J. Eur. Opt. Society-Rapid Publ. 21, 22( 2025) 219
Figure 7. Transmitted laser power in dependency of roughness for sample heating by solid state laser( 500 W for 30 s). Figure 10. Calibrated values of the emission coefficient e for different temperatures and two surface qualities.
Figure
8. Comparison of temperature values obtained with thermography camera and contact thermometer for the same laser heating process.
Figure 11. Deviation of measured temperature values for different emission coefficient settings.
Figure
9. Measurement set up for calibration of the emission coefficient e.
be entered into the calculation software for the evaluation of a thermographic measurement to achieve correct absolute values. This is dependent on material properties, observation angle, temperature range itself and surface properties. For the laser investigations shown, the value was set constant to e = 0.85.
For better understanding of the effect of this coefficient, measurement or calibration of the emission coefficient were carried out using a calibration radiator for a defined heating of a fused silica sample. In this calibration procedure, target temperature and measured temperature with the thermography camera are compared and the value of e is manually adjusted until both temperatures are equal. To take the surface roughness into consideration, two different surface qualities of fused silica workpieces were investigated: Rq = 1 nm( polished) and Rq = 1 lm( ground). The measurement set up can be seen in Figure 9.
The evaluation of the emission coefficient shows strong differences for this value depending on the surface quality of the heated sample and the actual, specifically set temperature of it. The coefficient e differs significantly between the polished and the ground surface quality. For higher temperatures, the values vary more between the two surface qualities( Fig. 10).
Therefore, deviations from the correct emission coefficient can cause significant discrepancies between the temperature measured by thermography and the real temperature of the fused silica sample. Figure 11 shows the deviation of the measured temperature value for different settings of e, all for a set calibration radiator temperature