# lumens vs. candlepower



## BugLightGeek (Jun 3, 2002)

So, I'm doing some shopping and trying to determine which light is brighter. One manufacturer rates everything in lumens and one rates in candlepower. 

How can a newbie figure out which will be brighter?


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## Darell (Jun 3, 2002)

Unfortunately, it is almost impossible to compare.






Your best bet is to buy them both, then tell everybody here what you've found. "But them both" is somewhat of our unofficial motto around here...


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## BugLightGeek (Jun 3, 2002)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by darell:
Unfortunately, it is almost impossible to compare.





Your best bet is to buy them both, then tell everybody here what you've found. "But them both" is somewhat of our unofficial motto around here...<HR></BLOCKQUOTE>

Sorry, I wasn't specific. OF COURSE I'll probably end up buying them both, but for now, I'd like to have a sense of comparison between candlepower and lumens...


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## rlhess (Jun 3, 2002)

You can use a (walk-in) integrating sphere, or you can integrate the light output over area and do the math.

I haven't done it yet, but I might try it on the plane tomorrow--or maybe I'll sleep on the plane.

Cheers,

Richard


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## BugLightGeek (Jun 3, 2002)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by rlhess:
You can use a (walk-in) integrating sphere, or you can integrate the light output over area and do the math.

I haven't done it yet, but I might try it on the plane tomorrow--or maybe I'll sleep on the plane.

Cheers,

Richard<HR></BLOCKQUOTE>

Whoa...you lost me on that one. But, if you could provide me with some sort of conversion, that would be appreciated.


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## Chris M. (Jun 3, 2002)

Greetments herr Doppelganger....


There is *no easy or direct comparison between Lumen and Candlepower ratings*.

Lumens are a measure of the total amount of light emitted in all directions, and while not entirely appropriate when applied to directional light sources like flashlights, it still can be used to measure all the light poured out the end of your torch, regardless of what direction it`s going in.

Candlepower or Candela is a peak spot measure- the intensity of the very brightest point of light within the focussed beam, usually the very center. It does relate with Lumens, to the _half angle_ of the beam- in other words, the angle from the centre at which the brightness is exactly half of the peak value at the centre. For an identical intensity light source (ie, lumen value is the same) the narrower the beam the higher the Candlepower rating. But the dispersion of the beam plays a big role. It depends largely on the width of the hotspot, what portion of the light goes into the main beam, and what portion spills off to the side.


Unless you know the precise dispersion of the individual beams, with an accurate 2 dimentional plot of intensity (or a simple 1-dimention graph if the dispertion is perfectly circular and not lopsided or oval), and then apply advanced mathematics that would make most highschool students heads spin, you cannot convert between the two. Can`t be done, sorry.

Regrettably there is not a simple home-friendly way to measure lumen outputs. A simple photographic Lux meter can be used to give you candela (candlepower) readings but Lumens require advanced methods. You need lots of space, lots of money and lots of Barium Sulphate paint in order to do it. Hence why most manufacturers just use candlepower. Additionally (and maybe more influencially), candlepower figures are often high and look great on fancy packaging and web sites- that thing about narrow beams again. Many Pelican lights and all Maglites for example, are rated quite highly as far as cp goes, but the overall light output is low compared to some, just the fact that they can focus pretty narrowly. A narrow beam is not always a useful beam so candlepower ratings cannot be soley relied upon. Similarly Lumen ratings cannot be soley relied upon either. A high-lumen beam _could_ be so wide and dispersed that it does not throw more than a few feet, or a low lumen beam _could_ be so narrow that it shines a very long way. 

It`s all down to what you`re looking for in a lignt and until flashlight manufacturers get their act together and embrace a universal, standardised and informative rating method they will all use, the only real way to tell if one torch is better for you than another is to try it out yourself. Hence the official CPF motto "Get them both



". We reviewers try our best but however detailed we try to be in our writeups, you still cannot beat seeing the thing for yourself.


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## Darell (Jun 3, 2002)

Wow!





Chris made my "it is almost impossible to compare" look really concise, huh?





That Chris M. really knows his stuff!


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## rlhess (Jun 3, 2002)

Hi, Chris,

I was suggesting that we could approach a ballpark total lumens output by, for example, taking illuminance measurements at every (let's say) five degrees, determining the area over which that is falling and do that until we get to a five degree ring that is at say 1% of the center output.

Yes, we'd have to do cosine correction (depending on how we held the light meter) and use s different radius value in the inverse square compensation (or move the light meter in a spherical path) for the outer rings, especially, and we'd have to come to some agreement as to the level/area for the center hotspot, but that would start to create a figure that could be considered "pseudo lumens" and a first-order approximation to the integrating-sphere measurements.

I wasn't meaning to say that we could approach laboratory conditions, but, for example, the illuminance charts that Craig does might be able to be used to provide an approximation of total light output.

I don't think the "integrating ceiling" approach is valid as there are too many other variables there.

But, what I'm proposing is a fair annoyance with trig functions and other calcs for only a rough approximation. 

Since I "know" that my SF E2e puts out 60 lm, I thought it would be interesting to use that as a model for the hypothetical rough-order-of-magnitude conversion. 

If I have time, I'll build the spreadsheet.

I doubt I'll have time to take all the measurements for a week or more.

Cheers,

Richard


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## treek13 (Jun 3, 2002)

Ok, first I had this whole lumen vs. candlepower thing to try to sort out and now I've got this ChrisM vs. Chris M. problem.

Now I'm really confused; heck I'm not even sure if I have their names straight here.


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## BugLightGeek (Jun 4, 2002)

First, congrats on the web page contest.

Wow, Chris M, a most excellent response. Even though it just boils down to GTB! I was hoping for some sort of conversion formula but oh well.


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## Chris M. (Jun 4, 2002)

_I was suggesting that we could approach a ballpark total lumens output by, for example, taking illuminance measurements at every (let's say) five degrees, determining the area over which that is falling and do that until we get to a five degree ring that is at say 1% of the center output...._


_....I wasn't meaning to say that we could approach laboratory conditions, but, for example, the illuminance charts that Craig does might be able to be used to provide an approximation of total light output._

That`s an interesting idea- assuming you had a perfectly round beam it could work. Lopsided beams have too many variables that would make for some horrendous 2 dimentional analysis with hundreds of measurement points and enough calculations to fill all 65000 rows that MS-Excel provides per-spreadsheet! 
All I can say is- Try it and see! The mathematics involved is waaaaay over my head though so you`re on your own there I`m afraid. I`m now a two-times university dropout resitting and failing the final year of my Electrical Power Engineering twice. The mathematical element of the course was my biggest fall-down point. That and the fact that now I know what it`s all about, I don`t actually want to be an electrical power engineer and was tired of the course



(don`t worry, I`m glad to be out of that place now and am in a job I quite enjoy for now so it`s all OK).


_Since I "know" that my SF E2e puts out 60 lm, I thought it would be interesting to use that as a model for the hypothetical rough-order-of-magnitude conversion._

Here`s the big fall-down point, you would need a few lights that you knew the _exact_ lumen output of, in order to "calibrate" and otherwise prove or deny your measurments and calculations. I am not 100% sure that SureFire`s MN03 lamp does put exactly 60 lumens. Think about it- the E2 light has always been rated at 60 "real out-of-the-reflector" lumens, but so is the new E2e. But the E2e uses a Pyrex lens with anti reflective coating that has some 97% efficient light transmission. The old E2 uses Lexan and has only 85% efficient light transmission. Both use the same lamp and probably the same reflector so which one is the 60 lumen output? I think probably 60Lu is only an approximation.

A friend of mine has access to a big proper Integrating Sphere that has recently been calibrated to Japanese industry standards. He said I could send _a small amount_ of lights to him for measurment, so possibly I can get a few of my Surefires and a specific brand MR16 lamp properly lumen-measured, we could work from there then (assuming you had the same SureFires and batteries). No guarantee that I`ll be able to have this done any time soon, he is a very busy man and acess to the Sphere is probably controlled too.

Definately interested in your results though. If we could get this to work, approximate "home" lumen measurements could be taken, if only for ball-park figures.

I have plans here to build an integrating sphere. I`ll say no more at present but if it all goes to plan, I could build my own for more accurate measurements






_First, congrats on the web page contest.
Wow, Chris M, a most excellent response. Even though it just boils down to GTB! I was hoping for some sort of conversion formula but oh well._

Thanks





I agree it is a shame that there is no easy comparison between the two, but that`s just the way things are- it`s kind of like equating gallons to feet or something. Photometrics is a complicated subject indeed. It`s enough to make your head spin








But you can never have enough torches so I`m all for an excuse to buy more!


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## hotfoot (Jun 4, 2002)

With regards to an integrating sphere, maybe we can wing it. If a sphere can be set up with ordinary white paint, or perhaps completely mirrored/chromed inside, an initial measurement can be first taken with a reference light of known and reliable output, then the deviation noted and used to correct readings against other lights - ya know, kinda slide rule style. I'm just guessing here, so please ignore this post if you recognize it as rubbish


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## Chris M. (Jun 4, 2002)

Mirrored, no. White paint, maybe- but it would have to be matte finish and very even. All`s I can think of is white primer spray- that has good bright-white matte properties, or at least the stuff I last had did.


The big troubles after that are:
- finding a large hollow sphere- or better, two hemispheres that can be bolted/hinged together. Minimum reccomended diameter is 10 times the diameter of the size of the lamp- in this case it would be at least 10 times the size of the lens of largest torch you plan to be able to measure. It needs to be openable for painting, cleaning (dust, etc) and to install an internal deflector that stops light falling on the sensor directly from the incoming lamp (all light on the sensor must be reflected from the surface of the sphere, none directly or it ruins the result)
- finding a suitable sensitive and linear light sensor. Maybe a Lux meter would be OK, it must have a remote sensor though (one built into the top or front would be impractical), and probably not be too big of a sensor.
- finding an adjustable iris for the "incoming" window through which you shine your torch. Different torches have different diameter heads of coruse so you need a variable iris that can be sized to fit the torch, stopping all external light getting in. Ideally white coloured on the inner surface but black apparently can work, it`s all down to the calibration.


Eventually I`ll try and build one (hopefully with Barium Sulphate paint). Anyone here who fancies a go, please feel free but I cannot guarantee the success or accuracy of anything you try. If it sort-of-works, it`s a step in the right direction at least....


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## brightnorm (Jun 4, 2002)

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Chris M.:
*..... But the E2e uses a Pyrex lens with anti reflective coating that has some 97% efficient light transmission. The old E2 uses Lexan and has only 85% efficient light transmission....*<HR></BLOCKQUOTE>

Chris M,

I am very surprised to see that the pyrex lens passes 14% more light than the lexan. I thought it was more like 8%.

Perhaps it is the antireflective coating that makes light transmission so efficient. Since many plastic prescription lenses have such a coating why not apply it to Lexan? It should raise costs nominally, and I would think that Lexan lenses might be an advantage during very rough usage or LEO action where even Pyrex could be vulnerable. 

Any info on this?

Brightnorm


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## **DONOTDELETE** (Jun 4, 2002)

I could try to check whether it is possible to convert between the candlepower and lumen readings using that ugly photometric math Chris referred to.

All I need is the following data for any of the surefire lamps:
- lumen output of the lamp
- lux readings at the hotspot of the beam at the distance of 1 and 2 meters
- diameter of the reflector
- diameter of the beam at the distance of 2 meters (ideally, such "diameter" would correspond to the lux reading being 50% of the maximum, but an eyeballed value would suffice). Is such data avilable anywhere? If not, does anybody out there have a Surefire AND a lux-meter?

If the computed lumen value lies in the neighborhood of the actual known lumen value, it may be possible to implement a lumens<->candlepowers converter (as a Java applet, for example) that would perform the conversion based on the specified beam profile.


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## **DONOTDELETE** (Jun 5, 2002)

I wish someone would produce a cheap (mebbe coupla hundred bucks?) integrating sphere. Exact accuracy not necessary, mebbe just within a +/- 10% margin of error?

I'd even go in with a few someones locally and then we'd have a reference machine -- and everybody could send us their beloved lights temporarily and we'd measure'em all and have a handy-dandy reference chart from which to decide on our purchases. In fact I wouldn't mind sending in $10 or $20 bucks to one of the CPF administrators, and if we had enough participation, we could have our very own CPF IS (Integrating Sphere) which we could send our addiction items for testing.

Whatsay, a Group Buy of *one* IS? Anybody?


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## Chris M. (Jun 5, 2002)

_I wish someone would produce a cheap (mebbe coupla hundred bucks?) integrating sphere. Exact accuracy not necessary, mebbe just within a +/- 10% margin of error?_

If only it were that simple. The specialised Barium Sulphate paint (extremely pure white) that is used to coat the inside of integrating spheres costs some £400 per litre (US$600) and it needs re-painting every couple of years as it degrades over time. It also needs constant re-caliabration against a reference light source, and such precision-made lamps and their associated power supplies cost big bucks too. Every year, the sensor and display/measuring equipment will need proffessionally recalibrating too- $$$





Got $1000 spare? If about a hundred of us got together with that amount each we could probably buy a properly constructed "CPF Sphere" and have some put aside for maintenece and calibration. Unfortunately it doesn`t seem practical. They can also be quite large.

DIY Lumen measuring is currently not too simple, which is a shame.


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## rlhess (Jun 5, 2002)

Hi, Jonathan,

That took time!

The integrating sphere is actually calibrated, as I understand it, from a standard lamp which is calibrated using a photometric goniometer or some such named device.

One needs to keep the sensor perpendicular to the light source to eliminate the need for reverse cosine correction.

It is convenient if the sensor moves in a sphere and not a plane, then you don't have to do inverse-square law distances.

I think we're in agreement here.

Great summary!

Cheers,

Richard


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## Jonathan (Jun 5, 2002)

Hmm. I suspect that if one uses the integrating ceiling approach, that you explicitly _wouldn't_ need to bother with inverse cosine correction nor inverse square law correction, and furthermore, you would want the sensor in the plane of the ceiling.

My reasoning is that as you move further off axis, the solid angle covered by a unit area patch of the ceiling will get smaller and smaller. This will be precisely matched by the error introduced by having the sensor further away as well as off axis. 

The net result will be that the off axis lux measurement will no longer correspond to the candela in that direction, but that the _total_ lumen number that you calculate will be correct. (Well, for the total lumen going toward the wall.)

So if your goal is to figure out the spatial distribution of candela, then you will need to point the sensor at the light source and correct for inverse square law...and it would be easier to simply move the sensor about on a spherical surface, but if you just want total lumen output, then the 'integrating ceiling' will work with pretty simple maths.

-Jon


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## rlhess (Jun 5, 2002)

Jonathan,

As I recall the setup described (by Mr. Bulk (tm) I believe) for the integrating ceiling, you aim the light at the ceiling and you measure the light falling off the ceiling with an illuminance meter at the light location (but no direct light shining on the meter).

Could you please try one more time to explain how the rays at 0 degrees and 20 degrees (for example) become integrated at the meter.

I'm not visualizing what you are--and since you're in Boston, I'm starting to worry that you're from MIT and I'm just not getting it.












Richard


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## Jonathan (Jun 6, 2002)

Wow. I am _very_ surprised at how an integrating sphere works. When I heard the name ' integrating sphere' and a description of what it is supposed to do, I envisioned an entirely different mode of operation. What I'd envisioned is exactly what rlhess suggested: measure the intensity of the light source from many different directions, and then numerically integrate to get the total light output.

IMHO the math is _not_ all that difficult, at least not if you break it down into small steps. Of course, there will be _lots_ of these small steps, but them's what computers are for. Below is my stab at the math that one would need:

First step: Units. We need to know the units we are working in.

Candela (or candle power): This is the base unit for the _intensity_ of a light source in a given direction, measured in terms of _human_ perception. Intensity will tell you 'at distance D, so much light will fall in area A'. The definition of the candela say that a 1 candela light source has the same intensity as a light source which produces 1/683 watts of photons of a particular frequency per steradian.

Steradian: This is a unit of solid angle. Consider a sphere of radius 1, and imagine a cone growing out of the center of the sphere. The cone will mark off a circle on the surface of the sphere. When this surface has an area of 1 square unit, then the solid angle of the cone is 1 steradian.

Lumen: Is a _quantity_ of light, rather than _intensity_. This is analogous to the difference between _volume_ and _depth_ in a pond. A lumen is the total quantity of light which an even 1 candela source would pour into a single steradian. Because of the way the units work, you could think of a 1 Candela light source as one which produces 1 Lumen per steradian...but the SI folk start with the Candela and derive the Lumen.

Lux: This is a measure of illuminance. It is measured in lumen per square meter. What is the difference between illuminance and intensity? Well, a light source _could_ be a point producing the light, so the unit for intensity describes light per unit solid _angle_. Illuminance is used to describe the light hitting an _area_, so you measure it in terms of light per unit area. You can also use Lux to measure the light coming off a light emitting surface.

The steradian ties this all together. A 1 candela source puts out 1 lumen per steradian. There are 4*pi steradians in a sphere, so an even 1 candela source will put out about 12.5 lumen. If you were to surround this light source with a 1 meter radius sphere, then each square meter of the sphere would intercept 1 steradian, and the total flux of light falling on the sphere surface would be 1 lumen per square meter, or 1 lux.

Second step: Tie the unit definitions to a physical measuring device.

Now, lux are measured in lumen per square meter, but you don't actually use a light detector that is 1 m^2. Instead you use a small light detector, measure the total number of lumen which hit that detector, and then divide by the area of the light detector to get the lumen/m^2. If your light detector were 1cm^2, and it collects 1 lumen of light, then the illuminance would be 10,000 lux.

So if you have a lux meter, and you place it exactly 1 meter from your light source, then the reading in lux is numerically equal to the intensity of the light source in candela, in the direction that you've placed the lux meter.

Third step: Describe the math

The sum over all directions of the intensity of the light source (in Candela) times the solid angle for that direction (in Steradians) is the total luminous flux of the light source (in Lumen).

This has to get written out as an integral, and to solve the integral requires lots of multiplication and addition, but not much complexity. A computer could take the light source, and generate a list of angle at which to measure the light, multiple the lux result by an appropriate area factor, and then just add up the appropriately scaled product from each direction.

Now, instead of an integrating sphere, we need a dark rook, a light sensor, a device for setting light source angles (called a goniometer).

Finally, I believe that if your lux meter properly measures light coming from all directions, that you wouldn't need to do any sort of cosine correction in order to run the 'integrating ceiling trick. Simply point the light source at a wall (or ceiling), and take a lux measurement over a grid (say every 10 cm), multiply the lux measurement by the area (in this case 10cm x 10cm is 0.01 m^2) and add up the results. The total should be the total number of lumen falling on the area that you measured.

Now remember that the the beam from something like a flashlight covers only a small total solid angle, but the space outside of the beam has a very large solid angle, so even though things are dim outside of the beam, there are probably lots of lumen there. But I think that the 'integrating ceiling would produce results which are useful at the CPF level.

-Jon


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## Jonathan (Jun 6, 2002)

Ahh. I was envisioning something along the lines of placing the light source 1 meter from a wall in a dark and _light absorbing_ room (eg. black curtains or the like). The light meter would then be placed against the wall, facing in the general direction of the light source.

You would then move the light meter to various grid targets along the wall, and measure the light falling on each patch of wall. 

So the integration step would be done mathematically by combining measurements taken at lots of locations, rather then mechanically by having a light reflecting surface and a single measurement.

-Jon

P.S. I'm not at MIT, though I do play with MIT kids on their robot submarine team.


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## brightnorm (Jun 7, 2002)

I'm not a math/sci person so please don't make fun of this idea.

What if you take a short tube/enclosure of pliable opaque material, fit it snugly around the bezel of a light to be measured, have a light meter or light sensitive device placed a finite distance from the light within this sealed enclosure, and an external means of reading the meter. Heat build up would not be a problem since a reading could be taken in a few seconds.
Since everything is enclosed the total amount of light, regardless of lamp type or reflector, could be measured in lumens or a figure easily converted to lumens.

Once you know the total amount of light you can convert to candlepower by specifying beam diameter at a specific distance.

As I said, I have virtually no math skills and what I am proposing may be humorous, but it is a sincere attempt to contribute to this subject.

Brightnorm


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## stuffdone (Apr 19, 2008)

*Re: lumens vs. candlepower -IDEA*

Idea.

Since the idea here seems to be to be able to make comparisons rather than arrive at an actual "output" number why not try this.

Make a black panel with a series of holes radiating out from a center point. Place a cheap light meter or photo transister behind each hole and shine a known new lamp as a reference. Note the level at each hole and record it.

With this in hand shine any other light from same distance to the center. Note the level at each hole compared to the reference source. This will provide a comparison as well as some idea of beam divergence. The more holes the more accuracy for comparison.

It would be easy and pretty cheap to duplicate and if everyone agrees on a standard reference lamp, comparisons to that reference would be pretty accurate.


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## d_huff78 (Oct 15, 2010)

I found this article after I was looking for new headlights for my car and found the article informative.


"Candlepower is a rating of light output at the source, using English measurements.
Foot-candles are a measurement of light at an illuminated object.
Lumens are a metric equivalent to foot-candles in that they are measured at an object you want to illuminate.
Divide the number of lumens you have produced, or are capable of producing, by 12.57 and you get the candlepower equivalent of that light source. "


So, isn't the whole thing moot? What I'm getting out of this is that the measurements are apples to oranges. 

Maybe it's just me.


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## Bullzeyebill (Oct 16, 2010)

Wow!! This is an old thread with an added post in 2008 and now 2010. Much information has been accumulated on CPF regarding this thread subject matter, and I am not sure we can really add to it today, but I will let it run for awhile, or let it die of natural causes.

Bill


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## FenixIlluminated (Oct 16, 2010)

1 candlepower = 12.57 lumens, but it should still be understood that these two forms of measuring are not truly identical.

Credit to 
Roy Nakatsuka, a guest on another forum I viewed.


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