A new method of assessing surface quality is described in which the light scattered from the surface is scanned by the edges of a chopper disc. The electrical output from the photocell is differentiated twice to give a signal which is a measure of the maximum rate at which the intensity of the reflected light falls off. It allows a simple, cheap, and robust instrument to be made which is insensitive to ambient illumination and can be used as a surface comparator either ‘on-line’ or as a bench instrument.
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