The term Reststrahlen was coined following the observation by Heinrich Rubens in 1898 that repeated reflection of an infrared beam at the surface of a given material suppresses radiation at all wavelengths except for certain spectral intervals, and Rubens detected wavelengths of sizes around 60 μm. The measured intensity for these special intervals (the Reststrahlen range) indicates a reflectance of up to 80% or even more, while the maximum reflectance due to infrared bands of dielectric materials are usually  compared to the intensity of the incident radiation, while the light in the Reststrahlen range can maintain 40% of its original intensity by the time it reaches the detector. Obviously, this contrast increases with the number of reflections and explains the observation made by Rubens and the term Reststrahlen (residual rays) used to describe this spectral selection.
Reststrahlen bands manifest in diffuse reflectance infrared absorption spectra as complete band reversal, or in infrared emission spectra as a minimum in emissivity.