In bright field, backlight illumination, light is either stopped or transmitted by the surface if the material is opaque (Fig. 14)or transparent.
In the first case, we see the outline of the object (black object on white background - see Fig. 16 and 18). In the latter, the non-planar features of the transparent object show up dark on a white background; in this second case, contrast is usually low unless the transparent surfaces present sharp curvatures (e.g. air bubble inclusions in plastic).
These lighting techniques can be achieved using diffuse backlights (Fig. 15a, 15b and 16) or telecentric illuminators, specifically designed for high accuracy applications (Fig. 17 and 18).