GB2248153A - Thermal imaging apparatus - Google Patents

Abstract

Thermal imaging apparatus comprises a linear array of detectors 4 arranged to receive a thermal image scanned by an oscillating mirror 6 from a wide field of view during a first time period and from a narrow field of view during a second time period, the mirror oscillating with large amplitude and low frequency to provide the wide field and small amplitude and high frequency to provide the narrow field. A plurality of first signals are derived each dependent upon the mean value of a respective one of the resulting image signals over a plurality of pixels during the first period, a plurality of second signals are derived each dependent upon the mean value of a respective one of the image signals over a plurality of pixels during the second period, and a plurality of offset signals eg gain control signals, are derived and applied to the image signals during the second period, each offset signal being such as to maintain a predetermined relationship between the corresponding second and first signals, and thermal references being preferably provided within the wide field. <IMAGE>

Description

THERMAL IMAGING APPARATUS This invention relates to thermal imaging apparatus.

One conventional form of such apparatus comprises a scanning optical system which causes a thermal image of a scene to be scanned across a detector or detector array. Electrical signals derived from the detector or detector array are then processed and applied to a display device, such as a cathode ray tube, to produce a visible image of the scene. The scanning optical system may include one or more movable mirrors, such as oscillating and/or rotating polygonal mirrors, whereby horizontal and/or vertical scanning of the image across the detector or detector array is achieved.

One problem which arises with such apparatus is that, where the scene under observation contains a rapidly moving object, the thermal image is not updated sufficiently frequently.

In one aspect, the present invention solves this problem by providing means for changing the rate of scanning, preferably with an associated change in the sizes of the field of view scanned. For example, where the scanning element includes or comprises an oscillating mirror, both the frequency and amplitude of the rotary oscillations of the mirror may be changed.

It is also conventional to include in thermal imaging apparatus, so-called thermal references at opposite sides of the field of view, which references are elements maintained at predetermined temperatures and images of which are scanned across the detector during portions of each scanning cycle. The signals produced by the detectors during the scanning of the thermal references are used to process the signals from different elements in the detectors to compensate for different response characteristics of the different elements. Where the above-mentioned first aspect of the invention is employed, the signals generated by the thermal references may be unavailable during times when the amplitude of oscillation of the mirror is reduced.

Further, in thermal imaging apparatus employing so-called "staring" arrays of detectors, problems may arise in processing the signals to compensate for differences in the response characteristics of the different detectors in the array.

A further aspect of the invention provides a thermal imaging apparatus in which thermal references are provided and the apparatus is operable such that energy from the thermal references is directed to the detectors relatively infrequently or only occasionally. Thus, signal processing may be carried out on the basis of data obtained from signals generated in response to energy from the thermal references without the necessity for repeatedly scanning the thermal references, for example once every frame. By way of example, gain control of the signals may be carried out over a number of frames or fields utilising data obtained from a previous observation of the thermal references by the detector elements and offset control may be carried out utilising data derived from the image signals themselves, for example the average brightness over the whole image.

The invention is described further by way of example with reference to the accompanying drawings in which: Fig. 1 is a block diagram of thermal imaging apparatus according to a preferred embodiment of the invention; Fig. 2 is a diagram illustrating the relationship between a thermal image and a linear detector array of the apparatus of Fig. 1; Fig. 3 is a wave form diagram illustrating operation of an oscillating mirror included in the apparatus of Fig. 1; and Fig. 4 is a block diagram showing part of the apparatus of Fig. 1 in more detail.

With reference to the drawings, a thermal telescope 2 directs radiation to a thermal detector 4 via a mirror 6, which oscillates about an axis 8 to perform horizontal scan, and an optical system 10. The detector 4, as best seen in Fig. 2, comprises a linear array of thermal detector elements, one for each horizontal line of the image, and extends from top to bottom of the thermal image which is diagrammatically indicated at 12 in Fig. 2. By way of example, the detector 4 may comprise a charge coupled device or devices having a total of 384 thermal detector elements made of cadmium mercury telluride. The effect of the oscillation of the mirror 6 is to scan the image 12 repeatedly across the detector 4 in the horizontal direction as indicated by arrow 14 in Fig.

2.

Thermal references 16, 18, which as is conventional may comprise metallic elements maintained at precisely controlled temperatures, are located in the telescope 2 such that energy from these references 16, 18 is directed to the detector 4 at the beginning and end of each horizontal scan performed by the mirror 6. Thus, in Fig. 2, images 16a and 18a of the thermal references 16 and 18 are indicated at opposite sides of the image 12.

The oscillation performed by the mirror 6 will be further understood by reference to Fig. 3 which includes a wave 19 illustrating the oscillatory rotation of the mirror during a first mode of operation of the apparatus. During a first portion 20 of the cycle, energy from the reference 18 is received by the detector 4. During a second portion 22 of the cycle, the image 12 is scanned across the detector 4 for the derivation of image signals. During a third portion 24 of the cycle, the second thermal reference 16 is scanned. Portion 26 of the cycle is a flyback portion during which the mirror moves in the opposite direction in preparation for beginning another horizontal scan.As is well known, the purpose of the thermal references is to enable adjustments to be made to the signals derived from individual ones of the detector elements to compensate for differences in the responses of those elements.

The mirror 6 is driven by a motor 28 under control of a timing and control circuit 30 which also clocks the detector 4 so that the signals produced by the detector elements thereof are read out, in series, repeatedly during the scanning movement of the mirror 1.

The circuit 30 includes an input 32 whereby the apparatus may be caused to operate in either the first mode as described above or in a second mode in which the amplitude of oscillation of the mirror 6 is reduced and its frequency increased as represented by the dotted line curve 34 of Fig. 3 such that only a central vertically extending band 36 (Fig. 2) of the thermal image i2 is scanned across the detector 4.

The second mode of operation is used, for example, if a rapidly moving object is in the field of view and it is desired to provide more frequent updating on the position of the object than would be provided by operation in the first mode. It will be seen from Fig. 2 and Fig. 3 that, in the second mode, the thermal references 16 and 18 are not scanned. In this mode, as will be described in more detail below, an alternative form of signal processing takes place to compensate for differences in the responses of the different detector elements.

As illustrated in Fig. 1, the signals from the elements of the detector 4 are supplied, via preamplifier/buffer 38, to an analogue to digital converter 40 which converts the signals into digital form and supplies them, via a data bus 42 to a signal conditioning circuit 44. Different ones of the detector elements of the detector array 4 have different response characteristics. The function of the circuit 44 is to apply different offsets and gains to the signals from the different detector elements in order to compensate for the differences in the response characteristics of the detector elements. The output of the signal conditioning circuit 44 is supplied to a scan conversion device 46 in which the received digital signals are temporarily stored and then output on bus 48 in a sequence appropriate for display on a conventional TV monitor. A digital to analogue coverter 50 converts the signals to analogue form and supplies them to a display device 52 in the form of a TV monitor. The scan conversion device 46 includes supplementary outputs 54 and 56 for making the digital signals available for other circuitry either in the pre-conversion format or the post-conversion format respectively.

As can be seen in Fig. 1, circuits 40, 44, 46 and 50 are controlled from the timing and control circuit 30.

With reference to Fig. 4 the signal conditioning circuit 44 comprises a digital adder 60 which receives on bus 62 the digital signals output by the analogue to digital converter 40 and adds to them an offset supplied in digital form on bus 64. The resulting digital signal is applied via bus 66 to a digital multiplying circuit 68 which adjusts the gain of the signal supplied by adder 60 by multiplying that signal with a further digital signal applied to the multiplier 68 via bus 70. The multiplier 68 outputs the offset and gain adjusted digital signal on bus 72 for supply to the scan conversion circuit 46. As already indicated, it is necessary that the signals derived from different detectors should have different offsets and gains applied to them to compensate for the different response characteristics of the different detectors.The different values of the offset to be applied to the signals from the respective different detectors are stored in a reference store 74 and the different values of gain to be applied to these respective different signals are stored in a gain store 76. A microprocessor 78 with associated memory 80 is coupled to the output bus 66 of adder 60 via buffers 82 and 84 and, as seen in the drawing, is also coupled to memory 74 via buffer 84.

A buffer 86 couples the microprocessor 78 to memory 76. An addressing circuit 88 controlled by the microprocessor 78 determines the locations in memories 74 and 76 at which reading and writing takes place.

The arrangement is such that the offset and gain values stored in the stores 74 and 76 are continuously being read out in appropriate timed relationship to the arrival of the signals from the different detector elements so that the appropriate level of offset and gain is applied to each successive pixel signal.

Also, the signals output by the adder 60 are supplied to buffers 82 to enable the microprocessor 78 to calculate average brightness of the entire scene.

The microprocessor 78 is programmed to calculate the values of offset and gain to be applied by circuits 60 and 68 respectively and to store the appropriate values in appropriate addresses in stores 74 and 76.

During the first mode of operation, wherein the thermal references 16 and 18 are scanned during each cycle of the mirror 6, the required calculations are performed as follows.

Let Vij be the voltage from the i'th detector on the j'th pixel, and Uij be the corresponding output voltage of the multiplier 68. Then Uij = Gi (Vij + Ai) (1) where Ai is the offset added for the itth detector, and Gi the corresponding gain or multiplier.

Next suppose that the thermal references produce signals Via and Vib where a means one reference and b means the other, and that all channels are to produce output voltages Wa and Ub at these times. Then Ua = Gi (Via + Ai) (2) Ub = Gi (Vib + Ai) (3) The microprocessor calculates Gi = Ua - Ub (4) Via - Vib for each detector and places these values in the store 76.

Similarly, the microprocessor calculates Ai = Ua/Gi - Via (5) and stores these values in the store 74.

With reduced field of view (the second mode described above) there is no longer access to Via and Vib, but Gi is established by a preliminary period of wide angle operation. Also during the wide angle operation, the average of Vij across the scene or across part nf the scene, at least in approximat ony during the flyback phase is obtained:

There will be a corresponding output level Uiz = Gi (Vif + Ai) (7) During narrow angle operation, the system takes a flyback average, say Vii, to get an output voltage.

Ui = Gi (Via + Ai') (8) where the prime on Ai' indicates that whilst Gi may be presumed to remain stable, (though different from detector to detector), Ai may well be subject to drifts with time. To maintain mean scene brightness, the system maintains Ui# = Uig i.e. the displayed brightness is maintained.

Therefore Uik = Gi (Vit + Ai') (9) or Ai' = Uig/Gi - Vis (10) Thus the offset is calculated from known quantities; viz Ui&gamma; = Gi (Vi&gamma; Ai) determined earlier in the wide field of view mode, as is Gi, and ViS is measured as often as needed in the second mode. This formula can be rewritten.

Ai' = Ai + Vit - ViS (11) Thus, during the mode of operation in which the thermal references are scanned, both offset and gain control may be carried out utilising values obtained by calculations utilising the signals from the thermal references. However, in the second mode of operation, in which the thermal references are not scanned, gain control may be carried out utilising values of gain obtained from a previous scanning of the thermal references and level control may be carried out utilising offset values obtained from averaging the brightness signals from the whole field of view in this mode of operation i.e. the restricted field of view. Such values preferably being calculated from signals produced by the detectors during flyback.

Claims (10)

CLAIMS:

1. Thermal imaging apparatus comprising: optical means for producing a thermal image; an array of detectors arranged for receiving thermal energy from said image; means for producing from said detectors a plurality of respective image signals representing said thermal image; means for causing the detectors to receive energy from a wide field of view during a first time period and from a narrow field of view during a second time period; means for deriving a plurality of first signals each dependent upon the mean value of a respective one of said image signals over a plurality of pixels during said first period; means for deriving a plurality o second signals each dependent upon the mean value of a respective one of said image signals over a plurality of pixels during said second period; and offset- control means for deriving a plurality of offset signals and applying said offset signals respectively to said image signals during said second period, each offset signal being such as to maintain a predetermined relationship between said corresponding second and first signals.

2. Apparatus according to claim 1, wherein said first signals are each dependent on the mean value of the respective image signal over those pixels which are included in the narrow field of view and are substantially independent of the values of said image signals during pixels not included in the narrow field of view.

3. Apparatus according to claim 1 or 2, wherein each said offset signal is such as to maintain the corresponding said first and second signals substantially equal.

4. Apparatus according to any preceding claim, including thermal references within said wide field of view but not said narrow field of view; and gain control means which derives a plurality of gain control signals dependent respectively upon the values of said image signals during said first period and applies said gain control signals to said respective image signals both during said first period and during said second period.

5. Apparatus according to claim 4, wherein said signal producing means is operable for producing said image signals in digital form; and including microprocessor means for digitally calculating said offset signals and said gain control signals, digital storage means for storing said offset signals and gain control signals and digital adding means and digital multiplying means controlled by said microprocessor means for performing said offset control and gain control respectively utilising said stored offset signals and gain control signals, in timed relationship to production of said respective image signals.

6. Apparatus according to any preceding claim, wherein said detector array is a linear array extending substantially from top to bottom of said thermal image and wherein said optical means comprises a scanning mirror operable in a first mode to provide said wide field of view and in a second mode to provide said narrow field of view.

7. Apparatus according to claim 6, wherein said scanning mirror is an oscillating mirror and including means to oscillate said mirror during said first time period with a relatively large amplitude and relatively low frequency and during said second period with a relatively small amplitude and relatively high frequency.

8. Apparatus according to claim 7, wherein said first and/or second signals are derived from the values of said image signals arising during flyback periods of said oscillating mirror.

9. Thermal imaging apparatus comprising optical means for producing a thermal image, an oscillating mirror for scanning said thermal image in the horizontal direction thereof, a linear array of detectors extending in the vertical direction of the thermal image substantially from top to bottom thereof, and control means for causing said oscillating mirror to operate in a first mode with a relatively large amplitude and relatively low frequency to provide wide field of view and a second mode at relatively small amplitude and relatively high frequency to provide narrow field of view.

10. Thermal imaging apparatus substantially as herein described with reference to the accompanying drawings.