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I have found something interesting, and want to share with you:
Playing arround with transparent (or invisible) plain emiter, I found that certain colours might get into conflict with the transparency.

The material I am talking about would be a plain emiter, assigned on a polygonn that would be hidden to camera. The result is a light source that would render invisible.

Of course if we want this light not to be noticeable we must hide iit from reflections too.

I found that if the emiter colour of such a light (in HSV system) has a saturation under 127 it will render totally invisible, but if I use a saturation over 128 it gets visible under certain circumstances.

My gess is that this could be realistic.
I am not sure the reason, but suspect that real emiters cannot be too saturated since they will not be emitters. I imagine that a totally saturated (for instance) red glass, could not be transparent, but visible.

Anyway I am just guessiing.
The only I know is that if you want a really transparent emiter , you must use a colour that would not be that saturated.

Ernesto

Hello Ernesto,

Trying to reproduce your issue, I'm using a pure saturated red emitter, hide it from camera, and as expected I don't see it in the render.
See below the result of the experiment.

In addition to that, as you are interested in not visible emitters, you may also be interested in checking this page, where it explains the difference between hidden emitters (like the one you are doing now) and invisible emitters (not even casting shadows).

http://support.nextlimit.com/display/tu ... e+Emitters

Greetings

Dario Lanza