# Spectral Variability and Filter Leakage

The uncertainty in Пf/nfref is mainly attributed to the spectral variability of

illumination lights and spectral leaking of optical filters. Possolo and Maier (1998) observed the spectral variability between flashes of a xenon lamp; the uncertainties in the absolute pressure and pressure coefficient due to the flash spectral variability were 0.05 psi and 0.01, respectively. If optical filters are not selected appropriately, a small portion of photons from the excitation light and ambient light may reach a detector through the filters, producing an additional output to the luminescent signal.

7.1.3. Pressure Mapping Errors

The uncertainty in pressure mapping is related to the data reduction procedure in which PSP data in images are mapped onto a surface grid of a model in the object space. It is contributed from the errors in camera resection/calibration and mapping onto a surface grid of a presumed rigid body. The camera resection/calibration error is represented by the standard deviations ax and a y of the calculated target coordinates from the measured target coordinates in the image plane. Typically, a good camera resection/calibration method gives the standard deviation of about 0.04 pixels in the image plane. For a given PSP image, the pressure variance induced by the camera resection/calibration error is

var(p) ~ (dp/дx) aУ + (p/дy) a2y.

The pressure mapping onto a presumably non-deformed model surface grid leads to another deformation-related error because a model may undergo a considerable deformation generated by aerodynamic loads in wind tunnel tests. When a point on a model surface moves by AX = X’ – X in the object space, the pressure variance induced by mapping onto a presumed rigid body grid without

correcting the model deformation is var(p) = | (Vp )surf • (AX )f, where (Vp )surf is the pressure gradient on the surface and (AX )sulf is the component of the

displacement vector AX projected on the surface in the object space. To eliminate this error, a deformed surface grid should be generated for PSP mapping based on optical model deformation measurements under the same testing conditions (Liu et al. 1999).