This paper brings a brief review of LED parameters that can be changed during the life of these types of light sources. Firstly the change of quantitative parameter of emitted light is presented (luminous flux) and following part is dedicated to change of quality parameter (spectral distribution) and change of electrical parameters. Modification of these parameters may affect the use of LEDs in lighting systems for general lighting and also in some applications with high demands on luminous flux and spectral distribution stability. Current attitudes to modelling and measuring methods and their fundamentals and deficiencies are reviewed briefly.

Paper describes deficiencies of conventional method for luminous intensity curve measuring and it brings comparison with newly used methods of near field photometry. There is also presented demonstration measurement of problematic type of luminaire with use of near field photometry.

IntroductionUsing of white LEDs (Light Emitting Diodes) for general lighting is on rise. It is caused by several factorscontinuously decreasing prices of LEDs, variable construction designs of luminaires, LED luminaires are suitable for implementation to advanced lighting systems due to their good controllability and others. But this rise brings new challenges for both researchers and manufacturers in the area of understanding the behaviour of LED parameters under real operating conditions. Current state of light parameters stability research is mostly focused on decrease of luminous flux during lifetime and influence of junction temperature on luminous flux. However using white LEDs especially for general lighting is conditioned by fulfilling of other parameter limits -operating voltage, luminous efficacy, colour rendering index, chromaticity temperature. Therefore the data based on spectral distribution changes of white LEDs respecting possible operational conditions are much more convenient for evaluation of possible influences on LED light sources.

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