The distance between diametrically opposed points in the cross section of a circular beam where the intensity is reduced by a factor of 1/e (0.368) of the peak level (for safety standards). The value is normally chosen at 1/e2 (0.135) of the peak level for manufacturing specifications.

The distance between diametrically opposed points in that cross section of a beam where the power per unit area is 1/e times that of the peak power per unit area

The diameter of that portion of the beam containing 86% of the output power.

(Machine Safety) The portion of a beam that must be blocked to cause an individual photo receiver to change state. One of the factors in determining resolution of a safety light screen. Also known as effective beam diameter.

The beam diameter of an electromagnetic beam is the diameter along any specified line that is perpendicular to the beam axis and intersects it. For this purpose, the diameter is often defined as the distance between the two diametrically opposite points at which the irradiance is a specified fraction of the beam's peak irradiance. The most common choices for the fraction are ½ and 1/e2.