lightlover
Flashlight Enthusiast
The subject of Candlepower versus Lumens arises quite frequently.
The difference between the two forms of light output rating took me some time to understand. I studied the different comparisons and examples members gave, and then it *finally* sank in. This is a summary of the explanations that made most sense (to me personally).
There are a lot of other ways to make analogies, so perhaps members will post the examples that they like best, their preferred explanations. There are also some much more technical approaches, and plenty of people who understand them, so there's a good chance that we can get posts to suit every level of interest.
So, my contribution is:
Basically, a Candlepower measurement, (shortened to "CP") is done at a one single point in the beam, and is a brightness at that particular point.
While Lumen ratings, (shortened to "Lu") are a measurement of the total, whole amount of light produced across the entire beam.
A CP measurement is taken by placing a small sensor in the beam, usually choosing the "best", brightest little point. As far as I know, there isn't any standard for the distance from the light-giving lamp/lens/LED/thing, it's a question of the technician hunting out the most bright point whether at 10" or 100".
A Lumen measurement is taken by putting the light into or at the entrance of an "Integrating", or "Ulbricht" sphere. This is a spherical container with special highly-reflective paint inside, and a sensor system which picks up all the light from all angles. Then, the measurement is integrated to Lumens.
Just as an example of the relationship between the two, without pretending to any particular accuracy in the numbers chosen -
Let's say that a 1,000 Lumen beam, when tightly focused, could be rated at 250,000 CandlePower.
But if you take that same lamp, so it's still the same amount of light, and change it over to a wide focus, the CP would drop to a figure of perhaps 25,000. It would still be at 1,000 Lu, because it's the same lamp.
Still the same amount of light, but not nearly the same measurement of light at just one chosen point.
Any diffusers fitted to the lens will cut down on the amount of light a little, because a diffuser will lower the amount of light getting out of the torch. Let's say for the sake of this example that a diffuser cuts the light output by 10%. So you will of course get a 10% lower Lu figure. But the effect of a diffuser on a CP measurement would be to lower it even more dramatically than the Lu figure, because there isn't a concentrated spot to test. It could go down to say 5,000 CP, but because of the diffusion, that CP would be a measurement accurate for a large part of the beam, not just a small point.
However you measure it, it will still be the same amount of light emitted by the same lamp and diffuser, but with the difference between Lumens and CP exaggerated.
At another extreme, a laser will project a bright point of light for a good distance. Even an ordinary keychain-type red laser has a very high CP rating, because it's a bright spot. But it's Lu rating is minute - you don't have any real illumination, you couldn't light up a room to see very much with it.
I have a beautiful turbo-boosted red laser. It will place a point of red on a building 3/4 of a mile away, so the CP rating is Huge, Enormous. But I would guess that it's Lu output is less than 1, (one !!) ......
However, if you are lucky enough to get a Green laser, you can project a point of light well over a mile. And if you shine that one at the ceiling of a room, it will throw a reasonable, small amount of reflected light you can see with. So there, you have a combination of a massive, enormous CP rating, and a low Lu rating.
I reckon the Green laser I borrowed from Telephony, PHD. has about 3+ Lu. (If you wanted to use it as an emergencies-only low-output EDC for reflective areas only, it wouldn't be too out of place !)
(Just picking these numbers at random). Say you have a light that measures mostly at 2000 CP, but 3-5,000 CP in parts, and 16,000 CP at one small point. How do you express that as one figure ?
It's difficult, so manufacturers often fudge the issue by claiming the highest reading at one particular point.
How would you quantify and report these "CP" readings ?
Quote from (Doug P) Quickbeam's WriteRight review -
"The maximum highest ... reading on the tightest focus ... was a whopping 815. However, that was in a very isolated tiny spot at the very center of the beam. As soon as I moved the meter away from that spot, it dropped into the 100's - 200's. At 1/2 the central beam radius the reading was around 100 - 140; a dramatic drop!"
@ http://thelightsite.tripod.com/reviews/writeright.htm
For actual practical use by anyone:
CP is such an inefficient way to indicate the actual power of a light, because in real life, you never could use a tiny single spot in the beam to see by. CP can't really be trusted on it's own.
Lumens are more useful, a more appropriate and accurate method of measurement , as the entire output of the light in question is measured. I suppose it's far more like the *light averaged out*, a more overall story.
But Lumens by themselves don't give the whole picture.
For example, the SureFire M2 (using a P60 lamp) is rated at 65 Lu, and the 8AX is rated at 110 Lu. But a lot of people feel that their 65 Lu M2's are brighter than the 110 Lu 8AX. Obviously, they aren't saying that SureFires Lu figures are wrong, but, they are saying that there is a difference. The fact of that is that the difference in the beam patterns is quite strong, because the "hotspot : surround" sizes and ratios are different. So, to some people the 65 Lu is preferable, brighter than 110 Lu.
That's because of another complication, beam distribution ......
As flashlight users, we need more info, including the angles of the central and surround beams, and the ratio of light between them, etc. etc.. No-one does that yet.
(I hope that someday SureFire will lead the way in this. There is some guidance from them: a "typical" ratio for a SureFire beam is 70% of the light output is given to the "hot spot", and 30% is given to the surround beam.)
Another example, the Cat Eye Stadium 3 bike light. Quote: "21 watt Metal Halide bulb ... the equivalent of 80+ watts of halogen light"
(I don't think that 21W of HID is quite the equivalent of 80+ Watts of incandescent, but let's not dwell on that.)
That has to be an enormous amount of Lumens - I seem to remember it's about 1,800-2,000 Lu. But they state only 3,500 candlepower. If that seems very low indeed for a light of that huge power, it's only because the beam is set at a very wide angle. I suppose that both ratings are accurate, but seeing it as only a 3,500 CP light would be misleading in that case, where a massive 21W of HID is available. http://www.cateye.com/ph_0108HL-STADIUM.htm
The Lu rating alone isn't really enough, but it does have some relevance to real life. Traditionally, CP has been used to rate lights, and it took me ages to realise that it really had no practical meaning to a torch user rather than a flashlight company marketing manager.
Quote from ike, in Topic: Newbie with an HID question... -
"Don't be impressed with Candlepower specs ... they are a marketing guys dream (I'm a marketing guy). They can be manipulated to give enormous numbers to sell flashlights. Candlepower is the lumens output divided by the solid angle of the beam. Thus, 1 lumen of parallel light has infinite candlepower! Go for raw lumens and see if the beam is tight enough for your application... forget CP unless you want to be prey for yet another incredible flashlight promotional claim."
@ http://www.candlepowerforums.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=1&t=000918&p=3
So CP ratings are practically irrelevant.
As for Lu ratings, we recently discovered that they can be exaggerated too. Even when using an Ulbricht sphere (Integrating sphere) to measure output, it's possible to apply very "odd" methods and arcane formulae to calculate Lumen readings. It's also possible to do misleading things like measuring the output of the lamp without a lens in front of it, or without a reflector, (so no losses due to those) and some other disreputable practices which increase the LU measurement.
Thankfully, SureFire don't exaggerate like this - their company policy is to understate specifications. They also take Lu output from the flashlight in "actual use" configurations.
It seems to be generally agreed that to see a big improvement in a beam, you need to double the output. So to see a big improvement from 100 Lu, you would need to increase output to 200 Lu. To improve the 200 Lu rating, you would have to go to 400 Lu. Human perceptions seem to need a light twice as powerful to make the increase much brighter. An increase from 100 to 120 Lu, would still be noticeable (if the beam patterns were exactly the same) but it wouldn't be 20% brighter, just a small increase.
lightlover
Resources.
One of the best resources for light measurement details is the justly famous "International Light Handbook", @ http://www.intl-light.com/handbook/ch07.html
Can be downloaded too, and well worth it. It's quite technical, but also clear on a lot of the issues. The illustrations make things clearer.
SureFire's Candlepower versus Lumens FAQ is @ http://www.surefire.com/cgi-bin/main.pl?pgm=co_disp&func=displ&pgrfnbr=65&sesent=0,0#1
Streamlight have an essay "Light Measurement" is @ http://www.streamlight.com/2001/releases/light_measurement.htm
DonL has a good analogy @ http://66.113.176.74/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=9;t=001380
(Don, can you post your analogy here ?)
Quote re: Pelican Nemo™ 8C submersible:
"14.4 Watts, Tested Lumen value 276.5 - Peak Beam Candlepower 100,000"
Quote re: Laser Devices:
"The 6-volt light is rated at 95 lumens and generates approximately 6,500 candlepower."
[This post subject to editing.]
Before you post to this, please read the original idea in Topic: "Archive/Reference" Topics (Draft Procedural Note). @ http://www.candlepowerforums.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=22&t=000383
The difference between the two forms of light output rating took me some time to understand. I studied the different comparisons and examples members gave, and then it *finally* sank in. This is a summary of the explanations that made most sense (to me personally).
There are a lot of other ways to make analogies, so perhaps members will post the examples that they like best, their preferred explanations. There are also some much more technical approaches, and plenty of people who understand them, so there's a good chance that we can get posts to suit every level of interest.
So, my contribution is:
Basically, a Candlepower measurement, (shortened to "CP") is done at a one single point in the beam, and is a brightness at that particular point.
While Lumen ratings, (shortened to "Lu") are a measurement of the total, whole amount of light produced across the entire beam.
A CP measurement is taken by placing a small sensor in the beam, usually choosing the "best", brightest little point. As far as I know, there isn't any standard for the distance from the light-giving lamp/lens/LED/thing, it's a question of the technician hunting out the most bright point whether at 10" or 100".
A Lumen measurement is taken by putting the light into or at the entrance of an "Integrating", or "Ulbricht" sphere. This is a spherical container with special highly-reflective paint inside, and a sensor system which picks up all the light from all angles. Then, the measurement is integrated to Lumens.
Just as an example of the relationship between the two, without pretending to any particular accuracy in the numbers chosen -
Let's say that a 1,000 Lumen beam, when tightly focused, could be rated at 250,000 CandlePower.
But if you take that same lamp, so it's still the same amount of light, and change it over to a wide focus, the CP would drop to a figure of perhaps 25,000. It would still be at 1,000 Lu, because it's the same lamp.
Still the same amount of light, but not nearly the same measurement of light at just one chosen point.
Any diffusers fitted to the lens will cut down on the amount of light a little, because a diffuser will lower the amount of light getting out of the torch. Let's say for the sake of this example that a diffuser cuts the light output by 10%. So you will of course get a 10% lower Lu figure. But the effect of a diffuser on a CP measurement would be to lower it even more dramatically than the Lu figure, because there isn't a concentrated spot to test. It could go down to say 5,000 CP, but because of the diffusion, that CP would be a measurement accurate for a large part of the beam, not just a small point.
However you measure it, it will still be the same amount of light emitted by the same lamp and diffuser, but with the difference between Lumens and CP exaggerated.
At another extreme, a laser will project a bright point of light for a good distance. Even an ordinary keychain-type red laser has a very high CP rating, because it's a bright spot. But it's Lu rating is minute - you don't have any real illumination, you couldn't light up a room to see very much with it.
I have a beautiful turbo-boosted red laser. It will place a point of red on a building 3/4 of a mile away, so the CP rating is Huge, Enormous. But I would guess that it's Lu output is less than 1, (one !!) ......
However, if you are lucky enough to get a Green laser, you can project a point of light well over a mile. And if you shine that one at the ceiling of a room, it will throw a reasonable, small amount of reflected light you can see with. So there, you have a combination of a massive, enormous CP rating, and a low Lu rating.
I reckon the Green laser I borrowed from Telephony, PHD. has about 3+ Lu. (If you wanted to use it as an emergencies-only low-output EDC for reflective areas only, it wouldn't be too out of place !)
(Just picking these numbers at random). Say you have a light that measures mostly at 2000 CP, but 3-5,000 CP in parts, and 16,000 CP at one small point. How do you express that as one figure ?
It's difficult, so manufacturers often fudge the issue by claiming the highest reading at one particular point.
How would you quantify and report these "CP" readings ?
Quote from (Doug P) Quickbeam's WriteRight review -
"The maximum highest ... reading on the tightest focus ... was a whopping 815. However, that was in a very isolated tiny spot at the very center of the beam. As soon as I moved the meter away from that spot, it dropped into the 100's - 200's. At 1/2 the central beam radius the reading was around 100 - 140; a dramatic drop!"
@ http://thelightsite.tripod.com/reviews/writeright.htm
For actual practical use by anyone:
CP is such an inefficient way to indicate the actual power of a light, because in real life, you never could use a tiny single spot in the beam to see by. CP can't really be trusted on it's own.
Lumens are more useful, a more appropriate and accurate method of measurement , as the entire output of the light in question is measured. I suppose it's far more like the *light averaged out*, a more overall story.
But Lumens by themselves don't give the whole picture.
For example, the SureFire M2 (using a P60 lamp) is rated at 65 Lu, and the 8AX is rated at 110 Lu. But a lot of people feel that their 65 Lu M2's are brighter than the 110 Lu 8AX. Obviously, they aren't saying that SureFires Lu figures are wrong, but, they are saying that there is a difference. The fact of that is that the difference in the beam patterns is quite strong, because the "hotspot : surround" sizes and ratios are different. So, to some people the 65 Lu is preferable, brighter than 110 Lu.
That's because of another complication, beam distribution ......
As flashlight users, we need more info, including the angles of the central and surround beams, and the ratio of light between them, etc. etc.. No-one does that yet.
(I hope that someday SureFire will lead the way in this. There is some guidance from them: a "typical" ratio for a SureFire beam is 70% of the light output is given to the "hot spot", and 30% is given to the surround beam.)
Another example, the Cat Eye Stadium 3 bike light. Quote: "21 watt Metal Halide bulb ... the equivalent of 80+ watts of halogen light"
(I don't think that 21W of HID is quite the equivalent of 80+ Watts of incandescent, but let's not dwell on that.)
That has to be an enormous amount of Lumens - I seem to remember it's about 1,800-2,000 Lu. But they state only 3,500 candlepower. If that seems very low indeed for a light of that huge power, it's only because the beam is set at a very wide angle. I suppose that both ratings are accurate, but seeing it as only a 3,500 CP light would be misleading in that case, where a massive 21W of HID is available. http://www.cateye.com/ph_0108HL-STADIUM.htm
The Lu rating alone isn't really enough, but it does have some relevance to real life. Traditionally, CP has been used to rate lights, and it took me ages to realise that it really had no practical meaning to a torch user rather than a flashlight company marketing manager.
Quote from ike, in Topic: Newbie with an HID question... -
"Don't be impressed with Candlepower specs ... they are a marketing guys dream (I'm a marketing guy). They can be manipulated to give enormous numbers to sell flashlights. Candlepower is the lumens output divided by the solid angle of the beam. Thus, 1 lumen of parallel light has infinite candlepower! Go for raw lumens and see if the beam is tight enough for your application... forget CP unless you want to be prey for yet another incredible flashlight promotional claim."
@ http://www.candlepowerforums.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=1&t=000918&p=3
So CP ratings are practically irrelevant.
As for Lu ratings, we recently discovered that they can be exaggerated too. Even when using an Ulbricht sphere (Integrating sphere) to measure output, it's possible to apply very "odd" methods and arcane formulae to calculate Lumen readings. It's also possible to do misleading things like measuring the output of the lamp without a lens in front of it, or without a reflector, (so no losses due to those) and some other disreputable practices which increase the LU measurement.
Thankfully, SureFire don't exaggerate like this - their company policy is to understate specifications. They also take Lu output from the flashlight in "actual use" configurations.
It seems to be generally agreed that to see a big improvement in a beam, you need to double the output. So to see a big improvement from 100 Lu, you would need to increase output to 200 Lu. To improve the 200 Lu rating, you would have to go to 400 Lu. Human perceptions seem to need a light twice as powerful to make the increase much brighter. An increase from 100 to 120 Lu, would still be noticeable (if the beam patterns were exactly the same) but it wouldn't be 20% brighter, just a small increase.
lightlover
Resources.
One of the best resources for light measurement details is the justly famous "International Light Handbook", @ http://www.intl-light.com/handbook/ch07.html
Can be downloaded too, and well worth it. It's quite technical, but also clear on a lot of the issues. The illustrations make things clearer.
SureFire's Candlepower versus Lumens FAQ is @ http://www.surefire.com/cgi-bin/main.pl?pgm=co_disp&func=displ&pgrfnbr=65&sesent=0,0#1
Streamlight have an essay "Light Measurement" is @ http://www.streamlight.com/2001/releases/light_measurement.htm
DonL has a good analogy @ http://66.113.176.74/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=9;t=001380
(Don, can you post your analogy here ?)
Quote re: Pelican Nemo™ 8C submersible:
"14.4 Watts, Tested Lumen value 276.5 - Peak Beam Candlepower 100,000"
Quote re: Laser Devices:
"The 6-volt light is rated at 95 lumens and generates approximately 6,500 candlepower."
[This post subject to editing.]
Before you post to this, please read the original idea in Topic: "Archive/Reference" Topics (Draft Procedural Note). @ http://www.candlepowerforums.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=22&t=000383
