GB2231222A

GB2231222A - Axis harmonisation in imaging sensor arrangements

Info

Publication number: GB2231222A
Application number: GB9009291A
Authority: GB
Inventors: Gunther Riedl
Original assignee: Messerschmitt Bolkow Blohm AG
Current assignee: Airbus Defence and Space GmbH
Priority date: 1989-04-26
Filing date: 1990-04-25
Publication date: 1990-11-07
Anticipated expiration: 2010-04-25
Also published as: GB2231222B; GB9009291D0; FR2646523B1; FR2646523A1; DE3913644A1; DE3913644C2

Description

2:2 3:L 2 2 -:> JSS270390 A METHOD FOR AXIS HARMONISATION The invention

relates to a method for the axis harmonisation of optronic sensors.

The imaging of test marks into sights for the harmonisation of axes of optronic sensors is already known. Furthermore, it is known to reduce the losses in brightness in the image paths between collimator and sensor by, in the case of thermal imagers, increasing the source temperature. This increase in the source temperature brings, however, only a slight improvement, since in the 8 to 12,wm region the radiated power rises only approximately proportioned to the use in temperature and the melting or oxidation temperature respectively of the heated materials used as the source sets limits on the increase of temperature.

As a consequence of these factors, there is posed, in all of the known prior art devices, the requirement that, the respective thermal imager must not during harmonisation procedure be acted upon by scene radiation. This is in order to avoid a merging of the test marks and any warm objects which are present in the scene, thereby to preclude an influencing of the harmonisation procedure.

JSS270390 The problem underlying the invention is to provide a method, of the type mentioned at the beginning hereof, in which the observation of the scene is not interrupted nor the scene itself impaired.

With this object in view the present invention provides a method for the axis harmonisation of optronic sensors by means of test or target marks which are generated by radiation sources in collimators, characterised in that the method includes the steps of forming the images of one or more target marks in a sensor for a short time and synchronously with the image generation procedure of the sensor, storing away in a digital store of a computer the composite targetmark/scene image which has been thus formed, subtracting from the target-mark/scene image a scene image recorded immediately before or after the imaging thereof and supplying the residual image pattern in accordance with its coordinates and the processing thereof to a video reticle generator and to a monitor.

In a preferred embodiment at least two target marks of specific shape are formed in the upper third of the scene image field. Additionally the harmonisation procedure is preferably repeated several times each with a changed postiion of a diaphragm in the collimator.

JSS270390 Advantageoulsy the radiation source used in the collimator is controlled by a shutter which is synchronised with the sensor image generation. Further, the radiation source is preferably a laser or thermal source.

Preferably the method corrects for displacement of the scene, which may have occurred between recordal of the composite target mark/scene image and of the scene image, by measuring the movement of the sensors carrier by, for example, correlating the scene image and the composite target mark/scene image using that part of the composite image which does not contain the target mark to ascertain the displacement thereof.

The invention will now be described further, by way of example, with reference to the accompanying drawings in which:

Fig. 1 shows a diagrammatic view of a sight with radiation source, collimator, thermal imager and computer/store unit; Fig. 2 shows different test patterns generated in scene-image fields;

Fig. 3 shows a diagrammatic view of a target-mark JSS240490 - 4 generator with a thermal radiation source; Fig. 4 shows a block diagram of the described exemplified embodiment in diagrammatic representation.

As has already been described, when using thermal sources in axis harmonisation mechanisms the relatively low brightness of the test or target mark(s) 50 (Fig. 2) is a problem which the method described hereinunder seeks to overcome. Since points of the scene radiation frequently have similar or even identical brightness to that of the test or target mark(s) 50 the procedures for harmonisation of the optronic sensor axes are in part considerably disturbed. The method outlined hereinunder minimises the susceptibility to trouble, by mirroring-in the target mark(s) image 50 for only a short time and synchronously with the image-generation procedure of the sensor 20 or 30 respectively, for example into the scanning rhythm of a charge coupled device or into the scanning rhythm of a scanner in a thermal imager. The arising image ZSB, which contains both the scene and the target mark or marks 50, is stored away in a digital store 41a, 41b of a computer 40 in which respect, of course, also the storage of a partial region which contains the target mark(s) 50 and its immediate surrounding is sufficient.

From this stored-away target-mark/scene image UB JSS270390 there is subtracted a scene image SB recorded immediately therebefore or thereafter. This scene image SB contains no target marks(s) 50. After the substraction there is left as result only the target mark(s) 50, disregarding any noise constituents and/or fluctuations. Displacements of the scene between recording of the target-mark/scene image (M) and scene image (SB) respectively, which can arise through movements of the carrier or wearer, are measured and corrected prior to the substraction. For this purpose it is proposed to determine the movement of the carrier of wearer by means of a correlation of the scene image (SB) and the targetmark/scene image (M) to ascertain the displacement itself. For this purpose it is sufficient to use only that part of the target-mark image which does not contain the target mark.

These target mark(s) 50 are further processed, with customary methods of calculation, in order to determine their centre. A synthetic reticle can then be superimposed in the video signal onto the scene image at the location of this centre.

If the target mark comes to rest on a saturated image portion, it is cancelled by subtraction in the same way as the scene. However, in order to be able to carry out a reliable harmonisation,in the case of JSS270390 saturated image components or a slight signal-to-noise ratio of the target-mark signal, several target marks 50 are used, which preferably have conspicuous characteristic shapes, such as crosses, rings, triangles, etc., which by means of the correlation allow further improved separation of scene remainders.

By using at least two test or target marks 50, which preferably also lie in the upper part of the scene image field SBf, a reliable, problem free axis harmonisation can be achieved, especially in view of the fact that the axis drift proceeds with a time constant of many minutes and therefore the harmonisation procedure can be effected with high redundancy. The variation of the scene through movement of the line of sight or respectively of the carrier or wearer of the sight thus allows sufficient undisturbed measurements, as the target marks 50 do not come to rest on saturated zones of the image field during each measurement.

An additional and further guarantee of accuracy for the harmonisation can be achieved if the harmonisation procedure is repeated with a changed position of the diaphragm 13 in the collimator 18, for example by rotation of the diaphragm 13 through a known angle. The harmonisation is, in this case, evaluated as valid only when the axis position as calculated from t JSS270390 both measurements agrees.

In order to act upon only a single image generated by the sensor 20, 30 including the target mark, the source 10, 11 used in the collimator 18 is controlled by an optical shutter 12 synchronised with the image generation of the sensor 20, 30 by a mechanism. 30d. Further a disturbance of the observer is precluded on account of the time constant of the observer's eye. Moreover, there is the additional possibility of darkkeying the one image which contains the target mark(s) 50 and dropping or replacing it by the preceding stored image.

Furthermore either the thermal source 11 or the light sources 10 can be replaced by a laser source. Also LED's are usable. The advantage of such a measure is that a laser can generate without problems powers which saturate the signal and thus permit test marks 50 to be generated which are at least as bright as the brightest objects in the scene. Thus, not only are fewer test points necessary, but also a lesser redundancy of the harmonisation procedure is required. Furthermore, in the case of the LED's or the laser the optical shutter 12 can be replaced by pulsed stimulation of the source.

JSS270390 The proposed method is summarised by the block diagram of Fig. 4 which shows an exemplified embodiment.

The respectively selected radiation source 10 or 11 - in the examplified embodiment shown here it is a thermal source 11 generates one or more test or target marks 50 which, as shown previously in Fig. 2, can be fashioned either as single or multiple points or as crosses, rings or triangles. Appropriately shaped diaphragm apertures are used to produce this result.

The wave length of the radiation of these test or target marks (50) corresponds to the various wave-length regions of the sensors 20, 30 that are to be harmonised. The radiation for these sensors 20, 30 is blocked or released by an electromechanical or electro-optical shutter 12. However, also one sensor (here, for example, the TV-set 20) can be supplied permanently with radiation, whilst the other sensor 30 (here a thermal imager) receives the radiation by way of a shutter 12.

In accordance with the proposed method, the shutter 12 is synchronised with the scanning 30a, which can be electronic or mechanical, of the thermal imager 30 by way of the synchronisation mechanism 30d, so that the test or target-mark image 50 of the radiation source 11 is superimposed in each case on only a single scene 1 JSS270390 image SDB. In the case of images with interlace it is in each case on only a half-image.

The store 41 of the computer 40 is laid out so that it can store the relevant image part - or additionally the entire scene image - in both stores 41a and 41b, so that, for example, one store 41a contains only the scene image SB without the superimposed image of the test or target marks 50 and the other store 41b contains the composite scene image with the target marks ZSB.

Then, by way of the subtraction mechanmism 42 of the computer 40 the two image store contents are subtracted from one another, so that only the image of the test or target marks 50 is left, in which respect so called "dirt effects" are negligible.

The output signal of the subtraction mechanism 42 is supplied to a contrast tracker 43 or alternatively to a correlator, which determines the centre of the target-mark/scene image M and supplies the corresponding coordinates to a reticle projector 44. This generates, at the location defined by the coordinates x and y, a reticle of suitable shape and delivers it to the video mixer 45. This mixes the cross with the image without a target mark of the thermal JSS270390 imager 30 and supplied it to the monitor 46 for presentation.

The use of a correlator instead of the contrast tracker 43 is expedient when, as a result of line-ofsight movement or upon severely fluctuating scene, as is afforded for example in the case of fires, the scene in a first image (in sensor 20) does not sufficiently resemble the scene in a second image (in the thermal imager 30) and thereby the output of the subtraction mechanism 42, in addition to the test- and target-mark image, includes scene relics or remainders. Also the use of a correlator can be of advantage upon saturation of the thermal imager 30. The harmonisation image pattern - and thus the test- or target-mark image - is here so shaped that easily correlatable shapes, which do not occur naturally in the scene or the image thereof with very high probability, are used alone or in combination. These shapes, such as crosses, rings, triangles and so forth have already been mentioned. They are stored as patterns in the reference part of the correlator and sought in a specific "expectation space" around the position of the image pattern known earlier from preceding measurements. If the stored-away pattern registers with the corresponding pattern from the thermal imager 30, the correlation coefficient reaches a maximum value. The level of this value is a measure of 1 Z JSS270390 the similarity of the stored patterns and those from the thermal imager. The position of the maximum yields the coordinates at the location whereof the synthetic reticle is generated.

For the sake of completeness it should be stated that in Fig. 4 the optical systems of the sensors 20 and 30 are designated by 14 and 15 and the deflecting mirror of the radiation source is designated by 16.

JSS270390

Claims

CLAIMS - 12 1. A method for the axis harmonisation of optronic sensors by

means of test or target marks which are generated by radiation sources in collimators, characterised in that the method includes the steps forming the images on one or more target marks in a sensor for a short time and synchronously with the image generation procedure of the sensor, storing away in a digital store of a computer the composite targetmark/scene image which has been thus formed, subtracting from the target-mark/scene image a scene image recorded immediately before or after the imaging thereof and supplying the residual image pattern in accordance with its coordinates and the processing thereof to a video reticle generator and to a monitor.
2. A method as claimed in claim 1, characterised in that at least two target marks of specific shape lie in the upper third of the scene image field.
3. A method as claimed in claim 1 or 2, characterised in that the axis harmonisation procedure is repeated several times each with a changed position of a diaphragm in the collimator.
4. A method as claimed in claims 1 to 3, characterised 1 11 JSS270390 in that, the control of the radiation source used in the collimator is for the generation of only a single target-mark/scene carried out by a shutter which is synchronised with the image generation of the sensor.
5. A method as claimed in claims 1 to 4, characterised in that thermal sources are used as radiation sources.
6. A method as claimed in any preceding claim in which the radiation sources are light sources.
7. A method as claimed in any preceding claim in which at least one of the radiation sources is a laser source.
8. A method as claimed in any preceding claim characterised in that the residual image pattern generated in the subtraction device is supplied to a correlator to determine the x-y coordinates.
9. A method as claimed in any preceding claim characterised in that displacements of the scene which have occurred between recordal of the composite targetmark/scene image and scene image are measured and are corrected prior to the subtraction, in which respect for the measurement the movement of the carrier is detected by means of correlation of scene image and targetmark/scene image using only that part of the targetJSS270390 mark/scene image which does not contain the target marks) to ascertain the displacement.
10. A method for the axis harmonisation of optronic sensors substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

published 1990 at The Patent Office. State House, 6671 High Rolborn, LondonWC1R4TP.Fwther copies maybe obtainedfrom The Patent Office. Sales Branch. St Mary Cray, Orpington. Kent BR5 3RD. printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1187