The way that the illumination interacts with the target surface is of paramount importance in obtaining a high quality image.
Surface reflectivity – shiny
If the surface of the target object is shiny, special care needs to be taken when selecting the type of illumination. A spot illuminator will cover the target in light, but will create numerous reflections that may adversely affect the resulting image. In the example of a flat shiny surface shown here, the light will create a localised, intense spot of little use for further software interpretation.
The next drawing shows how changing to a larger and more diffuse light will create a far better image, as long as the area of interest lies within the diffuse circle of illumination. A larger light would be required to illuminate the entire surface.
Surface reflectivity - scattering
If the object tends to scatter the light as it does with dull or matt finishes, the light that hits the surface will be dispersed in all directions leading to a much more evenly distributed pattern of illumination. The image shows the most basic set-up, where light is scattered evenly across the entire surface.
Surface geometry - angular
If the surface of the target object is angular, shadows may be produced. To counteract this, the object will need to be lit from different angles. In cases where only a single feature on the surface is of interest, it may be necessary to concentrate the light from a particular angle in order to obtain the best possible results.
The most difficult surface to illuminate correctly is one that has an undulating, shiny surface. Examples of this could be ball bearings, polished components or foil wrappings. These types of surfaces will produce multiple, direct reflections that would make subsequent image analysis very difficult and processor intensive. The best technique in this situation is to use a form of advanced coaxial or dome illumination that floods the entire surface with even, diffuse light from all angles.