GB1575492A - Aperture correction in a thermal imaging apparatus - Google Patents

Description

(54) APERTURE CORRECTION IN A THERMAL IMAGING APPARATUS (71) We, ELTRO G.M.B.H. GESBLL- SCHAFT FUR STRAHLUcSTFCHNIK, a German limited liability company, of 6900 Heidelberg 1, Kurpfalzring 106, Federal Republic of Germany, 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 a method of aperture correction for a modulation transfer function in a thermal imaging apparatus which scans a scene and passes signals picked up by way of an optical system to detectors and by way of a multiplexer to an image screen, and to a circuit arrangement for carrying out the method In a conventional thermal imaging apparatus detector signal amplitudes of high local frequencies are reduced, particularly because of the dispersion diameter of the optical system, the detector geometry, or the spatial extent of the detector and the display unit, so that the image reproduced is frequently blurred. For this reason it is known for the modulation transfer function (MTEi) to be corrected. Such correction may be effected by connecting into the amplifying channel a filter which accentuates the high frequencies. However, in a thermal imaging unit operating with a large number of detectors, this would entail the use of a correspondingly high number of separate filters, since each separate detector signal has to be corrected.

The many filters merely by themselves would involve costly production and stockholding. A thermal imaging unit equipped in this way would be inefficient in operation and susceptible to failure. For example, if the image angle were to be reduced to increase sensitivity in an apparatus operating with an oscillating mirror, then by changing the oscillating mirror the characteristics of the filter would also have to be changed with it. In a unit containing a hundred detectors, for example, this would require changing a hundred filters as well.

A filter is known from German published application No. 24 59 242 wherein the signal of a single detector is passed through a delay line and thereby delayed by a period of time which corresponds approximately to that of a single picture element.

This delayed signal is then subtracted from the direct signal, the amplitude of the delayed signal being adjusted so that a desired frequency characteristic is created.

It is an object of this invention to provide a relatively economical method of correcting detector output signals in a thermal imaging apparatus having a plurality of detectors. According to this invention a method of processing signals in a thermal imaging unit for providing aperture correction of the modulation transfer function comprises the steps of:- scanning a field of view and passing thermal radiation therefrom via an optical system to a plurality of detectors; feeding parallel electrical output signals from the detectors to a time-division multiplexing device; in the multiplexing device, converting the parallel signals sequentially into serial form; and feeding the serial signals to an image display means via a filter which is common to all the detectors and has a frequency response contour providing the required degree of aperture correction.

In a preferred embodiment of the invention, the filter is assembled from (in the flow direction of the signals): an analogue shift register to store and delay serial signals supplied directly from the multiplexer, a potentiometer to adjust the amplitude of the delayed serial signals, and a difference amplifier to subtract the delayed serial signals from the direct serial signals. A filter of this design is relatively compact, especially when compared with an array of RC filters having a separate filter for each detector, and has the advantage that it can be designed for as many detectors as required. This is as useful in the general construction of the thermal imaging unit as in its maintenance.

The individual filter elements may be assembled together to form one structural unit.

The invention will now be described by way of example with reference to the drawings, in which: Figure 1 is a block circuit diagram in cluding a filter in accordance with the invention Figures 2a to 2c are diagrams of three signals of differing amplitude and frequency, which signals are supplied to a multiplexer from three detectors; Figures 3a and 3b are diagrams respectively of a direct serial signal and a delayed serial signal obtained from the signals of Figure 2 after multiplexing; Figure 4 shows a serial signal corresponding to the difference between the serial signals of Figure 3.

The scene is scanned with the aid of an oscillating mirror as in a conventional thermal imaging unit, which is not shown in detail in the drawings since it is already known. The radiation picked up during this process is conducted via an optical system to a plurality of detectors 10 and is there converted to proportional ,analogue voltages. As shown in Figures 2 to 4 and as will be explained further in the following, these voltages are recalled one after the other via a multiplexer 2 and occur at its output as a serial signal. This serial signal is supplied via the line 12 to an analogue shift register 3 having at least (m x n) memory places. A timing generator 7, is arranged to control the multiplexer and the shift register, the latter being connected to the generator 7 via the line 12, for controlling the entry of the multiplexed signal, and via a delay stage 4 and the line 11 for controlling the shift register readout. The serial signals occurring at the output 9 of the multiplexer 2 appear at the shift register output with a delay corresponding to the quantity (m x n), where " n " is the number of detectors and "m" is the number of readout cycles by which the signal is to be delayed. The choice of m depends on how many readout cycles are allotted to one display picture point in the thermal imaging unit involved.

The delayed serial signal is fed to a potentiometer 5 and to one input of a difference amplifier 6, whose other input is coupled back to one phase of the multiplexer output. The amplitude of the delayed signal can be adjusted by the potentiometer 5 to produce a required frequency response, the delayed signal being subtracted from the direct signal. Due to the time delay, which corresponds to a phase displacement of 1800 at the highest occurring signal frequency from the detectors, the individual voltages of this highest frequency are actually added during the subtraction of the two signals, whilst low frequency signals are subtracted one from the other, since at low frequencies the delay corresponds to an insignificant phase shift. These mathematical processes are shown in Figures 2 to 4.

Figures 2a lo. 2c show three detector signals a, b and c which differ in amplitude and frequency, plotted on subdivided time axes.

In order to obtain usable signals from the multiplexer and the shift register, the rate at which each detector output is sampled by the multiplexer must be at least twice the highest occurring detector output signal frequency. Similarly, for each detector signal, at least two readouts must be made from the shift register per cycle of the said highest occurring frequency.

In Figures 2 and 3, which show signals occurring in a three-detector system (i.e.

n=3), the signal of the highest occurring frequency is shown by Figure 2a and the dotted envelope in Figure 3a. Thus, in Figure 3a the hatched strips are the portions of the serial signal obtained at the multiplexer output which correspond to samples of the detector signal a, which is at a relatively high frequency.

Figure 3b shows the same curve as in Figure 3a, delayed by six time subdivisions, six being a whole number multiple of n.

Figure 4 shows the result of the subtraction of curve 3b from curve 3a by the difference amplifier 6. In this diagram, more emphasis has been laid on the working out of the curve characteristics than on geometrical accuracy. Due to the minus sign, it is also helpful when actually carrying out the subtraction process if the serial signal in Figure 3b is first inverted so that its individual strips can then be added to the corresponding strips in Figure 3a. It is important to notice that the subtraction process is carried out between direct samples (Fig. 3a) of signal a and delayed samples (Fig. 3b) of signal a, and between direct samples of signal b, and delayed samples of signal b, and so on. Thus the delay introduced by the stage 4 is (m x n) time subdivisions or multiplexer output bits, where m is a whole number.

As shown in Figure 4 by the doted envelope curve corresponding to the hatched strips symbolising signal a, a large ampli tude occurs after subtraction, since for signal a a relatively high frequency was present originally, as can be seen from Figure 2a (solid line) and Figures 3a and 3b (dashed line), and the delay corresponds to a phase shift of almost 1800. On the other hand, the envelope curve (solid line) of the strips symbolising signal b shows that in this case there is a small amplitude after subtraction, since in this case a low frequency was present originally, as can be seen from Figures 26, 3a and 3b (all solid lines), and the delay corresponds to a relatively small phase shift. As Figure 2c shows, the frequency of signal c lies between the frequencies of signals a and b, so that the level of the amplitude at the filter output will also lie between those of signals a and b. However, in the interests of clarity, this case has not been shown in Figures 3a to 4. Therefore it will be seen that the apparatus shown in Figure 1 operates as a filter which suppresses low frequency signals and emphasises high frequency signals (up to a maximum frequency corresponding to the highest frequency signal expected from the detectors).

WHAT WE CLAIM IS:- 1. A method of processing signals in a thermal imaging unit for providing aperture correction of the modulation transfer function, comprising the steps of:- scanning a field of view and passing thermal radiation therefrom via an optical system to a plurality of detectors; feeding parallel electrical output signals from the detectors to a time-division multiplexing device; in the multiplexing device, converting the parallel signals sequentially into serial form; and feeding the serial signals to an image display means via a filter which is common to all the detectors and has a frequency response contour providing the required degree of aperture correction.

2. A method according to claim 1 wherein the step of feeding the serial signals to image display means via a filter includes feeding the serial signals from the multiplexer output to two inputs of a differencing stage, the signals being fed to one of the said inputs being delayed by a predetermined time interval.

3. A circuit arrangement for carrying out the method of claim 2 wherein the filter comprises, in the flow direction of the signals, an analogue shift register to store and delay serial signals supplied from the multiplexer, a potentiometer to adjust the amplitude of the delayed serial signals relative to serial signals fed directly from the multiplexer, and a differencing stage to combine the delayed serial signals and the direct serial signals.

4. An arrangement according to claim 3 including a timing generator arranged to control the multiplexer and the analogue shift register in synchronism.

5. An arrangement according to claim 3 or claim 4 including a delay unit connected to the analogue shift register to govern the time delay introduced by the latter.

6. An arrangement according to claim 5 wherein the delay unit is arranged to impose on the shift register a delay corresponding to (m x n) bits of the multiplexer output signal, where m is a whole number and n is the number of detectors.

7. A circuit arrangement according to any of claims 3 to 6 wherein the filter elements constitute one structural unit.

8. A method of aperture correction, in a thermal imaging unit, substantially as herein described with reference to the drawings.

9. A circuit arrangement constructed and arranged substantially as herein described and shown in Figure 1 of the accompanying drawings.

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

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. tude occurs after subtraction, since for signal a a relatively high frequency was present originally, as can be seen from Figure 2a (solid line) and Figures 3a and 3b (dashed line), and the delay corresponds to a phase shift of almost 1800. On the other hand, the envelope curve (solid line) of the strips symbolising signal b shows that in this case there is a small amplitude after subtraction, since in this case a low frequency was present originally, as can be seen from Figures 26, 3a and 3b (all solid lines), and the delay corresponds to a relatively small phase shift. As Figure 2c shows, the frequency of signal c lies between the frequencies of signals a and b, so that the level of the amplitude at the filter output will also lie between those of signals a and b. However, in the interests of clarity, this case has not been shown in Figures 3a to 4. Therefore it will be seen that the apparatus shown in Figure 1 operates as a filter which suppresses low frequency signals and emphasises high frequency signals (up to a maximum frequency corresponding to the highest frequency signal expected from the detectors). WHAT WE CLAIM IS:-

1. A method of processing signals in a thermal imaging unit for providing aperture correction of the modulation transfer function, comprising the steps of:- scanning a field of view and passing thermal radiation therefrom via an optical system to a plurality of detectors; feeding parallel electrical output signals from the detectors to a time-division multiplexing device; in the multiplexing device, converting the parallel signals sequentially into serial form; and feeding the serial signals to an image display means via a filter which is common to all the detectors and has a frequency response contour providing the required degree of aperture correction.

2. A method according to claim 1 wherein the step of feeding the serial signals to image display means via a filter includes feeding the serial signals from the multiplexer output to two inputs of a differencing stage, the signals being fed to one of the said inputs being delayed by a predetermined time interval.

3. A circuit arrangement for carrying out the method of claim 2 wherein the filter comprises, in the flow direction of the signals, an analogue shift register to store and delay serial signals supplied from the multiplexer, a potentiometer to adjust the amplitude of the delayed serial signals relative to serial signals fed directly from the multiplexer, and a differencing stage to combine the delayed serial signals and the direct serial signals.

4. An arrangement according to claim 3 including a timing generator arranged to control the multiplexer and the analogue shift register in synchronism.

5. An arrangement according to claim 3 or claim 4 including a delay unit connected to the analogue shift register to govern the time delay introduced by the latter.

6. An arrangement according to claim 5 wherein the delay unit is arranged to impose on the shift register a delay corresponding to (m x n) bits of the multiplexer output signal, where m is a whole number and n is the number of detectors.

7. A circuit arrangement according to any of claims 3 to 6 wherein the filter elements constitute one structural unit.

8. A method of aperture correction, in a thermal imaging unit, substantially as herein described with reference to the drawings.

9. A circuit arrangement constructed and arranged substantially as herein described and shown in Figure 1 of the accompanying drawings.