DE2940536C2 - Arrangement for recording a thermal image - Google Patents

Description

The present invention relates to an arrangement for recording a thermal image with a thermal imaging device. Thermal imaging devices, which are often called FLIR night vision devices (FLIR = Forward Looking Infra Red) are used when depicting a scene in the invisible wavelength range lying infrared radiation use. There is also a certain contrast in the picture Sunset due to the fact that there are different substances, such as metal, earth, rock and Plants due to different material properties, such as heat capacity and thermal conductivity, have heated up differently due to the sun's rays. The night vision with the best contrast is therefore possible immediately after sunset. As a result of the general cooling, the Temperature differences are getting smaller. Towards morning the temperature differences reach and with it the contrast a minimum. With regard to the detectors of such thermal imaging devices, there is a lower limit for the barely resolvable temperature difference, which is also due to an increase in the gain factor of the downstream electronic circuit cannot be improved because of the inherent noise of the Reinforcement elements are limited.

From US-PS 39 53 667 an arrangement consisting of one with a thermal imaging device coupled laser, known to operate at low temperature differences leads to an improvement in the contrast of the recorded thermal image a modulated continuous wave laser for use, whose laser beam after an expansion with the help a tilting and swiveling mirror is drawn over the object to be recorded by the thermal imaging device. A receiving circuit has two separate channels, the one being the received channel

ίο emitted thermal radiation and in the other channel the reflected modulated laser radiation is processed. Due to the constant scanning of the record to be recorded Object with a relatively large laser beam there is a risk that the location to be recorded Object the fact of the active scanning can be determined.

It is therefore the object of the present invention to improve this known arrangement in such a way that that the risk of discovering active scanning

2i> is decreased. This object is achieved according to the characterizing features of claim 1. Further advantageous configurations of the arrangement according to the invention can be found in the subclaims.

For this purpose, the invention preferably makes use of a pulsed laser which is successively aligned with the image points of the image and triggered when its position corresponds to the scanning position of the thermal imaging device. The laser preferably emits light with a wavelength in the spectral sensitivity range of the thermal imaging device. Due to the different IR reflection behavior of different materials such as metal and wood, when the observed scene is illuminated with an IR pulse, different reflections and thus an improved contrast are obtained. Since the contrast improvement takes some time, an image memory is expediently interposed in a television-compatible thermal imaging device with a downstream TV monitor. This also results in the possibility of computer-aided processing of the stored contrast signals.
In the following the invention is explained in more detail with reference to an embodiment shown in the single figure of the drawing.

The thermal imaging device 10 according to the invention consists of a commercially available FLIR night vision device 11, with which a commercially available laser 12 is coupled. The laser 12 is operated in a pulsed manner, A CO2 laser is particularly suitable for pulse lighting of the scene because its IR wavelength in the maximum sensitivity range of 10 μπι of the observing FLIR night vision device 11 is located.

A bundling optics 13 is connected upstream of the laser 12. The bundling optics 13 is adjusted so that the The spot illuminated by the laser 12 overlaps 2-3 scanning lines of the night vision device 11. The laser 12 has Drive devices 14, 14 ', and position sensors 15, 15', which are assigned to the vertical and horizontal movement are. The drive devices 14, 14 ′ are controlled by a laser scanning control unit 16 in terms of height and side controlled, which can be switched between manual and automatic mode via an operating device (not shown).

The actual position values of the laser 12 are reported back to the laser scanning control unit 16.

A FLIR scanning control unit 17 is used to drive and detect the position of the movable elements

in the FLIR night vision device 11, which can consist in a known manner of a horizontally rotating and a vertically tiltable mirror which cast the received radiation line by line and point by point onto appropriate detectors. The position signals picked up by the FLIR scanning control unit 17 are fed together with the position signals from the laser position sensors 15, 15 'to a comparison circuit 18 which, when the position ^; Signals a pulse control unit 19 which in turn triggers laser pulses. The duration of these laser illumination pulses preferably corresponds to at least the scanning time for three image points of the night vision device. This measure keeps the electronic and optical adjustment effort relatively low.

The laser 12 is then triggered only once per complete scan of the entire thermal image the laser will point to the next. Pixel aligned and at the next complete thermal image scan, when its new position coincides with the scanning position of the FLiR night vision device 11 again drawn, etc. Since it can therefore take a few minutes until a certain scene field with laser pulses is illuminated, it is necessary to save the scanning result at the respective steps. To this The purpose is an image memory 20 arranged whose capacity must be sufficient to store the information to save the largest possible test field selected in the observed scene. The saved The image is displayed on a TV monitor 21. Via an intermediary, not shown The computer can then process the stored video image. The when using a television-compatible FLIR night vision device 11 formed IR video signals can both via the image memory 20 as well as directly to a video amplification and clock control unit 22 will. This unit 22 supplies the information required for displaying images on the TV monitor 21 Video signals and synchronizes the display on the monitor with the via a central clock FLIR scanning by the control unit 17.

An auxiliary character generator 23 can, for example, provide the laser 12 with a specific line of sight predetermined and a certain scanning area can be set for this according to a test field, and it alphanumeric characters can be displayed on the TV monitor 21. The auxiliary character generator 23 is controlled accordingly by an operator via a control device (not shown).

The arrangement described above not only enables an improvement in contrast with regard to the entire image displayed on the TV monitor 21; it is rather thought about saving in the image memory 20 through only a certain section from a scene, which corresponds to a field Improve pulse lighting in its contrast and display the rest of the image in real time. Of the The advantage of such an operating mode is that even during the time-consuming contrast improvement normal operation of the FLIR night vision device

ίο is guaranteed outside of the test field. One Movement of the night vision device must, however, be avoided in this mode of operation.

With regard to the determination of the suitable length of a laser illumination pulse, the following can be made

Give thought:

In the case of a serial scanning TV-compatible FLIR night vision device, the recorded image is broken down Image, as already mentioned, line by line with one horizontally rotating and one after the

2 <) Pass through each line of vertically tilting mirror. According to the German television standard, 25 frames or 50 fields are scanned every second. Each picture consists of 625 horizontal lines. The ratio of image height to image width is 3: 4.

In an optimized image transmission system, the vertical and horizontal resolution, ie the pixel density, should be the same. The number of horizontal pixels ημ results as follows:

n _h = 4/3 χ 625 = 833

This number of pixels results in the following length of time for recording a line:

'z =

25 x 625

64 X 10 " ⁶ p

The time per horizontal pixel results as follows:

64

'"" 833

X 10 " ⁶ s = 76 x 10""s

Is done with regard to the already mentioned

reduced electronic and optical adjustment effort the laser illumination pulse so long that it If at least three pixels overlap, the following minimum pulse duration results:

r / M /> = 3 χ 76 χ 10- ⁹ s = 228 χ 10 ~ ⁹ s

so If one also assumes that the illumination spot overlaps 2 to 3 scanning lines, it makes sense to use the Select a field frequency of 50 Hz as the refresh rate.

1 sheet of drawings

Claims (5)

Patent claims: 1. Arrangement for recording a thermal image with a thermal imaging device and one with it coupled laser to improve the contrast of the recorded thermal image by using a pulsed laser (12), triggering the laser (12) Coincidence of its alignment with the scanning position of the thermal imaging device (11) and Further movement of the laser (12) by one pixel width after each complete thermal image scan. 2. Arrangement according to claim 1, characterized by vertical and horizontal drive devices (14, 14 ') acting on the laser (12), with which position sensors (15, 15') are coupled, the signals of which correspond to the respective scanning position signals of the thermal imaging device (U) be compared in order to trigger a laser pulse if the signals match. 3. Arrangement according to claim 1, characterized by one connected to the thermal imaging device TV monitor (21) with an assigned image memory (20). 4. Arrangement according to claim 3, characterized in that in the image memory (20) one in his Contrast to be improved test field is stored within the recorded thermal image that on the TV monitor (21) next to the other image recorded in real time for display got 5. Arrangement according to claim 1 or one of the following, characterized in that the temporal The duration of a laser pulse extends over the scanning of more than one pixel.