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U.S. Department of Energy Publishes Report on Color Retention of LED Products
Introduction: The US Department of Energy (DOE) recently released a new report on the color retention of LED products in laboratory and field applications. In a statement, the U.S. Department of Energy explained that while much of the focus on LED lighting products has focused on lumen maintenance, color shift is the cause of early failure in some products, especially those for applications with high visual appearance requirements. Not only can color casts cause changes in the color of emitted light, it can also change the appearance and color of illuminated objects, the report states.
Tags: U.S. Department of Energy LED Lighting Color Preservation GATEWAY Program
The U.S. Department of Energy (DOE) recently released a new report on the color retention of LED products for laboratory and field applications. In a statement, the U.S. Department of Energy explained that while much of the focus on LED lighting products has focused on lumen maintenance, color shift is the cause of early failure in some products, especially those for applications with high visual appearance requirements. Not only can color casts cause changes in the color of emitted light, it can also change the appearance and color of illuminated objects, the report states.
The report discusses field data from the Department of Energy's GATEWAY program. The GATEWAY program found that many of the LED lights installed in museums had developed a severe color cast before reaching their rated life, beyond reasonable limits.
In addition, the U.S. Department of Energy said laboratory data from its CALiPER program showed that many early LED products experienced unacceptable color shifts after only a few thousand hours of use. However, the CALiPER results also show that, after testing, the commercially available LED products that have won the LPrize award have color stability unmatched by traditional light sources.
The report outlines the metrics used to measure color casts and explains for end users how to monitor chromaticity and look for manufacturer warranties. It also includes physical changes that have been shown to cause color shifts in some types of LED devices. As far as lighting fixtures are concerned, LED device data reveals a wide variety of products to choose from. The DOE says more detailed and standardized information is necessary to help consumers make informed choices.
The report defines color stability as the ability of a lamp to maintain a certain spectral power distribution over time. It is worth noting that color stability is not the same as color consistency, which refers to the change in the spectral power distribution of the initial luminaire. According to the report, changes in the spectral output caused by changes in the surrounding environment are a unique problem, like the depreciation of radiant flux. The DOE says the issues are interrelated.
Chromaticity coordinates, a basic principle of the CIE chromaticity system or an index derived from it, are often used to describe the color of a light source. The chromaticity coordinates of a light source provide a numerical representation of the color of the light, but give little indication of how the light source will render the color of a particular object. Therefore, color casts are the most commonly mentioned when reporting differences or changes in coordinates.
The U.S. Department of Energy inspected many different light fixtures installed in the Smithsonian museums. The GATEWAY program also collected and measured light fixtures at the Harley Ford Museum of Art at Willamette University.
The ENERGY STAR project stipulates that the change in chromaticity coordinates should not exceed 0.007 for Δu'v' after 6000 hours of work. The DOE says the standard is a reasonable starting point, and it doesn't have strict requirements to ensure high-quality lighting, especially since LED products typically last well over 6,000 hours. A change of 0.001 corresponds to a single step of the Mac Adam Ellipse.
The report notes that many factors can affect color casts, so it is especially complex at the full product level. As a result, some products have a useful life that is significantly shorter than their rated life in applications where color quality is critical. This may in turn affect the expected return on investment. The report notes that there may be less focus on manufacturer warranties, but color casts are rarely included in currently available warranties.
In the report's concluding paragraph, the U.S. Department of Energy advocates: continued development of LED packaging technology to improve color stability; development of criteria for predicting long-term color shift performance from a more limited set of measured data; Investigate the interaction of multiple factors; teach the lighting industry about color shifts and how to properly state them; more broadly include color shifts in product warranties, documenting color shifts beyond established values following an agreed-upon approach.