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
The mesh light is provided as an alternate translator for a mesh. This means that shape attributes, such as visibility flags and the mesh light attributes are accessible from the Attribute Manager for the selected mesh.
Mesh light works with polygon objects only, so you will have to turn the object into a polygon object first. To do this, select the object and press C or right click it and select Make Editable. Once you have a poly object you can add the Arnold Mesh Light tag if you want to use the object as a light source.
Arnold Mesh light available as a C4DtoA tag for a polygon object
You can find a description of common light parameters on the lights page. Additional attributes are described below.
Makes the light source visible to the camera.
If Custom visible intensity is enabled then the Visible intensity and Visible exposure settings control the intensity of the light source visibility separately from the actual light intensity.
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.
The subdivision settings can be found in the Arnold tag of the object.
You can see that even with Diffuse samples = 16, 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 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.