Physical Properties of Paints
In order to quantitatively measure air pressure with PSP, the relationship between the luminescence signal (intensity, lifetime or phase) and air pressure should be experimentally determined by calibration. Apparatus for calibration of PSP over a temperature range of 90-423 K are described in Appendix A. A generic calibration set-up consists of a pressure chamber, excitation light source and photodetector. A PSP sample is placed in the pressure chamber where pressure can be adjusted from vacuum to high pressures. The surface temperature of the PSP sample is controlled using a heating/cooling device and measured using a temperature sensor. The PSP sample is excited by an illumination source (e. g. UV lamp, LED array or laser) through a window of the pressure chamber. The luminescent emission from the paint sample, after filtered by a band-pass optical filter, is measured using a photodetector (e. g. photodiode, PMT or CCD camera), and the photodetector output is acquired with a PC over a range of pressures and temperatures. Therefore, the correspondence between the luminescence signal and pressure, which is usually described by the Stern-Volmer equation, is established over a range of temperatures. Typical calibration results for a number of PSP formulations based on Platinum Porphrins, Ruthenium complexes and Perlene/Pyrene are given in the following sections. The calibration set-up for PSP can be used for TSP calibration when the surface temperature of a paint sample varies while pressure in the chamber is kept constant. Calibration data for TSP are typically presented as an Arrhenius plot over a certain temperature range; typical calibration results for TSP formulations are given in Sections 3.3 and 3.4.
The most common calibrations for PSP and TSP are based on measurements of the luminescent intensity as a function of pressure and/or temperature. As discussed before, however, the luminescent lifetime (or phase) is also a function of pressure and/or temperature. In a lifetime calibration apparatus, a pulsed (or modulated) excitation light source is used, and after an exciting pulse light ceases the exponential decay of the luminescent intensity is measured using a fast – responding photodetector and recorded with a PC or an oscilloscope. The luminescent lifetime can be determined by fitting data with a single exponential function or multiple-exponential function for certain paints over a range of pressures and temperatures. Early instrument for measuring the luminescent
lifetime was described by Brody (1957) and Bennett (1960), and the current state of luminescent lifetime measurement systems in photochemistry and medical applications was comprehensively reviewed by Lakowicz (1991, 1999). The Stern-Volmer relations between the lifetime and oxygen partial pressure (or concentration) for oxygen sensitive luminescent materials were determined by Gewehr and Delpy (1993), Gord et al. (1995), Sacksteder et al. (1993), Xu et al.
(1994) , and Papkovsky (1995). Lifetime calibration results for TSPs and thermographic phosphors were also reported by Sholes and Small (1980), Grattan et al. (1987), Bugos (1989), Noel et al. (1985), and Lakowicz (1999). A more detailed discussion on the lifetime and phase methods is given in Chapter 6.