Torus shape converted to a 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 a mesh light
When selecting an Arnold mesh light from the light menu, you will be asked to pick a mesh object.
The light is then created, and the picked object set as the light's shape in the light primitive ppg. Also, the light is pose-constrained to the mesh, so to visually match its position, scale and rotation.
The light is assigned a shader called arnold_mesh_light. It is this combination (the area light shape set to object and the arnold_mesh_light assigned as light shader) that makes SItoA understand that this light is to be treated as a mesh light.
Note that the mesh, besides becoming a light source, maintains its shading properties. This means its material stays untouched and therefore reacts to the light emitted by itself as well.
If enabled, you will be able to tweak the light softness by changing the area of the mesh, without affecting the amount of emitted light. This is very handy for artistic control. Otherwise, if not enabled, the amount of emitted light is proportional to the mesh's area, as dictated by physics.
It is worth noting the the effect of the sampling control with the mesh light:
The light color can be textured. In this case, the main UV set is used to lookup the color image.
If the mesh light color texture is clearly visible in specular reflections you may need to increase the number of subdivision iterations for the mesh light. 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, 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 has 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.