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Light Fall Off

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Kevin Camin
Light Fall Off
on Dec 26, 2017 at 6:09:08 pm
Last Edited By Kevin Camin on Dec 26, 2017 at 6:11:06 pm

Hello,

I have a question regarding the inverse square law in regards to different types of light. I understand how light quadruples when the distance is halved and how light quarters when the distance is doubled.

1. What about lights with lenses like a fresnel? How does the inverse square law get affected?
2. And just to split hairs, would a softbox and a barebulb exhibit identical fall off? If not, why?

Thank you. Any insights would be greatly appreciated. : )

Best regards,

Kevin Camin


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Rick Wise
Re: Light Fall Off
on Dec 26, 2017 at 7:47:42 pm

All abide by the same inverse-square law of optics. The moon too..... With a spot meter measure the brightness of a full moon. Then measure with an incident meter the light falling on the earth. A stunning difference, all due to the inverse square law.

Rick Wise
Cinematographer
MFA/BFA Lighting and Camera Instructor Academy of Art University
San Francisco Bay Area
http://www.RickWiseDP.com


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Kevin Camin
Re: Light Fall Off
on Dec 26, 2017 at 8:55:14 pm

I would think though that a focused beam would start to mess with the inverse square law. Imagine a spot light in the sky, the shaft of light seems to maintain itself longer than one would think if it were subject to the inverse square law. Or a laser which is highly focused light.

Thoughts?

Best regards,

Kevin Camin


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Todd Terry
Re: Light Fall Off
on Dec 26, 2017 at 9:05:27 pm
Last Edited By Todd Terry on Dec 26, 2017 at 9:05:40 pm

I think the inverse square law only applies to undirected (unfocused) point light sources (i.e., a bare bulb, candle flame, etc.). Once you start putting any kind of "concentrators" in the path (i.e. lenses) those exact physics no longer apply.

But that's all over my head.....

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Rick Wise
Re: Light Fall Off
on Dec 26, 2017 at 9:18:43 pm

You can prove it for yourself with the lights you own. Take a reading with the light, say, 4' from the subject, any light. Now move the meter another 4' away so the distance is 8'. Measure again. Did you lose 2 stops of light intensity? If so, the law holds. (Hint: these laws always hold.....)

Rick Wise
Cinematographer
MFA/BFA Lighting and Camera Instructor Academy of Art University
San Francisco Bay Area
http://www.RickWiseDP.com


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Kevin Camin
Re: Light Fall Off
on Dec 27, 2017 at 3:34:02 am

Thanks, Rick. I believe you but I will do a test in the coming days just for sh*ts and giggles. Thanks for taking the time to respond. You're work looks great BTW.

Best regards,

Kevin Camin


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Robert Olding
Re: Light Fall Off
on Dec 27, 2017 at 8:46:47 pm

The source and/or modifier won't matter. The inverse square law will apply equally.

Robert Olding
http://www.8streetstudio.com
Minneapolis, MN


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Mark Suszko
Re: Light Fall Off
on Jan 2, 2018 at 7:43:40 pm

The Fresnel lens may direct the photons in a tighter beam, but aren't they are still the same number of photons?


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