Talk:Radiance

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Contents 1 Impulse? 1.1 Luminance 2 spectral radiance & flicks 3 Meaning 4 Merger request 5 cosine term

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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:

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)

[edit] Merger request

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)

[edit] 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: .--Srleffler (talk) 23:18, 15 May 2008 (UTC)