Aberrations caused by inhomogenities of the optical components introduce
distortions in both the measuring and the comparison wave which are eliminated, since interference patterns result only home difference of their optical
paths. For the test section streak free sheet glass is recommended. Acrylic
glass with a large coefficient of volume expansion should be avoided because
a change of temperature would cause additional interference fringes.
Generally as few mirrors as possible should be used in order to avoid light
power losses through reflection and absorption; and to enhance mechanical
stability. A problem free adjustment of the laser beams in relation to the collimators should be also possible.