Diffuse reflection occurs when an incident ray of light strikes a surface and the light is scattered. In perfect or ideal diffuse reflection, all the light will be perfectly distributed in a hemisphere of even illumination around the point the light strikes the diffusion surface. The diagram shows the way light is scattered by a diffusion surface. Although the diagram is only two dimensional (2D) the light scatter forms a hemisphere (3D) around a light strike-point.
Photographers should remember that the perfect diffusion surface does not exist. In reality some of the light may be absorbed by the surface itself and the energy of the light will be transferred to the surface material. Some of the light will be reflected as diffused light and will be scattered in a hemisphere of illumination and a percentage of the light may be reflected in a reduced specular reflection. The latter depends on the degree to which the surface has any mirror-like characteristics.
A partially absorbing, diffusing and reflecting surface might be white gloss paint. Such a surface might absorb some of the light energy (eg. slightly warmed by the sun); reflect some light according to the law of reflection (specular reflection); and diffuse some light creating an hemisphere of illumination around the point where the light strikes.
- An absorbing surface minimises all reflection (eg. a painted matt black surface like a blackboard).
- A diffusion surface creates an illuminated spot (eg. a white unsurfaced cardboard, like artists mounting card).
- A reflecting surface creates specular reflection (eg. a mirror).