The _confocal parameter_ tells us the distance the collimated waist region of [light beam](Light-field.md) extends. Thus this is sometimes referred to as the _collimated range._ Where as the [Rayleigh range](Rayleigh%20range.md) is defined at a distance to one end from the beam waist, the confocal parameter is the length associated with the Rayleigh ranges in both directions from the waist. Thus it is twice the [[Rayleigh range]], $z_R.$ It is therefore given as $b=\frac{2\pi n w_0^2}{\lambda}=kw_0^2=2z_R$ ^01d9dd The places the confocal parameter appear in the [Gaussian beam](Gaussian%20beam.md) model is in the denominator of the [Gouy phase](Gaussian%20beam.md#Gouy%20phase) argument as well as in the expression for [beam radius](Gaussian%20beam.md#beam%20radius). # Derivation of the confocal parameter The criterion with which we derive the confocal parameter is determined by the diameter of the aperture.  (Image adapted from Siegman A. _Lasers_) #Electromagnetism/Optics/waveOptics/GaussianBeamOptics