Luftspalt

Luftspalt, a German term meaning air gap, refers to a small space filled with air between two solid surfaces. In optics it is a common element in devices that use interference or optical cavities, such as etalons, interferometers, or systems used to study thin-film interference.

In an air-filled cavity, light undergoes multiple reflections at the facing surfaces. The interference pattern depends on the thickness t of the gap and the refractive index n of the medium (n is about 1 for air). The approximate optical path difference is 2 n t, so changing the gap changes the transmitted and reflected light spectrum. This principle underlies devices like Fabry-Pérot etalons and Newton’s rings experiments, where precise control or measurement of the gap yields high spectral or radial interference information.

Quality and geometry are important when using a luftspalt. Uniformity and parallelism of the faces are required for predictable, high-contrast interference. A wedge-shaped or nonuniform gap produces varying fringe patterns rather than a uniform response. Environmental conditions such as temperature, atmospheric pressure, and humidity can alter air density and refractive index, and thus influence measurements or device performance.

Applications of luftspalten extend to optical metrology, calibration of instruments, and laboratory experiments where a tunable or well-characterized air gap is needed. The concept also appears in broader contexts where a spacer or cavity filled with air defines spacing between components in precision optics and instrumentation.