Such a device and a corresponding method are extensively described in U.S. Pat. No. 4,652,131, FERCHER/CARL ZEISS, and corresponding German Published, Non-Examined Patent Application DE-OS 33 18 678 or in the reference book "Laser Speckle". J. C. Dainty, Springer Verlag, Berlin, Heidelberg, New York, 1975. In the known device, the light emitted by a laser is split into a reference and a measuring beam, and a device shifts the frequency of the reference beam. The measuring beam is reflected on the rough surface which is to be measured and is then shown in the interferogram plane, together with the reference beam. With rough surfaces of this kind, a speckle pattern is obtained in the interferogram plane, instead of the customary interferogram. It is then possible to draw interferences as to the geometric shape of the rough surface by determining the phase difference such as described in more detail in the aforementioned prior art.
A complicated optical system with lenses, mirrors and prisms, as well as a frequency converter, not further described, is used for generating the different wavelengths or frequencies of the two beams. However, not only is such a system complicated and expensive to manufacture, but also hard to adjust to the required exactness. Furthermore, this arrangement requires such a large space that it is hard to contain it in a small and handy device.