|Citizen LED binning scheme|
White LEDs require strict color control of the chromaticity range, especially in white LEDs for general lighting. Usually, the products need to be ranked and sorted according to the requirements before shipment, as the color always varies even if the products are produced in the same lot.
The chromaticity of a white LED is determined by a combination of many elements including the LED chip, phosphor and encapsulation resin. Complicated process control is required to maintain control of these factors.
Without giving away any details, Citizen says that it has made innovations in its high-precision processing technology that will allow the company to achieve mass production with a chromaticity standard based on 3-step MacAdam ellipses. This is about one-ninth the size of the chromaticity ranges defined by the ANSI C78.377 standard—these are quadrants based on 7-step MacAdam ellipses.
Citizen says that the new binning scheme will be achieved by controlling the deviation from specific chromaticity coordinates and “elaborating” the products. It is not clear exactly what this means. However, Citizen also says the process will enable it to produce white LEDs with low variation in chromaticity while keeping the production cost low by discontinuing the “sorting” process before shipment. (See diagram).
The CL-L103 Series, CL-L251 Series and CL-L233 Series LED packages will all be available using 3-Step MacAdam ellipse binning at color temperatures of 4000K, 3500K, 3000K and 2700K. Samples will be available in April 2010 and mass production will begin in June, 2010. Product demonstrations will be held during Light+Building 2010 (Frankfurt am Main, Germany, April 11-16) and LightFair International 2010 (Las Vegas, NV, May 12-14).
Citizen says that it considers that it is very important to offer quality LED products, in a stable manner, with no variation in color, as well as improving the luminous efficacy.