12 Light Sources
In order for objects in a scene to be visible, there must be a source of illumination so that some light is reflected from them to the camera sensor. This chapter first describes different physical processes that lead to photon emission and then introduces the abstract Light class, which defines the interface used for light sources in pbrt. The implementations of a number of useful light sources follow. Because the implementations of different types of lights are all hidden behind a carefully designed interface, the light transport routines in Chapters 14, 15, and 16 can operate without knowing which particular types of lights are in the scene, similar to how acceleration structures can hold collections of different types of primitives without needing to know the details of their actual representations.
This chapter does not include implementations of all of the Light methods for all of the types of lights that are introduced. Many of the quantities related to complex light sources cannot be computed in closed form, and so Monte Carlo integration is needed. Therefore, the remainder of the Light methods are implemented in Section 14.2, after Monte Carlo methods have been introduced.
A wide variety of light source models are introduced in this chapter, although the variety is slightly limited by pbrt’s physically based design. Many non-physical light source models have been developed for computer graphics, incorporating control over properties like the rate at which the light falls off with distance, which objects are illuminated by the light, which objects cast shadows from the light, and so on. These sorts of controls are incompatible with physically based light transport algorithms and thus can’t be provided in the models here. As an example of the problems such lighting controls pose, consider a light that doesn’t cast shadows: the total energy arriving at surfaces in the scene increases without bound as more surfaces are added. Consider a series of concentric shells of spheres around such a light; if occlusion is ignored, each added shell increases the total received energy. This directly violates the principle that the total energy arriving at surfaces illuminated by the light can’t be greater than the total energy emitted by the light.