LED array: Where does far-field begin?

Any cluster of light-emitting diodes (LEDs) can be modeled as a directional point source if the far-zone condition is met. A general condition is derived for the distance beyond which the far-zone approximation can be used in measuring or modeling propagation of light from an LED array. A simple equation gives the far-field condition in function of parameters of influence, such as LED radiation pattern, array geometry, and number of LEDs. We calculate the nearzone extension of clusters with LED radiation patterns of practical interest; for example Lambertian-type, batwing, and side emitting. The far-field condition is shown to be considerable shorter for high packaging density LED arrays. Moreover, the far-field dramatically changes in function of the beam divergence of the LED radiation pattern. For example, the near-zone of a square LED array with highly directional LEDs (small half-intensity viewing angle) can extend to more than 70 times the cluster size. This value is far from the classical rule of thumb (5 times the source size).