Color quality is an important consideration when evaluating LED-based products for general illumination, a specially for white LEDs. Individual LED dies, often referred to as chips, emit light in a narrow range of wavelengths, giving the appearance of a monochromatic source. LED lamps and luminaires combine multiple spectral components, which may be produced directly or through phosphor conversion, to create a mixture that appears white to the human eye. In comparison, incandescent lamps have a broad distribution and fluorescent lamps typically rely on a limited, fi xed set of phosphors with specific emission characteristics. Figure given below compares the spectral power distributions of several light sources, adjusted for equal lumen output.
Currently, white light is most often achieved with LEDs using phosphor conversion, but mixed systems that use a combination of colored LEDs-RGB are also available. For LEDs, phosphor down conversion is most commonly based on a blue or near-ultraviolet emitting die that is combined with a yellow-emitting phosphor, or a combination of different phosphors that produce a broad energy distribution. The phosphor(s) may be incorporated into the LED package, or may be located remotely. Fluorescent lamps also utilize phosphor down-conversion. Earlier versions of the technology relied on broad emitting halophosphors, but most current lamps, called triphosphor fluorescent, utilize a combination of red-blue- green emitting phosphors.
Mixed LED sources have a higher theoretical maximum efficiency, potentially longer life, and allow for dynamic color control . However, they generally have less color consistency both initially and over time, require more elaborate optical systems to ensure proper mixing, and are generally more complex and expensive to manufacture. Check more informations here. Currently, mixed LED efficacy is typically lower than PC LED efficacy, limiting their use in general lighting applications. It is important to remember that even within a family of LED sources, or within the products offered by a given manufacturer, color characteristics can vary widely based on the choice of primaries or phosphors. Each individual product should be evaluated on its own merits, regardless of the technology.
What is Binning with respect to LEDs?
The practice of binning is designed to maximize effective utilization in the production of LEDs. This process is most important for luminaire manufacturers to specify and control since it has serious implications on performance, cost and lead time. It is also important as a point of general awareness for specifiers and end use customers so they understand how the manufacturing supply chain is ensuring high quality and consistency, specifically with regard to critical performance attributes such as light output and color. To understand binning, it is helpful to first review the process of LED production.
In the production of LEDs, a single round wafer is coated with various materials (epitaxial growth) to create the semiconductor which forms the heart of the blue LED. This is then sliced into extremely small rectangles (die). Wire bonds are inserted and the phosphor is added either as a coating or suspension within the enclosure. The assembly is then encapsulated to create a finished white light LED package.
The coating processes (epitaxial growth and phosphors) create significant inherent variations that impact the lumens, color temperature and voltage of the LEDs. Even with all of the R&D efforts underway and the billions of dollars spent within the semiconductor industry to minimize this production variation, the end result is a process that is not capable of producing highly consistent and tightly controlled production of LEDs. So, in an effort to maximize yields (and with a knowledge that the lighting industry has a wide range of needs), LED manufacturers sort their production into lumen, color and sometimes voltage bins. This allows luminaire manufacturers to select only those LEDs that meet their acceptable performance ranges while maximizing the overall usage of each of the bin ranges for the LED supplier.
Achieving Consistent Color with Binning
During production, LEDs vary in color, luminous flux, and forward voltage. Since the differences are significant, LEDs are measured and delivered to the market in subclasses, or bins. Binning makes it possible to select LEDs that conform to stated specifications. For instance, to select LEDs for traffic signals with the specific color required to meet the European standard.
Lighting fixture manufacturers devise methods of selecting bins of LEDs in such a way as to minimize differences in color that might be visible from fixture to fixture or from production run to production run. To understand how a bin is defined, we return to the diagram of the CIE 1931 color space, and zoom in on the black-body curve.
Because each ANSI-defined nominal CCT quadrangle allows for readily perceptible color variations, LED manufacturers subdivide each quadrangle into multiple smaller areas. These smaller areas are the available bins for LEDs of a given nominal CCT. A leading LED manufacturer, for example, sells a certain number of bins for a color temperature, each of which falls within the area that conforms to the ANSI standard for that nominal CCT. The diagram below shows an example binning plan for the manufacturer’s white-light LEDs at 2700 K.
As stated previously, there are several ways LEDs are binned. The most critical bin criteria that impact product performance are light output and color temperature. Binning for light output is a very straight forward linear function. LEDs are individually measured and sorted by lumen output into prescribed ranges. LED suppliers create their own standard set of lumen bins and provide clear information on the expected lumen performance of each of their bin ranges. So, luminaire manufacturers can easily select the bin (or set of bins) that best meets the lumen performance requirements of the fixture.
Binning for color temperature is a more complex process. Color temperature bins are defined by (x,y) coordinates on the CIE 1931 Chromaticity Diagram. These bins are grouped as quadrants around the standard chromaticity lines (shown below in yellow) for a specified color temperature. The larger the bin size, the more variation around the standard color temperature is accepted. Conversely, smaller bin sizes maintain a tighter control of color variation.
In 2008, ANSI and NEMA collaborated to establish a bin standard ANSI C78 377A1 which has become a minimum requirement for Energy Star® certification. This standard specifies a bin size that approximately correlates with the degree of color variation we experience today with commercial CFL sources. This allows for some degree of perceivable variation in color temperature among white light sources.
This is achieved through an additional sub-binning quality process that ensures chromaticity of the entire module falls within the smaller bin size. The RTLED refractor facilitates this process by diffusing and integrating the color of individual LEDs to create a uniform, aggregated chromaticity for the entire module.
For more information about color binning, bin sizes and the ANSI C78 377A, consult: http://www.nema.org/media/pr/20080221a.cfm.
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