# Newbie Q: Lux vs Lumens vs candlepower



## Solomon (Jan 26, 2005)

Could someone please give me an explanation about...

Lux?
Lumens?
Candlepower?

...and how they are measured? For example, does a 500 lumen flashlight equate to 500,000 candlepower?

Thanks in advance!


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## Doug S (Jan 26, 2005)

[ QUOTE ]
*Solomon said:*
Could someone please give me an explanation about...

Lux?
Lumens?
Candlepower?

...and how they are measured? For example, does a 500 lumen flashlight equate to 500,000 candlepower?

Thanks in advance! 

[/ QUOTE ]
This is complicated and confusing. Here is the fairly rigid explanation. Candlepower is a measure of luminous intensity when specifically measured in units of candelas. This is like "mileage" is a measure of distance when the unit of measure is in miles. Luminous intensity is a measure of light [flux] passing through a unit of *solid angle*. A candela is the intensity produced where there is one lumen [flux]per steradian [the measure of solid angle]. Lux is a unit of measure of illumination which is a measure of light [flux] *falling on a flat area*. One lux is the illumination produced when there is one lumen [flux] falling on a square meter. Lumens are a measure of total light [flux]. Think of a focusable flashlight. As the beam is adjusted from broad to narrow, the total flux [lumens] stays the same but the illumination of a surface or intensity [lux or candelas] within the beam increases. Illumination [Lux] or candlepower [candelas]can be measured with relatively simple instruments. Flux [lumens] is much more complicated. Most commonly an integrating sphere is used to capture all of the light emitted by a source and then the illumination is measured at a fixed point in the sphere. Through complicated calibrations this illumination measurement is then correlated with a flux value.

Some folks incorrectly think that candelas [candlepower] and lux are units of measure of the same thing but this is not quite correct. It is true that a point source with an *intensity* of one candela will produce an *illumination* of one lux at a distance of 1 meter but this does not make them measures of the same thing.

>For example, does a 500 lumen flashlight equate to 500,000 candlepower?<

No. They are measures of different things.


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## Lexus (Jan 26, 2005)

This site might help too, it's on the bottom of the page:

http://www.flashlightreviews.com/qa.htm


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## SilverFox (Jan 26, 2005)

Hello Doug,

Do you have any idea how spot light manufactures come up with their claims?

I believe there is a basis for their claims of X million candlepower, but have not been able to figure it out.

I have a butane lantern that was rated (by a testing lab) at 80 watts. It (EDIT THIS OUT puts out) registers 30 foot candles at one foot (or 30 lux at one meter) on my light meter. I was told that it was rated by consumption in a similar way that household light bulbs are.

The X million candlepower came from somewhere, any ideas?

Tom


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## Doug S (Jan 26, 2005)

[ QUOTE ]
*SilverFox said:*
Hello Doug,

Do you have any idea how spot light manufactures come up with their claims?

I believe there is a basis for their claims of X million candlepower, but have not been able to figure it out.

I have a butane lantern that was rated (by a testing lab) at 80 watts. It puts out 30 foot candles (lux). I was told that it was rated by consumption in a similar way that household light bulbs are.

The X million candlepower came from somewhere, any ideas?

Tom 

[/ QUOTE ]
Short answer: No 
Almost as short: Probably out of their A$$

BTW, a lantern doesn't put out X foot candles or X lux. It can produce an illumination of X at some arbitrary distance. If it produces say X footcandles of illumination at say 167.3' distance it will produce an illumination of about 10.8X lux at this same distance.


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## Solomon (Jan 26, 2005)

I've been wondering the same thing.

So can a properly designed reflector actually increase the candlepower of a flashlight or spotlight?

Is that correct?


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## Doug S (Jan 26, 2005)

[ QUOTE ]
*Solomon said:*
I've been wondering the same thing.

So can a properly designed reflector actually increase the candlepower of a flashlight or spotlight?

Is that correct? 

[/ QUOTE ]
Correct. Squeezing the same amount of light into a narrower beam will increase the candlepower *within the beam.*


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## SilverFox (Jan 26, 2005)

Hello Doug,

I have edited my original comments. They should be correct now...

And here I have been trying to factor in the melting point of the filament as well as various other things in an effort to make sense of this.

Perhaps there is a conversion to go from all the energy involved in manufacturing the light to Peak Beam Candlepower. /ubbthreads/images/graemlins/smile.gif I can see it now... Since we consumed MegaWatts of power manufacturing this light, it MUST produce Millions of Candlepower... /ubbthreads/images/graemlins/evilgrin07.gif

OK, enough of this foolishness. Time for a beer.

Tom


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## Doug S (Jan 26, 2005)

[ QUOTE ]
*SilverFox said:*
Hello Doug,

I have edited my original comments. They should be correct now...

Tom 

[/ QUOTE ]

Now I am remembering why I had resolved to never post in one of these photometric units threads /ubbthreads/images/graemlins/banghead.gif /ubbthreads/images/graemlins/dedhorse.gif

*I have a butane lantern that was rated (by a testing lab) at 80 watts. It (EDIT THIS OUT puts out) registers 30 foot candles (lux) on my light meter. *

For this to be meaningful, we would need to know at what distance the measurement was made. For whatever that distance, it could have measured 30 foot candles (fc) or 30 lux but not both. 1fc is about 10.8 lux.

[ QUOTE ]
*SilverFox said:*

OK, enough of this foolishness. Time for a beer.

Tom 

[/ QUOTE ]

Something we can agree on. /ubbthreads/images/graemlins/grin.gif


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## CroMAGnet (Jan 26, 2005)

Here is a good link explaining the CP/LUX/LUMENS thing. LEDLIGHT


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## Doug S (Jan 26, 2005)

[ QUOTE ]
*CromagNet said:*
Here is a good link explaining the CP/LUX/LUMENS thing. LEDLIGHT 

[/ QUOTE ]

The link is good. The explanations, however, leave much to be desired IMHO.


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## SilverFox (Jan 27, 2005)

Hello Doug,

I was assuming that people would understand that when foot candle values are reported they are measured at one foot and when lux values are reported they are measured at one meter (unless otherwise stated). 

I have corrected my post to make it more meaningful.

I did in fact measure both 30 foot candles at one foot and 30 lux at one meter.

Tom


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## Doug S (Jan 27, 2005)

The reason some folks here on CPF have choosen the 1M for lux measurements and 1ft for fc measurements is that the numerical value obtained will be equal to the source intensity [candlepower] measured in candela.


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## SilverFox (Jan 28, 2005)

Hello Doug,

And all this time I though we measured foot candles at 1 foot and lux at 1 meter because that's the way it is defined...

Tom


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## Doug S (Jan 28, 2005)

[ QUOTE ]
*SilverFox said:*
Hello Doug,

And all this time I though we measured foot candles at 1 foot and lux at 1 meter because that's the way it is defined...

Tom 

[/ QUOTE ]
I'm happy to have cleared that up for even one person. The modern definitions of these two units have nothing to do with distance from a source. The lux is defined as 1 lumen per square meter and a footcandle is one lumen per square foot. As a historical note, a long time ago when standard candles were defined in terms of a flame burning so many grams/hour of a certain type of whale oil [seriously] a foot candle was defined in terms of the illumination that would result at one foot distance measured perpendicular to the axis of the candle flame of the standard candle.


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## SilverFox (Jan 28, 2005)

Hello Doug,

I have a SureFire G2. I show 3534 lux at 1 meter. That means I have 3534 lumens per square meter.

If I have an 9 degree beam, that would give me 0.0194 steradians. sr = 2pi[1-cos(9/2)]

Taking 0.0194 steradians * 3534 lumens/square meter, I end up with close to 68.6 lumens.

Am I close?

Tom


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## Doug S (Jan 28, 2005)

[ QUOTE ]
*SilverFox said:*
Hello Doug,

I have a SureFire G2. I show 3534 lux at 1 meter. That means I have 3534 lumens per square meter.

If I have an 9 degree beam, that would give me 0.0194 steradians. sr = 2pi[1-cos(9/2)]

Taking 0.0194 steradians * 3534 lumens/square meter, I end up with close to 68.6 lumens.

Am I close?

Tom 

[/ QUOTE ]
Yes! Assuming that the average lux thoroughout the 9 degree beam width is the value you measured, your calculation is correct.


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## Doug S (Jan 28, 2005)

Now you've gone and done it, Tom. As if this topic wasn't confusing enough. I can see the geometrically challenged now scratching their collective heads wondering "just what the hell is a steradian and will it run on rechargeables?" /ubbthreads/images/graemlins/wink.gif


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## SilverFox (Jan 28, 2005)

Hello Doug,

I'm still trying to get the geometry correct to obtain 6 million candlepower... /ubbthreads/images/graemlins/smile.gif

Those spot light manufactures must use some pretty fancy math. /ubbthreads/images/graemlins/evilgrin07.gif

Tom


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## Doug S (Jan 28, 2005)

[ QUOTE ]
*SilverFox said:*
Hello Doug,

I'm still trying to get the geometry correct to obtain 6 million candlepower... /ubbthreads/images/graemlins/smile.gif

Those spot light manufactures must use some pretty fancy math. /ubbthreads/images/graemlins/evilgrin07.gif

Tom 

[/ QUOTE ]
It's easy, Tom. Just tweek the reflector of your 68 lm G2 so that the entire beam is just over 1mm in diameter at 1 foot and I believe the math will work out.


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