Effect of optical component parameters on signal characteristics and measured parameters of thermal spray particles

The effect of the optical components on the voltage level and the characteristics of thermal radiation signals was analyzed to ensure the accurate measurement of the parameters of the thermal spray particles. An experiment was carried out with air plasma sprayed Mo powder. It is found that the voltage level of signal pulses is determined by the aperture diameter and the aiming distance for the specific materials. The radiant signal characteristics vary with the spatial position of particles, the aperture diameter, and the dimension of the modulating window. The relative theoretical voltage level and the theoretical signal characteristics agree well with the experimental results. It is obvious that the large aperture leads to increasing signal to noise (S/N) ratio and worsens signal characteristics. While the small aperture contributes to the formation of well-shaped signals and results in decreasing S/N ratio and larger deviation of measurement data. Consequently, the aperture diameter ought to be selected as small as possible in the system design to ensure the generation of pulse signal with fine characteristic pattern, and the opto-electronic sensitivity of the system be adjusted high enough to ensure sufficient amplitude and S/N ratio of signals. Thus, it is necessary to introduce the photo sensor with higher sensitivity, such as avalanche photo diode (APD), to increase the measurement accuracy.