# Sources of light

Traditional computer graphics definitions of the source of light in a scene are based on simplifications of position, direction and a simple model of the directed illumination provided by spotlights. Later light source models included geometric extent from which light was emitted, called area lights. As a special case of area lights, a sphere of infinite radius around a point, the environment, defined a mapping of position to light intensity, represented either as a texture map or as a function returning intensity values.

To these traditional lighting concepts, Iray adds the ability to define any surface as emissive through its material definition defined by the Material Definition Language (MDL). This definition applies the idea of a distribution function to light emission. The components of the definition describe how light is emitted at a single point on the surface. The combined emissive effect of all points defines the emissive behavior of the surface as a whole.

## Concepts of light representation in Iray

For a single point on an emissive surface, all light energy will be emitted into the hemisphere defined by the surface normal at that point. In Figure 1, this is represented in a simplified form in two dimensions (like a slice of the real three-dimensional scene). A single direction within this hemisphere represents the fundamental concept in MDL's light emission model.

Figure 1. The hemisphere over a point on an emissive surface Light from a point on an emissive surface that is emitted evenly in all directions is called diffuse emission (by analogy to diffuse reflection). In Figure 2, the evenly distributed illumination of diffuse emission is represented by arrows that are the same length in all directions.

Figure 2. Diffuse emission from a point on a surface In many emissive objects in the world, however, light will be emitted unevenly in different directions. MDL defines the way that light is distributed from a single point on an emissive surface by its emission distribution function (EDF). In Figure 3, the length of each arrow represents the amount of light emitted in that direction.

Figure 3. Variations of the emission distribution function (EDF) The emission distribution function only defines the distribution properties of a single point on the surface. The sum of the emissive values of all points (the integral over the surface) defines the total emission of the surface itself. This calculation is done by Iray; the specification of emission by the user of MDL is through the definition of the EDF and the radiant exitance, a measure of how much energy is available for distribution into the hemisphere at each point.

Figure 4. The total emission of the surface is the sum of the emission of the all the points on the surface MDL also can specify what coordinate system is used as the reference framework for the evaluation of emission at points on the surface (represented by the red lines in Figure 5). This capability provides directional control useful in the emulation of spotlight effects.

Figure 5. Light oriented by the local tangent space (left) and a global coordinate system defined for all emissive points (right). The following sections describe the types of emission distribution functions defined by MDL and the way in which scene elements can distribute light in the Iray scene.