Lighting Africa is implemented in partnership with: The Global Environment Facility (GEF), the Energy Sector Management Assistance

Program (ESMAP), Good Energies Inc.,

The United Kingdom, Luxemburg, The Netherlands, Norway, The Public-Private Infrastructure Advisory Facility (PPIAF), The Renewable Energy & Energy Efficiency
Partnership (REEEP), and the Asia Sustainable and Alternative Energy Program (ASTAE). For more information: www.lightingafrica.org

Issue 1 – January 2010

Briefing Notes
These briefing notes series will offer insights on key
issues of interest to off-grid lighting professionals. Light Emitting Diode (LED) Lighting Basics
Topics treated here will cover technical, policy and This Technical Brief reviews important photometric terms and concepts. It
business issues that are of particular relevance to the
industry, and provide practical guidance and advice.
addresses some common misconceptions in lighting and is intended to give
They will be published occasionally. In this first edition: retailers, distributors, and consumers a basic understanding of the
• LED Lighting Basics information that is included on an LED datasheet or LED lighting product.
• Thermal management of LEDs

Brightness and Luminous Intensity Basic Photometric Units

Intensity is often equated with how ‘bright’ a light Photometric Term SI unit Basic Units
appears, and was originally described using light from a
Luminous Flux Lumen lm = cd • sr*
burning candle. Such ‘standard candles’ were used to Illuminance Lux lx = lm/m2
define the candela, the basic unit of luminous intensity. Luminous Intensity Candela Cd = lm/sr
A small spot of light like a candle (or an LED) may
appear to be bright, but not produce enough overall light * sr = steradian = solid angle. A solid angle is a two dimensional angular
span in three-dimensional space, like a cone intersecting a sphere.
to cover a larger surface or illuminate a room very well.
Illuminance is the amount of light incident on a surface,
Luminous Flux and Illuminance measured in lumens per meter2 (lm/m2). The unit of
illuminance is lux; 1 lux = 1 lm/m2. A typical handheld
Luminous flux, measured in lumens (lm), is typically illuminance meter measures lux (or foot-candles in
used to describe the total amount of light that a light English units).
source produces in all directions. A lumen represents a
specific perceived amount of light, and takes into
account the sensitivity of the human eye (the eye is more
sensitive to green light and less sensitive to deep red and
deep blue/purple).

Illuminance = Lux = Light incident on a surface.

This is what you measure with an illuminance meter;
this is NOT luminous flux!
LED
Flux vs. Illuminance
Lumen output examples
The difference between lumens and lux is important. A
Standard candle = 12 lumens
focused LED can concentrate light onto a small area, and
Kerosene wick lantern = 8 - 40 lumens
the illuminance at this point can be very high. But the
Pressurized kerosene lamp = 330 – 1000 lumens
total lumen output (luminous flux) for the device can
60 watt GLS incandescent = 900 lumens
still be very low because the light is only emitted in a
23 watt compact fluorescent = 1000 lumens
narrow angle.

Lighting Africa, a joint World Bank and IFC program, seeks to accelerate the development of markets for modern off-grid lighting products in Sub-
Saharan Africa where an estimated 10 to 30 percent of household incomes are spent on hazardous and low quality fuel-based lighting products. The
goal is to mobilize and provide support to the private sector to supply quality, affordable, clean and safe lighting to 2.5 million people by facilitating the
sale of 500,000 off-grid lighting units by 2012 while, at the same time, creating a sustainable commercial platform that will realize the vision of
providing 250 million people with modern off-grid lighting products by 2030. This platform will provide an avenue for social, health and economic
development, especially for households and small businesses that will realize significant cost savings and increases in productivity.
Light Emitting Diode (LED) Lighting Basics www.lightingafrica.org

Color (Chromaticity) Efficacy

The human eye can see wavelengths between about 400 Efficacy is a term used to describe the lighting efficiency
nm (deep purple) to 700 nm (deep red) - this is the of an individual LED or an LED system. Efficacy is
visible spectrum (nm = nanometer). To make a white measured in lumens (total luminous flux) per watt, lm/w.
LED, a blue LED chip is covered with a phosphor that
An LED manufacturer makes efficacy measurements of
converts some of the blue light into other wavelengths.
individual LEDs off of the assembly line, and lists the
The resulting mixture is perceived as white light. The
results when they sell the LEDs to a manufacturer. The
chemical composition of the phosphor determines the
tests are quick and do not allow the LEDs time to warm
specific mixture, and white light of many different
up. If the lamp manufacturer lists these results on their
‘shades’, or color temperatures, can be produced.
packaging, the efficacy will be exaggerated and will not
include losses that occur in a real world LED system.
The color temperature of a white light source is defined
by the different colors of light given off by a heated To get a true picture of the efficacy of an LED system,
‘black body’ emitter (think of a heated filament in a light the entire system must be tested after the LEDs have had
bulb). At lower temperatures, the filament will glow time to warm up. The power measurement should be the
red, then orange, yellow, and white. Heat the filament input power, and all lenses/diffusers should be in place.
further, and the white glow will start to take on a bright
bluish color. These different ‘colors’ of white light are Efficacy values are sometimes included on the datasheet
referred to as color temperature or correlated color for an LED product. This will often be the efficacy
temperature (CCT). Color temperature is expressed in value for the bare LED, taken from the LED
degrees Kelvin (K) manufacturer’s datasheet, and will not include many of
the losses that are part of the completed product.
White LED light with a strong blue component will
appear cool or bluish in color. This is said to have a Efficacy Examples
high color temperature (corresponding to a very hot
filament). If the phosphor has more red component Incandescent GLS (bare) = 15 lumens/watt
added, the LED can appear much warmer and therefore Compact Fluorescent (bare) = 40 - 60 lm/w
has a low color temperature (the filament not glowing LED (bare) = 20 - 100 + lm/w
very far past the red) LED Light (complete system) = 10 - 80 + lm/w

The first white LEDs were high color temperature

(bluish). Recent advances have produced LEDs with Glare
lower color temperatures because some people prefer the
warmer feel of the light. Very warm LEDs, similar to Glare can be a problem with LEDs. Glare results from a
incandescent lights, have CCTs in the 2700K range, concentration of light coming from a small area, and is
while cooler LEDs have CCTs of 5000K, 6000K, or likely to occur any time an LED can be seen directly or
higher. LED reflections bounce off of mirrored surfaces.
LEDs emit light from a small area and are perceived as
This is a “warm” LED This is a bluish, “cool” LED being very bright. However, perceived brightness does
not necessarily mean a high lumen output. Glare, in
addition to being distracting or annoying, can actually
make it harder for people to see the illuminated area.
LED systems should be designed to avoid direct viewing
of the LED sources, either by using diffusers in front of
the LEDs to spread out the light (increase the luminous
surface area) or by shielding the light with reflectors.
Keep in mind, though, that diffusers and reflectors will
absorb some light and decrease the efficacy. A balance
should be struck between efficiency and visual comfort
Technical notes: Thermal Management for LEDs www.lightingafrica.org

Thermal Management for LEDs

Poor thermal management can lead to early LED product failure. Fortunately, these types of failures can be avoided in
the design phase with the use of simple, inexpensive thermal management techniques. This Technical Brief discusses
these techniques and is intended to give manufacturers, engineers, and product managers some basic tools for good
LED thermal system design. Note that LED manufacturer guidelines should serve as the primary guide for thermal
management of their LEDs.
Introduction
Thick copper layers help with heat flow. Use 2 oz. or
Light from an LED (light emitting diode) comes from
the LED chip (or die) within the LED package. Light greater (>70 um) copper whenever possible.
output increases with increasing drive current, but in The copper pads used to solder the LED to the PCB
addition to emitting visible light, the LED chip also should be as large as practical (Fig. 1). Pads should be
becomes hot. This thermal energy limits the amount of on both sides of the PCB. The copper pad soldered to
power an LED can ultimately handle and must be the heat sink side of the LED (usually the cathode) is the
conducted away from the LED chip and dissipated to the most important, but both pads can be large.
surrounding environment. Light output, light color,
lumen maintenance, and LED lifetime are all adversely
affected by excessive LED temperatures during
operation. Use large copper pads for good
thermal dissipation
The critical temperature of the LED is called the LED
chip junction temperature (Tj). Tj is a function of:
• LED component design. Some LEDs have
enhanced thermally conductive lead frames and
can handle higher drive currents and power levels.
• LED forward current and voltage (this is the
power dissipated in the LED). Higher drive
currents result in more power dissipation and
higher Tj.
• Printed circuit board (pcb) design.
• Thermal resistances of components in the system. Fig. 1 Through-hole LED on a Printed Circuit Board
• Ambient temperature (PCB) with copper pads for thermal dissipation

Good LED product design achieves a balance between

maximizing the light output from the LED (by LED heat sink pad thermal vias
increasing the drive current) while maintaining safe
power and temperature levels in the LEDs and other
system components. The designer must understand the top copper
basics of thermal heat transfer, employ proper thermal
techniques in the design, measure the temperature of the
LEDs, and reliably control the delivered power.

PCB (printed circuit board) Design

The PCB used to mount the LEDs is the first thermal

conductor in the system, and frequently has the largest
effect on heat transfer from the LEDs to the ambient bottom copper
environment. Fig. 2 Surface mount LED with thermal vias

Fortunately, copper is an excellent heat conductor and For surface mount LEDs, use large copper pads on both
can serve as both an electrical connection and a thermal sides under the heat sink of the LED. Connect pads with
transfer surface. Just as thicker copper has lower multiple vias (Fig. 2 above) to conduct heat from one
electrical resistance, a thicker copper cross section will side of the pcb to the other.
also exhibit lower thermal resistance.
Technical notes: Thermal Management for LEDs www.lightingafrica.org

Metal Core Printed Circuit Boards (MCPCB) Thermal Measurement Guidelines

Another type of circuit board is called a Metal Core Good thermal measurement results are vital to proper
Printed Circuit Board (MCPCB) that places an design engineering. Care must be taken when making
aluminum plate under the dielectric fiberglass layer (Fig measurements because mistakes will yield temperature
3). This ‘core’ facilitates heat flow and is often mounted readings that are lower than the actual temperatures.
onto a heat sink for use with higher power LEDs.
1) Check thermocouples for accuracy. Use ice water
and boiling water to make sure the thermocouple
measures 0 °C and 100 °C, respectively (these values
apply at sea level; adjust as necessary for altitude).
2) Use thin gauge thermocouple wire (30 gauge or
higher). The thermocouple mass should not be large
enough to conduct significant heat away from the
measurement point. This is particularly true of (5 mm)
leaded thru-hole LEDs.
3) Attach the thermocouple to the LED case location
with solder or a thermally conductive epoxy. Make
sure that the head of the thermocouple is in good thermal
contact with the metal lead. Type “T” thermocouples are
composed of copper based wires and are easier to solder
Fig. 3 Heat flow diagram of a metal core pcb than other thermocouple types. Note that electrical noise
can sometimes interfere with a thermocouple
measurement, which may require the thermocouple bead
Measuring LED Temperatures to be electrically isolated from the LED lead. Read
equipment instructions carefully and refer to
Direct contact measurements of the LED junction thermocouple measurement guides when setting up
temperature are not possible because the LED chip is thermal experiments.
encapsulated. Instead, thermocouples are commonly 4) Allow the LED light to warm up to a steady state
used to measure the LED case temperature Tc (also temperature before taking measurements. This can
known as the solder point temperature Ts or temperature be a half hour or longer depending on the luminaire.
measurement point TMP (Fig. 4)). Tc is specified by the 5) Take multiple measurements on different LEDs in
LED manufacturer, and should be close to the LED chip a luminaire. Some LEDs can be run hotter than others,
junction. For through-hole LEDs, the thermocouple and it is important to measure the hottest LED in a
measurements will be taken on the lead that attaches group. LEDs in the center of an array usually, but not
directly to the LED chip. always, run at the highest temperatures.

This is the lead that will conduct the heat from Heat Flow Basics
the LED chip. It is usually (but not always)
the cathode (-) side of the LED Management of heat in LED products requires careful
attention to heat transfer principles. Thermal energy
(heat) flows from a hot object to a cool object when the
two come in contact with each other. This is called
Tj (LED chip)
thermal conduction, and both objects will eventually
become warm (and equal in temperature) if no more heat
is added to the system. If the warm objects are then
allowed to come in contact with air, and the air is free to
flow around them, the objects will transfer their thermal
energy to the air by a process called convection. With
Tc location. Place
thermocouple
both processes (convection and conduction), the amount
close to LED of heat transferred from hot to cold is limited by the
epoxy surface area of contact between the hot object and the
cold object (or the cold air). This limit in heat transfer is
Fig. 4 Through-hole LED with Tj and Tc locations mathematically represented by a thermal resistance.
Technical notes: Thermal Management for LEDs www.lightingafrica.org

Thermal Resistance Thermal Design "Rules of Thumb”

The flow of heat from an LED chip to the ambient The goal of good thermal design is to keep the LED case
environment can be modeled as a series of thermal temperatures, and therefore the LED junction
resistances between the chip (at Tj) and the ambient temperatures, as low as practical. This means that each
environment (Ta). The sum of these resistances is the thermal transfer step should be designed to lower its
total thermal resistance for the system. The lower the thermal resistance. In general the following rules will
thermal resistance, the more effective the design will be apply to most LED systems:
in conducting heat away from the LED chip junction. 1) Space LEDs and power components as far apart
as the optics will allow. This lowers the energy
One popular method for thermal resistance (R) notation density in the circuit and prevents hot spots.
is to use subscripts to describe the beginning and end 2) Make contact areas between mated parts as large
points of a thermal path. For the entire path this is Rj-a as possible along the thermal path. An increase in
and means the thermal resistance from junction to area will allow heat to conduct more easily between
ambient. Including smaller individual steps along the parts. Use heat transfer grease and thermal pads
way gives the general equation for the thermal resistance where possible to eliminate microscopic air gaps
of the system: between surfaces.
3) Shorten the length of the thermal path and
Rj-a = Rj-c + Rc-hs + Rhs-a maximize conductor cross sections. The farther
heat has to travel from one side of a material to the
Rj-c = resistance from LED junction to LED case other, the greater the resistance. When heat does
Rc-hs = resistance from LED case to heat sink need to be transported a long distance, use thick
Rhs-a = resistance from heat sink to ambient conductor cross sections. As with electrical
conductors, heat flows better in short runs with large
Each thermal resistance step is given in degrees Celsius cross section conductors.
per watt (°C/W) and means the rise in temperature per 4) Use high thermal conductivity materials. Copper
watt of power dissipated. is a very good thermal conductor, followed by
aluminum. Steel, particularly stainless steel, and
The LED manufacturer should list the value for Rj-c on plastics are poor thermal conductors and should be
the LED datasheet. Measurements of this LED case avoided in the thermal path.
temperature Tc can then be used to estimate Tj. 5) Design the luminaire housing to allow heat sink
(see “Estimating LED Junction Temperature” below) components contact with external ambient air.
The final step in the heat transfer path will be the
Package type/ Typical Thermal
typical power resistance (Rj-c )
transfer of heat to the ambient air. This is a critical
step and often misunderstood. A large heat sink
T-1¾ or
5mm LED
may not perform well if it is insulated or isolated
Up to from ambient airflow. Air itself is a poor thermal
20 to 30 mA 300 °C/W
0.07 to 0.1 watts conductor, and heat sinks rely on the process of
Thermally enhanced convection to transfer thermal energy. For heat
through-hole sinks that are contained inside a plastic enclosure
70 mA 130-160 °C/W (the plastic case of the luminaire), the pocket of air
0.18 to 0.23 watts surrounding the heat sink will be responsible for
Surface mount conducting and convecting heat out of the heat sink.
SMT type “½ watt” If this pocket is isolated or sealed from the outside
100 to 350 mA 40 – 130 °C/W air, it may limit the transfer of heat and lead to
0.3 to 1 watt higher LED junction temperatures.
Surface mount 6) Measure LED temperatures in normal operation.
SMT power package Careful measurements will reveal hotspots in the
350 mA to 1+ Amp 3 – 20 °C/W design. Always include actual ambient conditions in
1 to 10+ watt the tests, and allow the LEDs time to reach a steady
state operating temperature. This can be 30 minutes
or more depending on the size of heat sink
Table 1. Typical LED thermal resistances Note: These are components.
approximate values only. Actual thermal resistances vary by
LED type and should be provided by the LED manufacturer.
Technical notes: Thermal Management for LEDs www.lightingafrica.org

Estimating LED Junction Temperature LED Lighting Product Lifetimes

To calculate LED junction temperature, we must know Under normal operation and with proper thermal design,
the LED case temperature, the wattage (power) of the LEDs can operate for thousands of hours. The light
LED, and the thermal resistance (Rj-c) (Example 1). output, however will decrease over time in a non-
reversible process called lumen depreciation. LED
The power (in watts) dissipated in an LED is given by lifetime is commonly given as the point at which the
P = I x V, where I is the current in amps and V is the LED produces 70% of its initial output. This is called
voltage drop of the individual LED. The voltage drop V the L70 lifetime.
can be measured with a multimeter when the LED is
running normally. The current can be measured with a Many manufacturers claim 100,000 hour lifetime ratings
meter placed in series with the LED string, or calculated for their LED products. These claims are often overly
from the voltage drop on a series resistor R using Ohm’s optimistic and not supported by experimental data or
Law (V = I x R). Allow the system to reach a steady actual product testing. The use of 100,000 hour lifetime
state operating temperature before taking measurements. ratings on product literature can spoil the marketplace by
creating unrealistic expectations from consumers eager
For LEDs in series, the current in each LED is the same. to try new LED technologies.
For LEDs in parallel, the current may not be equal and
some will have higher currents. In these cases it is To ensure adequate lifetimes, LED temperatures should
necessary to measure the current in each LED string be measured under real world operating conditions and
simultaneously by inserting multiple current meters or the measurement results compared to lumen depreciation
having individual series resistors on each string. data from the LED manufacturer. In addition, multiple
test products should be run continuously early in the
Absolute maximum temperature ratings should be design phase. Lighting Africa recommends at least 2000
available from the LED manufacturer and listed on the hours of test operation to rule out the possibility of early
LED specification sheet. A Tj of 125°C is a common failure.
maximum rating. While the LED can survive at this
temperature, its lifetime may be very short. Heat Flow Basics
Conduction – transfer of heat through matter by
LEDs that run at excessive temperatures will
communication of kinetic energy from particle to
have very short lifetimes and fail to produce particle. An example is the use of a conductive metal
adequate light after a few short weeks or months such as copper to transfer heat.
of operation.
Convection – heat transfer through the circulatory
Example 1: The LED Tc = 60 °C. The thermal motion of a liquid or gas in contact with a hot surface.
Air surrounding a hot object removes heat by
resistance for the LED is 200°C/W, and the LED
conduction and convection, where gas molecules flow
is being driven at 0.08 watts (P = I x V). A quick past the surface and remove heat energy. Good
calculation (200 °C/W x 0.08 W = 16 °C) shows circulation is important to good heat transfer.
that Tj is 16°C higher than Tc and so: Tj = 76°C
Radiation – energy transmitted through infrared
electromagnetic waves. Visible light LEDs do not
Dear readers: produce significant infrared radiation.
• We welcome your suggestions on the topics to cover
in these Briefing Notes. Tell us what are the issues Heat sink – any thermally conductive element
of interest to you and we will make an effort to designed to transfer heat from a heat source (the LED)
cover them in our upcoming editions. to the ambient environment. Heat sinks with fins are
• To subscribe to the Lighting Africa Briefing Notes, go common and work by creating a large surface area that
to: www.lightingafrica.org/user/register
increases the flow of heat through convection.
Regards

Lighting Africa, a joint World Bank and IFC program, seeks to accelerate the development of markets for modern off-grid lighting products in Sub-
Saharan Africa where an estimated 10 to 30 percent of household incomes are spent on hazardous and low quality fuel-based lighting products. The
goal is to mobilize and provide support to the private sector to supply quality, affordable, clean and safe lighting to 2.5 million people by facilitating the
sale of 500,000 off-grid lighting units by 2012 while, at the same time, creating a sustainable commercial platform that will realize the vision of
providing 250 million people with modern off-grid lighting products by 2030. This platform will provide an avenue for social, health and economic
development, especially for households and small businesses that will realize significant cost savings and increases in productivity.