An intensity distribution (intensity as a function of radial distance from the beam center) which is given by the Gaussian function:
where: I0 is the intensity at the beam center
r is the radial distance from the beam center
ω0 is the radius at which the intensity drops to a value of 1/e², or 13.5%, or its peak value.
Since, in mathematical terms, the intensity of a Gaussian beam never drops completely to zero, it is standard to define the beam radius as the distance from the beam center to the 1/e² point.