What is the directivity?
Assuming the axial luminous intensity on the LED to be 100%, the directivity refers to a ratio of the luminous intensity viewed from the direction tilted by θ against the light source. The angle of exactly 50 % luminous intensity is called a half-power angle θ1/2, expressing 2θ1/2 for both sides. It is used to indicate the extent of spread of light (see the figure below).
