Talk:Etendue

Wiktionary has an article
Why the removal of transwiki flag? This seems like a textbook definition that belongs in wikitionary. Feco 19:20, 9 Apr 2005 (UTC)
 * From reading the histories, it's apparent that the original author removed the transwiki tag because xe submitted the same article as etendue. I've removed it again for the same reason.  I strongly urge you to (a) use the correct tags (You used the wrong one.) and (b) follow the advice on checking Wiktionary before applying the tags at Things to be moved to Wiktionary. Uncle G 22:17, 2005 May 9 (UTC)

definition
Although I added the commentary on the origin of the term, I have some questions on the subjects. As I know it, "étendue géometrique" in french is not defined exactly as you define it, although it has the same properties (being constant, and a perfect system supposedly keeping it unchanged). The slight difference is that you count the surface of the cross section of the cone of light, we usually count the surface lighted by this cone, instead. The advantage of that definition is that we can define the "étendue géométrique" of a system in three different but equal ways, either from the source point of view (the area of the source times the solid angle the diaphragm of the system subtends as seen from the source), from the system point of view (the area of the diaphragm times the solid angle the source subtends as seen from the diaphragm), or from an 'intermediate' point of view (the area of the source times the area of the diaphragm divided by the square of the distance between the 2). These definitions are for elementary parts though, and have to be summed over both the source and the diaphragm. A basic illustration and the equations can be found there : http://www.bibsciences.org/bibsup/opt-coll/pub/3/pdf/l1t3p15_Ibis.pdf on page 12. The angles used are the angles between the normal of the area considered and the axis. One can also find another illustation in this wikibook : http://fr.wikibooks.org/wiki/Photographie_-_05_-_Notion_d'étendue_géométrique. In any case, an illustration would be, I think, a good idea, to better grasp what étendue is. Palleas 13:53, 16 April 2006 (UTC)
 * Ah, now that I look at this again I understand it. Your definition is certainly correct and agrees with the reference I have here (although the description is so different this was not immediately clear). The definition given in the article here is not really different, just unacceptably vague. I assume you actually mean entrance pupil where you wrote diaphragm. Am I correct?--Srleffler 23:35, 8 August 2006 (UTC)
 * I copied the French definition into the article for now, because it seems pretty clear to me. I will attempt to translate it in a little while.--Srleffler 00:04, 9 August 2006 (UTC)

I've been absent a long while, and couldn't come here. Of course, I meant entrance pupil, and thanks for re-writing the article, it's much clearer now, and I'm flattered you partly used my words :p Palleas 11:02, 17 October 2007 (UTC)

Refractive Index
I added a brief mention of entendue in systems that have a non-constant refractive index. The n^2 rule is very important to know if you're designing concentration systems, particularly Winston concentrators. (I don't know any good reference for this derivation, although I wouldn't be surprised if Winston derived it somewhere. Long ago I derived it for some specific geometries in my thesis (following earlier work by Eli Yablonovich), but in fact it's true for all geometries.  You can derive this easily enough using statistical mechanics, if you keep in mind that the density of states increases with refractive index.) Geoffrey.landis 02:19, 18 September 2006 (UTC)

No increase in etendue ?
I am confused by the statement that the etendue can not be increased, phenomena like aberrations or scattering can cause an increase, I think.

85.100.127.184 (talk) 11:43, 26 October 2008 (UTC)


 * Yes, I think you're right. The étendue never decreases, because the light can never become less "spread out" than it is initially. I changed the article. (Radiance never increases, and radiance is inversely proportional to étendue.)--Srleffler (talk) 23:32, 26 October 2008 (UTC)

French Origin
The french word etendue is rarely in french to mean extent but rather spread, stretch or grow. The term is this case should be to the familiar use of spread, not extent. —Preceding unsigned comment added by 69.139.236.185 (talk) 20:18, 28 February 2009 (UTC)


 * Interesting. I wonder if the usage has changed over the last century or so. I notice that my copy of Larousse's dictionary lists "extent" as the first meaning of étendue, followed by expanse, range, stretch, and scope.--Srleffler (talk) 04:01, 1 March 2009 (UTC)

Lagrange invariant?
This article and Lagrange invariant mention eachother offhand, but don't explain how these terms are connected. They also both mention optical invariant, which just redirects to Lagrange invariant. —Ben FrantzDale (talk) 14:37, 13 December 2011 (UTC)
 * The Lagrange and optical invariants are invariant relationships between the position and direction of a ray in an optical system. Etendue is a property of a bundle of rays (possibly all of the rays in the system). Etendue is conserved in an ideal system because each ray obeys the optical invariant.--Srleffler (talk) 17:53, 13 December 2011 (UTC)

Is this related to surface brightness?
Is this related to the concept of surface brightness, which an optical system cannot improve on? 82.46.253.56 (talk) 15:29, 21 December 2012 (UTC)
 * It sounds like it. Etendue is related to radiance and luminance, which may be what you are thinking of when you say "surface brightness". "Brightness" is kind of an ambiguous term.--Srleffler (talk) 01:03, 22 December 2012 (UTC)

Phase conjugate mirrors
I'm not comfortable with the new section on phase conjugate mirrors. Overarching my specific concerns, is the general concern that this section really needs to cite a reliable source, since what it is expressing is essentially an opinion about how to interpret what happens when a phase conjugate mirror reflects light. One can have a good philosophical argument about whether the etendue decreases after reflection, or whether it never actually increased in the first place. Perhaps one concludes in the end that etendue never increases in any system, since in principle all light paths are reversible. Because the section is advancing an arguable point of view without reference to sources, I am inclined to remove it altogether unless some sources are provided.

Beyond that, a discussion of different interpretations may be required, depending on what one finds in the literature.--Srleffler (talk) 02:49, 12 June 2013 (UTC)


 * Point well taken. I can't find literature about it, so I just deleted it. Sorry. --Steve (talk) 14:03, 12 June 2013 (UTC)

Beam emittance
I'm not 100% sure, but this seems to be closely related to beam emittance? This should probably be commented on and the pages cross-linked. See also my comment on the talk page there. Kyrsjo (talk) 13:36, 11 February 2016 (UTC)

This article is very dense
It feels like whoever wrote this doesn’t understand the strategies to providing simpler models for people to understand the overarching ideas presented; this article dives straight into jargon, a sign it’s writer isn’t deeply familiar with the content. It would be useful for the article to provide a layman description of the ideas presented. 159.196.132.71 (talk) 22:25, 22 November 2022 (UTC)