Stria measurement using Wave Front Phase Imaging on a transparent plate

Wave Front Phase Imaging (WFPI) is used to measure the stria on an artificial, transparent plate made of Schott N-BK7 (R) glass material by accurately measuring the Optical Path Difference (OPD) map. WFPI is a new technique capable of reconstructing an accurate high resolution wave front phase map by capturing two intensity images at different propagation distances. An incoherent light source generated by a light emitting diode (LED) is collimated and transmitted through the sample. The resultant light beam carries the wave front information regarding the refraction index changes inside the sample(1). Using this information, WFPI solves the Transport Intensity Equation (TIE) to obtain the wave front phase map. Topography of reflective surfaces can also be studied with a different arrangement where the collimated light beam is reflected and carrying the wave front phase, which again is proportional to the surface topography. Three Schott N-BK7 (R) glass block samples were measured, each marked in which location the wave front phase measurement will be performed(2). Although WFPI output is an OPD map, knowing the value of refractive index of the material at the wavelength used in the measurements will lead to also knowing the thickness variations of the plate.