Interreflection

Interreflection is the process by which light undergoes multiple reflections between two or more surfaces after an initial emission, such as light bouncing within a room or between panels. In enclosed or partially enclosed spaces, light does not simply travel from source to a receptor; it also reflects from walls, floors, and ceilings, with each bounce altering its spectral composition according to the surfaces' reflectance.

The phenomenon depends on geometry, surface roughness, and spectral reflectance. High-reflectance surfaces (near-white or pale colors) enable many bounces, producing a softer, more uniform illumination and a color shift toward the average reflectance of the surroundings. Low-reflectance surfaces damp out the light quickly and preserve higher contrast.

In rendering and lighting design, interreflection is described and computed as part of global illumination. Radiosity methods compute the steady-state distribution of radiant energy between diffuse surfaces using form factors. More general methods like path tracing simulate multiple bounces stochastically. Understanding interreflection is important for accurate color reproduction in interiors, daylighting studies, and architectural models.

Interreflection is distinct from direct reflection, which is a single bounce from a surface, and from specular reflection, which preserves the angle of incidence.