GB2095069A - Thermal image recording device - Google Patents

Abstract

The thermal image recording device employs imaging apparatus 11, such as a forward looking infra red noctovisor unit, which is coupled to a pulsed laser 12, the laser being triggered at the coincidence of its orientation with the scanning position of apparatus (11), and being stepped by the width of one image point after each complete thermal image scanning. <IMAGE>

Description

SPECIFICATION Thermal image recording device The present invention relates to a thermal image recording device using for example a FLIR noctovisor apparatus (FLIR = Forward Looking Infra Red), which apparatus makes a thermal image of a scene using infrared radiation in the non-visible range of wave lengths. For images taken after sunset a certain contrast is obtained due to the fact that different materials (such as metal, earth surface, racks and plants) due to their different material properties (i.e. thermal capacity and thermal conductivity), have been differently heated up by the radiation of the sun. For the best contrast therefore Noctovision images should be taken directly after sunset.Due to a general cooling down after sunset, the temperature differences of the different materials become smaller, and towards the morning, the temperature differences and therefore the contrast are at a minimum. The detector of such thermal imaging apparatus has a certain lower limit for resolvable temperature difference which limit cannot be improved by an increase in the gain factor of the electronic circuit connected to detectors due to the intrinsic noise limit of the amplifying components.

U.S. Patent No. 3.953,667 describes a device including a laser coupled to the thermal imaging apparatus for improving the contrast of the recorded thermal image at low temperature differences. With this device, a modulated continuous wave laser is used, the laser beam of which after expansion by means of a tilt-andswing mirror is directed at the object to be recorded by means of the thermal imaging apparatus. The device also includes a receiver circuit comprising two separate channels, the one channel of which processes the received emitted thermal radiation and the other channel processes the reflected modulated laser radiation. By a continuous scanning of the object by means a relatively wide-spread laser beam, the danger exists that, at the object location the active scanning may be detected.

It is therefore an aim of the present invention to provide an improved thermal imaging device which reduces the danger of the active scanning being detected.

According to the invention, there is provided a device for recording a thermal image, the device comprising a thermal imaging apparatus, a pulsed laser, means for triggering the laser at coincident of its orientation with the scanning position of the thermal imaging apparatus, and means for stepping the laser by the width of one image point after each complete thermal image scanning.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawing which is a block diagram of thermal imaging apparatus according to the present invention.

Referring to the drawing, the thermal imaging apparatus 10 comprises a conventional FLIRnoctovisor unit 11 to which a conventional laser 12 is coupled. The laser 12 is operated by pulses and the laser may be of the CO2 type since its IRwave length lies in the maximum range of sensitivity of the FLIR-noctovisor unit 11. In front of the laser 12, a focussing lens 13 is arranged.

The focussing lens 1 3 is adjusted in such a way that the spot illuminated by the laser 1 2 overlaps two to three scan lines of the noctovisor unit 11.

The laser 12 comprises drive means 14,14' as well as position indicators 15,15' which are related to elevation and azimuth movement of the laser beam. The drive means 14,14' are controlled in elevation and azimuth by means of a laser scan control unit 1 6 which may be switched from manual to automatic operation by means of a control unit (not shown). The measured position values of the laser beam are fed back to the laser scan control unit 1 6.

A FLIR-scan control unit 1 7 serves to drive and indicate the position of the moveable components in the FLIR-noctovisor 11 which comprise a horizontal rotating and vertical tilting mirror by which the reveived radiation is thrown line by line and image point by image point on corresponding detectors. The position signals taken from the FLIR-scan control unit 1 7 together with the position signal of the laser position indicator 15,15' are applied to a comparison circuit 1-8 which triggers a pulsed control unit 1 9 at coincidence of the position signals, thereby initiating laser pulses. The duration of the illuminating laser pulses preferably corresponds at least to the scanning time for 3 image points of the noctovisor, thus reducing the expense for electronic and optical adjustnients.

The laser 12 is initiated once per complete scanning of the whole thermal image. Then the laser is directed to the next following image point and at the next complete scanning of the thermal image at coincidence of its position with the scanned position of the FLIR-noctovisor 11, it is again initiated and so on. Since it may take some minutes till a certain field of the scene is illuminated by laser pulses, it is necessary to store the scanning results of the different steps. For this reason, an image memory unit 20 is arranged, the capacity of which must be sufficient to store the information of the largest possible test field chosen in the viewed scene. The stored image is displayed on the TV monitor 21. By means of a computer (not shown), the stored video image may be further processed.The IR-video signals produced by the TV-compatible FLIR-noctovisor 11 may be applied to a video-clock control unit 22 directly or via the image memory 20. The unit 22 provides the video signals required for image display on the TV-monitor 21 and synchronizes by means of a central clock the display on the monitor with the FLIR-scanning by means of the control unit 1 7.

By means of an axuiliary signal generator 23, a predetermined line of sight may be provided for the laser 12, and a predetermined scanning area may be adjusted for the laser according to a test field and there may be displayed alpha numerical signs on the TV-monitor 21. The auxiliary signal generator 23 hereby is accordingly controlled by an operator by means of a control unit (not shown).

The above described device not only allows an improvement of the contrast with respect to the whole image displayed on the TV-monitor 21, it is also possible to improve the contrast only of a certain portion of a scene corresponding to a test field by means of pulse illuminating and storing in the image memory 20, the remaining image being displayed in real time. The advantage of such an operating mode is to be seen in the fact that during the time consuming contrast Improvement the normal function of the FLIR-noctovisor outside of the test field is guaranteed. Any movement of the noctovisor at this operation mode, however, must be avoided.

With respect to the evaluation of the appropriate length of an illuminating laser pulse, the following considerations may be made: With a serial scanning TV-compatible FLIRnoctovisor, the scanning of the recorded image as mentioned is done line by line with a horizontal rotating mirror which is vertically tilted after each complete line scan. With the German TV-standard, each second 25 images respectively 50 half images are scanned. Herewith, each image consists of 625 horizontal lines. The relation of image height to image width is 3 :4.

At an optimized image transmission system, the vertical and the horizontal resolution, this means the density of image points, should be equal. Therefrom results the number of horizontal image points nH as follows: no= 4/3 x 625=833 With this number of image points results the time period for recording one line: 1 tz ------ = 64x10-6s 25 x 625 The time per horizontal image point therefore results as follows: 64 tnH= x10-6s= 76 x10-9s 833 For less expensive electronic and optical adjusting, the illuminating laser pulse should overlap at least three image points so that the following minimum pulse duration follows: : tlMp=3 x 76 x 10-9s=228 x 10-9s Taking into consideration that the illuminating spot should overlap two to three scan lines, it makes sense to choose the half image frequency of 50 Hz as image repetition frequency.

Claims (6)

1. Device for recording a thermal image the device comprising a thermal imaging apparatus, a pulse laser, means for triggering the laser at coincidence of its orientation with the scanning position of the thermal imaging apparatus, and means for stepping the laser by the width of one image point after each complete thermal image scanning.

2. The device of Claim 1, including vertical and horizontal driving means acting upon the laser and having position indicators, the signal of said indicators being compared with each of the scan position signals in order to trigger a laser pulse at coincidence of the signals.

3. The device of Claim 1 or 2, including a TVmonitor connected to the thermal imaging apparatus and having an image memory.

4. The device of Claim 3, including means for storing in the image memory a test fieid lying within the recorded thermal image, and means for displaying said test field on the TV monitor alongside the remaining image which is recorded in real time.

5. The device of any one of the preceding claims, wherein the laser is such that the time period of the laser pulse is extended over the scanning of more than one image point.

6. A device for recording a thermal image, substantially as herein described with reference to the accompanying drawing.