Beam spread, as defined by the Illuminating Engineering Society (IES), describes the angle between the two directions in a plane in which the intensity is equal to a stated percentage of the maximum beam intensity as measured in a plane through the normal beam centerline. Beam spread is a general term. In different application scenarios, it can refer to the beam angle or field angle. The beam angle of a lamp or luminaire refers to the angle at which the half-maximum intensity (50 percent of the maximum intensity) occurs. The field angle is defined as the angle between the two directions for which the intensity is 10 percent of the maximum intensity.
In interior directional lighting applications, the beam angle is used in the same sense as beam spread. In outdoor floodlighting applications, the field angle is used in the same sense as beam spread. In the National Electrical Manufacturers Association (NEMA) luminaire classification system, the beam angle is specified between Type 1 through Type 7, based on the field angle defined as 10% of the maximum intensity.
In interior directional lighting applications, the beam angle is used in the same sense as beam spread. In outdoor floodlighting applications, the field angle is used in the same sense as beam spread. In the National Electrical Manufacturers Association (NEMA) luminaire classification system, the beam angle is specified between Type 1 through Type 7, based on the field angle defined as 10% of the maximum intensity.
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