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Mathematical Definition[edit]

This page should give a mathematical definition of irradiance, for instance in terms of the electric field, even if some caveats are required. —Preceding unsigned comment added by (talk) 17:15, 23 January 2009 (UTC)


Twice I have entered a definition for irradiance and twice it has disappeared. Both were in my own words. I would think that if there were something wrong that someone would fix it, not delete it. What am I missing here? David Battle

It was deleted because it was too small. A sentence is often not enough, we like to make a Wikipedia:stub article to make a new page. All you said was "Irradiance is measured as power per unit area.". In which field is irradiance an important measure in? What makes irradiance useful? and so on. By all means, make a stub, we welcome it. Dysprosia 13:50, 27 Mar 2004 (UTC)

Then I will leave it to someone else. I just thought I would save someone the trouble of looking of the definition of the term, assuming that my text might be extended, but I did not realize that the policy was that "nothing is better than somthing". Is the deletion done automatically? David Battle

But you see, we need the pages to be at least sort of usable. Someone who doesn't know about irradiance, hoping to look up what irradiance means, may not be fully satisfied with what's there. If we get a good, minimum definition, whether it be small or not, then the page automatically becomes usable.
By the way, the deletion is done by other administrators here. Dysprosia 14:01, 27 Mar 2004 (UTC)

Might I suggest that you leave a comment if you delete the first article created by a new user? You came very near to alienating me permanently. I see that you have now turned my third attempt into a stub; thank you. David Battle

I am afraid it was not I that deleted your article previously. But then, this is the beauty of the Wiki system, that nearly all actions are reversible. Dysprosia 14:28, 27 Mar 2004 (UTC)

Comparison to intensity[edit]

What is the difference between "irradiance" and "intensity" for light? Both seem to be power per unit area, SI units Watt / m^2. Is one time-averaged, but the other instantaneous? —The preceding unsigned comment was added by (talkcontribs) 05:17, 26 August 2005 (UTC)

No, if you're talking about power per unit area they are the same thing. Unfortunately, in optics (particularly photometry and radiometry) "intensity" has another meaning, which is technically "correct" according to the SI standards. In this standard usage "intensity" is a quantity per unit solid angle. In particular, radiant intensity is the power per unit solid angle. Theoretically, it is wrong to use "intensity" in optics to mean power per unit area, but this usage is still very common, especially among laser physicists. I suspect people who do radiometry would be much more likely to mean power per unit solid angle when they say "intensity". There is also luminous intensity, which is measured in lumens per unit solid angle.--Srleffler 08:29, 5 November 2005 (UTC)


Removed from article:

Unintuitively (someone please explain this), irradience at a point due to two sources of mutually coherent light at the same wavelength and arriving at the point in phase with the same irradience will be quadruple the irradience of a single source [1].

[1] Pedrotti, Frank L. et all. Introduction to Optics; 3rd Edition. 2007 Pearson Education, Inc.

The phenomenon here is coherence. When light is coherent, the wavefronts move together—the oscillations of their electric fields are synchronized. If light from the two coherent sources arrives at one point in phase, then when the electric field of one source is at its peak, so is that of the other. Because of this, the electric fields add up, and never cancel one another. The amplitude of the combination of two waves is thus twice the amplitude of each one alone. Intensity is proportional to the square of the amplitude (or the electric field). Therefore, the intensity is four times that of each wave alone.

In contrast, if the two light sources are not coherent with one another, the electric fields are not synchronized. When one is high the other may be low, and the timing between the two fields' oscillations varies with time. When one is high and the other is low they cancel each other. When both are high or both are low they add up. On average, the amplitude of the electric field is times the amplitude of each alone, so the average intensity is twice the intensity of either source, as you intuitively expect.

Note that you don't get something for nothing. When the fields are coherent and overlap to give four times the intensity in one place, there will always be other places where they are exactly out of phase with one another, and cancel to give an intensity of zero. The total power in the combined beams is exactly equal to the sum of the powers of the individual beams, as you would expect.--Srleffler 12:32, 27 October 2006 (UTC)


The units and indices were inconsistent so I standardised them to the Wm-3 style Unusual Cheese 10:46, 31 August 2007 (UTC)

That's fine. I added dots between the component units: W·m-3, etc. This is clearer, especially for readers who are not as familiar with the SI system.--Srleffler 04:50, 4 September 2007 (UTC)

The rest of the story[edit]

This stub is "fine" up to a point, but maybe now is time to start filling in some details. There is no discussion of the angle of incidence of the EM radiation hitting the surface, there is no discussion of how the striking energy may be varying in time (with careful distinction between optical frequency oscillation and slower amplitude variations such as diurnal variations). There is nothing on how measurements are actually made, or how calculating irradiance is done for simple geometries. How about a reference to cosine-to-the-fourth falloff? And technically, the irradiance does not "drop off in proportion to" (I know this is the colloquial way to say it.), irradiance is inversely proportional to the distance (from the point source to a measurement position) squared (where a measurement plane is perpendicular to a line connecting the point source to the measurement position, and where other picky conditions can be added). And in an example that has spherical geometry, the potential for misunderstanding increases. blackcloak (talk) 06:03, 30 January 2008 (UTC)

Merger request[edit]

It is unlikely that Irradiance page can be expanded beyond two paragraphs, so is the case with other radiometric quantities. I suggest we should merge article related radiometric quantities under one article such as Radiometric quantities with one sub-section for each quantities. This will also make it easy to create equations where notations are shared across several definitions. pruthvi (talk) 14:22, 8 March 2008 (UTC)

SI units and quantitites of measure generally have their own articles. Short articles are not necessarily a problem. The template at the end of the article is intended to help standardize notation.--Srleffler (talk) 14:50, 8 March 2008 (UTC)

Requested move[edit]

The following discussion is an archived discussion of the proposal. Please do not modify it. Subsequent comments should be made in a new section on the talk page. No further edits should be made to this section.

The result of the proposal was no consensus. --BDD (talk) 18:28, 23 October 2013 (UTC)

IrradianceRadiant flux density – The article discusses two related (but different) quantities describing incoming (irradiance) and leaving (radiant emittance) radiation. None of these titles is therefore appropriate to refer to both of them, so a different, more general title is needed. Relisted. BDD (talk) 18:10, 16 October 2013 (UTC)Petr Matas 05:04, 8 October 2013 (UTC)

Oppose. Irradiance is a much more common term than radiant flux density. Irradiance and radiant emittance are not really two different things. It's sensible to cover both together in an article with the most common unambiguous name.--Srleffler (talk) 05:28, 8 October 2013 (UTC)
The above discussion is preserved as an archive of the proposal. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

E for energy[edit]

The article uses an "E" for irradiance. Is that really the convention? It seems silly, since E is always used for energy, and since the irradiance will often be used to calculate some energy.

And if I understand the first section correct, there they use an "I" for irradiance, and even use an E for smth else (which I don't understand). (talk) 14:42, 23 September 2014 (UTC)

Yes, that is one convention, used in photometry where (as the table shows) a variety of different quantities need to be discussed, too many of which would be intuitively represented by "I" or "E". In that convention the symbol for energy is Q. The first section, confusingly, follows a different convention, using I for irradiance and E for electric field. --Srleffler (talk) 01:10, 24 September 2014 (UTC)
I changed the symbols in the first section to match the rest.--Srleffler (talk) 01:25, 24 September 2014 (UTC)