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mathematical description might be possible for one or
two of these processes, the complex overlapping and
interdependence of the processes makes it impossible
at this time.
Standards: LM-80 and TM-21
The Illumination Engineering Society (IES) has
developed standards for testing LEDs so performance
and reliability can be characterized in a consistent
fashion to assess their practical life. One standard,
LM-80, provides a description of the method to be used
in determining the “Lumen Maintenance” of LEDs, that
is the light output as a function of time. The essential
features specified by the testing protocol are:
• Ambient and LED Case Temperature and Orientation
• Drive Voltage, Current and Waveform
• Instrumentation
The standard calls for measurement of light output
at an ambient air temperature of 25 o C, but LED case
temperatures of 55 o C, 85 o C and another temperature
selected by the manufacturer. The drive current is
specified by the manufacturer as this varies with the
die area of the LED. Measurements take place over a
minimum of 6000 hrs of operation (10,000 hrs is preferred)
and at intervals of at most 1000 hrs. It is common practice
among first-tier manufacturers to employ several different
drive currents. As we shall see later this is very important
to our method of ensuring LED lifetime.
Although LM-80 provides a uniform method for measuring
light output over time under standardized conditions, in
practice LEDs may be used at temperatures that differ
substantially from the LM-80 values. Moreover, because
this is not an accelerated testing method, very long times
are needed to reach a conclusion on reliability. Another
more recent IES Standard, TM-21 helps solve this
dilemma. The standard is effectively an “ad hoc” model
of LED degradation that allows the interpolation of timetemperature
data between temperatures and formalizes
the extrapolation of data into the future to predict output
over extended times. The major points of the standard
are:
• Exponential Decrease in Light Output (LOP) is
assumed
• LOP May be Extrapolated Maximum of 6x in Time
• Interpolation Between Temperatures Are Based on
“Activation Energy”
In mathematical terms the decrease in light output can
be stated as,
L(t) B[e ]
t -a
=
where L(t) is the Lumen output at time t, B is the
normalized light output at 0 or 1 hours, and α is the
decay rate, which is a function of temperature. The value
of α varies with temperature according to the Arrehenius
expression,
a(T)
= Ce
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E /kT a -
where Ea is the activation energy, k is Boltzmann’s
constant and T is the temperature in degrees Kelvin, and
C is a constant. The TM-21 standard only allows for the
interpolation of temperature data, so for two temperatures
T 1 and T 2 we can calculate the activation factor, E a /k as,
a1
ln[ a2
] Ea
= ( )
1 1 k
[ - ]
T2
T1
Once the activation factor is known then it is
straightforward to calculate the decay rate from the
Arrehenius expression for the intermediate temperature
and the resulting time dependence. Figure 3 shows
a graph of both real LOP data and the extrapolated
and interpolated values based on these calculations
(reference: Mark Richman LEDs Magazine).
Normalised Light Output
1.1
1
0.9
0.8
0.7
0.6
105C1 Amp Data
TM-211 Amp 105C
85C1 Amp Data
TM-211 Amp 85C
0.5
0 5000 10000 15000 20000 25000
Hours
Figure 3: LED degradation data (triangles and diamonds) used to
extrapolate and interpolate temperature vs time Lumen maintenance
information.
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