A Sine-Modulated High-Intensity UV-LED Light Source for Pressure-Sensitive Paint (PSP) Application using Fluorescence Lifetime Imaging (FLIM) technique

Abstract

Pressure-Sensitive Paint (PSP) measurement technique is based on the change of the intensity or decay time of its luminescence with pressure, brought about by oxygen quenching. For the so called “lifetime method” the pressure dependent luminescence decay time must be detected. Two approaches exist for this type of measurement: The time-domain lifetime method, for which a light pulse is used to excite the paint and the pressure dependent time constant is determined from the decay curve, and the frequency-domain based lifetime method, where sinusoidal modulated light is used to excite the paint and the emission light is used to calculate the pressure dependent phase shift and amplitude ratio. A new CMOS image sensor has been developed by CSEM and PCO used in the pco.flim camera for fluorescence lifetime imaging (FLIM) microscopy. The camera provides a sinusoidal voltage output for controlling an excitation light source. In this study, a high-intensity LED system was developed by DLR and HARDsoft providing a sine wave output of UV light for excitation of PtTFPP based PSP on larger scale surfaces like wind tunnel models using the pco.flim camera. This system, called illuminator, has been designed around a single, high-power LED emitting UV light of 390 nm. This LED has been specified for CW operation with a supply current of up to 18 A. However, it has been proven that this LED works with a satisfied linearity in the range up to 10 A, providing a maximum radiometric flux of about 11.5 W. The illuminator can be operated with CW or sine modulation. For the sine modulation it can be driven by a sine signal of 1 Vp-p directly from the FLIM camera in the range of 5-50 kHz. In calibration tests using small samples coated with PSP, the influences of parameters like amplitude and frequency of excitation light on the pressure sensitivity of phase angle and amplitude ratio are shown. Results from a test on a delta wing in the 1m low speed wind tunnel of DLR Göttingen.