Lighting calculations - LED flash

[Lars]

Hi forum,
I need to calculate exposure (shutter, aperture, ISO) for an LED flash, given output in lumen, subject distance, and medium gray reflection.
Exposure time will be shorter than flash duration, so for all practical purposes the light source is continuous.
Looking for resources on how to calculate this.
Thanks,
-Lars

 

[Lars]

I think I might have found the correct answer:

Referring to this paper:
http://www.eetkorea.com/STATIC/PDF/200912/EEKOL_2009DEC10_EMS_OPT_NT_88.pdf

The paper below describes the following formula for dimensioning a light source for proper exposure:

L = ( d2 x N2 x C ) / ( S x t )​

where

L is LED intensity in Lux;
d is subject distance in meters;
N is aperture in f-stops;
C is an arbitrary constant chosen by manufacturers of exposure meters to reflect their view of what represents "correct exposure". Common values seem to be in the 180-250 range;
S is sensor sensitivity in ISO;
t is exposure time in seconds.​

For example, given a subject distance of 2 meters, f/2.8, ISO 400, 1/30 sec exposure, and C=200, we need a light source producing 470.4 Lux.

 

[Bob]

Very nice.
I love a good formula in the morning.
-bob