Torus shape converted to mesh light (rollover image). Model kindly provided by Lee Perry Smith.
In situations where conventional light shapes will not suffice, mesh lights are more suitable. Mesh lights can be used to create interesting lighting effects that would not be possible any other way. For example, effects such as neon lighting or a car light motion trail effect can be achieved more easily with mesh lights.
Geometry converted to Mesh light
It is not currently possible to make a mesh light visible to camera rays. A workaround is to add Emission to a Standard Surface shader assigned to the object. This will give the impression that the geometry is incandescent. You should also set the Base Weight and Specular Weight to 0 and the object's visibility flags should be set to invisible in diffuse/specular.
As well as the settings that are common to all lights, the Mesh Light also has the following input.
Shows the RGB Color parameter.
Shows the Color Texture parameter. An image map can be assigned here to texture the mesh light.
Mesh light with texture assigned
Shows the Color Shader parameter with the path of a vopnet that is contained inside the mesh light for convenience.
A shader and images network can be added inside this vopnet and connected to the the color parameter of the Light Output.
Connect a polygon mesh to this parameter to create a mesh light.
You may need to increase the number of subdivision iterations for the mesh light if the color texture is clearly visible in specular reflections. For example, this may be evident in a scene where a TV screen is reflected in a glass window.
You can see that even with diffuse samples = 16, the emission is noisier than a mesh_light with diffuse samples = 2.
Below is another comparison test between a mesh light (left image), and a sphere with a highly emissive standard_surface shader assigned to it (right image). As you can see in the mesh light image, light is bounced around the scene, whereas there is no bounced light in the emissive sphere render. This is because the rays from the mesh light work in conjunction with the Diffuse rays, whereas, only the primary rays are considered when using emission only. Even with 6 Diffuse samples, the render using Emission only contains far more noise.
In the example below, Diffuse samples have been increased to ten to get a clean result using only Emission in the scene. With mesh light, the scene only requires three Diffuse samples.