# calculating brightness



## superliminal (Mar 21, 2011)

Hello photophiles,

I've got a technical question for you all, hopefully you can help me solve it.

I am trying to figure out the brightness of an RGBA LED light that is not in my possession. I have the a light sensor reading from 1 meter away from the light source:

R ~ 265 Lux
G ~ 185 Lux
B ~ 425 Lux
A ~ 125 Lux
RGBA ~ 1250 Lux

I am trying to figure out what these measurements would be for a 2.5 meter (I would settle for 2 meters). Doesn't have to be super precise, just a good idea of how bright it will be at that distance.

Any help would be much appreciated!

thanks,
erik

(obligatory emoticon: )


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## fadingrae (Mar 24, 2011)

In units of lux (LX), 1LX = 1Lm/m2.

So you can calculate.


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## videoman (Mar 26, 2011)

For 2 meters I would say 4X LESS than it is at 1 meter. Light intensity is reduced by a factor of 4 EACH time the distance doubles, hence at 4 metres it would be NOT 4 X less but 16X less (4X4) than at 1 meter value and at 8 metres it would be ( 8X8) 64 times LESS, 16 meter distance is 16X16 = 256 times LESS, and so on and so forth. To answer you question : the 1250 RGBA lux at 2 meters would be 1250/4 = 312.5 lux and at 2.5 meters approximately 25% less than that.
I think that is called the inverse square law of light value over distance. 2X2,4X4,8X8,16X16,32X32 etc.


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## HooNz (Apr 20, 2011)

videoman said:


> For 2 meters I would say 4X LESS than it is at 1 meter. Light intensity is reduced by a factor of 4 EACH time the distance doubles, hence at 4 metres it would be NOT 4 X less but 16X less (4X4) than at 1 meter value and at 8 metres it would be ( 8X8) 64 times LESS, 16 meter distance is 16X16 = 256 times LESS, and so on and so forth. To answer you question : the 1250 RGBA lux at 2 meters would be 1250/4 = 312.5 lux and at 2.5 meters approximately 25% less than that.
> I think that is called the inverse square law of light value over distance. 2X2,4X4,8X8,16X16,32X32 etc.



As a side reader that info i found interesting , and strange too as even though light obviously is sourced from somewhere (electrons/battery) i really never associated that until i saw the inverse square bit and remembered this http://en.wikipedia.org/wiki/Radio_propagation , so to put that into perspective , more gain at the Antenna and directional capabilities is quite similar .

Antenna would be the emitter surface and directional the collimator , thanks for the thread !.
Paul :wave:


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