A method is proposed to use the speckles created in front of a curved surface to approach the slope contours of this surface. By focusing the camera on a plane in front of the surface and photographing the speckles contained in this plane before and after changing of the laser light wavelength , a speckle interferogram is obtained. Various methods of analysing speckle photographs are presented to obtain some informations related to the form of the objects. Two methods use the diffraction properties of the speckle patterns in coherent optics. A third method of viewing speckle photographs is to use a small light source which may have a limitated spatial coherence and is often polychromatic. A fourth method is suggested to observe in real time the contour lines of the object.
"Two Wavelength Speckle Images Applied To Contouring", Proc. SPIE 0136, 1st European Conf on Optics Applied to Metrology, (18 April 1978); doi: 10.1117/12.956174; https://doi.org/10.1117/12.956174