3-dimensional measurement with stereo-microscope of telescope-type

Optical 3-dimensional (3D) measurement is a very effective tool of quality accurance. This method is very fast and contact-free. Usually a 3D measurement device has got two different optical channels which are taking two different pictures of the specimen. The differences of this two images allow to calculate a 3D reproduction of the specimen. But the differences between the two pictures must be not too big. It seems that a stereo-microscope must be the ideal tool for 3D measurement because both optical channels of this device are using the same objective. Therefore both images are only slightly different and have got the same magnification. Unfortunately a farther investigation shows that the stereo-microscope is not feasible. There is no conjoint projection-center for the image-points and because of this the 3D reconstruction of the specimen cannot be correct. The reason of the lack of conjoint projection-center is the necessarily fullfield sin-condition of the objective. Only in case of telecentricity such a center exists for all image-points. It is located at infinity. Under this condition a stereo-microscope of telescope-type is actually a perfect device for 3D measurement.