goniophotometric

Goniophotometric refers to the measurement and analysis of the angular distribution of optical radiation emitted by a light source or arising from a reflecting surface. In practice, goniophotometry characterizes how intensity varies with direction, producing data essential for predicting illumination patterns, brightness, and glare in built environments. The measured quantity is typically luminous intensity I(theta, phi) in candelas, or its derived quantities such as luminous flux distribution.

Measurement setup: A goniophotometer places the source at a fixed position and uses a detector that moves through defined angular coordinates. In rotating-arm systems, the detector traverses a circular arc around the axis; in hemispherical or spherical configurations, two-axis motion covers full solid-angle sampling. Detectors can be calibrated photodiodes, photomultipliers, or spectroradiometers; some systems employ an integrating sphere with a scanning mechanism. The geometry, distance, and angular step size are specified by standards.

Data and formats: The primary output is a set of luminous intensity values I(theta, phi) across angles, from which the luminous flux distribution, candela distribution, and polar plots are derived. For commercial luminaires and LEDs, the data are often stored in IESNA file formats (.ies) or in CIE-compatible spectra. These results support photometric design, glare assessment, and compliance testing.

Standards and applications: Goniophotometric measurements follow guidelines from the CIE and the Illuminating Engineering Society (IES), as well as national standards. They are used in product development, luminaire certification, quality control, and performance documentation for lighting installations.