Title: Pressure-Sensitive Paint

Brief Description: PSP allows global surface pressure measurements to be made using an optical detector (camera). The surface is coated with PSP that is made up of a luminescent probe molecule held in an oxygen permeable binder. The probe molecule is chosen such that its luminescence is quenched by oxygen. Thus, with more oxygen present, less luminescence is observed, all other variables remaining constant. Since air is made up of 28% oxygen, as the air pressure changes, the oxygen content changes by 0.28 of the pressure ratio. This change in intensity can be converted to a change in pressure via an appropriate calibration. Two images of a given surface must be acquired, one where the surface is at a known, constant pressure, and the other at the unknow nconditions. The ratio of the two images is related to the pressure through the calibration. Since a ratio is taken, the effects of non-uniform illumination are removed. During the acquisition of the images, the test surface is exposed to excitation light in only a narrow frequency band. The detector is then notch filtered to pass only the luminescence band for the particular probe molecule. Of course, these two bands must not overlap.

Parameter Space Covered: Measurements have been made from 90 mph in atmospheric tunnels to Mach 4.5 in heated blowdown facilities.

Limitations: Must be used in an air tunnel (no nitrogen facilities). High pressure, low speed applications are difficult. Due to the nature of the technique, absolute pressure is sensed, not differential. All surfaces where pressure is to be determined must be visible by the detector. Most PSPs are sensitive to temperature as well as pressure.

Uncertainty: Several percent of full scale (absolute).

Sample Applications: (1) Seven-camera test of 7 percent F-18E/F test in NASA Ames 11 foot transonic tunnel where loads were calculated from the pressures. (2) Three camera test of a minivan in an atmospheric tunnel at 90 mph.