At least one direct light source (a point light or spot light) must emit photons for global illumination or caustics to work.
To turn on photon tracing
- Select the light source you want to emit photons.
For example, spot light. (See also Direct light sources.)
- Open the Attribute Editor (
+a). - Select the light’s shape tab.
- Set Decay Rate to Quadratic.
This causes light levels in the scene to decrease in intensity based on the inverse square law (naturally).
- Set the photon emission attributes.
See mental ray attributes for lights (point spot directional area).
- Repeat this procedure for every light source you want to emit photons.