A Linnik interferometer works only when the two
path lengths are matched to within the coherence length of the light
For a low-coherence source such as an LED, the coherence length is on the
order of micrometers, so matching the path lengths requires precise
adjustment of both the target and reference arms.
In a Mirau interferometer, the interferometric system is customized for a
single microscope lens, so the path lengths are matched during the
Consequently, taking three-dimensional interferometric images is as simple
as changing focus on the microscope stage (in the example shown here, focus
is changed by moving the microscope objective).
As the path length is changed, the interferometric pattern shifts.
Measuring this change provides a calibration between brightness of the
image and height of the target.