Another source of light reaching our eyes is literally sources of light. Light sources are said to emit light, and depending on the nature of the source, this light can be emitted in all directions or in specific directions. The sun emits light in all directions, for example, all though our eye can only sample light reaching it along direct lines of sight. A torch, by comparison, tends to emit light along a focused path that we call the beam. The problem of how our visual system distinguishes between light reflected from surfaces and emitted light will also become a key consideration in this book.
The final major source of light variation that we will consider is light that passes through objects, either entirely or partially. We say that objects are transparent if they transmit light through their surfaces, and we say objects are translucent if light only partially penetrates the object before being emitted at some other point of the surface. This latter effect is called sub-surface scattering and it is a physical phenomenon that lies somewhere between light reflection and emission (Figure 3).
Thus, we arrive at the four major sources of variation in sensed light that inform us of the visual world: diffuse/specular light reflection and light emission/scattering.
dave