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
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
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.