the radiation of said imager body and for producing a second output signal corresponding thereto; means for comparing said first and second output signals and for producing an output signal corresponding to any difference between said first and second output signals; and means responsive to said third output signal for adjusting the output radiation of said source to cause said first and second output signals to be substantially equal.

7. A thermal imager as claimed in claim 6 wherein: said second infra-red sensor mean'includes a thermocouple responsive to said radiation of said imager body and means for comparing the output signal of the thermocouple with a reference voltage to provide said second output signal.

8. A method of reducing narcissus in a thermal imager of the type including at least one cryogenically cooled detector, which narcissus arises from reflection of radiation from the at least one or each detector at a partially reflective element or elements within an imager body; comprising; injecting infra-red radiation characteristic of a higher temperature than that of said detector from a source into the imaging beam on the detector side of, and directed towards, the partially reflective element or elements; directing at least a portion of the injected infra-red radiation, after reflection at the element or elements, to the detector; sensing the radiation reflected by said imager body; sensing at least the radiation from said source; comparing the sensed reflected radiation from said imager body with the sensed radiation from at least said source to determine any difference; and adjusting the output radiation from said source to substantially cancel any said difference.

9. A method as defined in claim 8 wherein said step of sensing at least the output radiation from said source includes sensing the sum of the output radiation from said source and radiation from said detector.

10. A thermal imager substantially as hereinbefore described with reference to, and as illustrated by Figure 2 of the accompanying drawings.

11. A thermal imager including a feedback system, for reducing narcissus, substantially as hereinbefore described with reference to, and as illustrated by Figure 3 or Figure 4 of the accompanying drawings.

12. A method of reducing narcissus in a thermal imager substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.