Abstract
The relation between numerical values of photometric characteristics (total luminous flux TLF, correlated color temperature CCT, color rendering index CRI) of white light emitting diodes (LED) and the variation of the spectral shape of their radiation during aging has been investigated. All the measurements were made on internationally adopted test methods, taking into account environmental conditions, electrical parameters and evaluated measurement uncertainty. Every piece of test and measurement equipment has actual verification or calibration with traceability to national and international references. It was demonstrated that in the luminescence spectra consisting of the “blue” band around 450 nm originating from the semiconductor heterostructure, and the broad “yellow” band from luminophor, the last band is nonelementary and consists of at least two bands: the “green” one around 530 nm and the “orange” one around 580 nm. The most unstable “green” band has the highest impact on photometric characteristics. As a consequence, further investigation should be performed on how instability of elementary bands and its quantity will link not only with photometric characteristics, but with production conditions and material properties of LED heterostructure and luminophor itself. In particular, for improvement of the color stability of white LED, the parameters of luminophor forming the “green” band should be stabilized. А unified method for accelerated testing of LED products and method for long-time lifetime prediction shall be developed, taking into account not only depreciation of TLF, but also shift of other photometric and spectral characteristics of white LED.
Highlights
White light emitting diodes (LEDs) are widely used for liquid crystal display (LCD) backlighting, including low cost color displays
total luminous flux (TLF) decreasing is correlated with common practice [1, 4,5,6] and are usually described with monoexponential or bi-exponential decay curve according to Arrhenius law [2]
Several experimental results [2, 7] point out initial segment of TLF growth that lead to deviation from both monoexponential and bi-exponential decay laws commonly used, and can be caused by luminophor stabilization, but omitted from long-time prediction of LED life
Summary
White light emitting diodes (LEDs) are widely used for liquid crystal display (LCD) backlighting, including low cost color displays. Experimental results and analysis of the drift of some photometric and spectral characteristics of white LED lamps within time and under impact of additional thermal and electrical acceleration factors are described, with the aim to obtain more information about the relation between the colorimetric and spectral data.
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