how to render emission of light of a transparent background Imagine the following scene, top down view with a simple object emitting light (cycle) on a large floor (plane).

Now what i'd like to achieve is to only render the object and the emission of light but not the plane (tranparent bg). of course if I hide the plane there is no emission visible since there is no surface to refect it.

I think maybe using green screen effect (making the plane green) could do the trick but I don't think it will be very clean for a effect like that.
I wonder if there is any other way it could be done ?
I tested transparent material but again no emmision visible then.

edit : picture to illustrate
what I do
what i see 
what i'd like to see in a way less ugly version (done with my awesome skill in gimp) (but since its a white glow on a white background you can't see it here damnit)
Note : this answer is obsolete due to the comments. I don't delete it as it still produces a result.

Use the compositor.

The setup can be like this :



On the left, the rendered input (here, all is black around, because the world has no material or light set to 0).

The color ramp is here to convert eventual colored parts of your scene to black and white.

Alpha over puts a transparency (the first image slot) over the rendered image, depending on the B&amp;W chanel.