Simultaneous ESPI measurements using multiple wavelengths and a color camera

Abstract

ESPI is typically done using a single monochromatic light source with a monochrome camera. This arrangement enables full-field measurements of a single deformation quantity according to the particular arrangement of the optical system. If a further deformation quantity is to be measured, then the associated optical arrangement must be used sequentially. Here, an alternative approach is described where multiple interferometric measurements are simultaneously made using a color camera imaging monochromatic light sources of different wavelengths. The Red-Green-Blue (RGB) sensors of a conventional Bayer type camera can be read separately, thereby providing three independent color signals and independent ESPI phase maps. An example case is demonstrated here where two Michelson interferometers are combined to form a shearography camera with surface slope sensitivity in two perpendicular directions. By the use of two colors, both surface slopes can be measured simultaneously. The availability of the two surface slopes gives the opportunity for the data to be integrated numerically to give the surface displacement shape. This application is of significant practical interest because the surface displacement measurement can be made under field conditions by taking advantage of the well-known stability of shearography measurements.