The basis for every optical measurement is to display the features being measured with a highly accentuated contrast. This can best be achieved on the outer edges of objects. In this case, measurement can be performed in transmitted light (Fig. 12a). Ideal conditions are offered by flat objects. On the other hand, where wide edges are involved, the interrelationship between the illumination, the test object and the imaging beam path must be taken into consideration. The manufacturer of a coordinate measuring machine must ensure that the aperture angles of the individual optical systems are mutually aligned and that the image processing software enables adequate calibration of the edge position algorithms. Other transmitted-light units featuring adjustable apertures are used for special applications such as the measurement of rotationally symmetrical parts (for example, cylinders in a horizontal position).