goniophotometer

A goniophotometer is an instrument used to measure the angular distribution of light emitted by a luminaire, lamp, or other optical source. It characterizes how luminous intensity varies with direction, yielding a photometric or radiometric distribution I(θ, φ) over polar and azimuth angles. The measured distributions are used to evaluate lighting performance, comply with standards, and inform optical design.

A typical goniophotometer arrangement places the source on a fixed mount or on a rotating goniometer, while a detector moves around it on a calibrated arm. Detectors may be photodiodes, photomultiplier tubes, or spectroradiometers; some configurations incorporate an integrating sphere to obtain absolute measurements of luminous flux. Measurements can be relative (normalized to a reference) or absolute (providing luminous intensity in candelas and total flux in lumens). Modern systems may combine goniophotometry with spectral analysis to capture color and spectral power distribution.

Two main configurations exist: rotating-source and rotating-detector, enabling 2π or full 4π angular coverage. Outputs include angular intensity distributions, polar plots, and tabulated data used to compute beam characteristics, directivity, and directional luminous flux. Advances in software enable rapid data processing and integration with colorimetric data for LEDs and multi-channel luminaires.

Applications span automotive, architectural, street and area lighting, display backlighting, and luminaire design and testing. Standards and guidelines from bodies such as the CIE and IEC provide procedures for photometric measurements, data formats, and calibration procedures. Limitations include measurement duration, angular resolution, stray-light sensitivity, and the need for careful calibration and environmental control.