All I want for Christmas is a Buchmann-Meyer pattern
Doing some reading for my current project (a history of the CD Audio format), I stumbled across a physical phenomenon that does not often crop up in discussions of the history of gramophone recording: the Buchmann-Meyer effect. This optical effect was once used to measure the quality of gramophone records, both qualitatively and quantiatively, by shining a band of light on a disc and capturing the characteristic “Christmas tree”–like pattern that is reflected back to the viewer. If the pattern is clear and distinct, the record is in good nick; if the “branches” of the pattern are ill defined, the disc surface has perhaps become subject to wear and disintegration, or, indeed, was never fabricated to a high standard in the first place. In a figure from a very interesting recent paper—interesting for other reasons!—we see three discs.1 Visible are Buchmann-Meyer patterns reflected from the A and B sides of an Audio Engineering Society test record from 2007 (well defined) and one from an undated lacquer transcription disc, probably produced in France (not as well-defined).