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.

Color Type

Simple Color

Shows the RGB Color parameter.

Texture

Shows the Color Texture parameter. An image map can be assigned here to texture the mesh light.

Mesh light with texture assigned

Shader

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.

Mesh

Mesh Light vs Emission