Angular distribution of the averaged luminous intensity of low power LEDs transfer standards

A goniophotometer to characterize the angular distribution of the averaged luminous intensity of LEDs has been developed at Optics Institute of CSIC. Distributions of for four low-power LEDs transfer standards (red, blue, green and white) for luminous flux and luminous intensity are shown. A proper characterization of the angular luminous intensity distribution allows measurement uncertainties to be better determined. The instrument has got a photometer placed on a ruled optical bench to vary the source-detector distance and a CCD camera to carry out the alignment of the LEDs respect to the centre of geometrical reference system. Measurements were taken at a distance of 316 mm (condition A of the CIE) to different LEDs by varying the polar angle between 0° and 90° with a step of 5° and the azimuth angle between 0° and 180° with a step of around 11°. The red LED is the most Lambertian, whereas the blue, green and white LEDs have a different behavior. Finally, to identify systematic errors, a comparison with the measurements of a similar goniophotometer in Physikalisch-Technische Bundesanstalt (PTB) was carried out.