For most purposes, it is sufficient to specify life in terms of ability to deliver adequate light, 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.
ADDITIONAL FLUX SPECIFICATION
For some applications, the standard lumen lifetime as defined may not suffice. Reporting failure in terms 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).
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.