White light interferometers
Light interference occurs when there is a difference in distance traveled by the light from the surface of a target object to a certain point; the white light interferometer uses this phenomenon to measure the surface roughness of a sample. The figure on the right is a structural diagram of an interferometer. The light emitted from the source is separated into reference and measurement beams. While the reference beam is passed through the reference mirror through a beam splitter, the measurement beam is reflected and guided to the sample surface. The passed beam is reflected by the reference mirror to the CCD image sensor and forms an interference pattern. The other beam is reflected off the sample surface, passes the beam splitter, and forms an image through the CCD image sensor.
The white light interferometer is designed so that the optical path length from the CCD element to the reference mirror and that from the CCD element to the sample surface are the same. The asperity on the sample surface causes these path lengths to be unequal, which results in forming an interference pattern at the CCD element. The number of lines in the interference pattern is translated to peaks and troughs (heights) on the sample surface.