Luminous Flux Calibration of LEDs at NIST

The luminous flux (unit, lumen) is one of the most important characteristics of Light Emitting Diodes (LEDs) along with the luminous intensity, and is commonly measured using integrating sphere photometers. However, large variations of measurement results are reported among different manufacturers and users of LEDs, which is causing difficulties in trade. CIE 127 [1] gave a good solution to the problems in luminous intensity measurements, but does not give sufficient treatment for the problems in luminous flux measurement. There are issues such as the LED mounting geometry (4, 2), treatment of backward emission, the best integrating sphere geometries, etc., which are being addressed in CIE Technical Committee 2-45. While these issues are to be resolved, there is an urgent need for standard LEDs in order to certify measurement accuracy in industrial laboratories. To address such needs, NIST has developed measurement procedures to calibrate total luminous flux of LEDs using the existing NIST 2.5 m integrating sphere [2]. Figure 1 shows the arrangement of the NIST integrating sphere for LED measurement. This arrangement is for integration over 4p – the normal definition of total luminous flux