# Whats the difference between lux and cd as measurements of throw?



## HighlanderNorth (Sep 20, 2012)

Most manufacturers and dealers give the lumen/brightness specs for the lights they sell or make, but only a few of them list the throw specs. When they do, some give a lux number, while others use something called "cd". I assume that is a measurement telling me how far it will throw, while still putting a small amount of light on a "compact disc"(cd). No? LOL....

What is the difference between cd and lux, and how do I interpret a 'cd' measurement?


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## TEEJ (Sep 20, 2012)

cd and lux at 1 meter are interchangeable in this context, so the same # is OK for both units.

Lux is how much light is reflecting back from the target (the light you SEE when you look at something), where lumens is how much is being emitted by the source.

So, the lumen number is the total supply of light leaving the flashlight...analogous to the gallons per minute gushing from a hose.

Lux would be more analogous to how deep that puddle the hose made was....if we think of the depth of the water the way we'd think of how bright the puddle of light looked to us.

So, a hose pumping out a lot of lumens might not make too deep a puddle, if the spray was spread out over a wide area. It could also be used to fill a more concentrated area, like a bucket instead of the entire yard, etc.

A thrower is the hose with your thumb over most of the hose opening, so all the water squirts out of a small hole...so the same gallons per minute are concentrated into a small area....shooting further.

A flood light is more like a sprinkler head at the hose end...the same gallons are spread over a larger area.


A light with high lux or cd has concentrated its lumens to make a small patch brighter. For the same lumen quantity, the light with a higher cd or lux value will throw further for that reason. 

They use lux or cd in the specs as a short version of "Lux at 1 Meter"....IE: What would the lux be on a target 1 meter away.

This might not be a REAL number, as many throwers might have their focus further out than 1 meter, and the beam convergence has not happened yet...so a MEASUREMENT at 1 meter might be lower than what the specs say.

The MEASUREMENT might have been taken at a longer distance more in line with the beam focus, and then back-calculated to see what they would be at 1 meter. This way, they can all use 1 meter as a standard.


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## HighlanderNorth (Sep 20, 2012)

So if someone gives a 55,550 cd measurement, that is also equal to 55,550 lux?


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## TEEJ (Sep 20, 2012)

HighlanderNorth said:


> So if someone gives a 55,550 cd measurement, that is also equal to 55,550 lux?



Yes Sir

As long as the context is the Lux at 1 Meter.


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## SemiMan (Sep 21, 2012)

Lux is the intensity of light hitting a surface, nit is the amount of light reflected or emitted.

Never heard LUX used as a shortform of lux at 1 metre. If lux is ever quoted, it is given with a distance as well.


Lux and Candela are only interchangeable (in value only) at 1 metre as a steradian of a 1 meter diameter sphere has an area of 1 square meter. 

Candela = lumen/steradian and Lux = lumens/square metre

Again, lux will always have a distance measurement if quoted ... cheap garbage lights the exception.

Lux is completely dependent on distance (inverse square law). Candela is the same no matter how far away you are.

Semiman









TEEJ said:


> cd and lux at 1 meter are interchangeable in this context, so the same # is OK for both units.
> 
> Lux is how much light is reflecting back from the target (the light you SEE when you look at something), where lumens is how much is being emitted by the source.
> 
> ...


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## rufus001 (Sep 22, 2012)

I like this explanation.

http://www.wiseled.com/guides_demos_videos_lumensExplained.htm


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