In photometry, a beam is the circular area at the base of a light cone where luminous intensity is at least 50% of its maximum value.
The term "beam" describes how light is distributed from a focused source such as a spotlight, floodlight, or other directional luminaire. A standard photometric beam is defined as the region where intensity reaches half of the maximum brightness, with peak intensity concentrated at the center and diminishing toward the edges. This 50% threshold is critical for defining beam angle and intensity distribution in lighting design. While most beams follow this profile, adjustable optics can create irregular or asymmetrical beams that affect coverage and uniformity, making accurate measurement essential for commercial and AV applications.
Beam definitions and measurements are aligned with the Illuminating Engineering Society (IES) and the International Commission on Illumination (CIE), which establish photometric standards and testing methods.
The 50% intensity definition of a beam was standardized in the 20th century as photometric practices advanced. This benchmark enabled engineers and designers to consistently compare lighting products and predict performance in real-world environments.