Invisible Emitters

Sometimes it is needed to get an "invisible emitter": An emitter not only being hidden to camera, but also not casting shadows in the scene, remaining completely invisible to the render but its emission. A very useful trick for photographers and cinematographers.

From left to right: Normal emitter, Hidden emitter (cast shadow) and Invisible emitter (cast no shadow)

Doing this is easy by following these two steps (one at material level and one at object level):

1. On the material for the emitter to be invisible, aside from the emitting component, add one single BSDF that must be set with Transmittance pure white (255) and Nd=1.0 (which is the recipe for the vacuum or air). This completely transparent and non-refractive BSDF corresponds to an "air" material, also known as "ghost" material 
2. Then just enable the Hide to Camera option on the emitter object visibility options (and additionally you can hide it also from reflections and refractions it you don't want to see it reflected on other specular objects). 

This way your emitter will be completely invisible and will cast no shadow. 

Videotutorial 

This videotutorial also explains how to set up an invisible emitter: (the video has been recorded in Maya, but this principle is completely 3D-platform independent)