I don't see a basis for a distinction, but I've heard several people distinguish between direct and indirect light for people, claiming that indirect light would be better in many cases. "Direct" means that the light goes from the source to your eye without intermediary, and "indirect" means that the light bounces one or more times off other objects before entering your eye.

An example of indirect light, for the purposes of this discussion, is sunlight or indoor artificial light, reading an old-fashioned print book or a non-backlit Kindle. An example of direct light would be any modern computer screen, or a backlit Kindle. The one context I've heard of direct light being preferred is a light alarm clock, where it was stated to be an advantage that light from the alarm clock goes directly to your eyes and is not exclusively bounced off objects in a room.

Now I can see one or two obvious differences; direct light tends to come from a small source (we don't tend to cover walls with LED's), while indirect light is more likely to bounce off everything or at least much more of what you can see. Furthermore, all other things being equal, direct light should be brighter as I am not aware of anything that reflects fully 100% of the photons that strike it (maybe some odd variant on superconductors)? However, I see only superficial distinctions between the two, and not a reason why, if all other things are forced to be equal (e.g. an indirect source being made brighter to compensate for the photons that physical surroundings absorb instead of being reflected), there should be the fundamental difference between direct and indirect light that I have repeatedly heard from people I trust.

Is there such a basic difference, or not, and if there is a difference, what does that difference consist of?


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    – user25568
    Sep 19, 2016 at 14:22
  • $\begingroup$ If the lights from 2 source, is creating same intensity of light for all points of retina, I don't think there could be any difference of effect. $\endgroup$
    – user25568
    Sep 20, 2016 at 10:11
  • 1
    $\begingroup$ @AlwaysConfused That thought is part of why I asked the question (instead of simply agreeing with my friends). I see possible differences in a (theoretical) blackbody room where everything was covered with low-light LED's so that theoretically there is only ambient light but it is 100% direct and 0% indirect; or light not visible to the viewer could bounce off a single object the light was shining on and that object would seem isomorphic to a direct light source. Part of the assumption that led me to post here rather than physics.SE is I'd expect any valid answer to be biology, not physics. $\endgroup$ Sep 20, 2016 at 16:59
  • $\begingroup$ I don't know if really such biological experiment/evaluation took place or not. So I didn't go to write any answer, and wrote as comment. Thanks. $\endgroup$
    – user25568
    Sep 21, 2016 at 15:02
  • $\begingroup$ I'm voting to close this question as off-topic because it is more suitable for Physics-SE $\endgroup$ Sep 26, 2016 at 9:06


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