This window helps you calculating the diffraction limited spot size of a focussed Gaussian beam. 

You first define your focussing lens with by entering its pupil diameter and its NA or f-number or focal length. 
The following relations apply:

• NA = refractive_index*pupil_diameter/(2*focal_length) 
• f-number = 1/(2*NA)

You then define the properties of your beam: wavelength and Gaussian widh (by either rim intensity on the above defined lens, FWHM or 1/e^2-diameter). 
Here the following relations apply:

You then get the diffraction limited spot size as FWHM and 1/e^2-diameter. Additionally you get the truncation losses induced by the lens. For beams being small compared to the lense's pupil diameter the spot will have a Gaussian shape and the FWHM and the 1/e^2-diameter have a fixed ration of sqrt(2/ln 2)=1.699. In this pure Gaussian regime the spot size is calculated with standard gaussian beam optics. For large beams, where truncation plays an important role, the spot's shape gets closer and closer to an Airy-disc. Hence the ratio between FWHM and 1/e^2-diameter changes gradually. In this truncated Gaussian regime the diameters are calculated from a numerical fit to more complicated theoretical calculations. 

From the menu "immersion" it is possible to change the index of refraction of the surrounding medium. The currently used index and the corresponding speed of light is always displayed in the status bar at the bottom of the window.