Radiosity (radiometry)

Radiosity, in the context of radiometry and computer graphics, is a measure of the radiant flux leaving a surface per unit area. It quantifies the total power emitted and reflected by a surface. In simpler terms, it's the total "outgoing" light energy from a surface. Radiosity is typically denoted by the symbol B and is measured in watts per square meter (W/m²).

Unlike radiance, which describes the light traveling in a specific direction, radiosity represents the total light leaving a surface in all directions. It includes both emitted light (if the surface is a light source) and reflected light (light from other surfaces that has bounced off the surface in question).

Key characteristics of radiosity include:

• Total Outgoing Light: Radiosity accounts for all light leaving a surface, regardless of its origin (emission or reflection).
• View-Independence: In the classical radiosity method, radiosity is a view-independent property. This means that the radiosity of a surface is the same regardless of the viewer's position. This is a key distinction from other rendering techniques that calculate light based on the viewpoint. Note that this view-independence applies specifically to the calculation within the radiosity algorithm itself, not to any subsequent shading or texturing that might be applied.
• Energy Conservation: Radiosity methods are based on the principle of energy conservation. The total energy leaving a surface must equal the total energy arriving at that surface (either directly from light sources or indirectly through reflections).
• Global Illumination: Radiosity is a global illumination technique, meaning it takes into account the indirect lighting effects caused by light bouncing off surfaces. This allows for the simulation of realistic lighting effects such as color bleeding (where the color of one surface affects the color of nearby surfaces).
• Form Factors: Radiosity calculations often involve the concept of form factors (also known as view factors or configuration factors). A form factor represents the fraction of light leaving one surface that strikes another surface. These factors are geometric properties that depend on the size, shape, and relative position of the surfaces. The accurate calculation of form factors is crucial for the radiosity method to work correctly.
• Applications: Radiosity is commonly used in computer graphics for creating realistic images and animations. It is particularly well-suited for rendering diffuse environments where indirect lighting plays a significant role. It can also be used in other fields such as lighting design and thermal engineering.
• Contrast to Radiance, Irradiance, and Exitance: It is important to distinguish radiosity from other radiometric quantities: Radiance is the radiant flux per unit solid angle leaving a surface (W/m²/sr), Irradiance is the radiant flux incident on a surface per unit area (W/m²), and Exitance (or Radiant Exitance) is the radiant flux emitted by a surface per unit area (W/m²). Radiosity combines Exitance and reflected Irradiance.

In summary, radiosity is a crucial concept in radiometry and a fundamental technique for achieving realistic global illumination in computer graphics. It captures the total light leaving a surface, accounting for both emitted and reflected light, and enables the simulation of complex lighting effects.