This tool creates an Light object and sets the Light type parameter to "Geometry" and sets the Geometry object to the object you select.
This object emits light into the scene from the surface of the selected geometry object. It uses the geometry object’s surface shader for the colors of the emitted light, so you can create very complex lighting effects using multicolored shaders.
Geometry lights can be expensive to render. For simple cases you may want to try to simulate the effect you want using point lights and/or area lights. However, the geometry light is very useful since it’s more straightforward than using tricks with simple lights, and it uses the object’s surface shader to color the emitted light.
To get geometry lights to emit color based on their assigned surface shader, you need to define an export variable named
Ce. This holds the emitted light.
Making a light in the shape of an object
Select the object you want to emit light, then click the Geometry Light tool on the shelf.
With nothing selected, click the Geometry Light tool on the shelf, then select the object(s) you want to act as portals for the environment light and press Enter.