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Different heat sink designs will mean that LEDs and the associated electronic circuits will likely see different operating conditions despite operating similar times under similar • temperature conditions.
Different materials used in secondary optics may age differently.
Different environmental conditions (including air quality) may cause materials in different •
Different luminaire designs may create non-uniform color characteristics, such as halos, or yellowish, bluish, or greenish hues around the edges of the beam, and these color • characteristics may vary over time.
Some manufacturing processes have tight initial selection criteria while others have loose selection criteria, which will complicate the determination of color shift over time.
• LM-80, Section 6.2.
6 Ibid., Section 8.0, item 13.
engineering terms used to discuss color, including “chromaticity,” “black body curves,” “LED bins,” or “MacAdam ellipse.” There is no standard consumer definition that can be used as an alternative, although the current Lighting Facts label describes color in terms of correlated color temperature (CCT) or, to be more precise, defines an ANSI bin limiting total color variation. For example, a CCT of 3000 defines a color space region defined in C78-377. This, along with qualitative descriptions such as “warm” or “neutral," may be adequate for many applications, as it is similar to descriptions now being used for conventional lighting.
But such expressions are not enough for more demanding applications. CCT has often been used to describe color and color shift, but that is insufficient. For a given CCT value there is a actually a wide range of chromaticity values along the isotherm (both above and below the black body locus) that will all have the same CCT. This means that, for example, a 4000K LED can look greenish-white to purplish-white.
The International Electrotechnical Commission (IEC) is considering a different approach to characterizing color and color shift, based on specific color coordinates, with tolerances defined in terms of numbers of “standard deviations of color matching” (MacAdam ellipses). Figure 5 illustrates one example of how such a concept might be applied to color matching and, by extension, to color lifetime. A product could be considered failed for excessive color shift if it moves outside a boundary defined in terms of n-SDCM steps.
Throughout this section, “color shift” refers to changes occurring under normal operating conditions and 8 after the luminaire has stabilized after warm-up (not a significant period of time for LEDs, but possibly for fluorescents and other incumbent technologies).
Unless the boundary is fairly tight, however, there can be quite perceptible color mismatches over time if different individual products drift in different directions with respect to one another.
understood, well-studied, or even commonly used as a metric, even for incumbent technologies.
That said, we believe it is worth studying and characterizing, because SSL products may remain in place for a long time, and color shift may well be an important reliability consideration for certain applications. Although LM-80 requires LED manufacturers to collect data on color shift over 6,000 hours of operation, there is no accepted, standard way to use this data to extrapolate color shift.
While the IES TM-21 committee is working to define a method to project long-term lumen maintenance of LEDs from LM-80 test data, developing a method to extrapolate color shift is outside the scope of this working group’s task. At present there does not appear to be any standardized color shift projection under consideration. It is also important to note that often the actual measurements of color shift for LM-80 testing are not done in situ or at steady state operation but rather in short-duration, relative photometry measurements at room temperature.
Additionally, color changes in luminaires with multiple types of LEDs may not be easy to characterize using single-LED LM-80 testing.
Factors that will make color shift so difficult to extrapolate include differences in LED design, materials, manufacturing process, optics applied to the LED, and the temperature and time the LED
relatively easily in an integrating sphere, testing and measurement of multiple samples under various operating conditions can become very time consuming and hence expensive. It may also require specialized equipment, such as a large thermal chamber to test a roadway fixture at different temperatures. While such expenses may be justified in limited professional applications sensitive to color, that is the exception rather than the rule.
SEGMENTATION OF THE LUMINAIRE MARKET
It is clear that considerable work remains before we will be in a position to accurately specify endof-life color shift limits for a specific luminaire design. Given this situation, and pending further work by standards organizations, we recommend that manufacturers designate products in one of three categories: lamp replacement, luminaire (standard grade), or luminaire (specification grade), and then treat color shift differently for each segment.
1. Lamp replacements are more amenable to LM-80 color shift measurements and projections since the design is consistently repeated, and sales volumes are high. Color can be specified on the Lighting Facts label in general terms for what is assumed to be a non-critical market.
2. Standard-grade luminaires would specify a maximum-warranted color shift by indicating an n-step ellipse within which the product will remain within its lifetime. It would be up to the manufacturer to determine what limits should be specified and for how long a period the warranty applies, which may or may not coincide with the lumen lifetime.
3. Specification-grade luminaires are intended for more discerning customers. More sophisticated color metrics may be included in the specifications, and the maximum color shift over the stated lumen lifetime would be provided. For these types of applications, the defined limit of color shift may need to be very specific, perhaps in terms of Δu’v’, the shift of the actual color point. Some professional-use lamp replacements might be included in this category as well.
All three categories require some means for the manufacturer to predict color shift over a period of time, but with greater or lesser precision depending on the classification. Additional work is needed outside the scope of this document to improve these methods.
A number of follow-up activities are suggested in order to firm up specifications to promote market
For consumer-directed products, there should be a short-term effort to develop standard, • A working group on bulb-replacement specifications should review these recommendations broad, qualitative descriptions of the degree of color shift over life.
and determine if other specifications are needed.
In summary, color and color shift are among the more complex issues that need to be addressed with the introduction of LED-based replacement bulbs and luminaires. Segmenting the market into the three broad areas listed above and then identifying work-around solutions for short-term problems appears to be the best approach. In parallel, individual working groups should begin to address each segment’s requirements in more detail to develop documented standards of color shift requirements and measurements.
SPECIFYING AND DEMONSTRATING LIFETIME
As the above discussions should make clear, specifying product lifetime for an LED luminaire can be complex. At the same time, customers require some understanding of lifetime in order to make intelligent decisions regarding the purchase of an LED product, which is often considerably more expensive than the incumbent alternative. Consumers, in particular, want a simple, easy-tounderstand estimate of product life, and recent rulings by the Federal Trade Commission 9 require it for certain replacement lamp products. Commercial customers understand the trade-offs among energy efficiency, maintenance costs, and first cost of an LED product, but a clear expectation for each of these factors is essential for a realistic calculation. Many buyers and specifiers have signed up as DOE Lighting Facts partners and have asked that lifetime be included among the information provided through that program.
LIFETIME SPECIFICATIONGiven the need, and recognizing the limitations, the DOE and NGLIA recommend the “standard” or default lifetime of an LED luminaire (or lamp) be defined only in terms of lumen output and be specified as the time when half the product population has fallen below 70 percent of average initial light output for any reason.
To be clear, while emphasizing lumen output, the definition above goes beyond lumen depreciation to include any mechanisms that lower the light output. It encompasses gradual lumen depreciation of the LED sources, depreciation due to interaction with other components or materials in the luminaire, and catastrophic failure of any component or subsystem, ranging from total failure with no light output to the failure of a subset of the LEDs leading to luminous flux below a specified threshold.
Whatever the stated lifetime of an LED lighting product, it is a statistical measure of the performance of a given design, in this case the mean time to failure because of low (or no) light output. In the specification above, the standard reference level of 70 percent of LED initial output is referred to as “L70”; the time at which half the LEDs have fallen below this level is “B50” and is the same as conventional rated lamp life.
LED Luminaire Lifetime Recommendations, June 2011 Page 22 The minimum threshold (70 percent) chosen as the “default” failure level by which we define a “standard” lifetime may not exactly suit every application. For example, in certain safety situations depreciation of this magnitude may be unacceptable, or in some cost-sensitive, non-critical situations a higher level of depreciation may be acceptable. Nonetheless, the definition above is recommended as a standard life designation by which different products may be compared to one another.
Additional specifications, as discussed in the next section, may be added to cover particular applications. These additional specs may cover other aspects of light quality that affect product usefulness over time, or they may provide what amounts to a different estimate of useful life such as B50/L50, for example. Generally this additional information would be most useful for professionally designed lighting solutions.
although as noted above, some high-end applications may require additional information regarding color shift or different depreciation thresholds. Given the absence of a good way to project color shift for a product population, such information should be handled carefully. Until color shift extrapolation methods are developed and proven, the best approach may be to state average color shift for the product only out to a time that has been measured, e.g., 6,000 hours. Two coordinates are necessary to accurately specify color, so both must be considered in defining shift. For example, the changes in CCT and Duv (distance from the blackbody curve) could be used, or the actual average color coordinates in a CIE color space could be reported initially and after a specified time interval of operation.
For some applications, the standard lumen lifetime as defined may not suffice. Reporting failure in
ADDITIONAL FLUX SPECIFICATIONterms of low light output regardless of cause may not give the designer enough information. If the lifetime is stated to be 40,000 hours, does that mean half the lights are at 70 percent of their initial output, or does it mean half the lights are nearly at full output and the others are completely out, or something in between?
To address this ambiguity, two numbers are needed: the lumen maintenance lifetime, e.g., B50 or B10, and the conventional electric failure lifetime, e.g., F10, when 10 percent of the luminaires fail in a conventional sense. Both times—B and F—must be measured for the complete luminaire because of the interactions among the components.
Together, these B and F numbers can describe three types of luminaire failure:
1. All LEDs light up, but at a reduced light level (defined by time to BXX).
2. There is one or more catastrophic LED failures, but other LEDs are still functional, perhaps running at a reduced light level (defined by time to Bxx).
3. No LEDs light up, due to system failure other than the LED (defined by time to Fyy).
LED Luminaire Lifetime Recommendations, June 2011 Page 23 The choice of xx and yy is up to the manufacturer and may vary by intended customer base or manufacturer; however, it should be explicitly stated. The examples of B50/F10 above might not suit high-performance applications, for example, but may be satisfactory for general use. Such a designation is probably neither necessary nor useful for consumer markets.
DETERMINING AND MAINTAINING SPECIFIED LIFETIMEThe means to determine lifetime are not fully standardized at this time, but additional observations concerning its determination and methods of projecting lifetime from shorter-term measurements are discussed below.
Ideally, the number(s) reported should reflect a sufficient set of measurements that can be stated with a reasonable degree of confidence. The reported lifetime should have sufficient measurement accuracy and sample size to provide at least a 50 percent confidence level. But while 50 percent may be a practical limit in the near term, 90 percent or higher is more desirable.