Optical measuring technology with image processing is used for non-contact dimensional measurement and form measurement of workpieces. There are universally applicable optical measuring devices as well as workpiece-specific devices for measuring shafts, punched strips or small parts present as bulk material. Image processing as a measuring technology is mainly suitable for 2D measurement of flat surfaces, which is carried out either in transmitted light as a shadow image or in reflected light. This also includes visually operated measuring devices. 
2.5D measurement is used for the detection of sectionally flat objects by means of an additional distance measurement via focusing in the Z-axis, tactile sensors or laser distance sensors. In practice, optical sensors are also combined with tactile measuring systems and other sensor types to form multi-sensor measuring machines. Using stereo image processing or image processing with active illumination, true all-optical 3D measurement is also possible. 
The detection of surface defects such as scratches, dents, chipping and open pores is not a task of optical metrology, but belongs to the field of visual inspection or surface inspection.

A common application is the 2D inspection of flat or rotationally symmetrical workpieces. For inspection at standstill, the measured object is typically placed on a measuring table with a glass plate, presented to the image sensors with appropriate handling, or even fixed in workpiece holders. From the known distance to the measuring object and via the size of the image section, the dimensions of the measuring object can be calculated. 
Moving objects can be inspected by presenting the test object to the image sensors in transmitted light on a glass slide, a rotating glass plate, as a punched strip or in a rotating movement, which allows the test object to be detected even with several sensors from different angles and at high speed. Similarly, the outer contour of a rotationally symmetrical test specimen, for example, can be captured piece by piece in a large number of exposures with a rotating movement about the longitudinal axis. By taking images with very short exposure times, a sharp image is achieved that "freezes" the movement of the test object.