Talk:Luminous flux

From Wikipedia, the free encyclopedia

	Portal This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
???	This article has not yet received a rating on the assessment scale. [FAQ]
???	This article has not yet received an importance rating within physics.
Help with this template Please rate this article, and then leave comments to explain the ratings and/or to identify its strengths and weaknesses.

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)

[edit] 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 540×10¹² 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)