Lighting Fundamentals
Types of Reflection
Objects reflect light in two ways. In specular reflection, light from each incoming ray reflects in a single direction (figure 12). A tinned circuit board trace or a mirror exhibits specular reflection. In diffuse reflection, light from each incoming ray is scattered over a range of outgoing angles. A piece of copier paper is a diffuse reflector.
In reality, objects exhibit the whole range of behaviors between the specular and diffuse extremes. A machined metal surface scatters light over a small range of angles, and scatters differently in directions parallel and perpendicular to the turning marks. Paper exhibits some specular properties, as anyone who has tried to read with a high intensity lamp can attest. Many objects have components that reflect differently. An electrical connector includes both shiny (specular) metal pins and dull (diffuse) plastic housing parts.
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Figure 12. Types of reflection
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Specular Reflections
Specular reflections are bright, but unreliable. They are bright because the intensity of the reflection is comparable to the intensity of the light source. In many cases, a specular reflection saturates the camera. Specular reflections are unreliable because a small change in the angle between the illuminator, the object, and the lens may cause the specular reflection to disappear completely. Unless these angles are well controlled, it is best to avoid depending on specular reflections. The best method for lighting specular parts is with diffuse lighting (figure 13). The large illumination solid angle means that the image remains almost constant as the reflection angle changes.
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Diffuse Reflections
Diffuse reflections are dim but stable. The intensity of the reflection is reduced from that of the source by a factor of 10 to 1000. The reflected intensity changes slowly with the angle (figure 14). Diffuse surfaces can be lit successfully with either diffuse or point-like illuminators. Other considerations, such as specular elements on the object or the influence of shadows, determine the best approach.
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Figure 13. Specular objects viewed with diffuse lighting
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Figure 14. Diffuse objects illumincated with point-like source
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