Near Field and Far Field

1. In diffraction optics,

Fresnel number:

F =

R2 λz

(1)

where R is aperture's radius.

Far field:
     • F << 1.
     • It is the Fraunhofer diffraction area.
     • The diffraction pattern remains its shape; only its size gets bigger at farther distance.  

Near field:
     • F ≥ 1.
     • It is the Fresnel diffraction area.
     • The diffraction pattern changes shape as a function of distance.

 There is also an "extreme near field" region where the field observed behind the aperture is simply a geometrical projection of the aperture. Geometrical optics zone is when F >> 1.

2. For laser beam,

Rayleigh range:

z0 =

πW02 λ

=

π w02 λ M2

(2)

where W₀ is 1/e² waist radius and w₀ = W₀/M is the 1/e² waist radius of the embedded fundamental Gaussian beam. (i.e., a Gaussian beam and its embedded Gaussian beam have the same Rayleigh range).

Near field is near the laser beam waist, within its Rayleigh range.
Far field is outside of the Rayleigh range.

References:
[1] http://en.wikipedia.org/wiki/Fresnel_number
[2] 黄婉云，《傅里叶光学教程》，1985年第1版. Page 95.