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)
The material for an emitter to be invisible must have (obviously) the emitting component plus one single BSDF that must be set with Transmittance pure white (255) and Nd=1 (mimicking the vacuum).
Then just hide that emitter to camera and your emitter will be invisible and 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 3D-platform independent)