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Linnik Interferometry

The resolving power of a microscope is typically worse in the direction of the microscope axis than the direction of the imaging plane. Linnik interferometry provides a method by which the brightness of a microscope image becomes a strong function of distance to the target. In such a system, a light source that is at least partially coherent (such as an LED) is split by a beam-splitter and sent along two paths. One path reflects from a reference mirror and is sent back up to the camera by the same beam-splitter. The other path reflects from the specimen being imaged before being sent to the camera. At the camera, if the two path lengths are matched to within the coherence length of the light source, light from the two paths combines interferometrically. If the path lengths are exactly matched, or if they differ by an integer multiple of the wavelength of the light source, then light from the two paths will recombine in phase and create a bright spot. If the path lengths differ by 1/2 wavelength from this condition, light from the two paths will combine out of phase and create a dark spot. Consequently, the brightness of the target is a sensitive function of the height of the specimen.