Patent ID: 7206079

Claim:
An apparatus for measuring by the Schlieren technique light beam deviations generated by a sample, said apparatus comprising: a source of light beam for lighting up said sample therewith so as to have a transmitted beam; imaging means for forming an image of said sample from said transmitted beam; a Schlieren filter for obtaining from said image, in operating conditions, an optical signal called Schlieren fringes; wherein the Schlieren filter comprises a periodic structure of a defined period; detecting means for detecting said Schlieren fringes in operating conditions; wherein the apparatus further comprises: means to shift the Schlieren filter with a shift φ by a fraction of the period of the periodic structure thereby shifting the Schlieren fringes, means to acquire a set of at least three successive phase-shifted images; means for reconstructing a mean image of the sample from said collected phase-shifted images using a phase-shifting algorithm used in interferometry; and processing means to calculate by phase-shifting analysis the optical characteristics, angle and phase of the transmitted beam from said reconstructed image so as to determine the beam deviation angle generated by the sample.