Thermal Imaging System For Material Processing

In the field of lasers for welding and surface processing, we need to measure the map of temperatures in order to control the processing in real time by adjusting the laser power, the beam pointing and focussing and the workpiece moving speed. For that purpose, we studied, realized and evaluated a model of thermal imaging system at 2 wavelengths in the mid-infrared. The device is connected to a 3 axis table and to a 3 kW CO2 laser. The range of measured temperatures is 800 C to 1 500 C. The device includes two AGEMA infrared cameras fixed to the welding torch each operating with a choice of filters in the 3, 4 and 5 micrometre band. The field of view of each is about 14 mm by 38 mm. The cameras are connected to an M68000 microprocessor family based microcomputer in which the images enter at the rate of 6. 25 Hz with 64 x 128 pixels by image at both wavelengths. The microcomputer stores the pictures into memory and floppy disk, displays them in false colours and calculates for each pixel the surface temperature of the material with the grey body assumption. The results have been compared with metallurgic analysis of the samples. The precision is about 20 C in most cases and depends on the sample surface state. Simplifications of the laboratory device should lead to a cheap, convenient and reliable product.