Laser Scanning System

A generic laser scanning system for PSP and TSP is shown in Fig. 1.5. A low- power laser beam is focused to a small point and scanned over a model surface using a computer-controlled mirror to excite the paint on a model. The luminescent emission is detected using a low-noise photodetector (e. g. PMT); the photodetector signal is digitized with a high-resolution A/D converter in a PC and processed to calculate pressure or temperature based on the calibration relation for the paint. When the laser beam is modulated, a lock-in amplifier can be used to reduce the noise. Furthermore, the phase angle between the modulated excitation light and responding luminescence can be obtained using a lock-in amplifier for phase-based PSP and TSP measurements. The laser can be scanned continuously or in steps; it is synchronized to data acquisition such that the position of the laser spot on the model is known. In order to compensate for a laser power drift, the laser power variation is monitored using a photodiode. The laser scanning systems for PSP and TSP measurements were discussed by Hamner et al. (1994), Burns (1995), Torgerson et al (1996), and Torgerson (1997).

Compared to a CCD camera system, a laser scanning system offers certain advantages. Since a low-noise PMT is used to measure the luminescent emission, before an analog output from the PMT is digitized, standard SNR enhancement techniques are available to improve the measurement accuracy. Amplification and band-limited filtering can be used to improve the SNR. The signal is then digitized with a high-resolution A/D converter (12 to 24 bits). Additional noise reduction can be accomplished using a lock-in amplifier when the laser beam is modulated. The laser scanning system is able to provide uniform illumination over a surface by scanning a single laser spot. The laser power is easily monitored and correction for the laser power drift can be made for each measurement point. The laser scanning system can be used for PSP and TSP measurements in a facility where optical access is so limited that a CCD camera system is difficult to use.