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Older comments[edit]

I would add a graphic representing a candela. —Preceding unsigned comment added by (talkcontribs) 16:20, 27 February 2004

Maybe include a comparison of how much a candela is? For instance, I was told by grenade that a flashbang produces a light of one million candela. Now I know by a combination of context and common sense that this is extremely bright, but if I had no context it could just as easily suggest the brightness of, say, a dim light bulb (one million of a unit isn't necessarily much; a million nanometers isn't remarkably big to the eye, if it can even see it). Moreover it doesn't show me how extremely bright it is. It's a million times brighter than...something. Or five hundred thousand times brighter than...something.

--Furrykef 08:37, 18 May 2004 (UTC)

This topic is simple enough to be easily understandable, however it is so poorly organized, useful information is there if you care to follow the almost circular reasoning. Profoundly weak information about

  • natural sunlight,
  • distance from light source and effects,

I am looking for a comparison of sunlight vs artificial lights. A greenhouse's natural lighting needs supplimental artifical lighting. That would be in lux, dependendent on distance from source, it may be here , but not very clear. Typical W-pedia nonesense. —Preceding unsigned comment added by (talk) 17:16, 19 March 2009 (UTC)

Perhaps you want the article Lux?--Srleffler (talk) 23:19, 19 March 2009 (UTC)

Fundamental unit[edit]

Could someone please clarify: Is the Candela a fundamental unit or not?
What I find is the following:

  • SI claims candela to be a SI base unit
  • Fundamental unit claims candela to be a SI base unit and a fundamental unit
  • Candela however, claims that it is not a fundamental unit — anymore; but that it is a SI base unit.

\Mikez 07:35, 6 Aug 2004 (UTC)

The page Fundamental unit was wrong, as was the page SI base unit. I have corrected both. This page is correct. The candela is by definition a "base unit", but is no longer a fundamental unit.--Srleffler 23:54, 7 February 2006 (UTC)
I'm confused too. The article says it's not fundamental because the definition contains the watt. But it says it used to be fundamental when its definition contained the square centimetre. I fail to see the distinction. Indefatigable 22:31, 12 Nov 2004 (UTC).
It is still defined as a base unit. Read: [1]. My guess is that it is based on, but not proportional to, the watt. -- Pgabolde 15:53, 7 February 2006 (UTC)
It is a base unit but no longer a fundamental unit, because it is entirely definable in terms of other SI base units. Previously, to determine what a candela was, you had to observe the brightness of a specified amount of platinum, heated to its melting point. Every possible SI measurement of luminous intensity would be made by comparing the brightness of something to the brightness of melting platinum, or comparing to something else that had been compared to heated platinum, or comparing to something that had been compared to something that had been compared to heated platinum, etc. True measurement is always a chain of measurements back to some defined standard.
Under the new definition, the chain of measurements ultimately goes back to the definitions of kg, m, and s. In principle, if you had available to you means of comparing masses, lengths, and times to the SI standards, you could construct an experiment in which you could make true candela measurements with no other references.
The candela was kept as a "base unit" primarily for historical reasons. I guess nobody wanted to eliminate a "base unit". It seems to me that one ought to also be able to eliminate one of {mole, second, ampere} as well, but that would be similarly unpalatable.--Srleffler 23:34, 7 February 2006 (UTC)
You are not wright. It is defined on other SI units (Just like ampere or some other units are defined), but to calculate it you must know eye sensitivity at some particular waveleight, and it is not included in SI. Stijak 03:41, 14 March 2006 (UTC)
You don't need to know the sensitivity of an actual human eye. That would be meaningless. The sensitivity of the eye as a function of wavelength differs from person to person. The candela is based on a defined response curve, that attempts to represent the average response of people's eyes. This is a static mathematical function. It's completely possible to obtain a fully-calibrated luminous intensity measurement using only calibrated measures of power, area, and wavelength or frequency. The candela is therefore, not a fundamental unit, although it remains a "base unit" by definition.
This is not the same as the ampere, which is defined in terms of a specific physical measurement, not in terms of a general combination of other units and simple mathematics. The BIPM acknowledges this difference in status for the candela in their description of it: [2]--Srleffler 04:01, 14 March 2006 (UTC)
The fact that we use sensitivity of defined response curve(luminosity function), doesn't change anything. The only thing I don't understand is why luminosity function wasn't included in the definition od Candela. Thinking in your logic, it seams to me that mole for instance wouldn't be fundamental unit, since it can be calculated knowing the number of molecules, using simple mathematic and knowing Avogadro number(in case of candela we have to know ). Stijak 04:47, 14 March 2006 (UTC)

It is base unit, because it is connected with waveleight, and sensitivity of human eye at that waveleight. Corect formula is where V(l) is the relative sensitivity of the eye at wavelength l. Values of V(l), defined by the International Commission on Illumination (CIE)[3], are available online from the Color and Vision Research Laboratories of the University of California [4] at San Diego and the University of Tübingen, Germany. read [5] Stijak 18:59, 13 March 2006 (UTC)

I think you mean [DELETED]. --Srleffler 23:36, 13 March 2006 (UTC)
I deleted the formula I had put here, because it was confusing and perhaps incorrect. A better statement would be that a 1 cd source of wavelength must have a radiant intensity given by --Srleffler 17:09, 14 March 2006 (UTC)
Yes, thank you. I tried to edit some other pages contaning this misfacts about Candela. Stijak 03:32, 14 March 2006 (UTC)
I have reversed your edits. The candela is a base unit by definition, but is in fact completely defined in terms of other units and an arbitrary mathematical curve (the luminosity function). One can construct an experiment to measure a light source in candelas purely using instruments calibrated to other SI base units, with no reference to a standard SI luminous intensity reference. This makes the candela definitely not a fundamental unit.--Srleffler 03:50, 14 March 2006 (UTC)

Add the Nit[edit]

The chart should add the term "Nit" to the "Candela/m²" entry, this term is used extensively in industry.

Karl Lang

Nits are not an SI unit, so they do not belong in that chart. They are mentioned in the article on luminance, though, and they have their own article too.--Srleffler 22:58, 30 January 2006 (UTC)


Can somebody check the formula? I have the impression it contradicts the one in luminosity function. Shouldn't the luminous intensity be proportional to y(λ), not inversely proportional? Thanks. -- Pgabolde 14:55, 14 March 2006 (UTC)

Yes, it was wrong. I took the formula from User:Stijak's formula above without thinking it through carefully.--Srleffler 17:03, 14 March 2006 (UTC)

My contribution to the correction of the formula is only a question. I am really looking for help. I need a quick ( perhaps approximate ) but fundamentally correct method to convert Lumen to Candelas.( when beam angle is known )and back to lumen when Candelas and Beam Angle are known. The conversion table does not seem to be doing the trick for me. Here is why. I took a PAR38 90 watt 30 degrees lamp and measured the lumen using my integrated lumen sphere. Testing protocol IESNA LM20. My finding was 1270 lumen. According to the conversion "factor table" referenced here should I expect to have 5930 candelas ? ( 1270 lumen times 4.67 ) My bench gonio only reads this lamp at about 2400 candelas Can any one please explain why the conversion table is not working for me? PS All my instruments are calibrated and traceable to NIST —Preceding unsigned comment added by The old Athenian (talkcontribs) 18:53, 5 December 2008 (UTC)


Please don't remove multiples, for consistency they are in all seven base SI units.—The preceding unsigned comment was added by (talkcontribs) 10:25, September 6, 2006 .

This has got to be the longest-running slow edit war on Wikipedia. At least the table of prefixes is smaller and less obtrusive than the complete list of multiples.--Srleffler 00:22, 7 September 2006 (UTC)

The Sun[edit]

I moved the following here from the article, because I dispute it as both factually incorrect and Original Research:

The Sun shines with about 9000 Ycd [6] <!--- 1.5 Gcd/m² average maximum, times phtospheric surface of 6E18 m²; 3000 Ycd in the astronomical V band --->.

Urhixidur seems to have found a value for the Sun's luminance in the literature, and has tried to convert it to luminous intensity by multiplying by the Sun's surface area. This is incorrect. While it has the correct dimensions, you can't get a luminous intensity from a luminance in this manner. I'm also confused by the comment that the sun "shines with" 9000 Ycd, but puts out "3000 Ycd in the astronomical V band". Candelas are explicitly a measure of luminous intensity, evaluated with respect to the CIE standard photopic luminosity function. Nothing else can be correctly quoted as a value in candelas. One can measure the Sun's output with any detector response function one chooses, but if the detector response isn't spectrally resolved and doesn't match the CIE curve, the result cannot be expressed in or converted to candelas.

The confusing edits here and at Talk:Sun are a good example of the importance of the policy on Original Research, and indicate that this is a good case in which to apply the policy strictly. You may not derive new quantities from published sources, and include the results in Wikipedia. If you can't find a published value for the luminous intensity of the Sun, then you can't give a value for it here.--Srleffler 23:03, 18 September 2006 (UTC)


The traditional candlepower is different form the candela (about .981 candela), we should say something about this. Chamberlian 01:31, 5 October 2006 (UTC)

Yes, given that candlepower redirects here. The article does already mention that candlepower is a similar but distinct unit, and briefly indicates how it was defined.--Srleffler 19:29, 13 October 2006 (UTC)

The article Automotive_lighting points here with Mean Spherical Candlepower, but that term is not explained here. 02:34, 11 February 2007 (UTC)

I'm not familiar with that unit, but it seems to be the candlepower you would have if the light from the bulb (in lumens) were evenly distributed in all directions. If you take the values given in the article in lumens and divide by 4π steradians, you get the values they give for "mean spherical candlepower". I removed those units from the article, because this is a dumb way to characterize the light output of a lightbulb. --Srleffler 03:15, 11 February 2007 (UTC)
I restored the description of candlepower to the article. It had been deleted some months ago.--Srleffler 03:28, 11 February 2007 (UTC)

convert footcandles and lux to candelas[edit]

"Candelas are equal to the square of the distance multiplied by the number of footcandles. For example, if your meter is ten feet away from the light source and your meter reading is 10 footcandles, the equivalent candelas equals 10 feet squared (e.g. 100) times 10 -- which equals 1,000 candelas.

The conversion of footcandles and lux to candelas is most accurately achieved in the context of a single point source of light measured in the dark. If the light source is diffused, you should take several readings at different angles in order to calculate an "average" candela measurement." [7] - 16:17, 24 May 2007 (UTC)

You can't really "convert" footcandles or lux to candelas. They are measures of different physical quantities. If you define what your source is (e.g. a point source), then you can calculate the value of one from a measurement of the other. If the source is not exactly as described, you may or may not get a decent approximation to the correct value, depending on your assumptions.--Srleffler (talk) 23:28, 14 May 2008 (UTC)
I got to this article from bicycle lighting where the link to here was highly unhelpful for figuring which lamp would help avoid a pothole and which would help avoid being run over by a truck. Knowing more about bicycling than light, I fixed the problem there with a rough elementary introduction to the theory as far as it relates to that context. What suprizes me about the present article and its discussion page is, nobody has found a good place in their text for a link to Photometry (optics) which, so far as my dim understanding can see, is where the relations amomg flux, lux, radiance, intensity, brightness and other concepts are explained, or ought to be explained. Jim.henderson (talk) 00:17, 15 May 2008 (UTC)
Thanks for pointing that out. I edited the article to make it clearer and more correct, and tried to put things in less technical language. The full technical treatment is best kept for articles like this one, with a simpler approach in more general-interest articles.--Srleffler (talk) 02:48, 15 May 2008 (UTC)

About the Chart[edit]

Because I am new to this subject, I usually find it difficult to translate what some of these graphs mean. Specifically, the chart on the very top of this page. It goes on extensively about the different elements constituting it, however it leaves out what the value of the y-axis is inside the descriptor box. What I want to know is: What is the simple value of the y-axis? Can somebody please edit the article such that this information is privy throughout it. Dark Observer 21:21, 24 May 2007 (UTC)

It's a dimensionless value, normalized to one at the peak of the curve. The curves represent the relative response of the human eye to light, as a function of wavelength.--Srleffler (talk) 23:31, 14 May 2008 (UTC)

Planck radiator[edit]

I restored the link to Planck radiator, and redirected the latter to Planck's law. This is a more appropriate link than to Planck.--Srleffler 17:22, 23 September 2007 (UTC)

"luminous efficiency function"[edit]

In the chart and opening parragraph it should read "luminous efficiency function" NOT "luminosity function"

In all reference texts I use, CIE and NIST standards this is called the "luminous efficiency function." It portrays a standard human observer's efficiency at converting light of various wavelengths to a signal. I would also argue steadfastly that the entry for "luminosity function" be redirected to "luminous efficiency function" with a mention that this term is an arcane term derived from astronomy.


Wyzecki, G. and Stiles, W.S., 1982. Color Science: Concepts and Methods, Quantitative Data and Formulae

See also the direct link in the current chart text to the stockman, sharp data.

Illuminant (talk) 20:16, 13 May 2008 (UTC)

A quick survey of books finds about 200 each way, and a few with both. I've added an "also known as" in the lead to make the connection. Dicklyon (talk) 21:28, 13 May 2008 (UTC)

Examples (Watts)[edit]

This page should include some examples. In particular, how many candela per watt does a typical incandescent bulb produce. This seems particularly relevant because compact fluorescent bulbs and LED light bulbs describe the light output in incandescent-equivalent watts. —Ben FrantzDale (talk) 21:51, 10 June 2008 (UTC)

How about a mention with link to luminous efficacy? Dicklyon (talk) 23:44, 10 June 2008 (UTC)
My mistake: this request should really be on lumen (unit). With that in mind, the various light-unit pages should have some pictures to make it clear which is which. e.g., a picture of a big an a little light source with the same lumen output and with the same candela output. —Ben FrantzDale (talk) 02:23, 11 June 2008 (UTC)
My mistake. It's lumens, not candela -- a standard incandescent bulb is about 15 lm/W so that's roughly conversion between lumens and watts that CFL makers use. —Ben FrantzDale (talk) 02:23, 11 June 2008 (UTC)
If you found a source that indicates that they use 15 lm/W to rate CFLs, that would be good to add some place. Dicklyon (talk) 03:47, 11 June 2008 (UTC)
Unfortunately, I didn't find any specific references saying that's what's used for CFL; I just took a nice round number in the middle of the pack of 120V incandescents from luminous efficacy. That page does have citations, but not to "this is how they rate CFLs". —Ben FrantzDale (talk) 10:35, 11 June 2008 (UTC)
I doubt that they use a single number, since the efficacy of incandescent bulbs varies with wattage. They design the CFL to have output (in lumens) comparable to one size of standard incandescent bulb, and then report the CFL as a suitable replacement for that size bulb. No "standard efficacy" of incandescents is needed—only typical ranges for how much luminous flux is produced by bulbs in standard sizes (25, 40, 60, 100 W, etc.). --Srleffler (talk) 16:04, 11 June 2008 (UTC)

Which luminosity function?[edit]

There is one thing that strikes me: The SI standard does not say, on which of the many luminosity functions the unit is based. Considering that V (1931) and VM (1978) differ by a factor of 7 (!) at 400 nm, this makes me really wonder. Well, I guess, since the standard is from 1979, they wouldn’t have used the highly inaccurate function from 1931, but I can nowhere find the answer to the question whether the definition of the unit has been updated in 2005, and if so, if the 2° or the 10° version is used and if the corrections of CIE (2008) are applied. These questions can still account for uncertainties of a factor about 2 at some wavelengths. —Quilbert (talk) 04:05, 16 November 2008 (UTC)

Good point. I'm pretty sure the SI specifies the Vλ 1931 photopic luminosity function some place, but I'm not sure where, and our articles don't seem to say. Dicklyon (talk) 04:31, 16 November 2008 (UTC)
This book says the 1931 is basis of the current standards in the United States, but I think that really applied to the SI. It also says that the CIE says the 1978 "would be the preferred function" sometimes. Dicklyon (talk) 04:44, 16 November 2008 (UTC)
I found those links: an appendix to the SI brochure and this one. They give some clues, but still no definite answer …
“The 1979 definition gives no prescription as to how the candela should be realized, which has the great advantage that new techniques to realize the candela can be adopted without changing the definition.”
“the CIE’s responsibility for the standardization of the action spectra of the human eye”
CIE: “… the 75 year old photometric system is still the internationally accepted basis for all visually significant light measurements. It defines one of the base units of the SI system, the candela, and as such it should have a permanent status also in the future.”
Now, the former suggests that the CIE has the authority over any changes to V(λ) as used for the SI unit, and the latter suggests that they still recommend CIE 1931. But that is only a side note and not very reliable. Maybe “photometric system” does not include V(λ) … —Quilbert (talk) 02:49, 17 November 2008 (UTC)
Considering this page, I now tend to be convinced that the candela still uses CIE 1931. —Quilbert (talk) 03:02, 17 November 2008 (UTC)

Hefner Unit[edit]

One of the units previously (late 19th – early 20th centurie) used was the Hefner unit

"A unit of luminous intensity, late 19th – early 20th centuries, equal to the horizontal intensity of the light from a lamp developed by the German engineer Friedrich Franz von Hefner-Alteneck (1845-1904) in 1884. Abbreviation, HK. Mainly used in Germany before 1942. One hefner unit is approximately 0.903 candela. [..]"

Since I'm not sure on how to site the above, could someone please incorporate it to the main page? —Preceding unsigned comment added by (talk) 10:33, 24 February 2009 (UTC)

Thanks, this is a great addition. I inserted a comment about the Hefner lamp into several relevant articles.--Srleffler (talk) 05:50, 25 February 2009 (UTC)


I am usually not a big fan of formulas in articles, but why is there a table and not also a formula for seeing relation between angle°, candela and lumens?Dorit82 (talk) 11:29, 13 December 2009 (UTC)

I don't think there is a good reason; the editor who added that section just happened not to have given the formula. The table was originally much shorter. I think it has become too long. Rather than tabulate so many entries, we should just give the formula and an example.--Srleffler (talk) 05:43, 14 December 2009 (UTC)

Candles vs candela[edit]

It says "A common candle emits light with a luminous intensity of roughly one candela." Then later says the word means "Candle" in Latin and Italian. Can we correct this to show it is EXACTLY one candela? or do our new fangle candles show a better light therefore causing us to change the way we measure the power of one candela???? Wait, that would mean they would still be the same.. —Preceding unsigned comment added by TruXter (talkcontribs) 16:07, 15 March 2010

Candles are not all the same. As the article says, a common candle emits roughly (i.e. approximately) one candela. Some emit a little more, some a little less. A candela is a specific, well-defined amount of luminous intensity.
At one time, scientists used specially-made "standard candles", that produced light with well-defined luminous intensity, for use as a reference when measuring other light sources. One could certainly still make standard candles that produce exactly one candela, but we don't do that anymore since we now have better ways to measure luminous intensity, and better (electric) light sources.--Srleffler (talk) 02:00, 16 March 2010 (UTC)


a source that emits monochromatic radiation of frequency 540 × 1012 hertz

i am no expert here, and i have a question: why is it that the definition is based on monochromatic radiation of freaquency 540 × 1012 hertz rather than 540 x 1012 Hz and the appropriate number of significant figures? what is the relevance of mentioning that it is monochromatic? —Preceding unsigned comment added by Eandronic (talkcontribs) 21:20, 4 December 2010 (UTC)

The light source has to have very narrow spectral width. Specifying the peak wavelength to many significant figures is not the same as specifying that the spectrum must be narrow. You can have a broad peak with a very precisely defined peak wavelength.--Srleffler (talk) 05:06, 5 December 2010 (UTC)


The Table says dimension of candela is Joule, but according to the text it is Watts (the text refers to power emitted by a light source). Also the formula below suggests its Watts. I think Watts is correct. Maybe someone should fix the table. --MArras (talk) 14:41, 8 February 2013 (UTC)

The "J" in the table is not Joules. The table distinguishes between units and dimensions. "Joule" is a unit, not a dimension. "J" is the dimension symbol for the dimension of luminous intensity, for which the SI unit is the candela. Yes, this is confusing.--Srleffler (talk) 18:36, 8 February 2013 (UTC)

Why does an SI unit depend on the human eye?[edit]

Can someone explain why the Candela has what appears to be such a ridiculous definition? Whereas all the other units have sensible numbers, and are defined in terms of physical constants, the Candela isn't; it seems to me unsuited to be a base unit. The luminosity function isn't well defined; it's just an approximate average of the human eye's response. Furthermore, a UV or IR source cannot be measured in candelas. It just doesn't seem like it ought to belong in SI at all, any more than the Cubit (the length of a man's forearm) would be a good SI unit of length. — Preceding unsigned comment added by (talk) 14:11, 26 October 2013 (UTC)

Unlike the cubit for length, in this case the physical quantity being measured itself depends on the properties of human vision. Luminous intensity is entirely a feature of human vision. A UV or IR source can certainly be measured in candelas. The luminous intensity of either is exactly 0 cd. If you want to do measurements that don't depend on the characteristics of human vision, you measure radiant intensity, which is measured in watts per steradian.--Srleffler (talk) 01:27, 27 October 2013 (UTC)

Incorrect example here?[edit]

A 25 W compact fluorescent light bulb puts out around 1700 lumens; if that light is radiated equally in all directions, it will have an intensity of around 135 cd. Focused into a 20° beam, it will have an intensity of around 18 000 cd.

Fairly sure that is incorrect, and the quoted figure of 18,000cd is too high. (talk)

Actually, it might be too low. From Solid angle#Cone, spherical cap, hemisphere, I get that the solid angle of a 20° (full angle) cone is 0.0477 sr, so the beam would have an intensity of 35,600 sr.--Srleffler (talk) 19:32, 27 April 2014 (UTC)
There is no way that a solid angle of 20 degrees is 0.0477 sr - 1 sr is just short of a 30 degree solid angle (talk) 13:14, 30 May 2014 (UTC)
I made an error somehow. I now get that a full angle of 20° gives a solid angle of 0.0955 sr, so the beam's intensity would be 17,800 cd, in agreement with the article. For comparison, I get that a 30° half-angle cone subtends 0.84 sr. A solid angle of 1 sr corresponds to a half-angle slightly larger than 30° (32.8°).--Srleffler (talk) 02:36, 31 May 2014 (UTC)

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