These are notional measures of the distance from a light source. A region from where the measured intensity distribution doesn't change with an increase/decrease in the distance from a light source is called as far field. It occurs when the observation plane is much further from the source than 10x the largest transverse dimension within the source. In this regime, sources can be approximated as point sources. Distances shorter than the far field are near field distances where the intensity distribution changes with the distance from the light source.
