Creating a "magic goggles" effect I would like to create a scene in which some simple objects make a simple scene, normally lighted, and with a transparent lens object looking through which some of the objects have an emitter material that also illuminates the area around them, but only when seen through the lens object. Is there any way to do this? E.g. with 2 cameras or scenes...
I guess the proper way to do this would be via compositing, but in Cycles, the Light Path node can take you some way.. it's a bit of a fudge, and may depend on your scene:

The torus in the little shot below uses this tree:

'Is Transmission' ensures that the torus material will emit whenever seen through another transmissive material, and is 'OR'ed by the Maximum with:
'Ray Depth &gt; 2', which means that surrounding objects appear illuminated when seen through the glass, but not otherwise.. the discrimination isn't perfect in all situations, there might be a tiny bit of bounce from the torus when not seen through the glass, but seems good here:


(The glass is 2-sided, it has thickness)