Determination of the Optical Beam Axis, Centre Beam Intensity, and Beam Angle of Directional Light Sources

CIE TN 010:2019
Division 2

Many regional standards and regulations require a manufacturer of directional light sources to publish the beam angle (or beam angles for non-symmetrical sources) and centre beam luminous intensity of their products as well as derived data such as the useful luminous flux or zonal lumen values. However, these data are calculated from the optical beam axis and therefore their determination requires a precise, repeatable and unambiguous method of identifying the optical beam axis of a light source.

This document provides recommended methods for determining the optical beam axis of a directional light source; for performing a coordinate transformation to orient the luminous intensity distribution in the direction of the optical beam axis; and for determination of the centre beam intensity and beam angle(s) from the re-oriented luminous intensity distribution.

The publication is written in English, consists of 12 pages and is freely downloadable from the CIE website.

The following members of TC 2-78 "The Goniophotometry of Lamps and Luminaires" took part in the preparation of this Technical Note. The committee comes under Division 2 "Physical Measurement of Light and Radiation".


  • Bergen, A. (Chair) Australia
  • Blattner, P. Switzerland
  • Klej, A. Netherlands
  • Li, Q. China
  • Menegotto, T. Brazil
  • Reiners, T. Germany
  • Vandermeersch, G. Belgium