Optical measurement refers to noncontact measurement using various light sources. It usually requires at least one lens, a light source and a detector. 

Readers of this column will be familiar with the subject of measurement uncertainty since I comment on it from time to time, as I did last month. Those readers that have not been that interested in it will certainly run across it on reports from their calibration sources.

As related to form metrology, a roundness measurement defines how closely the shape of a part approaches that of a perfect circle.