Lummer–Gehrcke interferometer

The Lummer- Gehrcke plate, named after the German physicists Otto Lummer (1860-1925) and Ernst Gehrcke (1878-1960), is an optical filter that is used for producing narrow-band spectral lines.

Design and operation

The Lummer- Gehrcke plate consists of a plane-parallel " Lummer plate " made ​​of glass or quartz and a gehrckeschen essay, a triangular prism for light coupling. In addition to this, another construction is referred consisting of a plane-parallel plate with a beveled surface on a front page as a Lummer- Gehrcke plate in the literature.

The coupled light is easily reflected several times in the plane-parallel plate below the critical angle of total reflection. At each reflection some of the light enters the plate at a grazing of the above and the lower interface. The reflection close to the total reflection ensures a high reflection coefficient, and thus the possibility that a plurality of beams may interfere with each other.

The exiting of the upper and lower side parallel beams are then combined with a suitable lens assembly and interfere with each other, respectively ( see also the diffraction grating ). This gives two beams with a sharp spectral line, similar to a monochromatic source.

A disadvantage of the Lummer- Gehrcke plate, that a plane-parallel plate having a high parallelism, and surface flatness is required. These are difficult to manufacture and therefore expensive. Furthermore, can be varied in this rigid structure with constant plate thickness, the resolution is not. For these reasons, Lummer- Gehrcke plates are now hardly used and replaced by more flexible Fabry -Perot interferometer.