The interference field is a result of the phase shift of the object wave caused
by a change of the Optical refractivity, the path length or both. The length of
the test section influences the number of the interference lines. A low number
leads to a large evaluation error. A test section which is too long results in
a high deviation of the laser beam according to Snell’s law. In the area. of
interest, for example the boundary layer, approximately 5 to 10 lines should
exist