Talk:Radiance

Impulse?

The following was removed:

The solid angle in radiance means a range of impulses of the particles involved. For small angles this impulse is transverse to a main impulse, but in the same direction as the area. The product is bound by the uncertainty principle. The minimal product is achieved by lasers.

This needs to use more standard terminology. I cannot even imagine what is trying to be conveyed here. PAR 18:01, 3 February 2007 (UTC)

Looking at this again, I'm guessing he meant momentum rather than impulse. (Note that the German word for momentum is impuls.) Viewed that way, there seems to be a valid point here: one can consider the components of the photons' momenta that is perpendicular to the direction of propagation, and find that the width of that distribution is constrained by the uncertainty principle. (Which, in this case, is the same as diffraction.)--Srleffler 04:02, 4 February 2007 (UTC)

I think I am starting to get it, but its still a mess. Is this what he is trying to say? -

For a beam of a given solid angle there is a range of momenta of the photons involved. For small angles, the momentum of each photon may be divided up into a large momentum component in the direction of travel, and small deviations perpendicular to the direction of travel. The product of these deviations times the width of a beam is bound by the uncertainty principle. The minimal product is achieved by a laser beam.

PAR

That's how I'm reading it. The next question is whether the article needs this. By coincidence, I had a discussion about an issue similar to this with a colleague recently. He advanced the position that the uncertainty principle is overused, when dealing with wave phenomena. The behavior of light being described here is perfectly well described by classical wave mechanics, and the uncertainty relation between transverse momentum and beam width comes out of the mathematics of wave propagation (and classical diffraction theory). It adds nothing to attribute it to "The Uncertainty Principle", unless one wants to emphasize the dual wave/particle nature of light by showing how this classical wave phenomenon can be understood in the particle point of view.--Srleffler 21:33, 4 February 2007 (UTC)

I agree. Its a classical wave effect that only becomes equivalent to the uncertainty principle when the theory is quantized and the energy comes in packets of hν. It doesn't need to be in an article on classical radiance. PAR 23:49, 4 February 2007 (UTC)

Luminance

How does this fit in with Luminance? 5:22a 19 Apr 2007 (UTC)

Luminance is how much the human eye responds to the radiation. The radiation may be in the infrared, and have a lot of radiance, because it carries a lot of energy, but it will have zero luminance, because the human eye cannot see it. PAR 06:37, 21 April 2007 (UTC)

This sounds incorrect. Both have to do with visible light. 155.212.242.34 20:37, 4 December 2007 (UTC)

No, PAR is right, although his answer is perhaps not clear enough. Radiance is a physical measure of the radiation, independent of wavelength. It doesn't have to be visible light. Luminance is just radiance "multiplied" by the response of the human eye. As an example, suppose you have three sources that emit the same radiance, but one is monochromatic green light, one is red light, and the third is infrared light. The second source will have lower luminance than the first, because the eye's response to red is less than to green. The third source would have a luminance of zero, because the wavelength is outside the visible spectrum.--Srleffler (talk) 23:54, 4 December 2007 (UTC)

You are right. And "multiplied" needn't be in quotes. It's an inner product. 155.212.242.34 (talk) 21:08, 6 December 2007 (UTC)

"multiplied" would only be correct for discrete wavelengths. The correct term here is "convoluted".84.180.126.52 (talk) 05:50, 6 September 2012 (UTC)

spectral radiance & flicks

Hi: Shouldn't someone add that W/sr/m3 is commonly called a "flick?" Thanks Donicecapade 17:22, 18 June 2007 (UTC)

Meaning

The meaning could be explained better by someone who really understands it. As I understand it, the importance of radiance is how concentrated a light source is and so its ability to damage the retina. For example, an incandescent bulb with clear glass would have higher radiance than a "soft" bulb with the same irradiance in that the "soft" bulb releases its light over a greater spherical angle from the point of view of an observer. Is that right? 155.212.242.34 19:37, 3 December 2007 (UTC)

Many of the photometry and radiometry articles could use a going-over by someone well-versed in the field. There are a lot of subtleties in the use of these quantities. Radiance is relevant for much more than just damage to the retina. As mentioned in the article, radiance is a measure of how bright a distant object will appear to an optical system.

Yes, I believe you are right that the clear bulb would have higher radiance, but not quite for the reason you stated. Both bulbs emit light into 4π sr (minus the solid angle of the stem of the bulb), but the clear bulb's apparent emission area is smaller. Note that the solid angle here is the solid angle at the source, not at the observer.--Srleffler 04:36, 4 December 2007 (UTC)

When you say "solid angle at the source" do you mean the solid angle subtended by the viewer at the source or the other way around? 155.212.242.34 20:34, 4 December 2007 (UTC)

I mean a solid angle at the source, not at the viewer. I'm not as clear on what is subtended. In general, the radiance varies with direction, so for the most general case you have to use the differential form, which always confuses me:

$L={\frac {\mathrm {d} ^{2}\Phi }{\mathrm {d} A\,\mathrm {d} {\Omega }\cos \theta }}$

Note that solid angle does not appear directly—rather, you take a derivative with respect to solid angle. The form of the equation in which solid angle appears directly is a small-solid angle approximation. I think you would use that if you have a source that emits all its light into a limited solid angle (e.g. a laser). Radiance is very confusing, in general.--Srleffler (talk) 00:01, 5 December 2007 (UTC)

What is theta here? I take it that is an angle with respect to the normal of a surface, but what surface? Is it the surface of the radiating body or is it the surface upon which the radiation falls? A picture would be helpful. It says A cos(theta) is the projected area, but that term means diddly squat to the reader. Aflafla1 (talk) 17:48, 16 June 2022 (UTC)

It can be either the surface of the radiating body or the surface upon which the radiation falls, depending on whether you are calculating the radiance emitted by the source or the radiance received. For an ideal lossless optical system, the radiance received will equal the radiance emitted. --Srleffler (talk) 06:04, 18 June 2022 (UTC)

Merger request: Radiometric quantities

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) 14:24, 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)

cosine term

Moved following out of article. --Srleffler (talk) 23:11, 15 May 2008 (UTC)

I think the cosine term should be in the numerator see http://www.optics.arizona.edu/Palmer/rpfaq/rpfaq.htm. —Preceding unsigned comment added by 72.8.87.245 (talk • contribs) 13:28, May 15, 2008

I think you're mistaken. The article you mention gives radiance as equal to "dΦ/dω dA cos(θ)". This is ambiguous, because they have used a solidus to form a fraction involving more than two variables. One can't tell whether the dA and cos(θ) are supposed to be in the numerator or the denominator. I believe the form given in the article here is correct: $L={\frac {\mathrm {d} ^{2}\Phi }{\mathrm {d} A\,\mathrm {d} {\Omega }\cos \theta }}$.--Srleffler (talk) 23:18, 15 May 2008 (UTC)

A cosine term in the denominator would imply infinite radiance for an angle of incidence of 90° from the plane-normal. Is this physical? —Preceding unsigned comment added by Deathmare (talk • contribs) 13:55, 22 May 2009 (UTC)

The radiant flux for physical sources falls off at least as fast with angle as cos(θ). The cosine factor in the denominator reflects the fact that the apparent size of the source goes to zero as your angle of view approaches 90°. If you imagine a flat source, and you look at it at 90° from normal, you don't see the surface at all because it is parallel to your line of sight. The source cannot emit anything in that direction.--Srleffler (talk) 14:36, 22 May 2009 (UTC)

Merge in Intensity (heat transfer)

I have proposed to merge Intensity (heat transfer) into this article, because the former explicitly states that radiance is an alternate name for the same thing, and indeed they appear to be the same physical quantity. Wikipedia articles are organized by topic, not by name. When one thing has two names, both are covered in a single article. This is related to the fact that Wikipedia is an encyclopedia, not a dictionary.--Srleffler (talk) 04:23, 21 October 2008 (UTC)

Intensity and radiance are two different measurements, the difference being obvious because they have different units. blackcloak (talk) 03:52, 27 May 2009 (UTC)

"Intensity" is not a well-defined quantity, once one leaves the confines of a single field of study. Its units depend on the context. From the article linked above, it appears that in the study of heat transfer "intensity" has the same units as radiance, and in fact is a synonym for radiance. --Srleffler (talk) 03:12, 28 May 2009 (UTC)

General comments

There are a number of poorly worded/communicated ideas in this article that lead the reader to an incomplete/erroneous understanding of this key measure of an optical system. These should be fixed. There should be a mention of the term "brightness," a common substitute term. There should be a mention of the limitations nature places on the brightness, measured in an optical system (irradiance does not have this restriction). As light travels further from the source, and through any optical elements, the further along the optical system that the measurements are made, the brightness must always (well, except for some very special cases) fall to successively lower values. blackcloak (talk) 03:52, 27 May 2009 (UTC)

Brightness is mentioned. See the second paragraph of the "description" section. This alternate term should not be emphasized further, as its use is deprecated in general. If one means radiance, one should say radiance, not "brightness".

The following paragraph discusses the fact that radiance never increases.

If you have ideas about how to improve the article, feel free to edit it, or to make more suggestions here. I don't doubt that the article could explain this concept more clearly.--Srleffler (talk) 03:19, 28 May 2009 (UTC)

Obviously I did not read far enough to see the part about brightness. I tend to give up reading when information is not presented in a logical order. I expected to see the term introduced long before it was. Brightness is the commonly used term (at least informally) in general optics, although it may not be in the context of telecon. I do not agree that a subset group (US Federal Glossary of Telecommunication Terms (FS-1037C)) should dictate general term usage. Accordingly the term brightness should be introduced in the first sentence of the article. blackcloak (talk) 04:15, 22 July 2009 (UTC)

It is logically inconsistent to state "..., unless the index of refraction changes." There is no unless. This point is made in the text that follows. blackcloak (talk) 04:19, 22 July 2009 (UTC)

Radiance can increase when going from one medium to another. It is only invariant within a single medium. The "unless" qualification is correct.

It would be good to have some sources that document how the term "brightness" is used, and what bodies prefer to deprecate this term.--Srleffler (talk) 07:09, 23 July 2009 (UTC)

Put arbitrary barriers up if you like. All you have to do is read the first sentence of the Lambertian reflectance wiki article. And "deprecate?" That's a stretch. blackcloak (talk) 07:54, 24 July 2009 (UTC)

Slight Error? The body of the text has "This means that for an ideal optical system in air, the radiance at the output is the same as the input radiance."

I suspect the author intended this for radiation "in a vacuum" rather than "in air."

Would a knowledgeable person please check this and correct it if so, and explain why there is no loss in radiance in air - if not. —Preceding unsigned comment added by 69.22.179.156 (talk) 20:57, 24 February 2011 (UTC)

Not an error, an approximation. The absorption and scattering in air is negligible. The important point is that you cannot make the radiance higher than what you started with. No optical design will give you higher radiance at the output than you started with, assuming the same medium on both sides.--Srleffler (talk) 04:42, 25 February 2011 (UTC)

What is "D" in the formula ? Is this delta ? or distance ? or something else ? It's not in the legend... — Preceding unsigned comment added by 84.25.172.195 (talk) 14:13, 29 December 2011 (UTC)

I don't see a D. Lower-case d is the differential operator. If you're not familiar with calculus, you'll get roughly the right idea by thinking of it as a delta. The quantity "dx" can be thought of as an infinitely small range of x around some specific value x₀.--Srleffler (talk) 22:39, 29 December 2011 (UTC)

Definition of radiance

It's not clear to me what is that square factor doing in the equation of L:

$L={\frac {\mathrm {d} ^{2}\Phi }{\mathrm {d} A\,\mathrm {d} {\Omega }\cos \theta }}\approx {\frac {\Phi }{\Omega A\cos \theta }}$

Shouldn't it be simply:

$L={\frac {\mathrm {d} \Phi }{\mathrm {d} A\,\mathrm {d} {\Omega }\cos \theta }}\approx {\frac {\Phi }{\Omega A\cos \theta }}$

Can someone give a reason for that factor? Gaba p (talk) 14:59, 17 April 2012 (UTC)

It is correct. Nothing is squared. ${\frac {\mathrm {d} ^{2}}{\mathrm {d} x\mathrm {d} y}}$ is the notation for a second-order derivative with respect to x and y.--Srleffler (talk) 16:56, 17 April 2012 (UTC)

If that factor indicates a second derivative I'd say it does not belong there. This is the definition of a physical quantity, not an exact mathematical equation. See the definition of this same parameter here (eq. 4.27) There's no '2' in the energy differential (where dE = dΦ). Cheers. Gaba p (talk) 17:46, 17 April 2012 (UTC)

Ultimately, this is just a notational difference. Using d² makes it clear that the differential elements balance (L is finite). A real physical thing is reflected in this notation, though: d²Φ is the amount of flux that passes through an element of area dA, and is within an element of solid angle dΩ. To get the total flux, you must integrate over both area and solid angle:

$\Phi =\int _{A}\!\int _{\Omega }\mathrm {d} ^{2}\Phi$.

Two integrals requires two differential elements, or one that is second-order.

You may be interested in the article etendue, which treats a related quantity using similar notation, and includes some derivations.

I'm not sure what to make of your objection that "[T]his is the definition of a physical quantity, not an exact mathematical equation". Is acceleration not a physical quantity?

$a={\mathrm {d} ^{2}x \over \mathrm {d} t^{2}}$

Note also that radiance is kind of an abstract physical quantity. You cannot in general measure it directly, but rather infer its value from measurements of radiant flux or irradiance.--Srleffler (talk) 04:42, 19 April 2012 (UTC)

Please note I'm not saying it's incorrect per se, it's just not the way I'd expect it to be written. I agree this is merely a notational difference, but I've provided a reference for my way of writing it. Could you produce something similar please (from a source outside WP that is)? Thanks. Maybe we could add a line, something like "This equation can also be found in the form: (equation with no '2' in the differential)". Cheers. Gaba p (talk) 11:12, 19 April 2012 (UTC)

If I am reading your intention aright, the reference you give seems to be to a text that writes an equivalent of   $\mathrm {d} \Phi \ =\ L_{\nu }\ \cos \theta \ \mathrm {d} \nu \ \mathrm {d} A\ \mathrm {d} \Omega$ ?

As I read your reference, it does not actually write such a term as   $\mathrm {d} \Phi \over \mathrm {cos} \ \theta \ \mathrm {d} \nu \ \mathrm {d} A\ \mathrm {d} \Omega$ ?

Please say if this is what you mean by your reference.Chjoaygame (talk) 12:12, 19 April 2012 (UTC)

Yes, do you see a substantial difference between those two ways of writing that equation? I don't think it matters which side of the equation the differentials are. Gaba p (talk) 17:57, 19 April 2012 (UTC)

The writers of your reference, and others like them, do see a difference. As for quoting a reference, it is best to quote it as it stands rather than as you think it might equivalently stand on a question that seems substantial.

The reason the writers of your reference do it so is that they do not intend the equation to be manipulated as you have manipulated it. They expect the reader to understand that terms such as $\mathrm {d} \Phi$ and $\mathrm {d} A$ and $\mathrm {d} \Omega$ are to be read with a grain of salt.

By this I mean that they intend the reader to understand that the $\mathrm {d}$ symbol is not being used according to a strict notational convention, but is being used rather loosely. The quantities $\Phi$, $A$, and $\Omega$ are not real variables such as are the usual arguments of the operator $\mathrm {d}$. For example, the quantity $\Omega$ is solid angle which in a sense is a two-dimensional object, needing two independent real variables for its specification. One might say then it really needs a $\mathrm {d} ^{2}$ or somesuch. Likewise for $A$.

But the writers of your reference expect their reader to work this out for himself because for them to say it all explicitly would fill their page with hardly relevant verbiage without much benefit for their usual readership. To save chatter about this, the writers that you quote signal their intention by using the form they do, not the fractional form used in the Wikipedia article. I think the writer of the Wikipedia article also expects the reader to work this stuff out for himself, for the same reason as the writers that you quote; he puts the superscript index 2 in the numerator of his fraction to balance the number of $\mathrm {d}$ operators in his denominator, again expecting the reader to add the necessary grain of salt.

Thus I would not go quite so far as did Sreffler in his first response and call it "correct" as if some rigorous mathematics were being considered, nor would I go so far as you propose in the other direction and add a comment about the notation. I think the average Wikipedia reader will do well to go to the salt cellar and salt the present entry for himself.Chjoaygame (talk) 18:29, 19 April 2012 (UTC)

That's good enough for me. Thanks Chjoaygame. Cheers. Gaba p (talk) 12:49, 20 April 2012 (UTC)

Per unit projected source area?

The note accompanying Radiance in the table says "per unit projected source area" (my italics). Granted there is a symmetry between the projecting and projected area, but when considering radiation emitted from an arbitrarily shaped surface into space, with no particular absorber in mind (e.g. the situation described by Planck's law), doesn't it make more sense to use area for the emitter (hence the projecting area) and solid angle dΩ for the divergence of the radiation leaving an infinitesimal area dA? If radiometry is supposed to be neutral about the light field source and sink then "projected area" doesn't seem sufficiently neutral in that regard. --Vaughan Pratt (talk) 10:09, 2 May 2012 (UTC)

"Projected" refers to mathematical projection of the source area (or an element of area) onto the plane perpendicular to the receiver. You seem to be confusing this with the optical meaning of "projected".--Srleffler (talk) 17:05, 2 May 2012 (UTC)

Ah, thanks for clarifying this. Given that "projection" is far more commonly used with its optical than its mathematical meaning, should this clarification be made somehow in the article? --Vaughan Pratt (talk) 17:30, 2 May 2012 (UTC)

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"Orders of magnitude (radiance)" listed at Redirects for discussion

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Split off spectral radiance

At Talk:Specific radiative intensity#Requested move 8 October 2022 I have proposed that we rename that article Spectral radiance and move content on that topic from this article to that one. "Specific radiative intensity" is a synonym for "spectral radiance", and Wikipedia is not a dictionary—we need one article per topic. Srleffler (talk) 22:54, 8 October 2022 (UTC)