Talk:Candela

message: I would add a graphic representing a candela. —Preceding unsigned comment added by 81.203.89.187 (talk • contribs)  16:20, 27 February 2004   
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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)
replies: text: Perhaps you want the article Lux?--Srleffler (talk) 23:19, 19 March 2009 (UTC)
thread_title: Older comments

message: 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)
thread_title:  Add the Nit 

message: 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)
replies: text: 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)
thread_title:  Formula 

message: Please don't remove multiples, for consistency they are in all seven base SI units.—The preceding unsigned comment was added by 83.5.62.208 (talk • contribs) 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)
thread_title:  multiples 

message: 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 
.
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)
thread_title:  The Sun 

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

text: 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)

text: I restored the description of candlepower to the article. It had been deleted some months ago.--Srleffler 03:28, 11 February 2007 (UTC)
thread_title: Candlepower

message: "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."
-69.87.200.181 16:17, 24 May 2007 (UTC)
replies: text: 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)
replies: text: 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)
replies: text: 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)
thread_title:  convert footcandles and lux to candelas 

message: 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)
replies: text: 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)
thread_title:  About the Chart 

message: 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)
thread_title:  Planck radiator 

message: 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.
See
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.
http://www.cvrl.org/database/text/lum/ssvl2.htm
Illuminant (talk) 20:16, 13 May 2008 (UTC)
replies: text: 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)
thread_title:  "luminous efficiency function" 

message: 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)
replies: text: How about a mention with link to luminous efficacy? Dicklyon (talk) 23:44, 10 June 2008 (UTC)
replies: text:  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)

text: 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)
replies: text: 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)
replies: text:  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)
replies: text: 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)
thread_title:  Examples (Watts) 

message: 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.
[..]"
http://www.sizes.com/units/hefner.htm
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 195.134.77.189 (talk) 10:33, 24 February 2009 (UTC)
replies: text: 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)
thread_title:  Hefner Unit 

message: 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)
replies: text: 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)
thread_title:  Forumla 

message: 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 (talk • contribs) 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)
thread_title: Candles vs candela

message: 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 (talk • contribs) 21:20, 4 December 2010 (UTC)
replies: text: 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)
thread_title:  Definition

message: 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)
replies: text: 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)
thread_title: Dimensions

message: 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 87.194.171.29 (talk) 14:11, 26 October 2013 (UTC)
replies: text: 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)
thread_title:  Why does an SI unit depend on the human eye? 

message: Fairly sure that is incorrect, and the quoted figure of 18,000cd is too high. 84.92.87.70 (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)
replies: text: 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 89.168.209.236 (talk) 13:14, 30 May 2014 (UTC)
replies: text: 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)
thread_title:  Incorrect example here? 

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thread_title:  External links modified 

message: Part of the article says that ""ultra-bright" LEDs can reach 15,000 mcd (15 cd), or higher."  Possibly this should be updated to include modern high-power LEDS?  — Preceding unsigned comment added by Jazzlw (talk • contribs) 21:39, 29 November 2018 (UTC)
replies: text: The statement was not supported by any reference, so I simply removed it.--Srleffler (talk) 03:48, 30 November 2018 (UTC)
thread_title:  LED brightness 

message: "This made it easy for anyone to measure the standard, as high-purity platinum was widely available and easily prepared."
Surely this is a jest? How could high purity platinum be widely available to anyone, much less be prepared easily compared to previous methods that used wax or some other combustible?  — Preceding unsigned comment added by 68.148.146.18 (talk) 16:33, 19 November 2019 (UTC)
replies: text: It doesn't say that it was easy compared to previous methods. As the paragraph says "It became clear that a better-defined unit was needed." The platinum standard was more accurately repeatable than wax, and the required material was widely available, and easier to prepare than any alternative that would have been as accurate.--Srleffler (talk) 12:35, 20 November 2019 (UTC)
replies: text: Is there a citation for that claim? At what point in history has platinum ever been "widely available" by any reasonable standard? 157.93.254.217 (talk) 17:32, 21 March 2023 (UTC)
replies: text: Platinum is available in any developed country, and probably most others as well. You can order some high-purity platinum online right now if you like. You can even get high-purity platinum wire and sheets on Amazon. Are you confusing "widely available" with "cheap"? They are not the same thing.--Srleffler (talk) 17:14, 25 March 2023 (UTC)
thread_title:  Platinum 

message: I find this:
: when expressed in the unit lm W–1
what is this unit called "W"? And why doesn't it say what it is? Michael Hardy (talk) 22:24, 5 July 2021 (UTC)
replies: text: W is the symbol for watt. Jc3s5h (talk) 22:52, 5 July 2021 (UTC)
replies: text:   Thank you. (I actually realized that just after I posted.) Michael Hardy (talk) 00:19, 6 July 2021 (UTC)
replies: text: Whatt 155.4.128.66 (talk) 21:30, 9 January 2024 (UTC)
thread_title:  What is "W"? 

message: The following Wikimedia Commons file used on this page or its Wikidata item has been nominated for deletion:
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Participate in the deletion discussion at the . —Community Tech bot (talk) 22:38, 7 January 2022 (UTC)
thread_title:  A Commons file used on this page or its Wikidata item has been nominated for deletion 

message: Surely the dimension for Luminous Flux (and all derived quantities) should be Watts (ie Joules per second)? It even says "Luminous energy per unit time" in the notes. 62.3.117.190 (talk) 08:10, 1 August 2022 (UTC)
replies: text: Did you read the [nb 2] at the top of the column in that table?Jc3s5h (talk) 20:15, 2 August 2022 (UTC)

text: Luminous flux can't in general be converted to watts. It's a distinct quantity. If you know the full spectrum of the light you can calculate how many watts there are from the luminous flux at each wavelength, but if all you know is the luminous flux, you don't know how much power there is. Luminous energy is measured in lumen seconds, not Joules.--Srleffler (talk) 17:26, 7 August 2022 (UTC)
thread_title:  Dimension of Luminous Flux