Lummer-Gehrcke Plate

a multiple-beam optical interferometer in the form of a plane-parallel glass or quartz plate that is finished to a high degree of accuracy (see Figure 1); it has lateral entry of light. It was invented by the German physicists O. Lummer and E. Gehrcke.

Figure 1. Path of light rays in a Lummer-Gehrcke plate. Upon each successive reflection on the faces of the plate, a portion of the original beam is refracted and emerges from it. Thus, beams of parallel rays are formed that have a fixed path difference relative to one another and that interfere in the focal plane of a converging lens placed in their path.

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