# Candle Power vs Lumens, whats the difference?



## V8TOYTRUCK (Aug 3, 2001)

I was reading that my Ultrastinger has 75k candle power, then I find out its not the brightest flashlight out there. The M6 by SureFire is 250-500 Lumens. Since the M6 is almost triple to cost of the Ultrastinger, and doesnt have recharagable batteries. I think I would still perfer my Ultrastinger. So the question is, is there a fomula to figure out CP vs Lumens? Like 100 Cp = 1 Lumen?


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## Chris M. (Aug 3, 2001)

It`s not as simple as that. Candlepower is a measure of the peak beam intensity- the intensity at the very brightest spot, whereas Lumens, put simply, is a measure of the total radiated light output in all directions. A 100 lumen lamp could put out a 100 candlepower beam or a 100 000 candlepower beam depending on how tightly focussed it is. Take a cheap laser pointer for example. It shines for miles and would blind you if you stared straight at it- but you couldn`t use one to light a whole room bounced off the celing cos the total light (lumens) is so low.

Candlepower is a figure that manufacturers like to use because the figures are often really high and it looks good to say "half a million candepower" than it does to say, for example, 300 lumens. But on their own, they are not a good figure as in one case, a really high candlepower light could have a beam so narrow it is simply not practical. Similarly, lumens on their own don`t mean too much in a flashlight cos a 100 lumen flashlight will not necessarily shine as far as a 60 lumen one if it has a really wide beam in comparison.
For a fairer representation we need to know other things like the beam half-angle (the angle from the centre of the beam to the point where it is 50% of the peak brightness) or even, actual beam-profile shots at standard distances like me and Craig do with our test victims, er, subjects- they show the peripheral rings and spill light as well as the size and shape of the hotspot to give a better impression of the useability.


Confused? I think I am a bit too now.


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## Size15's (Aug 3, 2001)

I like to think of Lumens as the total amount of light the beam contains whereas Beam Candlepower is more a measure of how bright the beam is at a given point.

The _difference_ I believe is that a Lumens value is same measured at every distance from the lens whereas a Candlepower value decreases from the focus, the further away from the lens it is measured.

This Candlepower value would be great _if_ the distance from the lens was given.

Therefore, it is virtually impossible to trust a Candlepower value.

I would love to have the equipement to measure Lumens and Beam Candela. This would allow me to create a standardised measurement test that allowed flashlights to be more easily compared.

Al


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## Chris M. (Aug 3, 2001)

Measuring Lumens- apparently the stuff to do that costs an absolute fortune- you`d have to sell all your SFs to be able to buy _second hand_ photometric equipment needed, from what I`ve been led to believe, and the maths involved isn`t fun either.





Beam Candelas however, that one`s easier- at least as a good approximation. Find a light meter that will read in Foot-Candles. Set your lights 1 foot exactly away from the sensor (ideally 1 foot from the origin of the light- ie the bulb filament) with it perfectly face-on to the light. Gently move it round until you get the peak reading. The foot candle ,reading there is your Peak Beam Candelas. The accuracy depends on many things so without certified lab equipment I`d not take it as 100% perfect. I tend to leave things for 1 minute to settle down cos it changes so much initially, what with batteries running down or thermal runaway mildly toasting LEDs, though if your light has a burn time of 20 minutes, 1 minute is quite a long time, and heat.

My meter came from Farnell Electronics though you may have a bit of bother getting one yourself- apparently they only supply to business account holders and I had to order mine through work, and it was sort of special favour kind of thing to let me do it.

I`d offer to order one for you if you wanted one but I don`t think I can. I`ll ask, it can`t hurt...


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## Size15's (Aug 3, 2001)

Thanks for the offer.
However, I won't be going down that road for years yet me thinks.

Light Quality is far more important to me then CP or Lu values.

A good digital camera is far higher on my list!

Al


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## YeeDude (Aug 3, 2001)

Here are my observations to add to the continuing discussion of lumens vs. candlepower...

As has been said time and again, candlepower measures the "hottest" spot in a given light's beam whereas lumens reflect (no pun intended) the overall amount of light.

I witnessed an interesting demonstration of this recently. During a slightly foggy night I shined my UK Light Cannon out the window at a flagpole on a building across the street, down the block and into a heavily tinted minivan. (And no, nobody was in it, ya pervs!



) I then repeated this with my Turbocat S32 bicycle headlight system and then with a MagCharger.

Here are my observations. The S32 (used both high and low beam) and Light Cannon lit up the flag and down the street in a semi-diffuse way. In contrast to this, the MagCharger, despite being WAY less bright overall, was able to light up the flag better. The reason is obvious - the parabolic reflector.

This happened again in the third "tint test". Both the S32 and Light Cannon fully lit up the side of the minivan, but were only able to slightly light up the interior so that I can see through. With the MagCharger I was able to almost make out the fabric of the seats and not only the silhouette of the headrests.

So lumens and candlepower do have their uses so long as one understands the utility of each measurement and knows what one wants/needs a light to do.

(Hmm.. I wonder if UK would ever make a turbohead for Light Cannon - NOT!



)


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## **DONOTDELETE** (Aug 3, 2001)

Manufacturers are giving us ratings in 2 different ways;

1. measure light emitted from bulb separated from bulb housing and reflector
2. measure brightest concentration of resulting light from bulb and reflector

Both are confusing ways of measuring and rating because flashlights' lightbeam is the resulting light coming from bulb and reflector (including bright and dimmer rings).

As of now a picture of flashlights' lightbeam is still better than lumens or footcandle numbers until manufacturers or flashaholics come up with a standard to measure flashlights' lightbeam.

- verge -


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## **DONOTDELETE** (Aug 3, 2001)

YeeDude said: <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>As has been said time and again, candlepower measures the "hottest" spot in a given light's beam whereas lumens reflect (no pun intended) the overall amount of light.<HR></BLOCKQUOTE>

This is patently false. 

An isotropic (radiating in all directions equally) light of one candela will produce an illuminance of one lumen per square foot, which is exactly the same as one foot candle per square foot. NOTE: THEY ARE THE SAME!! 

The problem with real life comparisons is that beams from reflector equipped or LED devices is far from isotropic, and the fact that a lumen depends on the wavelength of the light being evaluated. 555 nm. is perceived as the brightest to the non dark adapted eye. 

"Full spectrum" devices rarely have a uniform level of light throughout the visible spectrum. This is why some lights are referred to in degrees Kelvin, which correspond to the radiation from a body of a given temperature.

Hope this helps. Go here for further information: http://www.intl-light.com/handbook/ch07.html 

Walt


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## Joe Talmadge (Aug 3, 2001)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by V8TOYOTATRK:
*I was reading that my Ultrastinger has 75k candle power, then I find out its not the brightest flashlight out there. The M6 by SureFire is 250-500 Lumens. Since the M6 is almost triple to cost of the Ultrastinger, and doesnt have recharagable batteries. I think I would still perfer my Ultrastinger. So the question is, is there a fomula to figure out CP vs Lumens? Like 100 Cp = 1 Lumen?*<HR></BLOCKQUOTE>

Not to distract from the interesting lumen vs candlepower discussion, but your assertion above isn't correct. I believe the Surefire 10X Dominator, which *is* a rechargeable, will do a little over 500 lumens, beating out the M6. If you want the brightest rechargeable and don't mind spending a small fortune, check out the Dominator.


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## lightlover (Aug 3, 2001)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Joe Talmadge:
*..........I believe the Surefire 10X Dominator, which is a rechargeable, will do a little over 500 lumens, ...................*<HR></BLOCKQUOTE>

Both a 110 Lumen and the 500 Lumen bulb can be run at the same time. See Tim Flanagan's review at - http://www.candlepowerforums.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=1&t=000324 

I think Brock reviewed it on his site.

Lightlover


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## V8TOYTRUCK (Aug 4, 2001)

Now I just have to decide, UK light Cannon or 10x?
This has to be my final flash light. No more after this. Well, maybe a few more.


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## pk (Aug 4, 2001)

I might sounds like a sales person,
However I like to inject other benefits of my 10X.

* dual light source.
* dual bulb for lamp failure.
* Duble step momentary switching. 
(ergonomic)
* Rugged construction.
* etc.

Thanks,
pk

BTW, I also have a Phillips Xenon Short Arc lamps too (2nd generation).
They are Impressive.


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## Brock (Aug 4, 2001)

PK forgot to mention another big plus for the 10X, it is rechargable, while the Light Cannon isn't.


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## Size15's (Aug 5, 2001)

Also, it's got a SureFire quality beam(s)

I believe with better quality light, it is quicker and easier to see things.

Al


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## Free (Aug 5, 2001)

You all forgot to mention one thing about the 10x....you can't get one right now! I paid for mine last March and I still don't have it because surefire is having problems with their bulbs. Who knows when it will be available??


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## Size15's (Aug 5, 2001)

Your dealer actually took your money?!!

I would not expect to be charged until the item is shipped.

Al


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## Free (Aug 5, 2001)

Yeah I bought something else and he said he had to charge the entire order to ship the one that was in stock. Az Gun Runners, great prices and I have ordered more from them since but I am still waiting for my 10x


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## YeeDude (Aug 6, 2001)

*Walt Welch* said: <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>"This is patently false."<HR></BLOCKQUOTE>

I apologize for the incorrect statements in my previous posting and defer to Dr. Welch's more accurate explanation of lumens and candlepower.

FWIW, UK claims the Light Cannon put out a beam of 6000 degrees Kelvin.


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## Size15's (Aug 6, 2001)

I read on a chart that 5250K is the colour temperature of sunlight.

The beam is not 6000K hot, it has the colour temperature of 6000K hot. This is most likely based on burning some metal????

So maybe I too was getting the Lumens/CP definition wrong.
Of course, we're talking about BEAMS of light.

How about this?

"Lumens is the whole light of the beam, and CP is the light at one point in the beam." (and that point is not usually given)

I think it's best to express the Lu & CP in terms we can relate to flashlight beams because that will make it easier for us (?)

Al


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## JoeyL (Aug 6, 2001)

I might be oversimplifying parts of it, but the degrees kelvin is a reference to the electromagnetic emission pattern of a "blackbody" radiator. For real world applications, opaque objects that can reach these temperatures emit a spectrum of radiation characteristic of a given temp in degrees kelvin. The sun's photosphere is about 6000 degrees kelvin. It looks yellow white. An object at 10000 degrees kelvin would look a lot more blue. The center of the sun is 15 million degrees kelvin. It emits mostly x-rays at that temperature.

What I'm not sure of, is whether the filament of a bulb with a 6000 degree kelvin whiteness actually reaches close to that temperature. I know that LED's do not, but they emit light on a different principle than heating a filament.


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## DavidW (Aug 6, 2001)

I believe it's referring to _color_ temperature. In referrence to the color spectrum and not _heat_ or lack of.


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## **DONOTDELETE** (Aug 8, 2001)

YeeDude; in retrospect, my wording was somewhat offensive. I apologize. Discussions between MDs tend to be somewhat, er, ah, powerfully and directly worded. I sometimes slip back into my old ways. Sorry.

Kelvin color temperature is based on the light which would be radiated by a black body which is at a given temperature in Kelvin (Celsius degrees above absolute zero; it is a scale used for convenience). The body (or filament) in question need not be (and is usually not) actually AT that temperature. For an excellent discussion of Kelvin temperatures, which incidentally are NOT equally bright in all wavelengths, go here: http://www.intl-light.com/handbook/ch05.html 

Now, one more attempt to clarify those who seem to think that foot candles and lumens are different in the qualities they describe. THEY ARE NOT!!! BOTH assume isotropicity (equal radiation in all directions). 

Illuminance is a measure of photometric flux per unit area, or visible flux density. Illuminance is typically expressed in lux (lumens per square meter) or foot-candles (lumens per square foot). 

A one candela light source emits 1 lumen per steradian in all directions (isotropically). A steradian is defined as the solid angle which, having its vertex at the center of the sphere, cuts off an area equal to the square of its radius. The number of steradians in a beam is equal to the projected area divided by the square of the distance. 

So, 1 steradian has a projected area of 1 square meter at a distance of 1 meter. Therefore, a 1 candela (1 lm/sr) light source will similarly produce 1 lumen per square foot at a distance of 1 foot (which is the same as a foot candle), and 1 lumen per square meter at 1 meter. Note that as the beam of light projects farther from the source, it expands, becoming less dense. 

If you wish to measure a non isotropic beam (such as that of most flashlights), then beam candela is the appropriate unit of measurement.

Beam candela, on the other hand, samples a very narrow angle and is only representative of the lumens per steradian at the peak intensity of the beam. This measurement is frequently misleading, since the sampling angle need not be defined. 
Suppose that two LED’s each emit 0.1 lm total in a narrow beam: One has a 10° solid angle and the other a 5° angle. The 10° LED has an intensity of 4.2 cd, and the 5° LED an intensity of 16.7 cd. They both output the same total amount of light, however -- 0.1 lm. 

Note that because a candela is a light source which is equivalent to one standard candle in luminous intensity (1.464 x 10-3 watts/sr at 555 nm), the intensity of the above described LEDs could be described in candlepower with equal validity. The reason that candlepower is not usually used is that it is an obsolete measurement.

Hope this helps, Walt


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## lightlover (Aug 8, 2001)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Walt Welch:
*........

Hope this helps, 

Walt*<HR></BLOCKQUOTE>

Walt, can you please explain that again ?

lightlover















(I posted this even before I finished reading Walt's no doubt excellent post - Oh well, back to studying his stuff yet again. Someday I will understand ...... )


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## **DONOTDELETE** (Aug 8, 2001)

Let's try this. Brock is in his flashlight testing room. He shines a flashlight at himself from a distance, and stands directly in the beam. He says, 'WOW; that has about 1,000 beam candela, according to my wavelength- weighted light meter!!' 

An old guy in a frock coat and a wig bends over Brock's shoulder, looks into the flashlight beam, and says, 'Verily, forsooth, 'tis bright as 1,000 candles as well!! You may perchance to say it has 1,000 candlepower!'

Hope THIS helps,



Walt


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## lightlover (Aug 9, 2001)

Close - but not That close !

Thanks Doc - I feel much better now ......

lightlover





[It must to be something like late onset electrical dysklectia]


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## PeLu (Aug 12, 2001)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by **Something Ridiculous**:
*Close - but not That close !
*<HR></BLOCKQUOTE>
One thing to add:
The luminous flux in Lumen does not necessarly mean that it is given for the bare bulb (as posted before). Honest companies give the 'real' lumen output which comes out in front of the flashlight (for example Surefire).

And the unit Kelvin does not need 'degree'.

Beside of this facts it is easy (but a lot of work) to calculate that the 'perfect' black body radiator will have maximum luminous efficacy of 96 lm/W at 6622 Kelvin.
Tungsten melts at about 3655 K. At this temperature a black body will have a theoretical efficacy of 43 lm/W. So this would be the max efficacy for a tungsten-filament bulb. Typical incandescent bulbs run at about 2900-3000 K, where the max efficacy is under 21 lm/W.


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