Image Acquisitionand Proces

Setting Up 39
Camera
Light Source
Object
(a)
(b)
FIGURE 2.29 (a) Ring schematic. (b) Ring
FIGURE 2.30 Reßective object illuminated with a ring light.
Ring lighting is often used in areas such as microscopy, photographic documentation,
medical and biological research, metallurgical and forensic inspection, semiconductor
etching and printing inspection. Ring lights are particularly useful when
illuminating objects at short working distances, otherwise its effects become similar
to a point source or area array.
2.4.3 DARK FIELD
The light emitted from dark Þeld illuminators (Figure 2.31) provides low-angle
illumination, enhancing the contrast of surface features, and displaying the contours
of the object, including engraved marks and surface defects.