Image and Data Analysis Techniques
This Chapter describes image and data analysis techniques used in various processing steps for PSP and TSP. For quantitative PSP and TSP measurements, cameras should be geometrically calibrated to establish the accurate relationship between the image plane and the 3D object space and map data in images onto a surface grid in the object space. Analytical camera calibration techniques, especially the Direct Linear Transformation (DLT) and the optimization calibration method, are discussed. Since PSP and TSP are based on radiometric measurements, an ideal camera should have a linear response to the luminescent radiance. For a camera having a non-linear response, radiometric camera calibration is required to determine the radiometric response function of the camera for correcting the image intensity before taking a ratio between the wind-on and wind-off images. A simple but effective technique is described here for radiometric camera calibration. The self-illumination of PSP and TSP may cause a significant error near a conjuncture of surfaces when a strong exchange of the radiative energy occurs between neighboring surfaces. The numerical methods for correcting the selfillumination are generally described and the errors associated with the selfillumination are estimated for a typical case. The self-illumination correction is usually made on a surface grid in the object space since it highly depends on the surface geometry.
A standard procedure in the intensity-based method for PSP and TSP is to take a ratio between the wind-on and wind-off images to eliminate the effects of non-homogenous illumination intensity, dye concentration, and paint thickness. However, since a model deforms due to aerodynamic loads, the wind-on image does not align with the wind-off image. The image registration technique based on a mathematical transformation between the wind-on and wind-off images is described to re-align these images. A crucial step for PSP is to accurately convert the luminescent intensity to pressure; cautious use of the calibration relations with a correction of the temperature effect of PSP is discussed. PSP measurements in low-speed flows are particularly difficult since a very small pressure change has to be sufficiently resolved by PSP. The pressure-correction method is described as an alternative to extrapolate the incompressible pressure coefficient from PSP measurements at suitably higher Mach numbers by removing the compressibility effect. The final processing step for PSP and TSP is to map results in images onto a model surface grid in the object space. When a model has a large deformation produced by aerodynamic loads, a deformed surface grid should be generated for more accurate PSP and TSP mapping. A methodology for generating a deformed wing grid is proposed based on
videogrammetric aeroelastic deformation measurements conducted simultaneously with PSP and TSP measurements.