As seen from the data presented above we can conclude that
the LED life time depends very strongly on the temperature and
driving current conditions under which the LED is operated. If
we take into account that a higher current leads to a higher
junction temperature, we can speak from the effect of intrinsic
heat. This is generated from two sources – the LED die and the
phosphor layer. It is important to examine which parts of the
LED package cause luminous flux degradation. An interesting
study is to measure the thermal resistance of the LED during the
test and try to find out if there is some correlation between
luminous flux decay and increase of the thermal resistance R .
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The usage of TM 21 for life time predictions seams to
deliver good results. A weakness of the methodology is that, if
another value different from L70B50 must to be estimated, then
in this case the influence of the outlier increases very strongly.
This is also connected with the assumption that the LEDs are
normally distributed which is not very often the case. During the
aging of the LEDs, the sample distribution may change
continuously causing a value fluctuation which is not based on
the aging but on the statistic. The usage of the median instead of
mean value for the averaged luminous flux can minimize the
distribution dependency effect.
Further work should be carried out related to the examination
of the reasons that lead to premature light output degradation.
The investigation of the causes can help find out the weakest
link in the chain.