How to do a lightcast only Layer in Cycles? I try to render a layer that only includes the light as if it was cast onto an object.

But it seams it is not possible to make a material that is invisible for the camera but bounces light from a certain light sources.

To put is simple I want to render a layer that only includes Emssionshader bounces from certain Lights.

best regards,
Wasabi

edit: I hope this makes it more clear what I want.


.blend for picture above

edit2:
And I already know how to do a Shadow/AO Layer but the light is difficult to distinguish as every pixel I render and is not Black is light. So i only want the Spotlight light but no world bounces or light that is 2-3 bounces "old".

edit3:
I faked the effect I am looking for with the asset I am working on.
This is maybe a better example than above.
I'm quite sure to not have fully understood your question, so before giving you some suggestions I would like to punctualize one aspect (as I have seen you are interested in compositing layers):

No transparency is involved in this scene, dark grey color of the plane/cube in shadow comes first of all from interaction of the plane/cube (diffuse white) with the world background light (which is colored dark grey). All the visible surfaces will produce a white alpha channel area.

You can remove the world influence on the scene by lowering the Background strenght to 0 or even completely remove the shader.



You can set the Max number of bounces of light allowed for each light source in the Lamp panel in the Object Data tab.



You could also tweak the "global" light path bounces, but you'll not have the precise control that the "per lamp" option allows.



By seeing your third edits I would suggest the following (more general) instructions:


Switch all lights off (Background too), except the light source you would like to isolate. All the light present in the scene is it contribution
Set the amount of bounces allowed for the light source (per lamp, or
globally if you are using an emission shader as a per object option
is not currently (2.75a) available)