Determination of the Rayleigh range
Beam path of focused beam.
For a Gaussian beam the depth of focus is defined by the Rayleigh range 2⋅zR in which the beam waist diameter Øspot does not increase more than a factor of 1.41.
λ = wavelength in µm
Øspot = beam waist diameter in µm
Spot size: Øspot = 7.1 µm
Wavelength: λ = 780 nm
Rayleigh range: 2zR=20.3 µm