This
level-4 vital article is rated C-class on Wikipedia's
content assessment scale. It is of interest to the following WikiProjects: | ||||||||||||||||||
|
A standard 100 watt incandescent light bulb emits approximately 1700 lumens in North America and around 1300 lumens in 220 V areas of the world. See luminous efficacy for the specific efficiency of various types of electric light sources.
At first glance, it makes absolutely no sense as to why a 100 watt bulb would be 1700 lumens in North America and why a 100 watts elsewhere would be any different. Voltage as the only controlled parameter has absolutely no affect on lumens per watt!
Lumens per watt is affected by filament temperature and envelope transmissiveness, but neither of those are controled by geographic location or voltage.
Thanks! -Jesse —Preceding unsigned comment added by 64.146.180.232 ( talk) 18:32, 4 December 2007 (UTC)
This issue about bulb construction is pointless in an article about lumens and in my opinion it should be removed. The original sentence was inserted as reference for a luminous flux from a typical standard bulb. Rather than a US-EU standard bulb competition, it would be more useful adding also halogen and fluorescent lamps data. BTW these standard bulbs burns all at 2700°K and as you can see the us version has higher luminosity than eu, but shorter life. There are no free-lunches or 120v miracles. -- Basilicofresco ( msg) 14:05, 2 January 2009 (UTC)
I moved the 230 V case out of parentheses. The article should not presume that the reader is in a place with a 120 V grid. The 120 V and 230 V cases are of equal importance. I removed the halogen lamp because I don't think we need three examples and because consumer halogen lamps vary a lot more in output than the others. Where I happen to live, the commonly available medium-base halogens are not significantly more efficient than a typical incandescent bulb, but they have long bulb life. The purpose here is to provide common examples, such that a reader can look at a light bulb in their own home, not think too much about the technical details, and get a feel for what a lumen is (or what 1500 lm is). I rephrased the CFL example to make the emitted lumens clearer, and added references for 120 V CFLs.-- Srleffler ( talk) 19:14, 4 January 2009 (UTC)
Differences between lumens and lux: This section has some poor grammar and is quite confusing to the reader. —Preceding
unsigned comment added by
211.30.168.79 (
talk) 12:43, 19 July 2009 (UTC)
What is the purpose behind this strange equivalency string?
1 lm = 1 cd·sr = 1 lx·m2
How is it useful or informative? What is the meaning of "cd x sr" or "lux x m2"? Wouldn't it be more useful to say that the lumen is defined by reference to a source with intensity of 1 cd in a given direction, where 1 cd = 1 lm/sr? An isotropic 1 cd source then emits 4pi lumens... Cluginbuhl ( talk) 02:12, 21 August 2009 (UTC)
The whole detailed discussion about lumen output of 120V vs 230V lamps is off-topic for this page defining and discussing lumen. Take it to a luminous efficacy page, if there is one. Here just give an example like a "typical" candle at about 10 lm, a 100W bulb at about 1500 lm, a streetlight at 20,000-50,000 lm. That's it. Don't get distracted. Cluginbuhl ( talk) 02:20, 21 August 2009 (UTC)
The reason that higher-voltage incandescent lamps have fewer lumens per watts is very simple. This was alluded to in the earlier ″Light output and lifetime″ section of this article. The following is a more explicit description of the difference in lumens versus operating voltage.
The main failure mode is evaporation of the tungsten filament. Evaporation causes the filament to become thinner. The higher-voltage lamp is designed start with a thinner filament than that of a lower-voltage, or it would draw too much wattage, operate at a much higher temperature and have a very short life. Thinning alone is not enough to make a product acceptable because that would make the life too short and also make it fail too easily from a minor mechanical shock. Dmc7 ( talk) 23:35, 18 August 2017 (UTC)
That is, a light source that uniformly radiates one candela in all directions radiates a total of 4π lumens. If the source were partially covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux—only 2π lumens.
I don't believe that this is correct. If the hemisphere is absorbing the light over half the sphere over which it radiates, then the light flux over the remaining half must be the same at 4π lumens. If the hemisphere redirected the light into the system forcing it to combine with the unshielded half, then it would become 8π lumens, but the source would remain a 1 candela source. 20.133.0.13 ( talk) 13:57, 9 September 2009 (UTC)
As the plural in English language is usually adapted from the language the word comes from e.g. radius -> radii, I changed that. Hope you're fine with that, otherwise please discuss here. example of a lighting firm's homepageThanks, Saippuakauppias ⇄ 23:48, 28 October 2009 (UTC)
{{
cite journal}}
: Cite journal requires |journal=
(
help)I reverted you again today, because you still have not provided a reliable source to support your claim that "lumina" is a valid English plural for "lumen", in the context of the unit of luminous flux. Wikipedia's verifiability policy allows uncited and disputed material to be removed by any editor. Such material must not be re-inserted unless a citation to a reliable source is provided.-- Srleffler ( talk) 00:11, 30 October 2009 (UTC)
Plurals aside, would it be appropriate to mention, in the introduction, the etymology of the name, "lumen (the Latin for light)" ? 84.215.6.238 ( talk) 14:46, 9 February 2013 (UTC)
Shouldn't Lumen be expressible in terms of photons per second (of a given wavelength)? Obviously the eye's spectral response and the energy per photon will make this depend on wavelength, but still, an order-of-magnitude number would be useful. According to Luminous efficacy, 100% efficiency would be 683 lm/W, so 683 lm = 1 J/s. Taking a value near the peak spectral response, λ=550 , gives 3.61e−19 J/photon by the Planck relation. Dividing by that gives
or
Again, this is just for 550 nm green light. Does that sound right? —Ben FrantzDale ( talk) 13:01, 13 September 2010 (UTC)
This is frustrating and the article is a terrible article. What is misleading about giving the reader a way to relate wavelength, lumens, and photons per second? Nothing does this, and the article gives volumes of useless information that are supremely confusing. Along the same lines as the question above, could we not write this?
Where E is the energy of a photon and N is the number per second? For heavens sake someone please explain this in the article because it's terrible without it. - Theanphibian ( talk • contribs) 18:18, 28 September 2011 (UTC)
As a reader of this page, I didn't find the thing I was looking for: a table relating incandescent bulb wattage to lumens. —Preceding unsigned comment added by 192.132.94.130 ( talk) 17:10, 11 January 2011 (UTC)
I removed the mostly-redundant kitchen light example, for two reasons: first, the flux given seemed a bit high for an actual fixture. Four 4' T8 fluorescent tubes output about 10 klx. More importantly, putting the same fixture in a larger room does not uniformly reduce the illuminance. Assuming the same ceiling height, the illuminance right under the fixture might be almost the same but the illuminance near the walls of the larger room would be lower. One could think about average illuminance, but that would be pointless. You don't measure the average illuminance across a whole room, nor is that what is important for use. -- Srleffler ( talk) 17:26, 23 June 2011 (UTC)
This is an article about lumen. There should be a related link, or a small paragraph about the comparison to nits too! — Preceding unsigned comment added by 98.254.64.146 ( talk) 14:01, 25 June 2011 (UTC)
This sentence (from the Explanation section) should be put in the introduction:
It immediately and clearly explaines what Lumen is. The introduction of this article is too complex as it is. While an in-depth explanation is needed, I think that the first paragraph should be easier to understand. - GeiwTeol 22:58, 15 July 2011 (UTC)
I came here looking for a section about the history/origin of the unit. -- oKtosiTe talk 09:29, 27 September 2011 (UTC)
Can we work some of this in under Lighting?
Thanks -- Jo3sampl ( talk) 04:49, 8 November 2011 (UTC)
There's a nice table at http://www.energy.ca.gov/lightbulbs/lightbulb_faqs.html. -- Jo3sampl ( talk) 13:19, 8 November 2011 (UTC)
This is a dead link Try this one https://www.rapidtables.com/calc/light/lumen-to-watt-calculator.html Jokem ( talk) 01:02, 30 October 2021 (UTC)
The source 3 "OSRAM DULUX energisparepærer" (pdf). Osram.dk. Retrieved May 25, 2013. is now a 404 error. Napishtim ( talk) 09:52, 22 July 2014 (UTC)
I have one LED lamp rated 420 lm at 8 W energy consumption and another one rated 806 lm and 9,5 W. So the second lamp seems to be much more efficient. But when you look a little closer, the second lamp only lights up 160 degrees while the first one has an emitting angle of 330 degrees (which seems also much closer to the characteristics of a standard incandescent light bulb). Wouldn't it be more appropriate to rate those lamps according to the total amount of energy they emit as visible light? BerlinSight ( talk) 22:52, 30 September 2014 (UTC)
Hello fellow Wikipedians,
I have just added archive links to one external link on
Lumen (unit). Please take a moment to review
my edit. If necessary, add {{
cbignore}}
after the link to keep me from modifying it. Alternatively, you can add {{
nobots|deny=InternetArchiveBot}}
to keep me off the page altogether. I made the following changes:
When you have finished reviewing my changes, please set the checked parameter below to true to let others know.
An editor has reviewed this edit and fixed any errors that were found.
Cheers.— cyberbot II Talk to my owner:Online 19:17, 11 February 2016 (UTC)
The comment(s) below were originally left at Talk:Lumen (unit)/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.
There appears to be little in the way of science on this page. Crucial things such as conversions and uses need to be here, as currently there is too much in the way of computer related buzz words. Either the two should be more separate, of the computer term 'Lumen' be defined more obviously.—Preceding unsigned comment added by Captainreuben ( talk • contribs) 06:19, 16 January 2007 |
Last edited at 19:24, 4 March 2009 (UTC). Substituted at 22:36, 29 April 2016 (UTC)
Hello fellow Wikipedians,
I have just modified 2 external links on Lumen (unit). Please take a moment to review my edit. If you have any questions, or need the bot to ignore the links, or the page altogether, please visit this simple FaQ for additional information. I made the following changes:
When you have finished reviewing my changes, you may follow the instructions on the template below to fix any issues with the URLs.
An editor has reviewed this edit and fixed any errors that were found.
Cheers.— InternetArchiveBot ( Report bug) 20:24, 27 May 2017 (UTC)
I get that the SI base unit is the candela, defined as: "The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian." But why perpetuate that, when, unless I made a mistake and not encumbered by a lot of experience in practical applications, the lumen can equivalently and far simpler be defined as: "The lumen is the luminous flux of a source that emits monochromatic radiation of frequency 540×10^12 hertz and that has a radiant flux of 1/683 watt." From there the candela can be defined as lumen/sr and the lux as lumen/m^2, instead of one's mind having to trampoline from candela, over lumen (by integrating over a sphere), and only then to lux. I don't think that a putative accident of history should bind us forever to have the issue presented to us along the same roundabout course, only making us wonder whether there is any logic to such madness, if the alternative is so much clearer, and it seems to me that the current direction of the SI derivation deserves little more consideration than a footnote. — RFST ( talk) 09:27, 7 February 2018 (UTC)
There isn't really an option here. The term "candela" has no meaning outside the definition given to it by the SI. The official definition defines what a candela is, and the definitions of other units follow from that. Presenting some other "definition" of the units would simply be incorrect. "Mathematical equivalence" is irrelevant, because this is not a math problem. Units are about physical measurement of the real world, and in the real world it very much does matter how you measure things. The purpose of standardized units is to enable measurements performed in widely-separated places to be compared, using units that are known to be the same everywhere. This is done by a chain of calibration that stretches from the equipment used to perform the measurements back to an actual implementation of the base unit measurement. This is fundamental to what standardized units are for.-- Srleffler ( talk) 05:19, 13 February 2018 (UTC)
Mathematical equivalence is irrelevant?! I give up... — RFST ( talk) 11:47, 13 February 2018 (UTC)
Good news: lumen is no longer defined in terms of candela. Why? I will explain below, but first let me explain why it used to be like this until 19 May 2019.
Until 1979, photometric units were defined in terms of "standard candles" (
Black Body,
Candlepower,
Hefner lamp). For calibration,
luminous intensities were compared. That's why candela had been declared as base unit in 1954. With the new definition as of 1979, the photometric units got based on luminous flux, i.e. power weighted by a photobiological factor. "Lumen" became a special expression for "
Watt" weighted by luminous efficacy. (Like the
sievert, which is J/kg times a biological factor). When this change was made, everybody agreed that lumen was now more "basic" than candela. There was also serious doubt whether there could be any base unit at all for photometry. (For details see
[4], the minutes of the
CIPM meeting of 1977, 66th session, especially pages 14 and 143 - all of this in French). The CCPR (Consultative Committee for Photometry and Radiometry) proposed candela to be replaced by lumen as base unit. However, this proposal was rejected, because people were afraid that any change in the list of base units would undermine the acceptance of SI and endanger the reform of photometric units planned for (and done in) 1979.
Now let's come my statement above that today lumen is no longer defined in terms of candela. The change came with the
2019 redefinition of the SI base units. In fact this was not just a redefinition of the base units but of the system itself. Previously, there had been 7 base units (including candela), each one them having its own, separate definition: "Base unit X is ...". As of 20 May 2019, there have been seven new definitions - but definitions of constants rather than base units. The seven fundamental definitions read: "Constant X has the value Y when expressed in SI units." The SI brochure clearly states (Chapter 2.3): "[...] this distinction [base units vs. derived units] is, in principle, not needed, since all units, base as well as derived units, may be constructed directly from the defining constants. Nevertheless, the concept of base and derived units is maintained because it is useful and historically well established [...]". So, the lumen is now defined directly as Watt × luminous efficacy. The intro of this acticle should be update accordingly. (by a native speaker, i.e. not by myself) --
Wassermaus (
talk) 13:13, 1 September 2019 (UTC)
There is no mention in the article of the ISO 21118 standard for a lumen - only the ANSI standard. There is confusion about the difference in methodology and in comparing the units. Gpurinton ( talk) 07:52, 22 July 2021 (UTC) Gpurinton
The article lists the lumen, expressed in base units, as lm = cd sr. But it seems to me it should either simply be lm = cd or lm = cd m^2/m^2. Certainly the steradian is not a base unit, it's a dimensionless quantity derived by dividing 1 square metre by 1 square metre, though dividing the square of any arbitrary unit of length by itself would give exactly the same quantity of solid angle. Ava Eva Thornton ( talk) 17:28, 29 August 2022 (UTC)
If a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian, the total luminous flux emitted into that angle is one lumen. The solid area is an essential multiplier; a candela over an entire sphere is an emission of 4π lumens. NebY ( talk) 17:23, 6 September 2022 (UTC)
This
level-4 vital article is rated C-class on Wikipedia's
content assessment scale. It is of interest to the following WikiProjects: | ||||||||||||||||||
|
A standard 100 watt incandescent light bulb emits approximately 1700 lumens in North America and around 1300 lumens in 220 V areas of the world. See luminous efficacy for the specific efficiency of various types of electric light sources.
At first glance, it makes absolutely no sense as to why a 100 watt bulb would be 1700 lumens in North America and why a 100 watts elsewhere would be any different. Voltage as the only controlled parameter has absolutely no affect on lumens per watt!
Lumens per watt is affected by filament temperature and envelope transmissiveness, but neither of those are controled by geographic location or voltage.
Thanks! -Jesse —Preceding unsigned comment added by 64.146.180.232 ( talk) 18:32, 4 December 2007 (UTC)
This issue about bulb construction is pointless in an article about lumens and in my opinion it should be removed. The original sentence was inserted as reference for a luminous flux from a typical standard bulb. Rather than a US-EU standard bulb competition, it would be more useful adding also halogen and fluorescent lamps data. BTW these standard bulbs burns all at 2700°K and as you can see the us version has higher luminosity than eu, but shorter life. There are no free-lunches or 120v miracles. -- Basilicofresco ( msg) 14:05, 2 January 2009 (UTC)
I moved the 230 V case out of parentheses. The article should not presume that the reader is in a place with a 120 V grid. The 120 V and 230 V cases are of equal importance. I removed the halogen lamp because I don't think we need three examples and because consumer halogen lamps vary a lot more in output than the others. Where I happen to live, the commonly available medium-base halogens are not significantly more efficient than a typical incandescent bulb, but they have long bulb life. The purpose here is to provide common examples, such that a reader can look at a light bulb in their own home, not think too much about the technical details, and get a feel for what a lumen is (or what 1500 lm is). I rephrased the CFL example to make the emitted lumens clearer, and added references for 120 V CFLs.-- Srleffler ( talk) 19:14, 4 January 2009 (UTC)
Differences between lumens and lux: This section has some poor grammar and is quite confusing to the reader. —Preceding
unsigned comment added by
211.30.168.79 (
talk) 12:43, 19 July 2009 (UTC)
What is the purpose behind this strange equivalency string?
1 lm = 1 cd·sr = 1 lx·m2
How is it useful or informative? What is the meaning of "cd x sr" or "lux x m2"? Wouldn't it be more useful to say that the lumen is defined by reference to a source with intensity of 1 cd in a given direction, where 1 cd = 1 lm/sr? An isotropic 1 cd source then emits 4pi lumens... Cluginbuhl ( talk) 02:12, 21 August 2009 (UTC)
The whole detailed discussion about lumen output of 120V vs 230V lamps is off-topic for this page defining and discussing lumen. Take it to a luminous efficacy page, if there is one. Here just give an example like a "typical" candle at about 10 lm, a 100W bulb at about 1500 lm, a streetlight at 20,000-50,000 lm. That's it. Don't get distracted. Cluginbuhl ( talk) 02:20, 21 August 2009 (UTC)
The reason that higher-voltage incandescent lamps have fewer lumens per watts is very simple. This was alluded to in the earlier ″Light output and lifetime″ section of this article. The following is a more explicit description of the difference in lumens versus operating voltage.
The main failure mode is evaporation of the tungsten filament. Evaporation causes the filament to become thinner. The higher-voltage lamp is designed start with a thinner filament than that of a lower-voltage, or it would draw too much wattage, operate at a much higher temperature and have a very short life. Thinning alone is not enough to make a product acceptable because that would make the life too short and also make it fail too easily from a minor mechanical shock. Dmc7 ( talk) 23:35, 18 August 2017 (UTC)
That is, a light source that uniformly radiates one candela in all directions radiates a total of 4π lumens. If the source were partially covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux—only 2π lumens.
I don't believe that this is correct. If the hemisphere is absorbing the light over half the sphere over which it radiates, then the light flux over the remaining half must be the same at 4π lumens. If the hemisphere redirected the light into the system forcing it to combine with the unshielded half, then it would become 8π lumens, but the source would remain a 1 candela source. 20.133.0.13 ( talk) 13:57, 9 September 2009 (UTC)
As the plural in English language is usually adapted from the language the word comes from e.g. radius -> radii, I changed that. Hope you're fine with that, otherwise please discuss here. example of a lighting firm's homepageThanks, Saippuakauppias ⇄ 23:48, 28 October 2009 (UTC)
{{
cite journal}}
: Cite journal requires |journal=
(
help)I reverted you again today, because you still have not provided a reliable source to support your claim that "lumina" is a valid English plural for "lumen", in the context of the unit of luminous flux. Wikipedia's verifiability policy allows uncited and disputed material to be removed by any editor. Such material must not be re-inserted unless a citation to a reliable source is provided.-- Srleffler ( talk) 00:11, 30 October 2009 (UTC)
Plurals aside, would it be appropriate to mention, in the introduction, the etymology of the name, "lumen (the Latin for light)" ? 84.215.6.238 ( talk) 14:46, 9 February 2013 (UTC)
Shouldn't Lumen be expressible in terms of photons per second (of a given wavelength)? Obviously the eye's spectral response and the energy per photon will make this depend on wavelength, but still, an order-of-magnitude number would be useful. According to Luminous efficacy, 100% efficiency would be 683 lm/W, so 683 lm = 1 J/s. Taking a value near the peak spectral response, λ=550 , gives 3.61e−19 J/photon by the Planck relation. Dividing by that gives
or
Again, this is just for 550 nm green light. Does that sound right? —Ben FrantzDale ( talk) 13:01, 13 September 2010 (UTC)
This is frustrating and the article is a terrible article. What is misleading about giving the reader a way to relate wavelength, lumens, and photons per second? Nothing does this, and the article gives volumes of useless information that are supremely confusing. Along the same lines as the question above, could we not write this?
Where E is the energy of a photon and N is the number per second? For heavens sake someone please explain this in the article because it's terrible without it. - Theanphibian ( talk • contribs) 18:18, 28 September 2011 (UTC)
As a reader of this page, I didn't find the thing I was looking for: a table relating incandescent bulb wattage to lumens. —Preceding unsigned comment added by 192.132.94.130 ( talk) 17:10, 11 January 2011 (UTC)
I removed the mostly-redundant kitchen light example, for two reasons: first, the flux given seemed a bit high for an actual fixture. Four 4' T8 fluorescent tubes output about 10 klx. More importantly, putting the same fixture in a larger room does not uniformly reduce the illuminance. Assuming the same ceiling height, the illuminance right under the fixture might be almost the same but the illuminance near the walls of the larger room would be lower. One could think about average illuminance, but that would be pointless. You don't measure the average illuminance across a whole room, nor is that what is important for use. -- Srleffler ( talk) 17:26, 23 June 2011 (UTC)
This is an article about lumen. There should be a related link, or a small paragraph about the comparison to nits too! — Preceding unsigned comment added by 98.254.64.146 ( talk) 14:01, 25 June 2011 (UTC)
This sentence (from the Explanation section) should be put in the introduction:
It immediately and clearly explaines what Lumen is. The introduction of this article is too complex as it is. While an in-depth explanation is needed, I think that the first paragraph should be easier to understand. - GeiwTeol 22:58, 15 July 2011 (UTC)
I came here looking for a section about the history/origin of the unit. -- oKtosiTe talk 09:29, 27 September 2011 (UTC)
Can we work some of this in under Lighting?
Thanks -- Jo3sampl ( talk) 04:49, 8 November 2011 (UTC)
There's a nice table at http://www.energy.ca.gov/lightbulbs/lightbulb_faqs.html. -- Jo3sampl ( talk) 13:19, 8 November 2011 (UTC)
This is a dead link Try this one https://www.rapidtables.com/calc/light/lumen-to-watt-calculator.html Jokem ( talk) 01:02, 30 October 2021 (UTC)
The source 3 "OSRAM DULUX energisparepærer" (pdf). Osram.dk. Retrieved May 25, 2013. is now a 404 error. Napishtim ( talk) 09:52, 22 July 2014 (UTC)
I have one LED lamp rated 420 lm at 8 W energy consumption and another one rated 806 lm and 9,5 W. So the second lamp seems to be much more efficient. But when you look a little closer, the second lamp only lights up 160 degrees while the first one has an emitting angle of 330 degrees (which seems also much closer to the characteristics of a standard incandescent light bulb). Wouldn't it be more appropriate to rate those lamps according to the total amount of energy they emit as visible light? BerlinSight ( talk) 22:52, 30 September 2014 (UTC)
Hello fellow Wikipedians,
I have just added archive links to one external link on
Lumen (unit). Please take a moment to review
my edit. If necessary, add {{
cbignore}}
after the link to keep me from modifying it. Alternatively, you can add {{
nobots|deny=InternetArchiveBot}}
to keep me off the page altogether. I made the following changes:
When you have finished reviewing my changes, please set the checked parameter below to true to let others know.
An editor has reviewed this edit and fixed any errors that were found.
Cheers.— cyberbot II Talk to my owner:Online 19:17, 11 February 2016 (UTC)
The comment(s) below were originally left at Talk:Lumen (unit)/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.
There appears to be little in the way of science on this page. Crucial things such as conversions and uses need to be here, as currently there is too much in the way of computer related buzz words. Either the two should be more separate, of the computer term 'Lumen' be defined more obviously.—Preceding unsigned comment added by Captainreuben ( talk • contribs) 06:19, 16 January 2007 |
Last edited at 19:24, 4 March 2009 (UTC). Substituted at 22:36, 29 April 2016 (UTC)
Hello fellow Wikipedians,
I have just modified 2 external links on Lumen (unit). Please take a moment to review my edit. If you have any questions, or need the bot to ignore the links, or the page altogether, please visit this simple FaQ for additional information. I made the following changes:
When you have finished reviewing my changes, you may follow the instructions on the template below to fix any issues with the URLs.
An editor has reviewed this edit and fixed any errors that were found.
Cheers.— InternetArchiveBot ( Report bug) 20:24, 27 May 2017 (UTC)
I get that the SI base unit is the candela, defined as: "The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian." But why perpetuate that, when, unless I made a mistake and not encumbered by a lot of experience in practical applications, the lumen can equivalently and far simpler be defined as: "The lumen is the luminous flux of a source that emits monochromatic radiation of frequency 540×10^12 hertz and that has a radiant flux of 1/683 watt." From there the candela can be defined as lumen/sr and the lux as lumen/m^2, instead of one's mind having to trampoline from candela, over lumen (by integrating over a sphere), and only then to lux. I don't think that a putative accident of history should bind us forever to have the issue presented to us along the same roundabout course, only making us wonder whether there is any logic to such madness, if the alternative is so much clearer, and it seems to me that the current direction of the SI derivation deserves little more consideration than a footnote. — RFST ( talk) 09:27, 7 February 2018 (UTC)
There isn't really an option here. The term "candela" has no meaning outside the definition given to it by the SI. The official definition defines what a candela is, and the definitions of other units follow from that. Presenting some other "definition" of the units would simply be incorrect. "Mathematical equivalence" is irrelevant, because this is not a math problem. Units are about physical measurement of the real world, and in the real world it very much does matter how you measure things. The purpose of standardized units is to enable measurements performed in widely-separated places to be compared, using units that are known to be the same everywhere. This is done by a chain of calibration that stretches from the equipment used to perform the measurements back to an actual implementation of the base unit measurement. This is fundamental to what standardized units are for.-- Srleffler ( talk) 05:19, 13 February 2018 (UTC)
Mathematical equivalence is irrelevant?! I give up... — RFST ( talk) 11:47, 13 February 2018 (UTC)
Good news: lumen is no longer defined in terms of candela. Why? I will explain below, but first let me explain why it used to be like this until 19 May 2019.
Until 1979, photometric units were defined in terms of "standard candles" (
Black Body,
Candlepower,
Hefner lamp). For calibration,
luminous intensities were compared. That's why candela had been declared as base unit in 1954. With the new definition as of 1979, the photometric units got based on luminous flux, i.e. power weighted by a photobiological factor. "Lumen" became a special expression for "
Watt" weighted by luminous efficacy. (Like the
sievert, which is J/kg times a biological factor). When this change was made, everybody agreed that lumen was now more "basic" than candela. There was also serious doubt whether there could be any base unit at all for photometry. (For details see
[4], the minutes of the
CIPM meeting of 1977, 66th session, especially pages 14 and 143 - all of this in French). The CCPR (Consultative Committee for Photometry and Radiometry) proposed candela to be replaced by lumen as base unit. However, this proposal was rejected, because people were afraid that any change in the list of base units would undermine the acceptance of SI and endanger the reform of photometric units planned for (and done in) 1979.
Now let's come my statement above that today lumen is no longer defined in terms of candela. The change came with the
2019 redefinition of the SI base units. In fact this was not just a redefinition of the base units but of the system itself. Previously, there had been 7 base units (including candela), each one them having its own, separate definition: "Base unit X is ...". As of 20 May 2019, there have been seven new definitions - but definitions of constants rather than base units. The seven fundamental definitions read: "Constant X has the value Y when expressed in SI units." The SI brochure clearly states (Chapter 2.3): "[...] this distinction [base units vs. derived units] is, in principle, not needed, since all units, base as well as derived units, may be constructed directly from the defining constants. Nevertheless, the concept of base and derived units is maintained because it is useful and historically well established [...]". So, the lumen is now defined directly as Watt × luminous efficacy. The intro of this acticle should be update accordingly. (by a native speaker, i.e. not by myself) --
Wassermaus (
talk) 13:13, 1 September 2019 (UTC)
There is no mention in the article of the ISO 21118 standard for a lumen - only the ANSI standard. There is confusion about the difference in methodology and in comparing the units. Gpurinton ( talk) 07:52, 22 July 2021 (UTC) Gpurinton
The article lists the lumen, expressed in base units, as lm = cd sr. But it seems to me it should either simply be lm = cd or lm = cd m^2/m^2. Certainly the steradian is not a base unit, it's a dimensionless quantity derived by dividing 1 square metre by 1 square metre, though dividing the square of any arbitrary unit of length by itself would give exactly the same quantity of solid angle. Ava Eva Thornton ( talk) 17:28, 29 August 2022 (UTC)
If a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian, the total luminous flux emitted into that angle is one lumen. The solid area is an essential multiplier; a candela over an entire sphere is an emission of 4π lumens. NebY ( talk) 17:23, 6 September 2022 (UTC)