The use of visual sensors usually requires reflected-light as well as transmitted-light illumination. A distinction must be made here between two types: bright field reflected light is projected onto the object parallel to the axis of the beam path. Ideally, this is achieved directly by the lens system of the imaging optics. This type of illumination results, for example, in direct reflections of light from metal surfaces. The object appears bright. Inclined surfaces reflect the light past the lens system and therefore appear dark (Fig. 13).

The dark field reflected light strikes the test object at an oblique angle to the imaging beam path. Ring-shaped configurations (or annular ring fiber-optic illuminators) are normally used for this purpose. The advantage of these components is the low amount of heat they add to the measuring volume. Via segmentation, illumination effects can be created from different spatial directions. Light-emitting diodes (LEDs) make it possible to optimally adapt dark field reflected light to the measuring job at hand. Different angles of illumination can be achieved by switching on various diode arrays. By using the MultiRing® illuminator combined with an adjustable working distance zoom lens, it is also possible to vary the angle from the optical axis within a wide range. In addition, this also make it possible to measure at a sufficiently large working distance away from the object.

In modern multisensor coordinate measuring machines, all light sources can be controlled automatically via the measuring software. The brightness can be controlled using the light reflected by the object. This enables practice-oriented application when measuring the surfaces of different materials. Mathematical correction of the lamp characteristics enables the continued use of the same CNC programs even after changing lamps.