The term “visual sensor” denotes all sensors which, similar to the human eye, pick up at least a two-dimensional image of the object being measured. The intensity distribution of this optical image is detected and evaluated by a sensor.

For many decades the human eye was the only visual “sensor” available for optical coordinate measuring instruments such as measuring microscopes and measuring projectors. Subjective error sources influencing measurements of this type include parallax (oblique sighting) and faulty measurement of bright-dark transitions on edges due to the logarithmic light sensitivity of the human eye. The results of such measurements therefore basically depend on the operator (Fig. 7) and are comparable only to a limited degree. The maximum measuring speed is also limited.

Because of all of its drawbacks, visual probing represents the last possible alternative for modern image processing systems. It is used in cases where the object structures to be measured show poor visibility and the geometric features can only be probed intuitively. Assuming that the human eye can resolve several tenths of a millimeter when sighting with a reticle, a final resolution of several microns can be attained by using this technique, for example, in conjunction with a 100 × optical magnification.