In-flight particle analysis via infrared sensing has proven helpful in the development and control of thermal spray processes. The aim of this work is to assess the effect of optical system design on thermal measurement accuracy. Through experimental testing and theoretical analyses, investigators show how variations in optical component sizes, alignments, and arrangements affect the amplitude and shape of sensor voltage waveforms and signal peaks. It is observed that only when photodetectors are in saturation, marked by the trapezoidal shape of their output signals, can they provide information about particle velocity, temperature, and size, and in some cases, even volume distribution. Correlation tests are carried out by means of plasma spraying molybdenum powder. Paper includes a German-language abstract.