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I've always wondered why, when light bounces into an object and illuminates its dark side, it would not also then bounce into the cast shadow itself and illuminate the shadow region. Is there a way to intuitively understand the 'falloff percentage' of light being bounced around an object and its surroundings?

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Take a look at this post, It will probably solve most of your doubts:

https://graphicdesign.stackexchange.com/questions/96192/what-are-the-mechanics-of-bounce-light-how-does-it-work/96248#96248

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.

enter image description here

If you have bounced light it is light reflected somewhere else, probably the floor itself.

enter image description here

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)

enter image description here

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:

  1. Distance from the light source

  1. Size of that light source

  1. 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.

enter image description here

(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.

enter image description here

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.

  • @ Rafael -- You get an A grade from me. Ever in Anaheim, let’s have lunch: A tip of the hat from Alan Marcus. – Alan Marcus Sep 5 '17 at 21:37
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Light bounced onto an object very well may also bounce into the shadow and lighten it as well. It can be difficult to understand the 'falloff percentage' because different objects absorb and reflect different amounts of light. Think of a mirror vs. a sheet of black curtains, for example.

Remember also that you're dealing with a multiplicative change. So a mirror might reflect 90% of the light that hits it. If you bounce from one mirrored surface to another, the second mirrored surface reflects about 90% of 90% = 81% of the original light. Still a fair bit. But a matte black material might only reflect 10% of the light. Bounce it again to another part of the surface, and it will only reflect 10% of 10% = 1% of the original amount of light. It might not be noticeable.

  • +1 for the exponential falloff per surface. It really adds up (or perhaps rather, the reduction really adds up). – scottbb Aug 27 '17 at 3:57
  • @scottbb "It really adds up"? Surely you mean "it really multiplies" ;-) – MathematicalOrchid Sep 4 '17 at 12:41
  • @MathematicalOrchid Not if you're already thinking in logarithms! It's where I prefer to stay. I'll take additive stops and decibels all day over iterated multiplications, thankyouverymuch. =) – scottbb Sep 4 '17 at 13:29
  • @scottbb Fair enough. ;-) – MathematicalOrchid Sep 4 '17 at 15:28

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