GB2225914A

GB2225914A - Heat image instrument with detector compensation

Info

Publication number: GB2225914A
Application number: GB8923439A
Authority: GB
Inventors: Wolf-Dieter Vogt; Wolfgang Weigel
Original assignee: Eltro GmbH and Co
Current assignee: Eltro GmbH and Co
Priority date: 1988-10-25
Filing date: 1989-10-18
Publication date: 1990-06-13
Anticipated expiration: 2009-10-18
Also published as: GB2225914B; GB8923439D0; DE58903280D1; DE3836294C2; EP0365948B1; DE3836294A1; FR2638243A1; ES2037365T3; FR2638243B1; EP0365948A1

Description

1 HEAT IMAGE INSTRUMENT WITH DETECTOR COMPENSATION The invention relates

to a heat image instrument with detector compensation, and in particular to such an instrument having an infra-red telescope, a scanning device, a detector objective lens, and a detector with compensation, the detector being coupled to opto-electronic conversion means for producing a visible image.

A combination of apparatus with a heat image instrument of the above description is known from DE 33 29 588 C1. The prior art combination has a compensation facility for the detector, although this is not actually stated in the relevant context. Such compensation can be used manually via potentiometers or automatically in a signal processing unit downstream of the detector. While in the various known individual detector devices manual compensation is, for practical purposes, no longer feasible by virtue of the amount of space and time it involves, so far no automated method is known with which several parameters can be taken into account simultaneously.

It is, therefore, an object of the invention to provide means for compensation which allow simultaneous and automatic adjustment of detector signals in a heat image instrument.

According to the invention, a heat image instrument comprises, in the direction of radiation, an infra-red telescope, a scanning device, a detector objective lens, a detector, and compensation means, wherein the compensation means comprises at least one reference temperature source for introducing radiation into the instrument in the region of the detector objective lens and which, in time intervals of the scanning device that are not used for imaging the scene, feeds to the detector radiation of at least two different strengths for the purpose of compensation. In consequence, it is possible to irradiate the amplifying zone 1 t 2 of a signal processing unit of the instrument with two (or if necessary more) radiation levels without the heat signals scanned from the scene being substantially affected.

The invention will now be described by way of example with reference to the drawings in which corresponding parts are identified by the same reference numerals:- Figure 1 is a simplified diagram showing a heat image instrument; Figure 2 is a graph of the oscillatory movement of a scanning device of the instrument of Figure 1 with time; Figure 3 is a diagram of a heat image instrument with a temperature reference source optically connected between optical imaging systems of the heat instrument; Figure 4a is a plan view of a mirror of partially reflecting 20 and partially transparent material for imaging of the signal from the reference source; Figure 4b is aplan view of an alternative mirror; Figure 5 is a diagram of a heat image instrument similar to that of Figure 3 but having a bent optical axis; Figure 6 is a diagram of a heat image instrument with two temperature sources; and Figure 7 is a diagram of a heat image instrument with two reference temperature sources mounted on a rotor.

Referring to Figure 1, a heat image instrument has an infra- red (i-r) telescope 1 from which heat radiation from a viewed scene is directed onto the scanning device 2 disposed, for example, at 450 to the telescope axis when in J N 3 the rest position as shown by continuous lines in Figure 1. The scanning device 2 reflects the incident rays at rightangles through two optical imaging systems 3 and 4 of a detector objective lens onto a detector 5 or individual elements thereof. In this example, the image may comprise two half-images. This means that when the scanning device operates according to a raster scanning pattern, the first scan covers all the odd lines 1, 3, 5, etc. while the second scan covers all the even lines 2, 4, 6, etc. (line offset).

After opto-electronic conversion, the signal passes through an amplifier 6 into a signal processing unit (not shown).

The scanning element 2 typically oscillates according to a ramp characteristic as shown in Figure 2, with a scanning period t,,. This characteristic is composed of a comparatively long active period t, and a very much shorter inactive period t2. In addition to the inactive period, there is also a reversal period t3. To give these periods some magnitude, it is possible, for example, to quote the following time values:

typically t,, 20 ms typically t, 14 ms typically t2 6 ms typically t3 2 ms This then is the known state of the art.

Referring to Figure 3, there is shown an instrument which 30 differs from that of Figure 1 mainly in that, between the objective imaging lenses 3 and 4, and in fact expediently in the region of the smallest radiation cross-section, through a disc or plate 7 in the form of a mirror, radiation from a reference temperature source 9,10,11 is reflected into the optical axis 14 of the heat image channel. This reflection is carried out via a lens 12. The source may be imaged in a diffuse or focal manner.

4 The reference temperature source 9,10,11 consists of a Peltier device 9 which has a radiator element 10 on the side which is towards the heat image instrument, and a cooling surface 11 on the side remote from the instrument for deflecting the heat. In the present embodiment, the cooling surface has a plurality of cooling fins, while the irradiator is a copper block. In other embodiments, it is possible to use other cooling surface and irradiator configurations without departing from the scope of the invention.

In the illustrated embodiment, the optical axes of the heat image instrument and of the reference temperature source 9,10,11 extend at 90' to each other so that the disc 7 is at an angle of 450 to these axes. As shown in Figure 4a, the disc has along its diameter a partially reflective zone 71 adjacent which there are two transparent zones 711 and 7111. Opposite the zone 71 there is likewise a partially reflective zone 71v. The reflective capacity of the zone 71 is around 80% while that of the zone 7,v is around 60%.

An alternative disc, which may be used in an alternative instrument as described below with reference to Figure 5, provides for the zones 7,1 and 7,11 to offer a reflective capacity of 100% and for the zones 71 and 71v to be of different transparencies (e.g. 80% and 60%).

In another alternative embodiment, shown in Figure 4b, the original disc indicated by broken lines, has only the zones 7, and 71v left, with their differing reflective capacities, whereas the adjacent segments are to a certain extent replaced by air.

It is also possible to conceive of an-alternative which would in turn be used in an instrument as described below with reference to Figure 5, in which the zones 711 and 7,1, i 1 are present and have a reflective capacity of 100%, the zones 7, and 7,v are replaced by air.

During operation of the instrument, scanning of the scene takes up the major part of the scanning period to (Figure 2), only the inactive time intervals t2 and t3 being available for the compensation process. The disc 7 is rotated about its centre in synchronism with the scanning device 2 by a motor 8. As a result, and due to the different reflective capacity or transparency of the zones 71 and 71v of the disc 7, radiation of two different intensities passes in the time interval t2 and/or t3 along optical axes 13 and 14 in the region of the amplifiers 18 where it is then available as a reference signal for the adjustment of the sensitivity and amplification of the individual detector elements. The two reference temperature sources, which are apparent, due to the construction of the disc 7 in the present embodiment, thus radiate alternately into the optical axis, in fact after each half image (inactive time).

Referring to Figure 5, an alternative instrument in accordance with the invention corresponds largely to that described with reference to Figure 3, but has an optical axis 14 which, for structural reasons, is bent through 90%, the disc 7 being used for deflection of the rays. It is conceivable in this embodiment for the high demands of optical accuracy not to be completely satisfied on account of the motor-driven movement of the disc 7. In such a case, it is advisable, of the two temperatur reference sources 9,10,11 and 91, 101,111 which appear to be available via the expedient of a special construction of disc - only to accept the two actually available sources as shown in Figure 6. In the inactive time intervals t2, t3 or t2 -- t3, these sources are pushed one after the other into the path of the rays, while for the time t, the middle portion of the disc llf ll' is transparent.

6 A further embodiment of the above-described construction is shown in Figure 7. Here, the temperature reference sources 9,10,11 and 91,101,111 are mounted in an electrically insulated manner in the outer marginal zones of the two arms or vanes of an impeller or rotor 15. The rotor is mounted to rotate on a shaft 18 disposed parallel to the optical axis 14 and during the above-mentioned inactive time, swings the sources into the path of the rays. A voltage supply 16 for the Peltier elements 9 or 91 is obtained via slip rings 10 17.

It will be understood that it is possible to combine the embodiments of Figures 3 and 5 with the two actually existing reference temperature sources of Figures 6 and 7. Furthermore, in all the above-mentioned embodiments an increase in the number both of the actually existing and also of the apparent temperature reference sources is conceivable, the latter case requiring a correspondingly modified construction for the disc 7.

In summary, the above heat radiation instrument comprises, in the direction of passage of radiation an infra-red telescope, a scanning device, a detector objective lens, a detector, and a compensating facility. In order to be able simultaneously and automatically to equalise the amplication and sensitivity characteristics of individual detector elements of the detector or detectors, at least one reference temperature source is provided in the region of the detector objective lens which, in time intervals of the scanning device movement cycle which are not used for passing heat radiation from the scene, feeds to the individual detector elements radiation of at least two different radiation intensities so that the detector elements can be equalised or compensated.

4 t Z1 i 7

Claims

CIAIMS

A heat image instrument comprising, in the direction of radiation, an infra-red telescope, a scanning device, a detector objective lens, a detector, and compensation means, wherein the compensation means comprises at least one reference temperature source for introducing radiation into the instrument in the region of the detector objective lens and which, in time intervals of the scanning device that are not used for imaging the scene, feeds to the detector radiation of at least two different strengths for the purpose of compensation.
2. An instrument according to claim 1, further comprising an amplifier downstream of the detector and optoelectronic conversion means for producing a visible image corresponding to radiation received by the instrument, and wherein the reference source is located in the region of the detector objective lens, and supplies radiation to individual detector elements so that they can be compensated.
3. An instrument according to claim 1 or claim 2, wherein the scanning device has a movement cycle including an active scanning period, a reversal period, and a return period, the reversal and/or the return periods being used as time intervals for compensation.
4. An instrument according to any preceding claim, wherein the reference temperature source is disposed, in the direction of radiation, upstream or downstream of the objective detector lens or between optical imaging components of the lens, preferably in the region of the smallest radiation cross-section or aperture.
5. An instrument, according to any preceding claim, wherein, if only one temperature reference source is used, its optical axis extends at right-angles at least 1 8 to one part of the optical axis of the heat image channel of the instrument and converges with the latter axis via a mirror.
6. An instrument according to claim 5, wherein the mirror is a circular disc and is driven by a motor in synchronism with the scanning device.
7. An instrument according to claim 5 or claim 6, wherein the mirror, along one diameter, has a partially reflective strip adjacent which there are, if necessary, bilaterally transparent disc sectors or wherein the bilateral segments are totally reflective while the sectors along the diameter are open.
8. An instrument according to claim 5 or claim 6, wherein the mirror, along a diameter, consists at least in half of a variously transparent strip, adjacent which there is on both sides a totally reflective disc segment.
An instrument according to any of claims 1 to 4, wherein, in the direction of the passage of radiation, parallel with the beam of the heat image channel and at least at two oppositely disposed locations, there is in each case a reference temperature source which, for purposes of compensation, can be alternately introduced into the path of the beams.
10. An instrument according to claim 9, wherein both reference temperature sources are mounted on two vanes of an impeller so as to be movable alternately into the path of the rays of the heat image channel.
11. An instrument according to claim 10, wherein both temperature reference sources are electrically insulated from the impeller, and have slip rings associated with the impeller for an electrical voltage supply.

1 h 9
12. An instrument according to any preceding claim, wherein the or each reference temperature source is a Peltier element having on the side which faces the heat image channel of instrument a radiator, and on the opposite side a cooling surface for deflecting heat.
13. An instrument according to claim 12, wherein the radiator comprises a copper block while the cooling surface comprises a plurality of cooling fins.
14. A heat image instrument constructed and arranged substantially as herein described and shown in Figures 3 to 7 of the drawings.

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