An example mesh light scene can be found here.

The mesh light is provided as an alternate translator for a mesh. This means that shape parameters, such as visibility flags, and the mesh light parameters are accessible from the Modify Panel of the mesh. 

The current limitations when using a mesh light are: 

Mesh light translator is available in the Modify Panel

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. Set the indirect_diffuse to 0 for the standard_surface shader assigned to mesh as the mesh light is already emitting light.

Setting indirect_diffuse to 0 disables GI bounces for the mesh

Mesh Light Vs Emission