Rayleigh length

The distance to the beam waist, until the beam diameter is increased by a factor of √2.

Unit: mm

M² = Beam quality (laser-dependent)
k = Correction factor, depends on A and D (ideally 1.27; more typically between 1.5 and 2.0)
A = Aperture of the scan system
D = Beam diameter prior to focusing (typically 6 – 70 mm)
λ = Wavelength (typically 193 nm – 10.6 μm)
f = Focal length (typically 30 mm – 2,000 mm)