The intensity-based method for PSP and TSP requires a ratio between the wind-on and wind-off images of a painted model. When a model moves in a nonhomogenous illumination filed during a test, the image-ratio method inevitably causes inaccuracy in determining pressure and temperature. A multiple – luminophore paint is designed to eliminate the need for a wind-off reference image. Generally, a two-luminophore PSP consists of a pressure-sensitive luminophore and a pressure-insensitive reference luminophore; similarly, a two – luminophore TSP combines a temperature-sensitive luminophore with a temperature-insensitive reference luminophore. The probe and reference luminophores can be excited by the same illumination light. Ideally, there is no overlap between the emission spectra of the probe and reference luminophores such that the luminescent emissions from the two components can be completely separated using optical filters. Theoretically, a ratio Ix /1^ between the probe and reference images could able to eliminate the effects of spatial non-uniform illumination, paint thickness and luminophore concentration, where Ix and I Xi
are the luminescent intensities at the emission wavelengths X1 and X2, respectively. However, McLean (1998) pointed out that since two luminophores cannot be perfectly mixed, the simple two-color intensity ratio I x /1^ cannot completely compensate the effect of non-homogenous dye concentration. In this case, a ratio of ratios (I Xj /1^ )/(Ix /1^ )0 should be used to correct the effects
of non-homogenous dye concentration and paint thickness variation, where the subscript 0 denotes the wind-off condition.
Besides the above combinations of luminophores, a temperature-sensitive luminophore, which cannot be quenched by oxygen, can be combined with an oxygen-sensitive luminophore. This two-luminophore temperature/pressure paint can be used for correcting the temperature effect of PSP. In particular, when the temperature dependencies of the two luminophores are close, a two-color intensity ratio between the two luminophores exhibits a very weak temperature dependency (Engler et al. 2001a). Figure 3.24 shows a ratio of ratios of a two-luminophore PSP (PtTFPP in FIB with a proprietary reference luminophore) as a function of pressure at different temperatures (Crafton et al. 2002). Clearly, the data at different temperatures overlap, and a ratio of ratios of this PSP is almost temperature insensitive in a range of 5-45oC. Furthermore, a multiple-
luminophore PSP can be developed to correct the temperature effect as well as the effect of non-uniform illumination simultaneously.
Oglesby et al. (1995b) used PtOEP or PtTFPP as a pressure probe luminophore and Fluorol Green Gold 084 (3,9-perylenedicarboxylic acid, bis(2- methylpropyl)ester) as a reference luminophore in the GP-197 polymer. Harris and Gouterman (1995, 1998) used PtTFPP as a pressure-sensitive luminophore and incorporated a solid-state phosphor BaMg2Al16O27:Eu2+ (BaMgAl) as a reference luminophore in an Acrylic copolymer. Since BaMgAl is insoluble, the reference luminophore was not uniformly distributed and therefore the paint suffers from the effect of the uneven layer thickness. TsAGI/OPTROD developed the proprietary two-luminophore PSP formulations, LPS B1 and LPS B12 (Bukov, et al. 1997; Lyonnet, et al. 1997). Three pressure sensitive paints with an internal temperature sensitive luminophore were also tested by Oglesby et al. (1996), where EuTTA, MgOEP and Ru(bpy) were used as temperature-sensitive reference luminophores. Hradil et al. (2002) used Ru(dpp) as a pressure probe molecule and manganese-activated magnesium fluorogermanate (MFG) as a thermographic phosphor. The preliminary results showed that two-luminophore PSPs indeed enabled point-by-point correction for the temperature effect of PSP.
Buck (1988, 1989, 1991) used a blue-green Radelin thermographic phosphor for aerothermodynamic testing that intrinsically exhibited two narrow-band emission peaks at 450 nm and 520 nm. It was found that a ratio of the blue to green emission intensity was a function of temperature, but independent of the UV illumination intensity. Another two-color phosphor system used a green-red mixture of rare-earth and Radelin phosphors for a broader range of temperatures. Buck (1988, 1989) gives a detailed description of the multiple-color phosphor thermography system developed at NASA Langley.