Patent ID: 8462356

Claim:
An apparatus for observing the optical appearance of a surface of a sample of a semitransparent material, the apparatus comprising: a light source for illuminating at least a region of interest of the surface of the sample from a predetermined direction, a camera for observing a response to the illumination of the region of interest, and an optical focus device for determining if the camera is in focus with at least a part of the surface of the region of interest, wherein the optical focus device provides for a first and a second focus light beam having a first and a second angle of incidence with the surface, wherein the first angle of incidence is different from the second angle of incidence, wherein the first and second focus light beam provide for a superposition of a first and a second image of a first and a second mask pattern on the surface of the region of interest, and wherein the focus is determined by the alignment of the first and the second mask pattern image.