Talk:Beam parameter product

I forget: is BPP defined in terms of diameter or radius? This article asserts diameter, while the same author's contribution at Gaussian beam asserts radius. Anyone know offhand which is correct?

By the way, something to be aware of: for a Gaussian beam, waist size and divergence are radial quantities, defined as the radius at which the beam intensity falls to 1/e² of the peak value. This is different from the statistical measure used to calculate size and divergence of real beams. If the correct constant is chosen (two, I think), the two measures are equal for the special case of a Gaussian beam.--Srleffler 21:44, 19 March 2006 (UTC)

Re-merging M squared article
Re: M squared

As the first edition author of the wikipedia m sqared article, I offer the following opinion on the merge to bpp discussion: M squared is a far more useful and popular laser parameter than is bpp. It should not be blended in as a subtopic on bpp. There is even an iso standard titled after it. Many new scientific papers have been published about m squared. M squared is related to bpp in a simple equation -that does not mean its definition should be confined to bpp. Is there suddenly pressure to limit the number of new independent articles in wikipedia? Of course not! I suggest (instead) the m squared sub-discussion in the bpp article be extracted and merged with the new m squared article. It should never have been put there in the first place. Apparently, I am the second author to initiate a seperate m squared article, as the history shows another attempt was made in 2006 --Kenmenard1983 (talk) 14:14, 8 February 2009 (UTC)
 * The 2006 version was created on the same day, and by the same author as the beam parameter product article. These two parameters are so closely related, that there should be only a single article on them. I'm open to the argument that we should merge Beam parameter product into M squared, on the grounds that the latter term is far more commonly used.--Srleffler (talk) 17:48, 8 February 2009 (UTC)
 * If your argument were valid, than one could also argue the following (for example): the term speed is the ratio of distance to time, so speed should be defined only within a sub-article about distance (since the "parameters are so closely related").--Kenmenard1983 (talk) 23:54, 8 February 2009 (UTC)
 * That isn't an equivalent example. The beam parameter product and M² are essentially the same thing, just normalized differently. They are precisely equivalent measures of beam quality, with the only difference being that M² is normalized such that the best value that can be physically achieved is 1 instead of λ/π. If we kept both articles and perfected both, they would contain exactly the same content, except for that difference in normalization. --Srleffler (talk) 05:27, 9 February 2009 (UTC)

Lagrange invariant
This seems closely related to Lagrange invariant and étendue. Should those be mentioned. —Ben FrantzDale (talk) 03:10, 3 April 2014 (UTC)
 * I see the similarity, but I am not sure that it is actually related. This is a diffractive effect, while Lagrange invariant and étendue are ray-based effects.--Srleffler (talk) 02:55, 7 April 2014 (UTC)