The Yellow-Ring Measurement of White LED by the Reflective Lightspot Method (RLM)

Abstract

The yellow-ring (YR) is a chromatism phenomenon which is caused by the inhomogeneous phosphor layer of the white-light LED (WLED). The characterized lightspot with YR is that the yellow light appears in the periphery of white lightspot zone. The lightspot image of WLED can be acquired from two approaches; projective lightspot method (PLM) and transmissive lightspot method (TLM). By the PLM system, the lightspot could be projected on the spot screen (wall or others) and its image is acquired by a CCD camera. However, the working distance between camera and sample WLED must be larger (~ 2 m) to obtain the lightspot image without tilt aberration. For the convenience of automatic inspection, the working distance in the TLM system can be modified to 0.3 m. Since the light travels through the spot screen, such as copier paper or acrylic plate, etc., the camera cannot obtain the real lightspot image in the TLM system. In practice, the material of spot screen will absorb and scatter the WLED light, and in the image color would be a bit different between the real lightspot and the acquired lightspot. In order to improve the above disadvantages, the study presents the reflective lightspot method (RLM) that the camera acquires the reflective lightspot image from the 50/50 beam splitter at the same optical axis. The RLM system is not only able to capture the tilt-less lightspot image, but also obtain readily the non-saturated image. The experiment results show the RLM system has the better evaluation of YR index (YRI), and is more suitable for the automatic inspection of WLEDs.