The major instrument used in this experiment is a Michelson interferometer. The experiment is a series of coherence-length measurements starting from a high-coherence source and ending with a broad-band light source. The three light sources used to illustrate optical coherence are an intensity-variable diode laser, a light-emitting diode (LED), and a white light source. 

 

The intensity of the diode laser can be tuned by varying its driving current, permitting the coherence length measurement near the threshold or high above the threshold of the diode laser. 

An LED’s coherence length is close to that of a diode laser near threshold. Therefore the coherence-length measurement of an LED is arranged immediately after that of the intensity-variable diode laser. 

A white-light source has the worst optical coherence. The last part of the experiment illustrates white-light interference with near equal optical paths in the two interferometer arms.