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A beam of light incident on a reflecting surface is reflected in a manner depending on the fine-scale character of the surface. If this is perfectly smooth, all the light is reflected in a direction determined by the familiar law of specular reflexion by a mirror, whereas, if it is perfectly rough - a completely matt surface - the light is diffusely scattered in all directions, there being no special concentration in the direction of specular reflexion. For surfaces of intermediate character the reflected light is still diffused in all directions, but is most strongly concentrated in and near the direction of specular reflexion. The fraction of the total reflected light which is comprised within a small cone whose axis is in this direction provides a measurable property which increases with the smoothness of the surface. The present paper describes an instrument which was developed to utilize this property for testing the quality of finish of flat or approximately flat metal surfaces. A beam of light is concentrated on a small area of the surface to be tested. All the reflected light is collected inside a small integrating sphere whose internal illumination is indicated by a photocell of the rectifier type connected to a galvanometer. A shutter is fitted within the sphere which enables the specularly reflected light to be distinguished from the diffusely scattered light, and the ratio of the specular light to the total reflected in all directions, termed the S/T ratio for the surface, can be determined. It is shown that this ratio affords a very sensitive means of comparing surfaces for smoothness, and a table of typical results is given to show where some familiar types of surface finish come on the S/T scale.
J Guild (Mon,) studied this question.