Led lighting outdoor design challenge dec2013

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2 Five rules for
designing roadway
lighting
9 LED modules bring
energy savings to
high-mast outdoor
lighting
15 Advanced thermal
characterization
improves LED street-
light design
EDITORIAL DIGEST
Outdoor lighting
challenges SSL
component and system
designers
Although outdoor lighting can benefit from
the advantages of solid-state lighting — such
as lifetime, reliability, and energy efficiency —
the application presents particular challenges:
the high lumen output required by high-
mast lighting, environmental conditions,
and avoiding inefficient distribution of light,
to name a few. This digest will address
how outdoor lighting system designers can
apply thermal management techniques and
materials to improve resistance to ambient
conditions, and integrate LED sources that
meet the mechanical design and luminous
efficacy requirements of a changing outdoor
environment.

LEDs Magazine :: EDITORIAL DIGEST
2
* This article was published in the April 2013 issue of LEDs Magazine.
Five rules for designing
roadway lighting
Effective use of LED sources and emerging
knowledge of human visual systems guide
best practices for SSL roadway lighting.
O
F THE MANY design challenges facing LED-based solid-state
lighting (SSL) applications, perhaps there is none greater than that
of expectations. There are expectations around the application.
There are expectations around the incumbent technology. There are
expectations around the way it has always been done, and, as a result, there are
expectations around the way it should be done going forward. What if we were
able, however, to design with a clean sheet of paper? Take roadway lighting as an
example. If we were to take that application, deconstruct it, and come at it from
a different angle, what might
we do differently, and how are
LEDs specifically suitable tools
in this redesign?
When we think about the
job of lighting a roadway,
we are conditioned to think
about what is happening
right in front of us. We think
about targets in the road and
response time in identification.
In fact, the entire series
of metrics for roadway
lighting is modeled around
these requirements. From this
Fig. 1. A driver’s view of a simulated roadway scene,
illustrating conventional forward auto lighting combined
with traditional roadway lighting.

Five rules for designing roadway lighting
3
standpoint, our examination of
roadway lighting is fundamentally
no different than our examination
of office lighting. The conditions
and demands of the tasks, however,
couldn’t be more different.
Rule #1: Zoom out and consider
the bigger picture.
Once we step back, one of the
things we can appreciate regarding
roadway lighting is that we are
invariably talking about night-time
situations. While the human visual
system has an amazing ability to
tolerate a wide range of conditions,
the mechanisms that allow
for those ranges vary for different lighting levels — night-time environments
especially. To better appreciate how those mechanisms come into play, we need
to consider the retina and its component parts.
The retina is incredibly complex, but its basic role can be summarized by two
types of photoreceptors: cones and rods. Cones are located predominantly in
the center of the retina in the fovea. Rods, which greatly outnumber cones,
surround the fovea and encompass the periphery of the retina. The retina is
in simplest terms a camera. It produces images for the central nervous system
(CNS) to interpret.
The CNS-to-photoreceptor pathways best define the photoreceptor’s role in vision.
Each cone, in effect, has its own direct path to the CNS. A quanta of information
is personally escorted to the brain for processing. This one-to-one relationship
defines its role in higher order perception such as fine detail discrimination and
color analysis. The peripheral vision pathways to the CNS are shared by large
groups of neighboring rods. Light that grazes one edge of the group triggers a
response on the far edge. Through this mechanism, rods preform their basic role
of gross peripheral motion detection.
Fig. 2. A driver’s view of a simulated roadway scene,
illustrating asymmetrical forward lighting for objects
on the roadway combined with peripheral roadway
lighting for detecting objects near the road.

5
Using night-time driving as an example of the mechanism, our eyes are directed
for the majority of time at the roadway, where the cones are aiding in the analysis
of detail. When something appears in the periphery, say a deer approaching the
shoulder of the road, this sight registers across many groups of rods, signaling
movement to the CNS. At this point, the eyes move and perhaps the head pivots, so
that the cones can be engaged for better detail analysis and subsequent reaction.
Rule #2: Appreciate the importance of peripheral detection in
night-time driving.
Our current metrics are concerned with foveal vision exclusively, yet the fovea
takes up a tiny percentage of the visual field. We essentially light the road
as depicted in Fig. 1. Mark Rea, director of the Lighting Research Center and
professor at Rensselaer Polytechnic Institute, has written extensively on the
subject. Rea has said that considering just the fovea in driving is akin to driving
while looking down a long, narrow tube. Given the choice, would we choose the
field of vision on the inside of the tube or the outside in order to drive? While
what is inside the tube is important, this example illustrates that the outside of
the tube — our peripheral vision, at the very least, deserves some consideration.
While rods work in groups, they are individually much more sensitive to light
than cones. Able to absorb and register even a single photon, one immediately
sees their advantage in night-time conditions. Indeed, as light levels drop, the
rod-to-cone activation ratio increases until rod sensitivities are at a peak level in
night-time conditions.
Rule #3: Consider the different sensitivities of the photoreceptors.
Where the spectrum of light is concerned, the rods and cones respond similarly
to higher wavelengths. Rods are, however, much more sensitive than cones
to lower wavelengths, especially after they have time to adapt to night-time
conditions. If one of our goals is to optimize the lighting to better aid in peripheral
target detection, we should be working with a spectrum that is optimized to that
task and optimized to the photoreceptors (rods) engaged in that task.
Rule #4: Eliminate double work.
Regardless of the importance of peripheral vision, we still need cones for sign
identification/reading and analysis of detail in the roadway. The metric that

6
matters, just as in office lighting, for example, is contrast. How do we present the
task in proper relief? Strong forward lighting (such as provided by car head lamps)
with narrow optics will optimally illuminate the vertical plane and present a
snappy, sharp shadow with an excellent dichotomy between light and dark. Current
roadway metrics, mostly concerned (again, like office lighting) with horizontal
illumination, don’t even consider the vertical plane. As written, the application
requirements only consider overhead lighting, which can have a deleterious effect
on contrast when combined with forward lighting on cars. Roadway lighting needs
to complement forward lighting on automobiles and aid in the creation of contrast
and clear, decipherable indicators to which our CNS can respond.
Rule #5: Light the edges.
More importantly, however, is the ability to identify hazards prior to them being
in the roadway. Rea has suggested, only partially in jest, that better viewing
conditions may be gained by simply pivoting roadway lighting 180o in order
to light the shoulder (Fig. 2). The job of lighting the roadway is then left to
headlights. The optimal solution is most likely a combination of that approach
and current practices, but the clues are there.
The issue with incumbent technology in roadway applications is the one-size-
fits-all limitations. We start with a high flux, high wattage, omnidirectional light
source, and we attempt to corral the beam to meet the application. The approach
is inherently inefficient from an optical perspective. There is no opportunity for
nuance or spectral shaping.
Fig. 3. Cree XSP street
lights installed in
Hollywood, CA, focus
light on the roadway,
limiting back light.

7
SSL in roadway lighting
With LED point sources, we build a fixture piece-wise until we have the perfect
distribution — no more; no less. As Fig. 3 shows, SSL fixtures can be designed to
produce almost no light behind the poles. Through proper binning, we are able to
spectrally shape the output in order to best match the visual needs. In the example
we have been using for roadway lighting, we can imagine many different designs or
a combination of attributes in one package.
We could have a component of the beam that lights the shoulder and surrounding
areas of the roadway for the optimal spectrum of the rods. We could concurrently
light the roadway with another spectrum ideal for foveal vision and contrast.
We could have peripheral lighting that stays on constantly in rural settings or in
areas of high deer traffic. Conversely, thanks to SSL instant start capabilities, we
could have peripheral lighting that comes on as a function of peripheral motion.
The fact is that a conversion of roadway lighting to SSL is happening at a rapid
pace, driven in many cases by energy efficiency and low maintenance. The city
of Los Angeles has retrofitted more than 115,000 street lights with LED fixtures
(see Fig. 4). However, SSL can go beyond saving energy by providing significant
enhancements to roadway safety.
Fig. 4. The City of Los
Angeles has replaced
more than 115,000
street lights with
energy-efficient LED
fixtures.

8
The options are open-ended. What is clear is that new technology allows
designers the opportunity to not only work with new tools but also return to the
applications themselves and rethink the way things are done. When we do that,
the value of lighting is optimized in its abilities to help people. We escape the
morass of expectations, and we evolve as an industry.
DON PEIFER is a senior product portfolio manager at Cree.

9
* This article was published in the June 2012 issue of LEDs Magazine.
LED modules bring
energy savings to high-
mast outdoor lighting
While LEDs have pervaded a variety of street- and
roadway-lighting applications, most owners of high-mast
lights have stayed with HID lamps. A Maine case study
indicates significant potential for SSL in the higher-power
lights used in places such as freeway interchanges.
W
E ROUTINELY COVER case
studies of LEDs used in outdoor,
street- and area-lighting
applications where solid-state
lighting (SSL) is delivering significant savings
in both energy and maintenance costs. But
repeatedly at conferences the prevailing wisdom
among speakers has been that the high lumen
output required in high-mast applications would
require SSL fixtures that cost far more than
metal-halide (MH) or high-pressure sodium
(HPS) sources – an even greater cost differential
than is the case with normal street lights.
Presumably the high cost can stretch the payback
beyond what municipalities or transportation
departments are comfortable with. The Maine
Department of Transportation (MaineDOT),
however, is testing LED-based lights in a high-mast retrofit and the results
are promising.
Fig. 1. Global Tech LED’s high-mast
retrofit module.

LED modules bring energy savings to high-mast outdoor lighting
10
High-mast lights are quite different
in nature from more typical street or
roadway lights. High-mast fixtures are
regularly mounted at 60 ft to more than
100 ft above ground level and occasionally
as high as 250 ft. Normal street lights are
typically mounted at heights lower than
60 ft, and many are in the 30-ft range.
The applications for high-mast lights
include installations at transportation
terminals, other large, outdoor
maintenance or storage yards, and
specialty roadway applications. The
aforementioned freeway interchange
installations are probably the most
common roadway application, although
you will find some high-mast lights within
municipalities in busy areas.
In street-light installations, the lighting
designer normally specifies a rectangular
beam distribution or pattern that directs the lumens precisely and eliminates
light spill. The pattern is designed to evenly illuminate the roadway with
maximum spacing between poles. High-mast applications rely on more of a
circular or square pattern and are designed to distribute light evenly over a
maximum-sized radius or area.
If you look at legacy lights installed in North America, you can generalize about
the two disparate applications in terms of energy usage. Municipalities typically
install 250-400W HPS lights individually on a pole in street-light applications.
High-mast installations regularly gang 2, 4, 6 or 8 1000W HPS fixtures spaced
evenly around a single pole.
Potential savings
Clearly there is potential for savings in such high-mast applications. Including
Fig. 2. Workers retrofit a lowered high-mast fixture.

11
the ballast, a 1000W HPS
light actually consumes as
much as 1200W. LEDs could
certainly cut that energy usage.
Plus consider the potential
maintenance savings. About
high-mast light owners, Jeffrey
Newman, president of Global
Tech LED, said “They have been
replacing lamps once per year.”
Global Tech manufactures LED
modules designed for use in
high-mast retrofit applications.
The modules include six
clusters of seven LEDs for a
total of 42 Philips Lumileds LEDs per module (Fig. 1). Global Tech has developed
customized lenses that cover each LED cluster to control the beam pattern.
Depending on the application, as many as four of the Global Tech modules might
be used to replace a single high-output HID lamp.
Newman is quick to attack the question of affordability of LEDs in the high-
mast application. He laid out a theoretical comparison where the LED alternative
dissipates 600W while the incumbent lamp is the 1200W HPS lamp and ballast.
According to Newman the 600W LED reference case is a very conservative
example, because most likely you would use a lower-power LED configuration.
The LED approach saves 600W. Based on a burn time of 12 hours per night, the
savings amount to 2628 kWh per year. At a rate of $0.12 per kWh, that electricity
saving equates to around $315 per year. The price a municipality would pay
for the retrofit would depend on distributor pricing, but Newman said that
MaineDOT is paying in the range of $1200 to $1300 per kit including credits
supplied by the state. So the payback is in the four-year range before you consider
maintenance costs, and perhaps a lower-power LED implementation.
Fig. 3. An LED-based high-mast pole in Waterville, Maine.

12
According to Newman, the LED
project in Maine is about more than
savings and payback and is focused
on keeping the lights on. He said,
“They were shutting the lights off
at 11 pm at night because of the
expense.” The LED retrofit will
allow the lights to burn all night,
although the long-term plan may
also entail dimming the lights late
at night.
Maine Interstate 295
Ron Cote with MaineDOT said
that he was doubtful that an LED-
based product could serve in the
high-mast application when Global
Tech first approached the state. But
after seeing the modular approach,
MaineDOT retrofitted one high-
mast pole with the Global Tech
modules eight months ago. Cote reports that the retrofitted fixtures on the pole
have been problem free.
The LED kits replaced 1000W HPS lamps. The project used four of the Global Tech
modules in place of the 1000W lamps. Each module dissipates 98W for a total of
392W per fixture. The retrofit relies on a metal mounting plate with four holes for
the modules, and the plate is attached to the reflector of the existing fixture.
Once installed Cote said that the LEDs provide 1 fc at ground level out to a
distance of 200-300 ft. He said, “Up until now, there hasn’t been an LED fixture
that could touch the light distribution of HPS.” But Cote said that the installed
LEDs are providing comparable performance.
After testing the one pole, MaineDOT is retrofitting eight additional poles at two
freeway interchanges. Cote reports that the retrofit process is relatively simple.
Fig. 4. LED high-mast lighting (top) compared with
HPS high-mast lighting (bottom).

13
Typically high-mast lights are mounted in such a way that cables can be used
to lower the fixtures to ground level as opposed to requiring a bucket truck
for service (Fig. 2). Cote said it typically takes about 20 minutes to lower a set
of lights and another 20 minutes to raise the fixtures back up the pole once
service is finished. He said it also takes workers about 20 minutes per fixture to
install the retrofit.
MaineDOT is able to afford to burn the LED lights all night. Cote said that the SSL
retrofit is delivering about 66% in energy savings. The energy cost per freeway
interchange has dropped from $800 to $266 per month.
The lights are also superior in terms of quality. Fig. 3. shows one of the Maine
LED high-mast lights. Cote said the broad-spectrum light and 5000K CCT
provide better visibility. Before and after photos weren’t available for the Maine
installation. But Fig. 4 shows LED and HPS high-mast lights from a Global Tech
project at a Florida shipping container terminal.
The energy-conservation-oriented Efficiency Maine organization also commented
on the quality of the SSL retrofit. “I went by the Waterville exits this morning
on the way in,” said Michael Watson, project engineer at Efficiency Maine. “The
Kennedy Memorial Drive exit is done, all four towers have the LED fixtures and it
looks great. They also had one done at the Main Street exit and what a difference
it makes compared to the HPS fixtures. The LEDs really light it up nice.”
Controls and dimming
Looking forward, Cote said that MaineDOT is contemplating a retrofit of 108
additional poles – the entire high-mast inventory along I-295. Moreover the
department may consider dimming the lights for five to six hours each night to
further reduce energy consumption.
Newman estimates that with dimming the energy savings could stretch to 80%.
Global Tech uses a combination of a custom microcontroller (MCU)-based control
circuit on each module along with a modular Philips Lighting driver. The MCU
can dim the lights to any level required. The MCU bases the dimming operation
on the photocell that is already used on each pole to turn the lights on. Newman
said a typical scenario is what he calls 561. Five hours after the lights come on,

14
the MCU dims the lights. The lights remain dimmed for six hours and are brought
back to full brightness for one hour.
Newman said Global Tech has also developed a wireless control network that
can optionally be installed in the retrofit modules. For now, MaineDOT is not
installing modules with wireless support.
Cote said that MaineDOT will likely test dimming at a single interchange in the
next phase of the project. The department will then seek input from the public
and other interested parties on the light levels.
The savings potential of LEDs on the Maine interstate system is significant. Cote
said that the state spends $750,000 annually on interstate highway lighting. Not
all of the lighting is high-mast. But Cote thinks the state could definitely save a
third of the total just through a move to LEDs on high-mast poles.
MaineDOT also expects to realize significant maintenance savings, although they
haven’t projected a figure. Cote said, however, that they were expecting 50,000
hours of life from the LEDs. That would certainly curtail the maintenance cycles
for replacing HPS lamps.
MAURY WRIGHT is the Editor of LEDs Magazine.

15
Advanced thermal
characterization improves
LED street-light design
A street light is hot-lumens tested in
compliance with JEDEC standards
S
OLID-STATE LIGHTING (SSL) designers who consider thermal properties in
their LED based design are more likely to produce luminaires with long-
term consistent light output and longer lifetime. In addition, in the case of
LED street lights, illumination often needs to be consistent over a range
of ambient conditions, which can be assured using the appropriate simulation and
thermal testing techniques.
This article demonstrates
how thermal simulation using
computational flow dynamics (CFD),
and thermal testing to the latest
Joint Electron Devices Engineering
Council (JEDEC) standards, can
provide the luminous flux of a
street-light luminaire under various
conditions. The test methods shown
can be used in prototype development,
product testing or failure analysis of
luminaires.
What constitutes good thermal
design?
LEDs, as one of the most efficient
light sources available today, are
* This article was published in the July/August 2012 issue of LEDs Magazine.
Fig. 1. Mentor Graphics’ LED characterization flow.

Advanced thermal characterization improves LED street-light design
16
becoming more widely used in indoor lighting, outdoor lighting and automotive
lighting. Good thermal design based on the application is essential to ensuring
the longevity of the LED luminaire because both LED lifetime and light output are
closely related to the LED’s junction temperature.
When an LED’s pn-junction
temperature is hotter, the
performance of the LED is
impacted in terms of shorter
lifetime and decreased light
output. In applications such
as headlights of cars or street
lighting where lives might be
at stake, lighting standards are
very strict. In addition to the
prescribed spatial distribution
patterns that are required,
illumination levels also need
to be provided consistently; for
example, even on hot summer
nights, luminous flux of LED-
based luminaires must meet
the lighting standards. This
necessitates having the appropriate knowledge about the thermal and light-
output properties of LEDs.
As of today, diligent lighting design with LEDs cannot be based solely on
a manufacturer’s data-sheet values. Information needs to be gathered
experimentally by physical testing of LEDs, and the gathered LED characteristics
need to be provided for thermal simulation using, for instance, CFD.
Thermal characterization of LEDs
From a semiconductor standpoint, LEDs are simple pn-junctions, thus it seems that
they should be easier to measure, when in actuality they are not. LEDs present a
number of thermal characterization challenges. They are often very small, and
measuring them un-mounted is difficult. Fortunately, parts can be mounted on
Fig. 2. Close-up view of the top cover of the housing
(heat sink) of the HungaroLux LED based street-
lighting luminaire.

17
Fig. 3. CFD thermal simulation results of LED-
based street-lighting luminaire where the applied
LEDs were all represented by their compact
thermal models obtained from T3Ster TeraLED
results.
two different substrates and the dual-
interface measurement principle can
be applied for obtaining their junction-
to-case thermal resistance. A greater
challenge comes from the fact that
LEDs, unlike other semiconductors,
emit light.
Light emission must be considered
when measuring the LED’s thermal
resistance. For the majority of
semiconductor devices, thermal
resistance can be calculated by simply
dividing the temperature rise by
the electrical power applied to the
package. This is because all of the
supplied electrical power is converted
to heat. However, this is not the
case for LEDs because a significant
proportion of the supplied energy is converted into and emitted as light, making
it an efficient light source. Depending on the LED, energy conversion efficiency
can be as high as 30-40%.
Based on these efficiency figures, if the supplied electrical power rather than the
correct (heating) power is used to calculate the package’s thermal resistance, the
thermal resistance value would be significantly lower, suggesting that the package
(of a less efficient LED) would be far better at dissipating the heat generated in the
LED than it actually is. The emitted optical power can be precisely measured to
account for the calculation of the real thermal resistance if thermal testing of the
LED in question is performed in a CIE 127-2007-compliant total-flux measurement
environment, such as a TeraLED system from Mentor Graphics.
In this system, the temperature of the LED under test can be precisely set to a
desired value by a temperature-controlled cold plate. Such a measurement setup is
also suggested by one of the most recent LED thermal testing standards, JESD51-52,
which provide guidelines on methods to measure LED light output in connection

18
Fig. 4. Luminaire surface temperature map as shown by infrared
imaging at an ambient temperature of 20°C.
with LED thermal measurements. This standard is one of a group of four new
international thermal test standards for LEDs that were published in May 2012.
As for the thermal characteristics of LED components, the junction-to-
case resistance is the most appropriate metric for packaged LEDs because it
characterizes the heat flow path from the point of heat generation at the pn-
junction down to the bottom of the case – exactly how LED packages are designed
to be cooled. A relatively new standard, JEDEC JESD51-14, for junction-to-case
thermal resistance measurement, is based on the latest thermal-transient
measurement techniques.
This method uses a
dual-interface approach
in which the thermal
resistance of the part
is measured against
a cold plate with and
without thermal grease.
The junction-to-case
resistance is determined
by examining where the
two measurements differ.
Very high measurement
repeatability is required
because the thermal
impedance curves for
the two measurements must be identical up to the point where the heat starts
to leave the package and enter the thermal interface between the package and
the cold plate. This ensures that the point where the curves deviate is clear. It
compares to LEDs mounted on a cold plate attached to an integrating sphere (as
the JESD51-52 standard recommends). This method provides the real junction-
to-case thermal resistance metric for LED packages if during the two subsequent
measurements the cold plate with LED under test is attached to an integrating
sphere (as the new JESD51-52 standard recommends).

19
Solutions for LED thermal characterization
The Mentor Graphics T3Ster thermal transient tester uses a smart
implementation of the static test version of the JEDEC JESD51-1 electrical test
method that allows for continuous measurement during a heating or cooling
transient, which also forms the basis of the JESD51-14 test method for the
junction-to-case thermal resistance measurements. This is also the preferred test
method in the LED-specific thermal measurement guidelines that are provided in
the JESD51-51 standard. The combination of Mentor Graphics’ T3Ster and TeraLED
products provide a comprehensive solution for LED testing which meets the
requirements of all the mentioned standards (Fig. 1).
In high-throughput bulk-testing applications (e.g., in large scale reliability
analysis), a multi-channel T3Ster system can characterize many thousands
of LEDs in an hour. T3Ster’s accurate measurements capture transient
responses of LEDs just 1 microsecond after switching the power off with a
temperature resolution of 0.01°C. This means that the earliest possible part of
the LED’s thermal response is captured; thus, you can see the influence of key
constructional features close to the heat source within the LED package, such as
the thermal resistance of the die attach, after a short time.
The T3Ster Master post-processing software fully supports the JESD51-14 standard
for junction-to-case thermal resistance measurement, allowing the temperature
versus time curve obtained directly from the measurement to be re-cast as
“structure functions” (described in JESD51-14 Annex A), and then automatically
determine the junction-to-case thermal resistance value. Structure functions are
also widely used in failure analysis as part of reliability studies mentioned earlier.
This combined with LM-80-compliant lifetime tests of LEDs helps establish
correlation between LED lifetime and degradation of different thermal interfaces
in the junction-to-ambient heat-flow path of LED components (see mycite.omikk.
bme.hu/doc/102602.pdf).
Because the JESD51-14 methodology yields the junction-to-case thermal resistance
as a side product, the step-wise approximation of the structure function up to
this thermal resistance value provides the dynamic compact thermal model of
the LED package automatically. The identified junction-to-case thermal resistance
values may be published on the product datasheet, and the automatically

20
generated dynamic compact thermal model of the LED package can be applied
directly in CFD analysis software such as Mentor Graphics FloTHERM.
The challenge of correct LED thermal characterization is compounded because an
LED’s efficiency is adversely affected by the junction temperature. This presents
a challenge for both LED vendors and SSL designers. The LED’s light output,
junction temperature, and power draw need to stabilize before measurements
can be taken. Consequently, the static measurement method used to capture the
cooling curve is the only correct approach to characterize LEDs.
The combination of the light output measurement (performed with equipment
such as TeraLED), and thermal transient testing allows measurement of the
light-output characteristics as a function of the temperature. Providing these
data as a function of the reference temperature of the cold plate is useful
information for SSL designers. But the same data is also available as a function
of the LEDs’ junction temperature, which is required for the correct physical
modeling of the light output of LEDs, in other words, the input data for hot lumen
calculations. Such a combined thermal and radiometric/photometric test setup
is recommended by the most recently published JESD51-5x series of LED thermal
testing standards.
Street-light luminaires
Hungary, was to develop street-lighting luminaires with the minimal number
of LEDs per luminaire such that all requirements of the rather strict European
street-lighting standards could be met for a wide range of road categories. The
two principal goals to reach were to obtain the required spatial light distribution
pattern (batwing pattern) and to reach the required level of luminance on the
road surface under all possible environmental conditions.
The first goal required careful optical design for which LED vendors typically
publish their LEDs’ so-called trace files.
Careful thermal design is required to achieve the second goal because the
required level of light output must also be ensured on a hot summer evening. For
this, reliable thermal simulations are needed that properly predict the junction
temperatures of LEDs assembled into the luminaire. Unfortunately, luminaire

21
vendors have not yet published LED thermal models. Thermal data on their data
sheet are sometimes questionable because, so far, no testing standard has been
explicit about the combined thermal and radiometric/photometric testing of LEDs
to be able to yield the real thermal metrics of LEDs. The solution to the thermal
design problem of HungaroLux was provided by the combination of Mentor
Graphics thermal testing and CFD analysis tools.
As described in the previous section, thermal testing of LEDs can yield compact
thermal models of their packages that are directly applicable in CFD simulation
tools. The CAD file of the HungaroLux street-lighting luminaire (Fig. 2) was also
directly used to build the final, detailed system-level thermal model. All 48 LEDs
were replaced by their compact models along with a compact thermal model of
the LED driver circuitry.
From the dissipation of the individual LEDs driven by the nominal forward current
(350 mA, 700 mA, 1500 mA), the driver’s dissipation is also calculated. In this way,
the luminaire-level CFD analysis is performed with real data that represent the
LEDs’ junction temperatures (Fig. 3). The CFD thermal simulation results have been
verified by measuring the surface temperature of the luminaire (Fig. 4).
Because the temperature dependence of the light output characteristics of the
LEDs was known from the same measurements that formed the basis of the
LEDs’ compact thermal models, the total luminous flux output of the luminaire
also could be calculated. Using this method, the luminaire could be properly sized
in terms of the number of LEDs needed to provide the required road luminance
level and for the LEDs’ junction temperature.
Conclusions
Recently published LED thermal testing standards and their commercial
implementations provide tools for comprehensive physical testing of power LED
components. Measurement results can be easily turned into LED compact models
that are directly applicable in CFD-based thermal analysis on the luminaire level.
The system-level CFD simulation results also allow the calculation of the hot
lumens of the entire luminaire because the combined thermal and radiometric/
photometric test setup used in the physical characterization of LEDs yields data
regarding the temperature dependence of the total luminous flux of LEDs. With

22
such a diligent and comprehensive characterization method, SSL designers can
be assured that their final LED-based products will meet the applicable lighting
standards and will provide the expected long lifetime.
ANDRÁS POPPE is a marketing manager at Mentor Graphics and an associate
professor at the Budapest University of Technology. ANDRÁS SZALAI is the chief
financial officer of HungaroLux Light. JOHN PARRY is a research manager at
Mentor Graphics Mechanical Analysis Division.

Led lighting outdoor design challenge dec2013

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Led lighting outdoor design challenge dec2013