Effect of aiming distance on full width at half maximum (FWHM) of signal pulses generated by thermal spray particles and corresponding calibration

The effect of aiming distance on full width at half maximum (FWHM) of signal pulses was theoretically analyzed for a novel measurement system based on window-modulating velocimetry and 2-color pyrometry to diagnose multiple parameters of thermal spray particles. It reveals that the optical leverage relationship and the variance of signal characteristics serve as two main factors affecting the average FWHM of the measured signal pulses. The two factors exert a superimposed effect on the average FWHM when the aiming distance is larger than standard 75 mm, or a neutralized effect when the aiming distance is smaller than 75 mm. Consequently, when the aiming distance varies from 45 mm to 120 mm, the average FWHM appears an asymmetrical U-shaped variation to well coincide with the experimental results. Calibrating with the presented coefficients, the error level of the measured velocity of in-flight particles at non-standard aiming distance lowers significantly.