Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
Figure 19: Directional front lighting.
3.2.3 Ring lighting
Ring lighting, as shown in Figure 20, is available in different configurations, using many of
the illumination sources described previously. This provides shadow free illumination
along the optical axis of the camera and is especially useful when imaging objects with
high reflectance. Some workpiece parts, subject to dimensional measurement, may not
have a sharp edge convenient for edge detection but a curved or chamfered edge. In these
cases, simple reflected illumination will not result in high accuracy edge detection since it
produces an image contrast with a gradual intensity curve. In extreme cases, no form of
edge detection may be attainable. Using ring light illumination makes it possible to
enhance the image contrast and perform highly reliable edge detection.
Figure 20: Ring light illumination.
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