The contrast of the light source does not affect the appearance of a diffuse reflection. It is worth proving this with one more picture of the same scene. The earlier photograph was lit by a small light. We could see that by the hard shadows cast by the objects in it. Now look at Figure 3 to see what happens when we use a large light instead.
Predictably, the large light source has softened the shadows in the scene, but notice that the highlights on the paper look about the same. The diffuse reflection from the surface of the paper is identical to that in Figure 2.
So we now have seen that neither the angle nor the size of the light source affects the appearance of a diffuse reflection. However, the distance from the light to the surface of the subject does matter. The closer the light gets to the subject, the brighter the subject becomes and, at a given exposure setting, the lighter the subject appears in the finished picture.
The Inverse Square Law
A diffuse reflection gets brighter if we move the light source closer to the subject. If we needed, we could calculate this change in brightness with the inverse square law. The inverse square law says that intensity is inversely proportional to the square of the distance. Thus, a light at any particular distance from the subject will light the subject with an intensity four times as bright as the same light twice as far away. Similarly, a light will have nine times the intensity of the same light moved three times as far from the subject. As the intensity of the light falling on the subject varies, so does that of the diffuse reflection.
Ignoring the math, this simply means that reflection from a surface gets brighter if we move the light closer and it gets dimmer if we move the light farther away. Intuitively, this seems immediately obvious. Why even bother to mention it? Because such intuition is often misleading. Some subjects, as we shall soon see, do not produce brighter reflections as the light moves closer to them.
Direct reflections are a mirror image of the light source that produces them. They are also called specular reflections. Figure 4 is similar to Figure 1, but this time we have replaced the white card with a small mirror. Both the light source and the observers are in the same positions as they were earlier.
Notice what happens. This time one of the three cameras now sees a blindingly bright reflection, while the others see no reflection at all in the mirror. This diagram illustrates the direct reflection produced when a light is directed at a polished surface such as glass. The light rays bounce from the smooth surface at the same angle at which they hit it. More precisely stated: the angle of incidence equals the angle of reflectance. This means that the point at which direct reflections can be seen is exactly determined by the angles between the light source, the subject and the camera viewpoint.