Cree XLamp
Long Term Lumen Maintenance
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 1
Outline
1. Testing LED Lamps per IES LM-80-08.
2. LED lamp lumen maintenance characteristics.
3. Errors in the ASSIST method of projecting Long Term Lumen Maintenance.
4. Developing an algorithm for projecting Long Term Lumen Maintenance.
5. Comparison of new projection method vs ASSIST.
6. Lumen Maintenance Graphs.
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 2
IESNA LM-80
• The IESNA has published the definitive method for long term lumen
maintenance testing of LED lamps – a method that is now THE INDUSTRY
STANDARD.
• IES LM-80-08, “Measuring Lumen Maintenance of LED Light Sources”
o
o
requires testing of LED lamps at two air temperatures: 55 C and 85 C. The
base of the lamp (solder point temperature) must also be held at these two
temperatures.
• It also allows LED manufacturers the flexibility to test the lamps at a third
temperature that will represent another condition in which the lamps may be used.
• The US Department of Energy recognizes the validity of this method and
requires it to be used for all luminaires that are Energy Star compliant.
• As a recognized Energy Star Partner, Cree has used and will continue to use the
methods defined in LM-80 to test XLamps.
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 3
Why is Air Temperature Important?
The silicone based encapsulants used in the industry degrade when exposed
to high temperatures. As they degrade over time, less light is transmitted
through the encapsulant. The higher the air temperature the more the
encapsulant will degrade.
Note: Tsp – Measurement point of solder pad of lamp
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 4
Cree’s Configuration for Lumen Maintenance Testing of XLamps per LM-80
During test, the temperature of the solder pad of the lamps (Tsp as
shown in previous image) and the air around the lamps is the same. Per
o
o
LM-80, for 55 C testing, the Tsp of the lamps and air are both at 55 C.
o
o
For 85 C testing, the Tsp and air are both at 85 C.
Temperature of ambient around lamps is actively controlled
by air flowing through chamber
Lamps are mounted to MCPCB’s.
Temperature of solder pad of lamps is independently actively
controlled by fluid flowing through heat sink.
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 5
High Junction Temperature Testing of Cree XLamps
• Cree tests Long Term Lumen Maintenance for XLamps in accordance with
the industry standard IES LM-80.
• The test results that Cree reports are based upon this industry standard
compliant testing.
• Some lamp manufacturers report high junction temperature operating
o
conditions for lamps (ie Junction Temperatures > 135 C with L70 of 50,000
hours).
• It should be noted that the test conditions used to develop these estimates do
o
not conform to industry standards as the air around the LED’s kept at 25 C.
• By keeping the air temperature low, the effects of encapsulant degradation
are eliminated during such non-standard testing.
o
• For comparison with other reports, similar 25 C testing at Cree is now
o
underway. In these tests the air around the lamps is maintained at 25 C while
the solder point temperatures (and resulting junction temperatures) are
maintained at much higher levels.
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 6
Other Configurations being used at Cree to study Long Term Lumen Maintenance:
Room Temperature Testing
o
Ambient Temperature is actively controlled at 25 C
Boards are attached to heat sink that is not
directly controlled. The resulting Tsp of the
lamps measured with a thermocouple will vary
depending on drive current.
High Junction Temperature Testing
o
Ambient Temperature is actively controlled at 25 C
Boards are attached to high power TEC
controllers for precise high temperature control
of Tsp. The resulting junction temperatures of
o
lamps under test are >160 C.
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 7
Behavior of Lamps During Lumen Maintenance Testing
In Low Air Temperatures
105
% LF
100
95
90
85
0
1,000
2,000
3,000
4,000
Hours on Test
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 8
5,000
6,000
7,000
Behavior of Lamps During Lumen Maintenance Testing
In Higher Air Temperatures
105
% LF
100
95
90
85
0
1,000
2,000
3,000
4,000
Hours on Test
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 9
5,000
6,000
7,000
Typical Test Results
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 10
Typical Test Results
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 11
Typical Test Results
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 12
Projecting Long Term Lumen Maintenance
The Alliance for Solid-State Illumination Systems and Technologies (ASSIST)
at the Lighting Research Center at Rensselaer Polytechnic Institute in the U.S.
recommended a method for extrapolating long term lumen maintenance of
LED’s. Cree has historically used this method to determine L70 for lamps.
The ASSIST recommendation is to test LED’s for a minimum of 6000 hours
and the 1000 hour data point is normalized to 100%.
An exponential curve is then fit to the 1000 hour data point and the 6000 hour
data point and projected forward to model the light degradation of the LED.
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 13
Projecting Long Term Lumen Maintenance
The ASSIST method of projecting Long Term Lumen Maintenance is now known
throughout the industry to be inaccurate as it significantly underestimates actual
long term lumen maintenance.
As lamps are left on test for longer and longer periods of time, the L70 values
project out to longer and longer times when using an exponential curve fit to
the data.
For example – this is actual test data for Cree’s white XLamp:
Cree White XLamp Long Term Lumen Maintenance
(Ambient Temperature = 25C, Junction Temperature = 65C)
110
100
90
80
% LF
70
60
50
40
30
20
10
0
0
5000
10000
15000
20000
25000
30000
35000
time (hours)
study name: Dem03WTest#52#53RTOL
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 14
40000
Projecting Long Term Lumen Maintenance
Using the actual 6000 hour test data, the projected L70 value is 63,917 hours.
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 15
Projecting Long Term Lumen Maintenance
Using the actual test data at 10,600 hours, L70 increases to 109,903 hours.
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 16
Projecting Long Term Lumen Maintenance
When using the 34,800 hour data, the L70 increases to > 200,000 hours!
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 17
Projecting Long Term Lumen Maintenance
It can be seen that the longer lamps are tested, the higher L70 becomes when using
the ASSIST method of curve fitting.
The IESNA SSL Testing Procedures committee (TM-21) is now working to
develop an accurate algorithm for modeling long term lamp behavior.
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 18
White XR-E XLamp Results in 45°C Air
Cree White XR-E XLamp Long Term Lumen Maintenance
Ambient Temperature 45C, Junction Temperature 53C and 65C
110
100
% LF
90
If = 350mA
80
If = 700mA
70
60
50
40
1000
2000
study name = PDWLT1106RTOL
Copyright © 2009, Cree, Inc.
Rev 5.0
3000
4000
5000
6000
7000
time (hours)
January 2009
pg. 19
8000
White XR-E XLamp Results in 85°C Air
Cree White XR-E XLamp Long Term Lumen Maintenance
Ambient Temperature 85C, Junction Temperature 93C and 105C
110
100
% LF
90
80
If = 350mA
If = 700mA
70
60
50
40
1000
3000
5000
7000
9000
time (hours)
study name = PDWLT1106HTOL
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 20
11000
Modeling Long Term Lumen Maintenance
“A”
“B”
110
% LF
100
90
80
70
0
2,000
4,000
6,000
8,000
5,000
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 21
10,000
12,000
14,000
Modeling Long Term Lumen Maintenance
“A”
“B”
110
% LF
100
Slope A =
90
∆yA
∆xA
Slope B =
∆yB
∆xB
80
70
0
2,000
4,000
6,000
8,000
5,000
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 22
10,000
12,000
14,000
Modeling Long Term Lumen Maintenance
16 individual data sets were used to
calculate the Slope of A with variables
that included:
“A”
110
• Solder Point Temperature (TSP)
% LF
100
• Drive Current (IF)
Slope A =
90
∆yA
∆xA
• Junction Temperature (TJ)
• Air Temperature (TA)
80
0
2,000
Copyright © 2009, Cree, Inc.
4,000
Rev 5.0
January 2009
pg. 23
Modeling Long Term Lumen Maintenance
“B”
The calculations for the Slope of B
included the same variables:
• Solder Point Temperature (TSP)
• Drive Current (IF)
• Junction Temperature (TJ)
Slope B =
• Air Temperature (TA)
6,000
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
8,000
10,000
pg. 24
∆yB
∆xB
12,000
14,000
Modeling Long Term Lumen Maintenance
• Using linear regression, it is possible to adjust the critical operating
parameters (TSP, IF, TJ, TA) for any lamp to define the “A” slope for any
conditions.
• Knowing the “A” slope it is then possible to determine amount of lumen
depreciation though 5000 hours.
• Using the same method, the “B” slope of any lamp under any operating
conditions can be calculated.
• With the 5000 hour lumen output and the “B” slope, the final L70 values
for lamps under any operating conditions can be determined.
Copyright © 2009, Cree, Inc.
Rev 5.0
January 2009
pg. 25
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Copyright © 2009, Cree, Inc.
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pg. 33