To put it simply
EL lamps or "high field electroluminescent" lamps use electric
current directly through a phosphor to make light. Unlike most lamps,
they can be shaped to be extremely flat, or in narrow wire-like shapes. Electroluminescence or "EL" is the non-thermal conversion
of electrical energy into light energy. This phenomenon is used in EL
lamps, LEDs, and OLEDs. In this page we talk about EL devices which
create light by exciting high energy electrons in phosphor materials
like ZnS:Mn. This type of device uses "high field electroluminescence".
-It is different from LEDs/OLEDs
in that OLEDs use a p/n junction (two semiconductive materials where
electrons and holes combine on the boundary). In EL there is a layer
called activator in which the whole layer is emitting light, not just
-It is different from incandescence.
With incandescence you send current through a material, this creates
heat and that heat emits light at a high enough temperature.
More on How it EL Works (below)
uses: nightlights, decorative luminescent clothing, watch
illumination, flat wall decorative illumination, durable waterproof
displays, medical tool display screens, and most recently computer monitors
Classic TV sets
use electroluminescence. CRTs (Cathode Ray Tubes) have an electroluminescent
coating made of rare earth oxides and oxysulfides. These materials glow
when struck by electrons fired from a cathode in the back of the tube.
credits and sources are located at the bottom of each lighting page
consumer may be familiar with brand names Panelescent and Indiglo
which feature lamps and watch lights that use EL. Planar
and Sharp Corporation have
pioneered the use of electroluminescence in displays.
One of the first EL lamps on the market: Panelescent by Sylvania.
120 V .02W nightlight, directly plugs into wall and provides
a soft green glow.
-No external circuitry required (no ballast needed to limit current,
it can be plugged directly into AC power and will self-regulate power
through it's own resistivity)
-Can be manufactured into flat flexible panels, narrow strings, and
other small shapes
-Can be made into waterproof computer monitors which are more durable
and light weight than LCDs or Plasma screens.
-Not directional like LCDs when used as a computer monitor, looks good
at all angles
-EL displays can handle an impressive -60 C to 95 C temperature range,
which LCD monitors cannot do
-Not practical for general lighting of large areas due
to low lumen output of phosphors (so far)
-Poor lumens per watt rating, however typically the lamp is not used
for high lumen output anyway
-Reduced lumen output over time, although newer technologies are better
than older phosphors on this point
-Flexible flat EL sheets wear out as they get flexed, durability is
being worked on
-The lamps can use significant amount of electricity: 60-600 volts
-Typical EL Needs a converter when used with DC sources such as on watches
(to create higher frequency AC power, this is audible)
per watt: 2-6
life: 2,000 - 50,000 hours
*Color Temperature - N/A
*Available in 0.01 - 3 W
EL background with LCD display,
marketed as "Indiglo" by Timex in the 1990s
electroluminescent exit sign, easy to operate on low power and very
long lamp life. Photo: Limelite
1. How it Works:
There are several
variations on how EL works depending on whether you are talking about
a flat panel light, rope light, DC EL technology, thin film EL display,
or other complex design.
are monocarrier devices which give off light due to impact excitation
of an optical center like the Mn atom. They do this by transporting
high energy electrons in the host matrix (commonly ZnS).
we will describe a simple EL lamp:
High voltage AC power passes through a thin layer of phosphor or semiconductor
and this causes emission of light. Two layers of solid material (one
being transparent) act as electrodes and a powder phosphor or semiconductor
in between glows when electrons pass through it from one electrode to
another. Light escapes the device on one side thanks to the development
of transparent conductors like indium tin.
phosphor powder EL Lamps are used in most simple lamps used
for illumination including the night lights and exit/safety signs. The
graphic below shows thick phosphor lamps.
Film and Thick Dielectric EL (TFEL, TDEL): this technology
is used in a variety of applications, EL displays (ELDs) are the most
common use. A display is not a "lamp" in the traditional sense,
however we cover it here due to its importance in the development of
EL. TFEL and TDEL often use rare earth materials such as Er, Tm, In,
- Thin Film Electroluminescent Devices
TFEL emerged in the 1950s and it different in that it contains thinner
active layers and a different construction. TFEL was an improvement
over thick powder construction, it allows for small devices and precise
control of pixels on a display. It was a challenge to develop ways to
deposit/grow thin polycrystalline films onto a substrate (the supporting
material) however many processes have been developed to allow TFEL technologies
to expand. Below we highlight the basic construction of a TFEL device. Note:
-TFEL has a maximum of 6 lumens per Watt as of 2012.
-TFEL typically requires 1.5 Megavolts per centimeter to cause the active
layer to make light
Video of TFEL
TFEL has a phosphor layer that emits light when a big enough electrical
field is applied. This thin film of a phosphor requires such a high
level of energy that there is a potential for a damaging short circuit
through imperfections in the phosphor. Insulating layers are used between
the electrode and the phosphor on both sides to limit current and make
the TFEL work properly.
TFEL devices behave like
3 capacitors in series: voltage rises and a breakdown voltage is reached
where current flows through the semiconductive layer (the phosphor)
which excites the phosphor and makes light. The insulator layers act
as capacitors, with voltage building and breaking through.
Before you see how the EL
device works you may want to review how a capacitor works in this video:
As voltage increases more and more Manganese centers become excited
and the device brightens. (6 lumens per Watt) After a while the increasing
voltage does not make the device brighter because the Mn centers become
saturated, efficiency goes down at this point (to 3 lumens per Watt
Making primary colors used in displays has been a major problem preventing
the use EL for TV or Computer monitors until recently. Engineers have
used filters to make light. Colors can be made by filtering white into
red, green and blue, but developing an efficient white phosphor has
been hard. Engineers have also worked on developing separate RGB phosphors.
ZnS:Mn makes a green, which is the most efficient phosphor. Many EL
displays are green and red because they can filter red from the green.
CaS:Eu creates red but it has not been bright enough. Making an efficient
blue phosphor with a good enough brightness has been the challenge.
0.1 lumens per Watt, achieved by Heinrich Hertz Institute is just not
good enough. Remember that this must compete with LCD technology in
order to survive on the market. BaAl2S4:Eu is a primary phosphor used
for blue. While TFEL has had trouble getting acceptable efficiency,
TDEL has achieved a more acceptable rate of 3 lumens p/W.
using EL to backlight liquid crystal number segments
Most ELs are made
with ZnS:Mn (zinc sulfide doped with manganese)
Other materials used to make EL lamps are
- Zinc sulfide with Cu or Silver
- Zinc sulfide with various alkali metals for blue, green, red,
-diamond with boron - blue color
-III-V semiconductors InP
-GaAs, and GaN
film EL uses a process of epitaxy to grow crystals on top of a substrate.
This process allows one to create a "film" or ultra-thin layer
of material (measured in nanometers (nm)) on glass or other flat surface
(this surface provides structure and is called the 'substrate').
TFEL epitaxy creates layers about 500 nanometers thick, although
the size varies depending on the product. Later on TDEL
(thick dielectric EL) was developed to produce a product with higher
luminosity than TFEL. TDEL uses a structure where electrodes are separated
from the thicker phosphor by a thin insulative layer. Both TFEL and
TDEL use epitaxy, there are many forms of epitaxy from MBE (molecular
beam) to ALE (Atomic Layer Epitaxy) (which was renamed ALD (Atomic Layer
Deposition)). Understanding epitaxy requires a bit of time, we recommend
online lectures and web sites for this area. Read more about ALD from
Tuomo Suntola here
and Non-transparent EL displays
way to build a non-transparent TFEL display is to use two layers for
plastic film or glass, one is coated with indium tin oxide (ITO) or
other semiconductor while the other flat surface has a reflective material.
Light will be produced in the 'active' layer of phosphors (ZnS Mn for
example). Light emitted in the wrong direction will be reflected off
the back plate and go through the opposite side which has the transparent
semiconductor, this way you achieve a higher luminosity. With many individually
controlled units and a controlling computer you can turn the unit on
or off, collectively this will make a display screen. In a multi-color
display filters applied over top of the units can control whether the
unit emits red, yellow or green light. Blue has not been developed yet,
and it is because of this EL displays cannot currently compete with
LCD technology for full-color consumer displays.
displays two layers of transparent conducting films (TCFs)
as the electrodes with the phosphor in between. Since they do
not have a reflective backing they do not currently produce
the same level of brightness as standard EL displays. Despite
this the display has some very interesting and unique applications
which have not become widespread yet.
conducting films (TCFs)
include indium-tin oxide (ITO) and fluorine doped tin or zinc oxide
(FTO)(FZO). ITO is also used in the thin-film solar industry. Carbon
nanotube technology is an organic conducting film which could replace
expensive rare earth materials like indium. Poly(3,4-ethylenedioxythiophene)
PEDOT films and other polymer films also have potential to replace ITO.
Making newer cheaper materials is important for seeing the growth of
EL displays and lights in the daily life of consumers.
This type of lamp makes light as electrons radioactively combined in
holes of a semiconductor. Understanding how semiconductors work on a
molecular level requires a long description or entire lecture. The Indian
Institute of Technology Madras has a multi-video lecture starting with
a 59 minute video on solid state
TDEL or thick film dielectric EL technology is known for providing a
solution to the blue problem. It provides the only full color RGB display
technology available at this time.
film dielectric displays have proven to be effective: they have a good
brightness (luminosity) and have a decent efficiency. iFire Group and
TDK Corporation currently hold the patents for this technology. The
phosphor in TDEL is 10K - 20K nanometers thick. Some TDEL like that
used in displays uses two layers of phosphors. The bottom thick layer
is resistant to dielectric breakdown, so it can transport a higher current
and make a brighter light. Above the thick dielectric layer is colored
phosphors of ZnMgS:Mn (green) and BaAl2S4:Eu (blue). With this system
RGB can be created.
Electroluminescence was used
as early as 1936 by scientist Georges Destriau. It wasn't until the
1950s when companies began developing the technology to be used for
- Georges Destriau, who
was an associate of Marie Curie in her lab in Paris began studying
electroluminescence. He coined the term as he worked with ZnS
powders. Paris, France
- Elmer Fridrich while working
for General Electric developed EL lamps, some of which where
quite sophisticated in design. Fridrich also became famous for
inventing the halogen lamp and advancing
fluorescent lamp technology.
He was a key member of engineering teams at
Nela Park, Ohio and Schenectady, New York.
- Nataliya Andreeva Vlasenko
and A. Popkov: Developed
the first TFEL prototype and worked on methods for boosting
luminosity. They pioneered early work on DC EL lamps as well.
- Aron Vecht develops DC
EL technology for lamps and watches. London,
University of Greenwich
- Tuomo Suntola develops
ALE Epitaxy for Thin film electroluminescent (TFEL) technologies.
This method of depositing thin semiconducting films on a substrate
has become a basis for TFEL production. Thin polycrystalline
films are about 500 nanometers thick. Thin films allow for a
more uses of EL than clumsy thick phosphor powders.
Lohja, Finland Photo:
- Hiroshi Kobayashi worked
over 30 years on inorganic electroluminescent devices with late
Professor Shosaku Tanaka. His work
helped with the commercialization of inorganic EL displays in
Japanese industry. A great deal of work was done at Tottori
University. He retired in 2003 and now lives in Tokyo. Tottori Prefecture / Tokyo, Japan Photo:
- Toshio Inoguchi develops
the first practical ELD (electroluminescent display) at Sharp
Corporation. He uses TFEL to make this possible. His displays
have long life and are brighter in luminosity. His work set
the stage for later advancements and kept Sharp at the leading
edge for the next few decades. The displays were used first
as displays for medical instruments. The displays were monochromatic,
but a better option than CRTs.
Toshio Inoguchi and Sharp Corporation
- Christopher N. King and
team* develop advanced EL displays which use thin film technology.
The team had started at Tektronix and launched spin-off Planar
Systems in 1983. The new displays increase the number of
available colors as time goes on. Increasing luminosity and
contrast to compete with LCDs became important in the 1990s
and 2000s. Since the 90s the engineers at Planar have improved
the EL display, they have achieved better luminosity, contrast
and efficiency. *Jim Hurd, John Laney, Eric R. Dickey
(ICEBrite) Beaverton, Oregon Photo:
- Xingwei Wu develops TDEL
technology. Thick Dielectric EL displays achieve blues bright
enough to be used in full color displays. TDEL is brighter than
TFEL, and uses "color by blue" method to achieve good
RGB. TDEL is the first full color capable EL technology. Dr.
Xingwei Wu is the primary engineer at iFire
Xingwei Wu. iFire Technology Ltd.
- You - Choose a career
in engineering and be the next pioneer! LEARN
2000s - EL lamps become
more affordable to the average consumer and are used in decorative clothing
and thin film application on various products. As a lamp for general
illumination EL technology is not preferred due to limited maximum lumen
production combined with low efficiency compared with LEDs. The unique
spatial aspect of the EL lamp (flat and flexible) allows it to maintain
a market niche.
EL displays have come a long
way since 1980 however a better blue phosphor which can be used in displays
is still needed. Developing a high-luminosity, high-efficiency blue
would allow a red-green-blue combination that would allow the EL display
to better compete with LCD.
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graphics and article by M. Whelan
Thanks to assistance from Chris King, Toumo
Suntola and Toshiyuki Matsumura
Greenwich University A
History of Electroluminescent Displays by
Jeffrey A. Hart, Stefanie Ann Lenway, Thomas Murtha. 1999
The General Electric Story. Hall of History. Schenectady Museum
"Electroluminescent Displays" by Christopher N. King
Edison Tech Center
Planar Systems www.Planar.com
Christopher N. King
iFire Technology Ltd.
Prof. Emeritus Hiroshi Kobayashi of Tottori University
Edison Tech Center Photos may be used/reproduced for educational purposes,
the photos may NOT be altered except for resizing. The Edison Tech Center
must be credited. Linking to this page is preferred for online publications.