Why doesn’t bounced light fully illuminate a cast shadow after multiple reflections?
Asked 8/25/2017
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When light reflects off nearby surfaces and fills the shadow side of a subject, why doesn’t it also keep bouncing enough to noticeably brighten the cast shadow itself? Is there an intuitive way to think about how much light is lost with each bounce, especially with different materials?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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Take a look at this post, It will probably solve most of your doubts:
I've always wondered why, when light bounces into an object and illuminates its dark side
A normal object, like a vase, does not bounce light into its own dark side. If you have nothing to bounce the light around your object, this side will be simply pitch black.
If you have bounced light it is light reflected somewhere else, probably the floor itself.
it would not also then bounce into the cast shadow itself and illuminate the shadow region.
The cast shadow can not be filled with the reflection of this same object (well, probably a little tiny bit), neither the object where this casted shadow is. It must be filled with the bounce from a third object. (Normally on this planet it is the sky or the walls of your room)
Is there a way to intuitively understand the 'fall off percentage' of light being bounced around an object and its surroundings?
The fall off percentage from a light source is given by some main variables:
- Distance from the light source
- Size of that light source
- Angles of the involved elements
But the falloff of a bounced light is given not only to sizes and distances but also by material, color, glossiness, and transparency.
(Rocket, planet, and spaceman from Pixabay)
Edition:
Is there a way to intuitively understand the 'falloff percentage' of light being bounced around an object and its surroundings?
Here is a quick test you can do if you have an incident light exposure meter or using your camera in some automatic mode, like Aperture Priority.
Put a large source light, for example, a softbox, and put in front of it the material you want to measure, in this case, a white paper, let us say like 50cm apart.
Take the two measures. I did a really quick test (very faulty) and I got these values.
Translating this data it means that that material at that distance reflected 4 stops less light than the light source. 4 stops means 16 times less light.
Originally by user37321. Source · Licensed CC BY-SA 4.0
user37321
8y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—bounced light can brighten both the shadow side of a subject and, to a smaller extent, the cast shadow area too. The reason it usually doesn’t brighten shadows very much is that each reflection loses energy, and the amount lost depends heavily on the surface.
A useful way to think about it is as a percentage kept at each bounce:
- very reflective surface: maybe about 90% reflected
- dark/matte surface: maybe about 10% reflected
That loss compounds multiplicatively. If a surface reflects 90%, then after two bounces you have about 0.9 × 0.9 = 81% of the original light. If it reflects 10%, after two bounces you have only 0.1 × 0.1 = 1% left. After a few bounces, the light may be too weak to notice.
Also, an object usually does not significantly light its own dark side unless there are other nearby surfaces—like a floor or wall—sending reflected light back into it. Likewise, a cast shadow may receive a tiny bit of indirect light, but often not enough to visibly “fill in” the shadow compared with the directly lit areas.
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