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Surface Characteristics Of Machined Optics

The light intensity diffracted out of a reflected laser beam as a function of angle can be used to determine a portion of the spectral density function (SDF) of the reflector's surface. These measurements cover the frequency range in which surface deviations are generally termed surface roughness (as opposed to surface contour). In addition to giving values of rms roughness, the SDF can be used to infer more specific surface and machining parameters such as tool feed rate, groove shape information and tool chatter. The SDF also provides an interesting way of comparing roughness measurements made by systems with different spatial frequency band widths. Results are given for a number of machined surfaces. The tendency for large amounts of low frequency roughness is discussed. Surface groove shape is shown to be different from the expected "circular cusp shape" calculated from tool radius and feed rate.