22
PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 84 NR 8/2008
Sandra TABI
ŠOVA
Slovak University of Technology in Bratislava
LEDs and their application in practice
Abstract. This paper handles with the properties and application of nowadays most forwarding light source, LED, particularly from the point of view
of lightning technology. There were done measurements on white, red and blue diodes.
Streszczenie.
W artykule przedstawiono problematykę cech i zastosowań obecnie najbardziej rozwijających się źródeł światła, LED, zwłaszcza z
punktu widzenia techniki świetlnej. Wykonano pomiary diod białych, czerwonych i niebieskich. (Diody elektroluminescencyjne i ich praktyczne
zastosowanie).
Keywords: light, LED, measurements, lifetime, luminaire.
Słowa kluczowe: światło, diody elektroluminescencyjne, pomiary, trwałość, oprawa oświetleniowa.
Introduction
LED was used more than 30 years only in special
applications as: signal-technique. The reason of their thin
application area was their low specific output and their
colour rendering. In the last years this light source features
beating up lighting and technical parameters, such as a
specific output, luminance and low consumption. LEDs are
moderately efficient, the average outputs is about 32 lm/W,
new technologies deliver specific output up to 80-100 lm/W.
Advantages and disadvantages of LED
LEDs are typically much smaller than conventional light
sources, allowing for dramatically different lighting designs
capitalizing on the unobtrusiveness of the source.
Designing with LED for example, can enable completely
hidden sources of light, eliminating large visual luminaries,
and creating a magical effect when illuminated.
Fig. 1. One Chip LED lamps.
In figure 1 there are two LED lamps which can be used
instead of standard bulbs or halogen lamps. Every lamp
consists of LED with power of 3W. The construction of
these lamps has also the function of cooling system.
Light emitting diodes provide the lighting designer with
additional options and choices compared to conventional
technologies. Instead of taking one very bright source and
mounting it in a reflector optic to distribute light, LEDs can
enable alternative lighting concepts where the light source
can be divided into multiple points of light, distributed
across a surface, or placed in multiple planes. What is listed
in figure 3.
These devices have no moving parts, no fragile glass
environments, no mercury, no toxic gasses, and no
filament. There is nothing to break, rupture, shatter, leak, or
contaminate. Unlike typical conventional light sources,
LEDs are not subject to sudden failure or burnout. There is
no point in time at which the light source ceases to function;
instead, LEDs gradually degrade in performance over time.
Conventional light sources (as well as some LEDs) contain
invisible radiation as well as the visible component of light in
the beam. This radiation can be very short wavelength blue,
known as ultraviolet light, or long wavelength red, known as
infrared, which causes heat. Ultraviolet light can, and will,
damage materials, cause colour changes and eventually
breakdown
many
materials.
Museums
and
other
applications where ultraviolet light is a liability use
expensive low flexibility light pipes to filter out this harmful
component of the generated light. Frequently the light
source used for these light pipes is a very bright, hot,
incandescent or halogen sources, generating most of their
light as heat. Infrared light can damage displayed objects,
increases air conditioning costs, decreases environmental
comfort, and when reflected off reading surfaces increases
eyestrain.
Since LED based light sources last at least 10 times
longer than a normal light source, there is no need to
replace the light source, reducing or even eliminating
ongoing maintenance costs and periodic relapsing
expenses. Because if we use LED instead of a conventional
light source, there we have to replace online one piece of
LED, not all LEDs which are in this lamp.
Light emitting diodes provide even greater advantages
in applications where location makes replacement difficult.
Radio towers, offshore marine buoys, aircraft warning lights,
and bridge and tunnel lights all require expensive
maintenance fees due to their prohibitive location.
Eliminating or dramatically reducing the frequency of
required maintenance can save thousands of euros per
maintenance call in these applications.
The long life of a LED solution also allows for trouble
free designs. For example, even with the temperature
extremes, high shock and vibration environment of trucks
and cars.
Light Emitting Diodes require no filters to create
coloured light, resulting in deeper saturated colours without
wasted light. Deep reds, greens, blues and other colours
can be produced in monochromatic form directly from the
solid state element. Many lighting applications, such as
signs, signals, and warning lights, are coloured. Usually this
light is created through filtering. These filters block the
undesirable portions of the white light and let only the
desired wavelengths through. The blocked light that is not
of the correct colour is therefore lost energy.
Therefore, to make a red sign, one can use a
monochromatic LED that generates only red light and
therefore does not require an additional colour filter. There
is no need for expensive filters, and there is no wasted
energy.
The light emitted from an LED is directional. Typical
conventional sources such as incandescent, halogen, or
fluorescent lights are omnidirectional, emitting light in all
directions. In order to direct the light onto the object to be
PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 84 NR 8/2008 23
illuminated, light that is not directed toward the desired
location must be redirected using secondary optics. Each
time a light beam is reflected and some of its intensity is
lost, resulting in a fixture loss. Typical fixture losses range
from 40 to 60%, meaning that in some cases less than half
of the light generated by the source is directed in the
desired direction. The directed nature of LEDs can result in
fixture efficiencies of 80 to 90%, requiring less total lumens
to provide the same level of illuminance. When considering
LEDs as a light source it is important to consider all factors
in determining the appropriate solution. For example, if you
are replacing a 500 lm source in a fixture with 50%
efficiency with an LED based source in a fixture with 85%
efficiency, less than 300 lm will be required from the LED
source for an equivalent illuminance. A side benefit from the
directionality of light emitted from the LED solution is the
reduction of light pollution.
Light emitting diodes are solid state devices containing
no moving parts and no filaments to break. As such, LEDs
handle rough environments including heavy vibration and
impact. Unlike conventional light sources, which typically
contain a fragile filament enclosed in a breakable glass
enclosure. The solid state natures of LEDs make them
extremely rugged and durable and excellent choice for
applications where reliability and dependability are
paramount.
Colour LEDs can be used for dynamic colour changing
applications. With fast turn on times (measured in
microseconds) it is easy to instantly change from one hue to
another. Further, you can generate more brilliant colours
than are currently possible on conventional LCD or CRT
monitors, these LEDs offers a larger colour range than is
possible using conventional lighting technologies. Not only
can you use LEDs for viewing applications (such as TVs or
computer screens); the endless choices of colour are
available for almost any application. Imagine everything
from a stadium replay panel, laptop screen, wall up lighters,
pools and fountains. As white light is the combination of red,
green, and blue (RGB), we can easily create any colour of
white (CCT) from warm to cool blue by changing the relative
intensity of each colour component.
LEDs switch on and off so quickly and have an
exceptional dimming range. Light emitting diodes are fully
dimmable with straightforward driver topologies. This
dimming ability, through the common technique of pulse
width modulation (PWM) of the drive current, allows for
environmentally friendly illumination without sacrificing
either the Colour Acceptance Index (CAI) or the Colour
Rendering Index (CRI). With PWM control, the colour
provided is independent of the set intensity. Therefore it is
easy to optimize the desired colour and then adjust the
intensity to suit the needs of virtually any application. The
use of an LED based lighting system enables a wide colour
gamut of saturated static and dynamic lighting effects.
Whether the desired effect is a tunable white source or a
single light capable of digitally producing any colour.
Many light sources in use today are not well suited to
cold environments. In some cases, such as walk in
refrigerators, cool outdoor climates, or automotive
applications expensive drivers are required to enable
ignition at low temperatures. LEDs do not require any
special equipment or drivers, regardless of the temperature.
Cold start is not a problem for this solid state technology,
down to -
40°C. This cold start ability allows for instant on/off
control without specially designed circuitry, simplifying
system design while lowering the cost of the electronic
driver [1].
Street lighting
For public lighting there are used high pressure lamps.
In older lighting systems are used mostly mercury high
pressure lamps. In this pressure lamp, like in every
pressure lamp is the generation of light based on another
principle than it is at the solid state device LED. Mercury
pressure lamps have relatively bad colour rendering Ra=40-
60, lifetime 8-12 000 hours and luminous efficiency 40-60
lm/W. High pressure mercury lamps are replaced by high
pressure sodium lamps, which have better technical
properties. The tube in this lamp is filled except other
materials mostly with sodium. This lamps have higher
lifetime up to 32 000 hours, high luminous efficiency 60-130
lm/W though the colour rendering is very low. In the past
few years there have been developed high pressure sodium
lamps with a good colour rendering Ra=30-80.
For public lighting is also possible to use fluorescent or
compact fluorescent lamps. These are low pressure lamps
with lifetime about 8000 hours under service with
conventional ballast. If we use electronic ballast, the lifetime
of a low pressure lamp is ca.18 000 hours. Colour rendering
of these light sources is very good it is between Ra=80-100.
Problem on this lamp is the dependence luminous flux on
the surround temperature. In the summer and winter
months there decreases the luminous flux. This problem is
solved by a specific construction of fluorescent lamps. The
luminous flux is not adequate for lighting of motorways. This
light source is better to use for lighting of parks or
pedestrian zones. Adequate light source for public lighting
is the metal halide lamp, which has a better colour
rendering and higher lifetime than high pressure sodium
lamp. Nowadays there are some lamp producers, which
produce and sell luminaries for street lighting with LEDs. In
figure 2 is an example for such street luminaire.
Fig. 2. MILLENIO LED street luminaire [2].
As I ment
ioned in paper from Lumen V4: „LED lamp for
public lighting” it would be much more expensive using the
LEDs instead of high pressure lamps. But now, there are
produced LEDs with higher outputs for lower prices per
piece then it was two years ago. Because of the sinking
price, there is given a possibility to produce exterior
luminaries for lower prices as it was mentioned in that
paper.
Fig. 3. LED exterior luminary and conventional used exterior
luminaries [4].
24
PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 84 NR 8/2008
In figure 3 it is shown a LED exterior luminaire. This
luminaire was designed and produced by our student Ing.
František Fülöp within the frame of his master thesis. As
we can see in the picture there were used only 20 LEDs,
what means a constant lighting even if one of them is no
more functioning. This luminaire can be used in parks,
these more sources in one luminaire provide more safety
for the pedestrians. The price for such luminaire is about
two times higher than a price for a standard street
luminaire. This diodes should light 100 000 hours, what is
given by the producer of this devices. This means almost no
costs for the service.
Factors influencing the lifetime of LED
To the factors which affect reliability and therefore also
the lifetime of LED is especially temperature. With the
increase of the temperature is the specific output sinking.
Maximum allowable temperature of the junction T
J
should
not be during the operation overruned (figure 4).
Junction Temperature T
J
(
°C)
R
e
la
ti
v
e
l
u
m
in
o
u
s
f
lu
x
Fig. 4. Relative luminous flux versus junction temperature of the
source [3].
Operating temperature is the primary condition,
depending on the surrounding. LED performance largely
depends on the ambient temperature of the operating
environment. "Driving" a LED "hard" in high ambient
temperatures may result in overheating of the LED
package, eventually leading to device failure. Adequate
heat-sinking is required to maintain long life. This is
especially important when considering automotive, medical,
and military applications where the device must operate
over a large range of temperatures, and are required to
have a low failure rate.
Another influence of the lifetime of a LED is for example:
mechanical influence as
a pressure or drawing force, which
increase mechanical tension in light emitting diode. Also
infrared radiance means always a supplementary heating
up of the source, whereby it is possible to overrun the
temperature range, at which can LED emit light. Ultraviolet
radiation is also unsuitable effect for the quality of radiance
of LED, because it is rich of energy and the consequence is
quenching of the resinous cover. Same like by virtue of IR
and UV radiance, comes by the contact of LED with
chemical gases to the decrease of the lifetime. Depending
of the used material can the lifetime through the service in
surrounding with a high dampness and temperature
drastically shorten.
Measurement
We decided to measure if there is any decrease in
luminous flux through the lifetime of LEDs, because this is
one important factor influencing the application in street
luminaires, as well as in another then only in street
luminaires.
We used 30 LED for the measurement of the lifetime.
These LED had different colour, there had ten pieces blue
colour, ten had red colour and ten were white. Every diode
was in the circuit with a resistance. Used were LEDs which
has these properties, given by the manufacturer:
-operating temperature range: -
30 °C to +80°C,
- red LED was made of AlGaInP, blue and white
semiconductor material was InGaN,
-luminance of red and blue LED was between 3000-4000
mcd and the white had 11 500-16 500 mcd,
- power is 100 mW at the white, at the blue and red LED is
10 mW.
This circuit consisting of 30 LEDs and 30 resistances
was connected to constant-current supply. At this way
connected LEDs did light constantly. Their lighting was
stopped only during the measurement, which took about
one hour. Measurement on LED was done in different time
periods. Through spectrophotometer USB 2000, notebook
and multimeter. LED was positioned in a measurement
apparatus, which was connected to the spectrophotometer.
The ambient temperature was also marked, it moved in the
temperature interval 21,5-
25ºC. Scheme during the
measurement is in figure 5.
Source
A
R
I
S
PC
+
-
Fig. 5. Scheme during the measurement (S- spectrophotometer, A -
multimeter, I - integrator, where the LED was inserted and R - serial
integ
rated resistance R=300 Ω, PC - notebook).
Up to this day were done ten measurements for each
type of used LED, together we collected more than 300
values. From the tables we can identify, that the decrease
of the luminous flux is visibly at every diode but the most
visible decrease of the flux is measured at the white LEDs,
as we can see in figure 7. Luminous flux did sink at these
diodes less than 60%, even on the LED was this decrease
near to eighty percents, what some manufacturer describe
as the end of the lifetime of light source.
The lowest decrease at the red LED was on the red
LED, it was 25% toward to the initial luminous flux.
Accordingly relative low decrease of the luminous flux
measured at the red LED, was on the blue diodes, where
the biggest ultimate decrease was 38%. One blue LED
diode at the last measurement ended her life after 1336,8
hours of lighting.
Fig. 6. Percentage luminous flux versus time.
Conclusion
From the measured values we did find out, that the
decrease at the white LED was average 60% of their
beginning luminous flux. At the similar, how were used LED
diodes for measurement, the producers indicate the lifetime
between 8000 to 10 000 hours. A good question is, if for
PRZEGLĄD ELEKTROTECHNICZNY, ISSN 0033-2097, R. 84 NR 8/2008 25
selling bidden diodes really do satisfy the demand of
maximum decrease of the luminous flux by 20% through
their lifetime, as many producer of lighting sources indicate,
seeing that after 1336 hours of lighting is the decrease of
luminous flux at the majority of LED bigger than 20% and it
was even showed, that one lighting source would be no
more useful in the practice. Interesting would be
measurements with more than 30 LEDs, as well as with
power LEDs.
LED could be effectively used in areas, where is not
heavy traffic, for example in the villa areas or in the
communities in switching schedule, as a reaction for coming
vehicle or pedestrian. Zero time of switching on and minimal
costs for such lamp by using LED as light source, would be
in this lighting system a great benefit opposite high pressure
lamps, which have a longer time for start and they cognize
of failing by switching service. Nowadays we provide
measurements on the university with LEDs, which are in
switching schedule. These results will be interesting for the
using in exterior lighting luminaires.
LED lighting can ensure a safe traffic and for pedestrian
can sink risk of crime delinquency.
Through this lighting which uses a tight viewing angle
will decrease in many incidents glare for the pedestrians
and it will decrease the values of luminance in
today’s often
bad illuminated parts of city communications.
REFERENCES
[1] Philips Lumileds lighting company,Benefits of Philips Lumileds
Solid State Lighting vs. Conventional Lighting. [online].
Available on internet: <http://www.luxeon.com/pdfs/AB17.pdf>,
2006
.
[2] HESS. Available on internet: <http://pdf.archiexpo.de/pdf/hess-
form-licht/millenio-led-strassenleuchte/9916- 1665.html>, 2008.
[3] GEMMER,W.: Light emitting diodes
. In: Handbuch für
Beleuchtung. Horst Lange, 5.Auflage, 2005. I - 6.13.1
– I –
6.13.8. ISBN 3-609-75390-0.
[4]
SITECO.Product
catalogue.
Available
on
internet:
<.http://www.siteco.de/de/produkte/aussenleuchten/chapter/15
46/category/6910/family/6920.html>, 2008.
Author:
M. Sc. Sandra Tabišová, Slovak University of Technology
in Bratislava, Faculty of Electrical Engineering and Information
Technology, Department of Electrical Power Engineering,
Ilkovičova
3,
812
19
Bratislava,
Slovak
Republic,
sandra.tabisova@stuba.sk