The Yellow Ring Measurement for the Phosphor-converted White LED

Since Nichia Corporation developed YAG-based phosphor successfully, which had provided the best performance of conversion efficiency, the phosphor-converted white LED is already regarded as the major manufacturing process for white light generation. In general, the principle of white light generation is that a blue chip emits yellowish green lights by pass ing through phosphor layer and then yellowish green lights mix part of blue lights to yield white lights. However, the consistency of the blue chip and the homogeneity of the phosphor layer are key factors to affect the quality of white LED. Due to the lim itations of current LED package, bracket or phosphor, the lightspot of a white LED projection has the common features that yellow lights surround the periphery of white lights. This phenomenon is so-called “yellow ring”. The study aims to identify the scope of yellow ring by the RGB proportion of lightspot. In addition, we create an analysis model of yellow ring to evaluate the degree of yellow ring in white light LED. From the experiment result, the yellow ring will appear when blue light decreases over 15%, and disappear in case the intensity of white light drop down to 10∼15%.