Equipment: Lamps and Gear - Road Lighting: Fundamentals, Technology and Application - page 113

Fig. 10.2 Examples of lamp
survival ( continuous line ) and
lumen depreciation ( dashed
line ) curves

100

%

90

80

70

60

50

L 70,12

Burning hours

lamp failure and lamp lumen depreciation is a decrease in luminous efficacy and
lighting level, which may reach a stage where it is economically more profitable
to replace all lamps by new ones, rather than to wait for their ultimate failure. The
concept of economic lifetime or rated life has therefore been introduced. It takes
both lamp failure and lamp lumen depreciation into account. The economic or rated
life of lamps in an installation is defined as: the time after which, due to lamp failure
and depreciation of light output of the lamps, the light output of the installation has
fallen by a certain percentage. Which percentage is relevant is especially dependent
upon the type of application and the actual costs of lamp replacement (including
labour cost). For road lighting installations percentages of 20-30 % are often used.
The corresponding economic lifetimes are called L 80 and L 70 respectively, 80 and 70
standing for the percentage of light that remains. Figure 10.2 gives an example of a
rated lifetime according to L 70 : a survival rate of 88 % and a lumen depreciation of the
remaining lamps of 80 % results in a total remaining light output of the installation
of 88

70 %.
The International Electrotechnical Commission (IEC) recommends that for LED
systems (especially those using a multitude of individual LEDs) the rated-life state-
ment should include a statement of the actual “catastrophic” (viz. complete) failures
in addition to the remaining percentage of light (IEC/PAS 2011a , b ). Rated life is then
given in terms of L x , the remaining light percentage as a result of both gradual lumen
depreciation and catastrophic failures, together with “F y ”, the actual failure fraction
at that rated lifetime. The rated life of the example of Fig. 10.2 thus corresponds to
L 70 F 12 . For a LED module (or LED luminaire) with a multitude of individual LEDs
this means that at the stated lifetime the remaining light output of a great number
of these modules is 70 %, while at that moment 12 % of the modules or luminaires
have to be considered as failures (viz. producing between “less than 80 %” and 0 %
of light output).
Given the long lifetime of LEDs it is impossible for manufacturers to actually
measure LED failures and lumen depreciation over the total period of the stated life:
it would simply takes years to collect the data. Keeping this practical consideration in
mind, IES document LM-80 “Measuring lumen maintenance of LED Light Sources”
has laid down norms for life testing (IES 2008 ). It requires testing of LEDs for at

× 80 =