# Template talk:SI light units

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## SI

the term "SI" is used but not defined in this article, nor does a content search of Wikipedia define this term. Will someone who knows what this means please add it to this article, or place a link here to another area of Wikipedia that defines this? 68.62.20.247 01:19, 18 October 2007 (UTC)

See SI. Dicklyon 03:00, 18 October 2007 (UTC)
SI is short for Le Système international d'unités, or in English the International System of Units. It is what is commonly called the metric system. Note that this is not an article, but rather a template to be used inside articles. Articles that include this template should normally have a link to the article on SI. A content search for "SI" does produce the correct article, but note that searches are case sensitive.--Srleffler 04:33, 18 October 2007 (UTC)

## Circular definition?

The definition for luminous flux and luminous intensity are circular. Isn't there a better way to do it that is not? It would be like cracking open a dictionary and seeing "Redundant: see superfluous" and then "Superfluous: see redundant." You get nowhere. —Preceding unsigned comment added by AZylman (talkcontribs) 08:46, 3 January 2008 (UTC)

No, they're not. But you need to go where they are defined. The base unit is candela, as the table shows, so that one is not defined in terms of any of the others. Dicklyon (talk) 16:00, 3 January 2008 (UTC)
As Dick notes, they aren't circular because they are not definitions. See the individual articles for definitions of these quantities. The relation cd=lm/sr is redundant, as you noted. This redundant equation is there for convenience.--Srleffler (talk) 18:04, 3 January 2008 (UTC)
I thinkk they are. If Candela is the base unit then it isn't defined in terms of other units. I've deleted the defination from Candela. If you want to know the relationship between cd and lm then look at lm.filceolaire (talk) 20:52, 7 June 2009 (UTC)
I reverted you. Yes, the information is redundant. So what? The template provides a collection of useful information. Sometimes it is useful to provide the same information in multiple forms. Redundancy is OK. Storage is cheap.--Srleffler (talk) 04:32, 8 June 2009 (UTC)

Unlike the name of the template, this is actually a collection of "SI photometry units", as per the table's caption. Compare to the table at Light intensity (as current at this time). Should radiometric variables be included, or the title of the template amended? It is inconsistent/incomplete as it stands now. —DIV (138.194.12.32 (talk) 00:15, 18 December 2009 (UTC))

The radiometric quantities are in Template:SI radiometry units. The title of this template is inaccurate, but this isn't really important since the title is only relevant to editors, not readers.--Srleffler (talk) 01:01, 18 December 2009 (UTC)

## Proposal: Add units in photons

I've referred to this table frequently over the past several years; for some reason, I have trouble making the meanings of these quantities stick.

If no one objects, I'd like to add a column listing the unit in terms of photons. I think many people have intuition for a photon as a little packet of light and then can build on what that means for other things. That is, luminous energy is a number of photons, luminous flux is a number of photons per second, luminous intensity is photons per second per steradian, illuminance and luminous emmitance are photons per square meter per second.

Luminance is photons per steradian per square meter, which I still have trouble wrapping my mind around... I'll ask on Talk:Luminance.

For this to be qualitative, it would obviously be in number of 555nm photons.

Thoughts? —Ben FrantzDale (talk) 15:49, 2 December 2010 (UTC)

You cannot correctly express photometric quantities in terms of number of photons. There is no fixed relation between the two. For the very restricted case of monochromatic light one can relate them, but what you propose would be highly misleading if not outright wrong.
Everyone has trouble getting their heads around these quantities, especially luminance. You're not alone. Photometry is a bit subtle. Probably the best way to get an intuitive understanding of them is to compare them to the corresponding radiometric units. --Srleffler (talk) 17:42, 2 December 2010 (UTC)

## Template SI photon units

FWIW, I created a template named Template:SI photon units in analogy to Template:SI radiometry units and SI photometry units aka Template:SI light units. It's still "raw", but perhaps useful to some. --Matthiaspaul (talk) 16:56, 17 August 2011 (UTC)

I'm all for having that template (see my question above) so long as these are actually SI units. Can you provide a reference that these are "real"? —Ben FrantzDale (talk) 13:23, 18 August 2011 (UTC)

## Too wordy

I disagree with this edit. It is sufficient to say that the suffix is v for "visual". "Visual" does not start with a Greek letter nu, so there is no danger of confusion. Adding "small Latin letter v" is unnecessary, overly wordy, and is a good example of overlinking. This is different from Template:SI radiometry units, where it is necessary to specify that the subscript is a nu, and to provide a link to the article for readers who may not be familiar with Greek letters. Readers of the English Wikipedia can be safely assumed to be familiar with the Western alphabet, and with the fact that the English word "visual" does not start with a Greek letter nu.--Srleffler (talk) 23:23, 19 August 2011 (UTC)

Answer and potential solution given here:
--Matthiaspaul (talk) 00:51, 21 August 2011 (UTC)

## Dimension column incorrect?

This table reports "J" (Joules, i.e. energy) as the dimension for luminous flux. Surely this should be power (energy per unit time) instead? Lumen (unit) says that one lumen = 1/683 watt at 555-nm green. See also radiant flux in Template:SI_radiometry_units, which is in watts. This error is repeated all the way down the table, i.e. all the "J"s should be "W"s. Am I missing something here? Reedbeta (talk) 17:27, 4 October 2011 (UTC)

I'm not sure that "J" stands for "Joules" there. Dimensions are usually denoted by describing the physical quantity not the units: M for mass, L for length, T for time, etc. Not sure what "J" stands for in this context.--Srleffler (talk) 03:17, 5 October 2011 (UTC)
Hi! "J" does not stand for the unit joule here at all. This column does not discuss units (joule, candela, lumen, watt, etc.), but dimensions. In the context of dimensions, "J" is the symbol for the dimension of the quantity luminous intensity Iv, measured in units of candela (cd). I was living under the impression that it is defined as such in the SI system (see, for example, German WP article de:Internationales_Einheitensystem or de:Lichtstärke (Photometrie)). Quite interestingly, I cannot find this covered in the English WP right now. --Matthiaspaul (talk) 11:38, 5 October 2011 (UTC)
Very interesting. If "J" really is the accepted symbol for the dimensions of luminous flux/intensity, then the table is correct. However "J" seems a very bizarre symbol to pick for this purpose, so I wonder whether this is correct for English or is perhaps a mis-borrowing from another language? --Reedbeta (talk) 17:38, 5 October 2011 (UTC)
Hm, given that the letter I was already used up for the dimension of the electrical current I, J does not look like a bad choice at all to me, as the letters I and J are often used as alternatives. --Matthiaspaul (talk) 19:08, 5 October 2011 (UTC)
The symbol doesn't seem obvious or well-known enough to use in this table without explanation. I have replaced "J" with "Iv" in the table, as a quick fix to the problem. While probably nonstandard, the latter at least has the merit of already being defined within the table, and eliminates the redundancy of using two different symbols for the same thing in one table.--Srleffler (talk) 02:33, 6 October 2011 (UTC)
I reverted your change and added a footnote, as I found an official source clearly stating that the SI standard recommends the usage of symbol J for the dimension of luminous intensity: [[1]] It's in German language, but the Physikalisch-Technische Bundesanstalt (PTB) is Germany's official organ to publish this kind of information, so this is basically from the horse's mouth. We should be able to find the same information in the actual SI standard. Since it is difficult to implant references into footnotes (WP limitation), I suggest that we incorporate the actual references into the International System of Units article. --Matthiaspaul (talk) 14:11, 6 October 2011 (UTC)
Here is the English and French version of the same document: [[2]] --Matthiaspaul (talk) 14:47, 6 October 2011 (UTC)
Thanks for adding the footnote, and for looking into the source of this notation.--Srleffler (talk) 23:50, 6 October 2011 (UTC)

The table is wrong!!! 1/683 Watt per steradian equals watts, not joules. The steraidian is dimensionless. — Preceding unsigned comment added by 68.84.232.225 (talk) 02:26, 8 April 2013 (UTC)

Hm, where in the table do you see watt per steradian equaling joule? Are you perhaps mixing up Luminous intensity (a photometry quantity in lumen per steradian) and Radiant intensity (a radiometry quantity in watt per steradian) as well as the concepts of units (watt, joule) and dimensions? If this doesn't solve your problem, please be more specific. --Matthiaspaul (talk) 03:06, 8 April 2013 (UTC)
I have to say, Matthias, that I am very tempted to remove the "dimension" column. It's not clear to me who it serves. It makes the template larger and more complicated, and as the discussion above shows it creates a lot of confusion, mostly due to the unfortunate choice of "J" as the symbol for the dimension of luminous intensity.--Srleffler (talk) 00:34, 9 April 2013 (UTC)
Hm, I like your improvement of the footnote, but I don't think removing the column would be a good idea. Let's face it, radiometry and photometry are difficult to understand subjects for many people, no wonder that some will mix up a few terms until they really dig into it. Studying the dimensions can help them alot recognizing the differences and similarities - and much easier than by comparing the units. Therefore, I think, this column is really useful - and important for completeness (this is exactly the kind of information why I would consult this Wikipedia table).
Yes, it is unfortunate that the letter J is used for multiple purpose, but that's how it is standardized and used internationally. Symbols for quantities, units and dimensions do not need to agree at all (in fact, they don't), as they are used in different workspaces. Perhaps the problem was that some readers did not recognize symbols as being associated with either quantities, units or dimensions; I hope, the modified table header will make these dependencies more obvious. Greetings. --Matthiaspaul (talk) 03:41, 9 April 2013 (UTC)
I like the change you made in the header a lot. It's a simple thing, but to my eye it seems to clarify the relationship between the columns. Hopefully these changes will reduce reader confusion about "J".--Srleffler (talk) 04:12, 9 April 2013 (UTC)
Thanks for the feedback! I'm glad that you like it. :-) --Matthiaspaul (talk) 04:34, 9 April 2013 (UTC)