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I saw this option mentioned in another answer:

How can I cast artificially darker shadows?

I want to try this but I do not know how to add a light that is only visible to the shadow path. My apologies if this has an obvious answer! I am trying to learn everything at once and I am missing some fundamentals right now.

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  • $\begingroup$ Well, you can very easily add a shadow whose caster isn't visible: go to the object's properties (the orange cube icon in the right-hand-side panel, scroll down and look for "Ray visibility" (or somesuch, I am not sure how it is worded). There you can disable visibility to everything except shadows. $\endgroup$
    – Mörkö
    Sep 16 '15 at 6:00
  • $\begingroup$ Adding light visible to only shadow rays seems to be a weird consept. As far as I understand it, that would produce light that illuminates the scene, but does so solely through direct illumination. $\endgroup$
    – Mörkö
    Sep 16 '15 at 6:01
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The Shadow Ray output is a little unique in that its type is determined based on its end point (a light source), rather than its start point (the material it last interacted with).

Making a light source invisible to everything except shadow rays will cause it to only block light from other light sources and not influence anything else.

However, it is possible to limit the number bounces a light source is allowed to influence. (I suspect this is what more you want to do)

Lamps have a Max bounces setting, which when set to 0 will only allow that lamp to illuminate directly (disabling "bounce lighting" for that specific lamp):

enter image description here

To do the same with nodes (e.g. for mesh lights), you can use the Ray Depth output of the light path node:

enter image description here

As always when working with light paths, it's important to keep in mind that rays are traced backwards (from the camera).

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  • $\begingroup$ Thank you, this is very helpful, but I am a little confused. Is this how to make a light that is only visible to the shadow path? Is that the same thing as what you write, "Making a light source invisible to shadow rays"? Sorry, all of this is taking me some time to understand. In the meantime I will try this and if I am just confused I will delete this comment later. :) $\endgroup$
    – WishyQ
    Sep 16 '15 at 18:59
  • $\begingroup$ @WishyQ Sorry, it seems I confused myself while writing that.. Hopefully it's clearer now. $\endgroup$
    – gandalf3
    Sep 16 '15 at 21:43
  • $\begingroup$ Thank you! I am also interested in this: "Making a light source invisible to everything except shadow rays will cause it to only block light from other light sources and not influence anything else." This seems like it would come in handy. How is it done? $\endgroup$
    – WishyQ
    Sep 17 '15 at 1:33
  • $\begingroup$ @WishyQ Post another question :) We try to keep to one question per post, in order to keep things as on-topic and organized as possible. $\endgroup$
    – gandalf3
    Sep 17 '15 at 3:41
  • $\begingroup$ OK, posting another question now! :) $\endgroup$
    – WishyQ
    Sep 17 '15 at 6:43

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