The Pressure Sensitive Paint (PSP) is very useful and easy way to obtain the pressure distribution on surfaces. Therefore the PSP has been adopted to various flow fields to obtain pressure distributions, showing good agreement with other methods, such as a pressure tap. Many kinds of flow fields have been used in the manufacturing process, like small scale, low pressure and/or high speed flows; for example, semiconductor manufacturing processes where a mean free path of gas molecules is large, and micro-/nano-systems where a characteristic length is small. Recently the PSP is applied to these high Knudsen number flows. Usually it is considered that the luminescent of the PSP represents the surface pressure, but it is not clear yet that this “pressure” means “static” or “total” pressure. The PSP is considered to be in the boundary layer, leading to the local equilibrium of oxygen molecular density between in the gas phase and in the PSP is achieved. This becomes a large problem especially in the high Knudsen number regime. The mechanism of the PSP has been mainly discussed focused on the luminescence intensity and the oxygen quenching of luminescent molecules. Here, in this study, we tried to relate the flux of oxygen molecules and the surface pressure to the luminescence intensity of the PSP, and clarify the limit of application from the molecular kinetics point of view. The main target of this problem was in the high Knudsen number and the high Mach number flow fields.

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