# Determining foot candles of bounce light

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### #1 Tyler Clark

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Posted 06 January 2016 - 02:29 PM

How do I determine foot candles of bounce light?

Without having the light and being able to meter it.

Edited by Tyler Clark, 06 January 2016 - 02:29 PM.

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### #2 Jay Young

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Posted 06 January 2016 - 02:53 PM

How do I determine foot candles of bounce light?

Without having the light and being able to meter it.

Magic?

In reality - Do you know the fc of the light hitting the bounce material?

Do you know the fc of the light at any distance?

Do you have another lamp that is close to the same?

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Posted 06 January 2016 - 04:40 PM

You'd use a light meter to read it.

I am sure you could apply some maths in an idealized situation figuring out the amount of fC of the source light at x distance and then the reflectivity of the bounce material, and then working out the FC of the light from the bounce at distance y-- but in reality, lightmeter.

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### #4 Tyler Clark

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Posted 06 January 2016 - 04:54 PM

Magic?

In reality - Do you know the fc of the light hitting the bounce material?

Do you know the fc of the light at any distance?

Do you have another lamp that is close to the same?

You'd use a light meter to read it.

I am sure you could apply some maths in an idealized situation figuring out the amount of fC of the source light at x distance and then the reflectivity of the bounce material, and then working out the FC of the light from the bounce at distance y-- but in reality, lightmeter.

Thanks guys! Was looking to see if there was a formula to rough it in using the photometrics of the lamp heads like with diffusion.

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Posted 06 January 2016 - 05:01 PM

i'm sure there is some formula; however, the main issue you'd run into is, well, it wouldn't be really precise-- angles would also come into play, rather strongly, as if you're hitting a bounce obliquely, you'd have a lot less transmission of light @90 degrees off of the bounce -v- if you were hitting it directly and measuring the bounce directly back, ya know?

So a lot of this comes with just experience, and knowing your lights and materials and what it'll get you.

When  in doubt, double the size of the head if you're concerned about exposure, as you can always scrim down.

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### #6 John E Clark

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Posted 06 January 2016 - 05:53 PM

How do I determine foot candles of bounce light?

Without having the light and being able to meter it.

One can get in to some amount of difficulty at this point... but one initially takes the 'reflectance' of the bounce and starts from there... if it is 50% reflectance... then whatever your incoming light level is, it will be 50% of that after the bounce. Also I'm assuming the surface behaves according the the Lambertian pattern shown below... In this example those %-ages are in terms of the 'reflected/bounced' intensity. So if the overall surface reflects only 50% then the resulting 'intensity' would be sa 50 * 75 = 37 %... for the one 'vector' labeled 75%.

Often the inverse square law is trotted out... but that does not hold in the 'near field' of the bounce. So depending on the size of the bounce, the near field approximation may be just that 50%...

But at some distance, the 'bounce' for photometric purposes, becomes a 'point source', and so the inverse square law becomes the rule.

I think it may be something like 2x the longest diagonal... but I could be wrong on that... it's been a while.

In any case... a meter sorts this out without having to go back to school for an optics and calculus refresher...

Edited by John E Clark, 06 January 2016 - 05:54 PM.

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### #7 Albion Hockney

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Posted 06 January 2016 - 06:15 PM

When I need to determine how big of lights I need for a bounce I always just take 2-3 stops off - it's a little drastic, but it keeps me safe as I have many times not had a big enough light.

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### #8 Gregg MacPherson

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Posted 06 January 2016 - 09:07 PM

Lambertian pattern.....

Some do say, that the idealized Lambertian reflectance distribution (pattern) gives the same intensity to an observer at any angle. looking at the pic that John posted, how is that possible? Acute angles give less relectance. But, if you look at the area of surface that presents itself to the eye at those acute angles, the area is greater at more acute angles.

It's a good distribution pic though. Sciency DoPs should draw it on the back of their grey card.
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### #9 Tyler Clark

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Posted 07 January 2016 - 09:30 AM

It's a good distribution pic though. Sciency DoPs should draw it on the back of their grey card.

Lol. Thats fantastic.

I do appreciate the pic however and appreciate all the info.

When I need to determine how big of lights I need for a bounce I always just take 2-3 stops off - it's a little drastic, but it keeps me safe as I have many times not had a big enough light.

The 2-3 stops off measurement is fantastic as well! Ill be using that alot more.

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