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Inverse Square Law, Does the Diffusion Become the Source


Michael O'Connor
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Hi All,

Just wondering if anyone can help put a question to bed for me while I don't have the space to test and experiment.

When using a lamp in conjunction with say a 4x4 frame of diffusion, does the inverse square law begin from the lamp or does the diffusion effectively become the source and therefore you start counting again from there.

Not suggesting I actually want to do the maths on every occasion, but essentially wondering if it's worth taking the lamp farther back and away from the diffusion if the space allows it in order to create a more gradual fall off from the diffusion onward. 

I understand if I make the diffusion larger and brighter I can pull that further away from the subject and have a more gradual fall off, I specifically mean if the distance between the staging area and the diffusion is fixed. In order to have a more consistent exposure across the staging area, closer and further away from the diffusion, does taking the lamp further away help that process.

I imagine, if the diffusion does have an impact, the density of it might play a role. Perhaps something lighter like opal has little affect and therefore taking the lamp farther back does help make the fall off from the point of diffusion less harsh. But with something heavier like full grid cloth, it's much more like a new source meaning you can effectively meter from the diffusion frame and still see the light level fall to a quarter every time you double the distance.

If that is the case, does anyone have any experience enough to suggest at what point roughly the thickness of diffusion makes the distance of the source irrelevant?

Would be great to get opinions on whether the distance between the lamp and the diffusion has an effect on the fall off stage side.

Thanks and all the best,

Mike

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Broad sources don't strictly obey the Inverse Square law, which really only applies to point sources. You can think of a broad source as being a nearly infinite number of point sources, all overlapping each other as they spread. Because of this, the fall off is not exactly as would be predicted by Inverse Square. 

This probably doesn't make a huge difference in 99% of situations, but it's worth knowing.

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I've been wanting to test this, whether very thin diffusers create both a fast and long fall-off. I'd need a large space though with a big lamp and then something like a window frame in the middle, then I'd measure the rate from the window to the far end of the room with no diffusion, then with heavy diffusion on the glass, and then with something very light and porous even, either a curtain sheer or 1/4 Hampshire Frost.

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This is interesting:

https://m.youtube.com/watch?v=8JrqH2oOTK4

This guy points out that video lcd screens have a fresnel lens layered in there that disrupts the inverse square drop off and extends the brightness range of the diffuse transmission, making for a more realistic "cloudy sky" look. I've always thought this could be a useful trick lighting an indoor set through fake windows.

TC

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42 minutes ago, Tim Chang said:

This is interesting:

https://m.youtube.com/watch?v=8JrqH2oOTK4

This guy points out that video lcd screens have a fresnel lens layered in there that disrupts the inverse square drop off and extends the brightness range of the diffuse transmission, making for a more realistic "cloudy sky" look. I've always thought this could be a useful trick lighting an indoor set through fake windows.

TC

Nah, panoramic molton bounce for me 🙂

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