Calculate focused spot size - Gaussian FWHM

[eliasds]

http://www.calctool.org/CALC/phys/optics/f_NA

Does anyone know how they calculate the spot size (Gaussian FWHM) at this website?

For a plane wave, the focused spot size it approx=2*wavelength*f/(pi*d0)
f=focal length of lens
d0=collimated beam diameter before lens

but how do you calculate the spot size of a gaussian?

 

[JeffKoch]

If you're asking how to derive the formulas used on the website, you'll have to look it up and wade through the details on your own - laser optics textbooks often go through it, and I bet it's in Born and Wolfe in one form or another, but generally it's a relatively simple and approximate formula that you just use.

 

[eliasds]

Anyone else have a more detailed answer?