Light-emitting diodes (LED) are the ideal choice for a variety of applications, including instrument-panel indicators, architectural accent lighting and general illumination. The challenge is that, despite careful process control, the output characteristics of LEDs from even the same wafer will vary, sometimes significantly. The industry continues to work on the problem, but in the meantime the issue is being addressed by sorting the finished devices into groups or bins.
To ensure that LEDs meet the design needs, it’s important to understand their properties. This means understanding how the manufacturer uses binning to build its product offerings, which is not as easy as it sounds.
LEDs consist of p-n junctions epitaxially grown on either p-type or n-type substrates. The band gap of the material determines the emission wavelength of the LED chip, which can range from the near-IR to the UV spectral regions. Compared to diode lasers, fitted with resonant cavities that only support lasing at narrow spectral line widths, LEDs generate relatively broadband output. Instead of talking about output wavelength, we talk about peak wavelength.
Although LED fabrication is a highly refined process, thermal-coefficient mismatch between the epitaxial layers and substrate can lead to strain-induced defects. These defects in turn cause holes and electrons to combine non-radiatively at the junction, which reduces the luminous flux generated by the device. As a result, even LEDs grown side-by-side on the same substrate might display different brightness.
The most common technique for producing white-light LEDs involves applying a yellow phosphor, typically cerium-doped yttrium-aluminum-garnet (Ce:YAG), to a blue LED. The phosphor down converts some of the blue emission to yellow, and human eyes perceive the mix as white light. The exact color temperature of the light is driven in part by the thickness and concentration of the phosphor. Again, even the best process control cannot yield a completely consistent product, so LEDs typically display unit-to-unit color variation. Colored LEDs also display variation, although it is generally less of an issue than for white-light devices.
To maximize yield in the face of these issues, manufacturers sort LEDs by luminous flux and either chromaticity (for white LEDs) or color (for colored devices), dividing them into bins. The approach helps suppliers manage inventory to ensure availability and gives users a clear idea of the output characteristics that can be obtained.
The ANSI_NEMA_ANSLG C78.377-2008 standard from the American National Standards Institute defines a series of chromaticity quadrangles that fall along the black body line in Commission Internationale de l'Eclairage (CIE) chromaticity space (For more information see the TechZone article on understanding the color white. Also see Figure 1 below.)
Figure 1: The black body line in CIE 1931 x, y chromaticity space (left) provides a quantitative method for comparing the color of LEDs to that of black body radiators. The ANSI_NEMA_ANSLG C78.377-2008 standard (right) specifies a series of chromaticity quadrangles that define the maximum allowable color variation for general lighting applications. (Source: ANSI)
The quadrangles essentially correspond to correlated color temperatures (CCT) that mimic white-light sources ranging from cool white to warm white, as seen in Figure 2. Manufacturers bin white-light LEDs by chromaticity to varying degrees of granularity, using subsets of the chromaticity quadrilaterals, again as seen in Figure 2. The smaller the bins, the more consistent the device performance. However, there’s a tradeoff involved. Purchasing by the single bin, assuming the supplier even offers it, involves a price jump of as much as two orders of magnitude. The more common, and economical, approach is to buy products that group multiple bins together.
Figure 2: Manufacturers divide ANSI chromaticity quadrilaterals into segments called bins, then group devices according to those output characteristics. (Source: Osram Opto Semiconductors Inc.)
Osram Opto Semiconductors, for example, specifies 62 distinct chromaticity bins for its DURIS E3 general lighting device, then groups them into products classed as warm white, neutral white or cool white. The datasheet gives the exact CIE x, y coordinates for each bin, and the bounding bin numbers appear at the end of the part ID. For all but the most demanding applications, mixing devices from multiple, related bins smoothes out variations enough to generate an output that appears homogeneous.
Even though products tend to encompass multiple bins, the die generally arrives sorted. A customer purchasing a warm-white LED product that incorporates eight bins, for example, would probably receive eight tape reels, each containing chips from a single bin. Although the output of the LEDs would vary enough from reel to reel to be discernible by the human eye, color mixing evens out the effect.
That’s fine for most uses, but some applications can’t tolerate any variation at all. For directional applications such as museum or retail lighting, the multi-die XLamp MT-G EasyWhite LED from Cree provides an alternative. The company groups the die into bins centered on, but only a fraction of the size, of the ANSI quadrilaterals. More importantly, the products are not sold as groups, allowing customers to choose the bin they need. In a museum, for example, where accurate color rendering is essential, the ability to choose a specific bin provides enormous benefit.
Manufacturers also bin LEDs according to variations in luminous flux. Compared to color sorting, screening devices by flux tends to involve only a handful of bins. The tables on data sheets typically sort LEDs by luminous flux first, then by chromaticity or color. The devices can be further subdivided by forward voltage, although this tends to be used mostly for colored LEDs.
To select the right LED, start by defining requirements in terms of luminous flux, and chromaticity or color, as well as allowable bandwidth. Check the luminous flux binning tables to ensure the device will supply the brightness required. Next, use the color data on the specification sheet to determine which chromaticity quadrangle will provide the right wavelengths. Finally, go to the product table and determine which selection includes the output characteristics needed. A number of manufacturers include the codes for flux and color bins in part numbers, but syntax varies, so read carefully.
Although the solid-state lighting industry continually works to improve LED fabrication steps, it appears unlikely that output variations will disappear entirely. Given the challenges inherent to the process, binning appears to be here to stay. Understanding the binning specifications for any product can help inform the best design and implementation.
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