The design of a low-coherence interferometer, as implemented by TRIOPTICS in the OptiSurf®system is described in Fig. 1. The light from a low-coherence light source is divided by a beam splitter into an object beam and a reference beam.
The object beam illuminates the lens system along its optical axis. A fraction of the incoming light beam is reflected back at each surface of the sample. This light is superimposed on a photo detector with the light from the reference arm. The light in the reference arm is varied by means of a delay section in the time span.
The length of the reference arm is varied by means of a movable mirror, and measured using an laser interferometer. When the resulting intensity of the back-reflected reference beam and object beam are analyzed as a function of the change in length/delay of the reference arm (Fig. right) then interference patterns are always observed when the optical path lengths coincide in the two interferometer arms. Thus, the position of each surface of the sample can be determined mathematically.