# Light power vs distance chart or apps calculator

Apps distance calculator

4 replies to this topic

### #1 Mathieu Elie

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Posted 07 August 2016 - 08:40 PM

Hello guys, is there a chart or an apps to calculate distance / aperture / power / diffusion? ex: 9K HMI / Full silk / 800 ISO at T2.8 = what distance? or any other way around?

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### #2 John E Clark

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Posted 08 August 2016 - 11:38 AM

ARRI has an online calculator and an iPhone app (may have an android one as well, but I don't have one...).

http://calc.arri.de/calculator

https://itunes.apple...d733009338?mt=8

Of course they are all targeted to ARRIs product lineup.

The problem is unless the manufacturer produces the chart, it is really difficult to determine from 'fundamental principles', especially with off brands that don't even bother to indicate the Footcandle/Lux values... but more often just indicate wattage consumed. To besure there's a Watts -> Lumen conversion factor, but the lamp shape, the reflector shape figure into the amount of light delivered on a subject at distance X...

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### #3 John E Clark

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Posted 08 August 2016 - 12:18 PM

It's early, but I think the following would give an approximation for the delivered light to a subject for an arbitrary lamp/reflector.

A rough approximation would be:

Assume the reflector is near 100% Take the beam angle A, and calculate the area of the base of a cone at distance X from the light.

The area of the base of the cone is BaseArea = pi * R ^2, where R = tan(A/2) * X

With the area then Lux or Footcandles = Lumen/BaseArea given the units used for distance X.

Edited by John E Clark, 08 August 2016 - 12:18 PM.

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### #4 Phil Rhodes

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Posted 08 August 2016 - 12:29 PM

I recently wrote a little javascript library to generate exposures from lux readings.

It might end up going online. I'll check back if it does.

P

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### #5 Stuart Brereton

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Posted 08 August 2016 - 06:08 PM

There is an app called Light Source which is available for the iPhone (don't know about Android). It has photometric data for hundreds of lamps from many different manufacturers.

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