Principal Component Analysis Calibration Method for Dual-Luminophore Oxygen and Temperature Sensor Films: Application to Luminescence Imaging Muhammet Erkan Ko ¨se, † Ahmed Omar, ‡ Christopher A. Virgin, ‡ Bruce F. Carroll, ‡ and Kirk S. Schanze* ,† Department of Chemistry and Department of Mechanical and Engineering Science, University of Florida, P. O. Box 117200, Gainesville, Florida 32611 Received April 14, 2005 Oxygen sensor films are frequently used to image air-pressure distributions on surfaces in aerodynamic wind tunnels. In this application, the sensor film is referred to as a pressure-sensitive paint (PSP). A Stern-Volmer calibration is used to relate the emission intensity ratio of a long-lifetime luminescent dye (the pressure-sensitive luminophore, PSL) to surface air pressure. A major problem in PSP measurements arises because the Stern-Volmer calibration of the PSL’s emission varies with temperature. To correct for the temperature dependence, a second luminescent dye that has an emission that varies with temperature (the temperature-sensitive luminophore, TSL) is incorporated into the sensor film. With such a dual- luminophore PSP (DL-PSP), it is possible to measure the surface-temperature distribution with the TSL emission, and this information is then used to correct the temperature dependence of the PSL’s pressure response. In the present article, we report the application of a DL-PSP to obtain high-resolution air- pressure distributions on a surface that is subjected to a 20 °C temperature gradient. Two different calibration methods are used to generate surface-temperature and air-pressure distributions from the luminescence imaging data, and a quantitative comparison of the results obtained from the two methods is provided. The first method is based on an intensity-ratio calibration that uses luminescence images collected at two wavelengths, one corresponding to the TSL emission and the second corresponding to the PSL emission. The second method is based on principal component analysis (PCA) of luminescence images obtained at four wavelengths throughout the spectral region of the TSL and PSL emission (hyperspectral imaging, 550-750 nm). The results demonstrate that the PCA method allows the measurement of surface air pressure with higher accuracy and precision compared to those of the intensity-ratio method. The improvement is especially significant at pressures near 1 atm, where the temperature interference is most pronounced. Surface-pressure distributions are measured with comparable accuracy and precision with the two methods. Introduction Luminescent oxygen sensor films have been widely used for the measurement of surface air-pressure distributions on aerodynamic models. 1-3 In this application, the oxygen sensor film is often referred to as a pressure-sensitive paint (PSP). When combined with scientific-grade CCD cameras, a PSP sensor affords air-pressure distributions with high spatial resolution and a wide dynamic range. The PSP method has many advantages over conventional approaches for surface air-pressure measurement (e.g., pressure taps installed at discrete locations on an aero- dynamic model) in terms of cost, labor involved with prototype model construction, and the amount of data collected. 1 Although PSPs offer many advantages, a major draw- back in current technology is the temperature dependence of luminescence intensity-pressure calibration of the oxygen sensor film. 4 When an aerodynamic test experi- ment is conducted in a wind tunnel, the temperature of the model surface may change during the experiment; consequently, the air-pressure distribution determined from the luminescence from the sensor film has errors associated with the nonuniform temperature on the model surface. To correct for the temperature interference, the model’s surface-temperature distribution must be mea- sured concurrently with the pressure-surface distribu- tion. One way to achieve this is to use a dual-lumino- phore pressure-sensitive paint (DL-PSP) that contains a temperature-sensitive luminophore (TSL) dispersed along with a pressure-sensitive luminophore (PSL) in the polymer matrix. 5-7 The general concept is that the emission intensity of the TSL varies only with temper- ature, whereas that of the PSL varies with pressure (and temperature). Thus, CCD images obtained at the wavelength corresponding to the emission from the TSL can be related to the surface temperature distribu- tion, and this information can be used to correct the pressure-intensity calibration used for CCD images obtained at the wavelength corresponding to the PSL emission. In the previous article in this journal, we describe a novel approach to the formulation of a novel DL-PSP as * To whom correspondence should be addressed. E-mail: kschanze@chem.ufl.edu. Tel: 352-392-9133. Fax: 352-392-2395. web: http://www.chem.ufl.edu/∼kschanze. † Department of Chemistry. ‡ Department of Mechanical and Engineering Science. (1) Bell, J. H.; Schairer, E. T.; Hand, L. A.; Mehta, R. D. Annu. Rev. Fluid Mech. 2001, 33, 155-206. (2) Morris, M. J.; Donovan, J. F.; Kegelman, J. T.; Schwab, S. D.; Levy, R. L.; Crites, R. C. AIAA J. 1993, 31, 419-425. (3) Gouterman, M. J. Chem. Educ. 1997, 74, 697-702. (4) Schanze, K. S.; Carroll, B. F.; Korotkevitch, S.; Morris, M. J. AIAA J. 1997, 35, 306-310. (5) Zelelow, B.; Khalil, G. E.; Phelan, G.; Carlson, B.; Gouterman, M.; Callis, J. B.; Dalton, L. R. Sens. Actuators, B 2003, 96, 304-314. (6) Mitsuo, K.; Asai, K.; Hayasaka, M.; Kameda, M. J. Visualization 2003, 6, 213-223. (7) Gouterman, M.; Callis, J.; Dalton, L.; Khalil, G.; Mebarki, Y.; Cooper, K. R.; Grenier, M. Meas. Sci. Technol. 2004, 15, 1986-1994. 9110 Langmuir 2005, 21, 9110-9120 10.1021/la050999+ CCC: $30.25 © 2005 American Chemical Society Published on Web 08/23/2005