What Is LED Binning?

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LED binning is a process of sorting LEDs according to specific criteria such as chromaticity, lumen output (luminous flux), and sometimes forward voltage (amount of forward voltage it takes to turn on an LED). Variability in the manufacturing process from wafer production and crystal growth to component packaging will inevitably lead to slight differences in intensity and color of LEDs even within a single production batch. Binning keeps variations of the performance characteristics of a particular group of LEDs within defined acceptance criteria. The allowable range of variation, or tolerance, depends on the application. The tighter the binning, the more uniform each LED in that bin will appear and perform to another. Binning tolerances remain essential to accent lighting applications such as retail display lighting and museum and gallery lighting. Other application scenarios that require light fixtures manufactured to tight tolerances include hospitality spaces, high end residences, and commercial/public facilities where there's a large installation of light fixtures within the field of view.

Among the three binning variables, luminous flux and forward voltage can be measured numerically and thus binning based on the numerical differences of these variables is straightforward. The challenge arises from defining groups of products by similarity in colors. Colorimetry is the science of the human perception of color. Not only the color appearance of light (chromaticity) cannot be numerically measured as with the flux and voltage values, the threshold at which a color difference becomes perceptible can vary between different viewers.

The chromaticity binning mechanisms for LEDs are based the CIE color spaces. The CIE 1931 (x, y) chromaticity diagram provides a methodology for mapping perceived color onto the unit plane of an x, y graph. A pair of chromaticity coordinates corresponds to a unique color point of light. While chromaticity coordinates are used to describe the hue and saturation of light, they lack one important quality: an intuitive system that allows people to easily visualize the color of a light source. The use of correlated color temperature (CCT) overcomes this limitation. CCT characterizes the hue of white light using a nominal designation, such as 2700 K, 3500 K or 5000 K. The nominal designation brings to mind to how “cool” (bluish) or how “warm” (yellowish) nominally white light appears. CCT, however, does not convey both dimensions of chromaticity. The chromaticity point of a light source can be located above or below the black body line (BBL). This means multiple light sources can have the same CCT but have different chromaticity points. To address this problem Duv and Δu'v'values were introduced to measure the distance from BBL in the CIE 1960 and CIE 1976 color spaces, respectively.

CIE-1931-chromaticity-diagram.jpg


The use of MacAdam ellipses as a measure of color differences has been a common practice in the lighting industry. A MacAdam ellipse defines a zone in the CIE 1931 (x, y) color space within which people are able to distinguish color variations within a specific range. The scale of a MacAdam ellipse is defined by standard deviation of color matching (SDCM). MacAdam ellipses are reported with step sizes, e.g. 1-step (1 SDCM), 2-step (2 SDCM), 3-step (3 SDCM), 4-step (4 SDCM), and 7-step (2 SDCM) MacAdam ellipses. The larger the step size, the bigger the tolerance. The human eye cannot distinguish color differences in a set of LEDs of which the chromaticity coordinates of fall within 1 SDCM (or a 1-step MacAdam ellipse). Although the chromaticity variation of LEDs binned to 2-step MacAdam ellipse tolerance extends to a zone that is twice as big as the 1-step MacAdam ellipse, the color difference is barely visible by the trained eye. LED light sources are generally binned to a tolerance of between a 7-step MacAdam ellipse and a 3-step MacAdam ellipse. A 6 or 7-step MacAdam ellipse tolerance is considered acceptable for general lighting in outdoor areas and residential interiors. However, a tighter control of color variation (3 - 5 SDCM) is required in applications where color consistency is critical.

MacAdam-ellipse.jpg


The LED binning method defined by the American National Standards Institute (ANSI) in the C78.377 standard uses color bins to create chromaticity categories. Color bins are defined as parallelograms which are sized and oriented to approximately enclose a 7-step MacAdam ellipse whose center follows the Planckian curve throughout the white color space in the CIE 1931 color diagram. The ANSI color bins are further subdivided by some LED manufacturers for progressively tighter specification and increased product uniformity.

ANSI-C78.377.jpg


The dominant wavelength of an LED chip is affected by the junction temperature. As the junction temperature increases, the energy bandgap becomes narrower, and thus the dominant wavelength of LED increases. Every 10°C rise in junction temperature will lead to a one-nanometer increase in the dominant wavelength. Therefore there's a color shift when the diode is heated at typical application conditions. The reduction of band gap energy at higher junction temperatures also causes a decrease in the forward voltage of an LED. Consequently, the input power will decrease and the lumen output will decline. To improve predictability of the performance of an LED in real-world applications, LEDs should be preferably tested and binned with the junction temperature set to a typical operating temperature.
 
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