The angle of half intensity of an LED is the angle at which the light emitted by an LED is 50% or higher than its maximum intensity. It is also called beam angle. The beam angle determines the size of the light circle appearing on the illuminated object. A full intensity (100%) is achieved only at the center of this light circle and the intensity goes on decreasing as we move away from the center.

Beam angle indicates the angle at which luminous flux i.e., light is emitted from an LED source. The source creates a light cone on the illuminated object and whose diameter depends on the source and the illuminated object. 

Beam angle is calculated using the following equation,

Here ø is the diameter of the object or the surface to be illuminated and d is the distance between the LED source and the object to be illuminated.

The outer area in the light cone is defined by the field angle where the luminous intensity decreases from 50% to 10% of its maximum value.