Profile projectors, also known as optical comparators or shadowgraphs, are normally employed in a manual mode for the inspection of engineering components. Traditionally the projected and enlarged shadow image of an object is compared with a large scale transparency of the desired form of an object profile. More recent versions employ digital transducer scales and 'edge-sensors' to allow measurements of X, Y coordinates of points around the profile. The development of reliable sensing devices has now allowed the measurement process to be carried out completely automatically. Although currently restricted to 2-dimensional operation, the system produces output data in a similar manner to co-ordinate measuring machines, which are now very widely used in the engineering industry.
"Automatic Inspection Of Components Using Profile Projection", Proc. SPIE 1010, Industrial Inspection, (15 February 1989); doi: 10.1117/12.949243; https://doi.org/10.1117/12.949243