In recent years the Pressure Sensitive Paint (PSP) technique has established as a reliable working tool in the field of optical pressure distribution measurements at many major research institutions like the DLR in Göttingen, Germany, the ONERA in France, the JAXA in Japan, and the NASA in the U.S.. The technique uses the deactivation of photo chemically excited colorant molecules with the help of oxygen as the fundamental measuring principle, i.e. the fact that the fluorescence of an excited colorant molecule can be erased with the help of oxygen. Since the oxygen concentration in the fluid at equilibrium is proportional to pressure, this process called quenching can directly be used for space-resolved pressure distribution measurements. For this purpose fluorescent molecules are stored in paint permeable to oxygen and are sprayed onto the model surface to be investigated after the appropriate pre-treatment. The sprayed on paint layer thereby has to be as thin as possible, so that a manipulation of the flow can be neglected. On the other hand the concentration of the fluorescent molecules - namely the thickness of the paint layer - has to be big enough for the signal to be recorded reliably with a CCD camera. After the application of the paint layer and the calibration of the measurement system at different reference pressures and temperatures the molecules are illuminated and the light from the molecules is recorded with the help of a suitable CCD camera within the frequency range of interest. In order to avoid that, for example, different thicknesses of the paint layer or a variation of the illumination intensity result in a spatially varying sensitivity, a reference camera is needed. Also the use of high-quality cameras is compulsory in order to be able to register even the smallest intensity differences. With a 12bit CCD camera a resolution of about ±10 mbar or sigma_cp=±0.05 can be achieved at sufficient temporal integration; with a 16bit camera a resolution of ±1 mbar or sigma_cp=±0.005.
The measurement technique of Pressure Sensitive Paint (PSP) still holds a lot potential for improvements despite its long lasting development. The Institute of Fluid Mechanics and Aerodynamics works on enhancements in the following fields:
• Data acquisition with CCD and CMOS cameras
• Illumination with flash lamps, diodes, and lasers
• Data evaluation and analysis in consideration of model deformation and vibration and of temperature drift
• Simultaneous application with other measurement techniques like PIV, deformation measurement technique, and IR thermography.