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Doing this is easy by following these two steps (one in the emitter at material level and one in the at object itselflevel):
- 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, material
- Then 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).
and 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)
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