 The Twyman – Green interferometer , invented by Twyman and Green in 1916 , is a variant of the Michelson interferometer widely used to test optical components . The basic characteristics distinguishing it from the Michelson configuration are the use of a monochromatic point light source and a collimator . It is interesting to note that Michelson ( 1918 ) criticized the Twyman @-@ Green configuration as being unsuitable for the testing of large optical components , since the light sources available at the time had limited coherence length . Michelson pointed out that constraints on geometry forced by limited coherence length required the use of a reference mirror of equal size to the test mirror , making the Twyman @-@ Green impractical for many purposes . Decades later , the advent of laser light sources answered Michelson 's objections . ( A Twyman @-@ Green interferometer using a laser light source and unequal path length is known as a Laser Unequal Path Interferometer , or <unk> . ) Fig . 14 illustrates a Twyman @-@ Green interferometer set up to test a lens . Light from a monochromatic point source is expanded by a diverging lens ( not shown ) , then is collimated into a parallel beam . A convex spherical mirror is positioned so that its center of curvature coincides with the focus of the lens being tested . The emergent beam is recorded by an imaging system for analysis . 
