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Abstract
Light Emitting Diodes are an integral part of modern illumination systems. While their long-term stability in terms of lumen maintenance used to be in the focus leading to drastically increased nominal lifetimes over the past decade, the color shift or respectively chromaticity shift is lately attracting more and more attention especially in lighting applications that demand for a high chromatic stability. Common ways of displaying and reporting color shifts in LEDs and LED products provide only limited possibilities to accurately classify changes in color according to the current understanding of the underlying degradation processes. This paper therefore presents a methodology to overcome the observed discrepancies in analyzing time-dependent chromaticity data in terms of their shift direction and distance within the CIE 1976 ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$u',v'$ </tex-math></inline-formula> ) chromaticity diagram. The use of an extrapolation function extending the shift towards the spectral locus enables the determination of the corresponding nearest spectral locus intersection wavelength. Thereby, it is possible to classify changes in color according to different shift regions, which basically allows for performing an automated comprehensive color shift mode analysis. As part of this paper five different algorithmic approaches for determining the intersection wavelength and shift region are discussed and compared in terms of their accuracy and ease in implementation.
Highlights
Light Emitting Diodes are an integral part of modern illumination systems
The current paper presents an enhanced methodology for displaying and classifying timedependent chromaticity shifts in Light Emitting Diode (LED), which goes beyond the rudimentary documentation that, if at all, is performed by some
The blue direction, which is expressed by a steady decrease in both chromaticity coordinates u and v and favored by low operational stress conditions, such as low LED board temperatures and/or low drive currents; ii) chromaticity-shift modes (CSM)-2 describes a chromaticity shift into the green direction, which is favored by low stress conditions and most likely related to oxidation occurring in the phosphor; iii) CSM-3 represents a persistent shift into the yellow direction (v increases significantly at only slightly changing u ) that usually occurs after an initial shift into the blue direction resulting in a characteristic hook pattern when plotted in the chromaticity diagram
Summary
Acceptability limits, the industry further lacks a consensus methodology for accelerated testing, analyzing and subsequent. In order to develop such long-term LED color-shift prediction models, it is crucial to gain further knowledge about the nature of the causal degradation processes and potential acceleration factors, such as current, humidity and temperature, affecting chromatic stability. In this context, the current paper presents an enhanced methodology for displaying and classifying timedependent chromaticity shifts in LEDs, which goes beyond the rudimentary documentation that, if at all, is performed by some.
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