Documentation Talk:Reference Section 6.2

The keyword pass_through causes photons to pass through the object unaffected on their way to a target object. Once a photon hits the target object, it will ignore the pass_through flag. This is basically a photon version of the no_shadow keyword. If you use the no_shadow keyword, the object will be tagged as pass_through automatically. You can then turn off pass_through if necessary by simply using photons { pass_through off }.

[clipka - Removed the mentioning of media still collecting photons, as it was wrong (fortunately) even for 3.6: on their way to their target, photons are never collected by other objects, whether they are pass_through or otherwise.]