# the inverse sqare law of light

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### #1 YongLee

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Posted 09 January 2008 - 01:21 AM

i know the"the inverse sqare law " of light ,but i think it is apply to spot light only, for large surface light , it is not right. who can tell me how the"the inverse sqare law " of light to use in lighting film sence? thanks!
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### #2 Michael Nash

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Posted 09 January 2008 - 02:45 AM

The inverse square law applies to a point source only. The larger the source (in surface area), the less the light falls off with distance. How much less depends on the size of the source, relative to the distance.
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### #3 Matthew Parnell

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Posted 09 January 2008 - 04:13 AM

The maths that specifically applies to the falloff of a diffused source has been discussed in the past, it is rather complex and confusing. just do a quick search of the forums.
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### #4 John Sprung

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Posted 09 January 2008 - 02:45 PM

The maths that specifically applies to the falloff of a diffused source has been discussed in the past, it is rather complex and confusing. ...

Nah, all you gotta do is treat every point on the large surface as a point source, and just do an integral in two variables. Piece o'cake. ;-)

-- J.S.
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### #5 Steve London

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Posted 09 January 2008 - 04:24 PM

Besides large sources the inverse square law doesn't apply to collimated sources either, where a reflector and perhaps lenses direct light (more or less) in one direction.

So, while our movie lights rarley rigorously obey the law it's a useful first approximation in practical circumstances, though.
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### #6 Steve London

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Posted 09 January 2008 - 04:30 PM

Piece o'cake. ;-)

-- J.S.

I got As in more than a year of Calculus and you took the words right out of my mouth. Trivial.

I'm a little busy though, so I'll let you post the equations and get the credit.

(Hope the humor is obvious, John.)
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### #7 YongLee

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Posted 11 January 2008 - 08:01 AM

thanks , everybody! :
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### #8 Tim O'Connor

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Posted 11 January 2008 - 08:50 PM

The inverse square law applies to a point source only. The larger the source (in surface area), the less the light falls off with distance. How much less depends on the size of the source, relative to the distance.

I know this and yet...everything is relative. The inverse square law doesn't apply to the largest soft light
on Earth and yet the largest point source in the world (to us anyway) is the sun, which is bigger than
any soft light or frame of diffusion that I've seen.

I'll never forget shooting for NASA (footage still classified) when we got so close in the rocket to the sun that
the light coming in the window went from casting shadows to being the biggest wrap around soft light.
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