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What is the minimum photometric distance? Introduction to photometric measurements

Photometry is the science of light measurement, which takes into account how the human eye perceive its brightness. It measures only the visible spectral range, where human eyes are sensitive. Photometric measurements are crucial in quality testing and new product development, starting with single LEDs through modules and lamps up to luminaires used in buildings and public areas.

by Miko Przybyla | GL Optic

Photometric measurements are crucial in quality testing

Photometry is the science of light measurement, which takes into account how the human eye perceive its brightness. It measures only the visible spectral range, where human eyes are sensitive. Gonio-photometers are used for measurements of angular distribution of light in order to fully characterize how the signal is emitted from the light source. LED, OLED and other modern lighting systems require both photometric and colorimetric measurements. Photometric measurements are crucial in quality testing and new product development, starting with single LEDs through modules and lamps up to luminaires used in buildings and public areas.

Minimum photometric distance

GL Optic was one of the first system providers to combine a goniometric system with a spectroradiometer for a full characterization of lighting fixtures including photometric and spectral performance.

The straight forward way to calculate the value of luminous intensity [cd] is using the lux values and distance, therefore most often far-field goniometers are used. Such a measuring system assumes the placement of a measuring device distant from the source at such a distance at which the shape and dimensions of the luminaire are not important and it is measured as a point source.

When using a goniophotometer to collect photometric data of your lamps, i.e. the so-called LID diagrams (Luminous Intensity Distribution) you need to measure your lamp from a specific distance. The appropriate distance is the so-called minimum photometric distance. It is the distance for which the inverse of the square law is true. When setting the minimum photometric distance, it is advisable to follow the recommendations of CIE standards. A rule of thumb suggest to put the test device in a distance of 5, 10 or 15 times the size of the luminaire’s illuminating surface depending on the type of the lamp or luminaire.

Watch this video explaining technical details on how to set the minimum photometric distance. Learn practical recommendations on how to measure LED lamps of different shapes and sizes.

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