Talk:Luminous flux

Untitled
The defintion given on this page is precisely the definition of lumen, the physical unit in which luminous flux is allegedly measured. --Smack 19:54, 4 Jan 2005 (UTC)

The caption which states that "one lumen is defined as the amount of light that falls on a unit spherical area at unit distance from a light source of one candela" is not correct.

The correct physical definition for the lumen is as follows:

"One lumen is the luminous flux produced by a point source with a luminous intensity of one candela over a solid angle of one sterradian".

To state this mathematically: PHI = I x OMEGA

In unit form: [lm] = [cd x sr]

Where PHI is the luminous flux, given in lumen [lm], I is the luminous intensity given in candela [cd] and OMEGA is the solid angle given in sterradian [sr].

The candela is one of the seven fundamental SI units and the sterradian is one of the two supplementary SI units.

The correction, as given above, regardind the definition of the lumen was provided by Jerry N. Reider jnreider@anahuac.mx on June 04, 2005 at 00:32 UTC

It would be good to provide some examples of typical luminous flux of common light sources - at least the luminous flux of a 100W incandecent bulb - to give reader a better understanding of different values of luminous flux. I, personally, am not an expert in bulbs so I would appreciate someone knowledgeble add this information to the article. Thank you!Matveims 01:30, 2 December 2005 (UTC)
 * I added as an example the luminous flux of a 100W light bulb. The page on incandescent light bulbs has more information on the efficiency of different light sources. A table of example values might be useful here too, if someone feels like putting one together.--Srleffler 04:09, 2 December 2005 (UTC)

Removed confusing words
I struck out some words in this passage: One lumen is defined as the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian.

My reason is that if we were to replace "candela" with its definiton, we would have the following passage:


 * One lumen is defined as the luminous flux of light produced by a light source that emits monochromatic radiation of frequency 540�1012 hertz and

that has a radiant intensity in that direction of 1/683 watt per steradian over a solid angle of one steradian.

Implicitly mentioning steradian twice is confusing. --Gerry Ashton (talk) 23:09, 10 March 2008 (UTC)


 * You're mistaken. Not all sources that emit radiation with a luminous intensity of 1 cd emit a total luminous flux of 1 lm. The source described emits 1 cd only in a limited pattern (perhaps a cone), with the total solid angle lit by the source being 1 cd. If instead the source emitted radiation uniformly in all directions (still with a luminous intensity of 1 cd), it would emit a total of 4π lumens.


 * More generally, substituting the definition into the paragraph and expecting the result to make sense was not very reasonable. Language is not mathematics. The correct substitution, if you must, would be:


 * "One lumen is defined as the luminous flux of light produced by a light source that emits monochromatic radiation of frequency 540 hertz into a solid angle of one steradian, and that has a radiant intensity in that solid angle of 1/683 watt per steradian."
 * One could cancel the steradians by saying that a 1 lm source at 540 THz has a radiant flux of 1/683 watts. The more convoluted version is the official definition, however.--Srleffler (talk) 02:36, 11 March 2008 (UTC)

i want to know how to calculate luminous flux,what i need to calculate it — Preceding unsigned comment added by 41.95.9.221 (talk) 07:59, 21 September 2011 (UTC)

J
Luminous flux dimension should be watt not joule. Who wrote this staff has no clue at all of physics. Shame on you. — Preceding unsigned comment added by 86.71.62.229 (talk) 17:19, 25 February 2012 (UTC)


 * If you're talking about the "Dimension" column of the table, you need to read the footnote (#3, I think). "J" in that column does not stand for Joules; J is the recommended symbol for the dimension of luminous intensity.--Srleffler (talk) 05:41, 26 February 2012 (UTC)

The confusion has been caused by the use in the table of the wrong style of letter. In the SI, dimensions must be represented by slanted (Italic) letters and units by upright (Roman) letters. The slanted "J" is, therefore, the symbol for the dimension "luminous intensity" and the upright "J" is the symbol for the SI unit "joule." Many persons, I think, do not realize that this absolutely crucial distinction exists and must be scrupulously followed. (Also easily confused are the slanted "T," that is the symbol for the dimension "thermodynamic temperature," and the upright "T" that is the symbol for "tesla," the SI unit of magnetic flux density; or the slanted "F" that is the symbol for the dimension "force," and the upright "F" that is the symbol for "farad," the SI unit of capacitance.)

The confusion probably is caused by there being three different style conventions for symbols used in the SI: one for dimension, one for quantity, and one for units. Dimension symbols are printed in upright, bold, uppercase, sans-serif letters, one of which is the "J" referred to above, the symbol for the dimension of the SI base quantity luminous intensity. Quantity symbols are printed in slanted (Italic) letters. Unit symbols are printed in upright, not bold letters. So, for example, In the case of the letter "T," the symbol for the DIMENSION of the SI base quantity "time" is the upright, bold, uppercase, sans-serif "T"; that for the base quantity "time" itself is the slanted lowercase "t"; that for the quantity "thermodynamic temperature" is the slanted uppercase "T"; and that for the SI derived unit "tesla" is the upright, not bold, uppercase "T."Wikifan2744 (talk) 16:53, 2 February 2014 (UTC)

Another convention that is part of the SI is that, while the symbol for a unit that is the name of a person always is, or always begins with, an upper case (capital) letter, the name of that unit when spelled out always begins with a lower case letter. So, for instance, the name of the SI unit for energy is the word "joule," beginning with a lower case "j," whereas the SI symbol for the word "joule" is an upper case (capital) "J." Other such names, always beginning with a lower case letter, include kelvin, newton, pascal, gray, sievert, becquerel, hertz, katal, weber, henry, tesla, coulomb, farad, and siemens, as well as the well known watt, volt, ampere, and ohm. The only exception is the word "Celsius" because it (unlike "kelvin") is still being used as an adjective modifying the word "degree" ("degree Celsius"). The word "kelvin," on the other hand, is no longer being used as an adjective but is now itself the noun that is the actual name of the unit of thermodynamic temperature and, therefore, now begins with a lower case "k," the phrase "degree Kelvin" having been purged in the 1960s from the SI, although many persons do not yet know it.Wikifan2744 (talk) 11:43, 5 December 2013 (UTC)

I withdraw the first of my above two paragraphs because it conflates dimension and quantity. The "J" in the Dimension Symbol column is, indeed, different from the "J" that is the symbol for joule, but not in the way I specified. Quantity symbols are those that are printed in slanted (Italic) letters. The symbols for the dimensions of the SI base quantities are printed in bold, upright, uppercase, sans-serif letters, one of which is the "J" referred to above, the symbol for the dimension of the SI base quantity luminous intensity. There are, thus, three, not two, style conventions for symbols that must be scrupulously followed: one for dimension, one for quantity, and one for units. In the case of the letter "T," the symbol for the dimension of the SI base quantity "time" is the bold upright, sans-serif, uppercase "T"; that for the quantity "thermodynamic temperature" is the slanted uppercase "T"; and that for the SI derived unit "tesla" is the upright, not bold, uppercase "T." Wikifan2744 (talk) 20:18, 5 December 2013 (UTC)

Relationship to Luminous Intensity
I would like to add a section on the relationship of Luminous Flux to Luminous Intensity (i.e. Lumens to Candela). The reason is that sometimes Luminous Flux is quoted for light sources and sometimes luminous intensity. Before I did so, I wanted to be sure that I was understanding the difference myself. As I understand it, if you had a 1 Candela bulb and arranged the optics of your lamp so that all of this is focused into a beam of 1 steradian then you would have a 1 lumen lamp. If you were to change the optics of your lamp so that the light is shining into a beam of only 1/2 a steradian then you would have a 2 lumen lamp. Am I understanding this correctly? Op47 (talk) 12:27, 11 October 2013 (UTC)


 * No. The luminous flux (in lumens) is the total amount of light the lamp puts out. The luminous intensity (in candelas) is a measure of how concentrated the light is, in angle. If you have a 1 lumen lamp and set up optics to focus the light evenly into a 1 steradian beam, then you would have a 1 candela light source. If instead you concentrate the light into a 1/2 steradian beam, you would have a 2 candela light source. Either way, the luminous flux emitted by the lamp remains 1 lumen (assuming the optics don't absorb any light).--Srleffler (talk) 13:55, 12 October 2013 (UTC)

The confusion of Op47 and others may be, in part, because in the SI the candela, not the lumen, is the base SI unit. Derived dimensions, I think, are easier to understand when they are ratios rather than products, examples being velocity, acceleration, power, and illuminance. If the lumen (luminous flux) were the base unit, then illuminance would still be lumens per square meter but the candela (luminous intensity) would now be lumens per steradian, rather than the lumen being candelas multiplied by steradians, a much harder concept to visualize. The reason that it's set up with the candela being the base unit probably has to do, I would guess, with the practical laboratory physics of measuring the one or the other.

The above paragraph by Srieffler is far and away the best explanation that I have ever seen of the difference between luminous flux (in lumens) and luminous intensity (in candelas) and of the relationship of the two dimensions as well as of their SI units. I believe it should be made a part of the article itself, if possible, but I would like to see included as well the precise mathematical relationship of the two dimensions and also of their SI units.Wikifan2744 (talk) 09:55, 5 December 2013 (UTC)

I am modifying my two paragraphs above to read as follows:

The confusion of Op47 and others may be, in part, because in the SI luminous intensity (in candelas), not luminous flux (in lumens), is the base SI quantity. Derived quantities, I think, are easier to understand when they are ratios rather than products, examples being velocity, acceleration, power, and illuminance. If luminous flux (in lumens) were the base quantity, then illuminance (in lux) would still be lumens per square meter, but the luminous intensity (in candelas) would now be lumens per steradian, rather than the luminous flux (in lumens) being candelas multiplied by steradians, a much harder concept to visualize. I would guess that the reason that the SI base quantity is luminous intensity rather than luminous flux probably has to do with the practical laboratory physics of measuring the one or the other.

The above paragraph by Srieffler is far and away the best explanation that I have ever seen of the difference between luminous flux (in lumens) and luminous intensity (in candelas) and of the relationship of the two quantities as well as of their SI units. I believe it should be made a part of the article itself, if possible, but I would like to see included as well the precise mathematical relationship of the two quantities and also of their SI units.Wikifan2744 (talk) 19:25, 5 December 2013 (UTC)

A thousand apologies, I intended to do that and got waylaid. I have put the explanation into a template : Template:Relationship of Luminous Flux to Luminous Intensity because there are probably several places that need this explanation. I would prefer it if the template were kept as the layman's explanation and any maths and such like were confined to this article. Op47 (talk) 15:28, 8 December 2013 (UTC)
 * I substituted the template into the articles. Typically a paragraph like this is going to need to be edited to fit the context of each article in which it appears. Simple things like whether luminous flux or luminous intensity is mentioned first, and not repeating things that are already covered earlier in the article make templates not so useful for paragraphs of text.--Srleffler (talk) 18:35, 8 December 2013 (UTC)

Suffix or subscript "v"?
'Standards organizations recommend that photometric quantities be denoted with a suffix "v" (for "visual")'

Should this refer to "subscript" rather than "suffix"? Gwideman (talk) 07:38, 17 February 2016 (UTC)


 * fixed — Wassermaus (talk) 07:09, 3 September 2019 (UTC)

Lumimous? really?
Check the spelling on the picture — Preceding unsigned comment added by 121.243.44.34 (talk) 00:30, 26 April 2016 (UTC)
 * I removed the image from the article, until it is fixed. These images have been discussed at Talk:Lux.--Srleffler (talk) 04:44, 26 April 2016 (UTC)

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