The evaluation of uncertainty about the shape of aperture on measurement of averaged LED intensity

Averaged LED Intensity was defined in CIE 127, in which two measurement conditions were proposed, both required the area of entrance aperture to be 100 mm2. The CIE 127 LED measurement method is widely accepted. Circular entrance aperture is usually used in commercial photometers, but the photosurface of silicon photodetector is square. However, when LEDs with a narrow beam angle are measured their Averaged LED Intensity, the result may vary due to various aperture shapes. In this paper, a mathematical measurement model is built, then the uncertainty of measurement introduced by shape of aperture is rigorously evaluated by comparing the prediction of model and the results obtained from measurement experiments. It is an effective quantitative analysis to the impact of aperture shapes on accurate measurement of Averaged LED Intensity.