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As anyone involved in the field of industrial image processing knows, the choice of lighting is extremely important for producing good images, because the better and richer in contrast the image is, the less effort is required from the software. However, lighting is not just lighting! There is a big difference in results if a coin is illuminated using diffuse lighting, side-lighting (dark field) or coaxial incident lighting. The basic rule is that the lighting should be tested on the object. In the following, different lighting techniques are briefly described and the general application possibilities are listed.
• Incident lighting, transmitted light or backlighting, and self-luminous. Incident lighting means that the object is directly illuminated, transmitted lighting means that the object is trans-illuminated, backlighting means that the back of the object is illuminated and self-luminous objects, such as melting glass or steel, do not normally require any extra lighting.
• Direct and indirect lighting. Direct lighting means that the light falls directly on the object (this may result in annoying specular highlights if shiny objects are being photographed). Indirect lighting prevents the light source from reflecting so that there are almost no specular highlights.
• The angle of incidence. Here, with direct lighting, all angles between 0° (vertical to the optical axis) and 90° (parallel to the optical axis) are possible. In the case of diffuse lighting, the light should fall as homogeneously as possible from all directions onto the object.
• Coaxial lighting is a special case. Here, the light is parallel to the optical axis. Coaxial lighting causes the areas of the object that are vertical to the optical axis to appear bright, and areas that are diagonal to the optical axis such as the edges of a scratch, for example, to appear dark.