IMPACT OF LUMINAIRE DESIGN AND MANUFACTURING PRACTICES
Color stability, like lumen depreciation but to an even greater extent, is not exclusively determined by the performance of the LED. Examples of how luminaire design and manufacturing practices will impact color quality and color shift include:
• 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 luminaires to behave differently.
• 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
•Finally, some luminaires address color shift with active color management, including sensors and controls. However, sensors and controls may themselves shift over time and affect color.
DESCRIBING COLOR SHIFT8
Consumers have no experience with, and cannot be expected to easily relate to, most scientific or 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.