EL.FEB–Electroluminescent Technologies–pm
EL lamps solidify their position
Steady improvements ensure the EL lamp's continued success despite
pressure from competing technologies
BY BRAD LIZOTTE Electroluminescent Technologies Corp. Horsham, PA
Since the first attempts to commercialize electroluminescent (EL) lamps
in the 1950s, the technology has metamorphosed from a curiosity to a
highly marketable product. Such is the degree of change that the
technology can now compete successfully with many of today's advanced
lighting technologies, including LED, incandescent, and fiber optics. The
success enjoyed by the EL lamp is derived from steady reductions in
moisture ingress, lamp thickness, and inverter size. The factor that has
been the focus of the most attention is moisture ingress, which causes EL
phosphors to decay, particularly when illuminated. This has constituted a
major headache for manufacturers who have, until recently, struggled to
devise a manufacturing process that would eliminate this problem.
Advantages of EL The motivation to cultivate and nurture the technology,
despite its inherent difficulties, stems from the EL lamp's ability to
provide unsurpassed resistance to vibration, shock, temperature, and
altitude. The technology is ideally suited to applications with low power
requirements, minimal space, and intermittent duty cycles. It is also
suited to applications that operate in near or total darkness. When
properly employed, EL lamps will last for many years with few catastrophic
failures. LED, incandescent, or fluorescent lamps, though they are
brighter and last longer, have the potential for catastrophic failure when
dropped or jolted. Fragile connectors or filaments frequently break,
particularly in portable instrumentation or aircraft. Applications The
many positive qualities peculiar to EL lamps allow for myriad
applications, including liquid-crystal display (LCD) backlighting (see
Fig. 1), exit signs, and automotive and military lighting. Perhaps the
largest commercial success for the EL industry has been LCD backlighting.
This involves placing an EL lamp behind the LCD glass to improve the
display's contrast dramatically. EL consumption by the LCD industry peaked
about 5 years ago with a total market segment size of about $50 million
worldwide. This dollar figure was about 80% of the total backlighting used
in the LCD industry. Today, EL consumption in LCD backlighting is at about
25% to 30% of the total LCD industry market. Exit signs consume large
quantities of EL lamps because of the lamp's low power consumption. EL is
typically 10 times more efficient than other light sources used in such
signs today, which results in dramatic savings to the end user. In fact,
in many areas of the U.S., electric companies are paying exit-sign users
to install low-power devices. In automobiles, EL lamps are used to light
instrument clusters, to provide low-level flood lighting, and as
ornamentation in the form of coach lamps and insignia backlights.
However, according to Tim Zeigler, sales manager at E-L Products Co., East
Aurora, NY, “the use of EL lamps in automobiles may be falling out of
fashion because of the international trend toward clean, sleek, automobile
body styles.” Nonetheless, Zeigler believes that applications will
continue to develop for automotive interiors. In the meantime, E-L
Products continues to produce value-added EL assemblies. Its “smart
panels” (see Fig. 2) have a miniature circuit built in that monitors the
status of the EL lamp on its life curve and adjusts power to the lamp
accordingly to maintain a desired output level. E-L Products claims a
lifespan of up to 50,000 hours for the panel and targets it at
manufacturers of radios and control panels. Because of its resilience, EL
is the lighting technology of choice in most helicopter lighting schemes
in the military. Also, night-vision goggles take advantage of the ability
of EL lamps to be easily filtered because of their narrow spectral output.
Active applications include the Black Hawk, Chinook, V-22, and the F-117
planes. Types of EL lamps As the EL industry evolved, two broad types
of lamp-manufacturing processes have been developed–the laminated or
screen-printed EL lamp, and the fusion-sealed, foil-based lamp.
Typically, the printing process uses reference points or targets for
registration between steps. The process also allows the printed layers to
be applied with excellent accuracy (typically +/-0.005 in.) over a large
nesting of lamps. Using the same registration targets, the nest of lamps can
be moved from the screen printer to the die-cut table, where multiple
lamps can be final-cut in one cycle (see Fig. 3a). Though this process
results in a lamp cost of about $0.20 to $0.25 ea/in.
performance of the lamp is relatively poor and is considered inadequate
for most applications. The primary reason for the poor performance is the
packaging materials used to process the screen-printed lamp. Polyester,
the typical base material, has virtually no value as a moisture barrier.
The screen-printed lamp can result in moisture-saturated phosphors or
dielectric failures. Though these problems exist, the screen-printed lamp
suffice for applications with intermittent duty cycles where the lamp
structure can dry out between lit cycles. Another type of EL lamp employs
a foil-based, fusion-sealed, encapsulation process using Aclar. Aclar is a
fluoropolymer film originally developed as a packaging film for
pharmaceuticals. Aclar has excellent moisture barrier properties and, in
some cases, is as much as 20 times better than other materials used in the
fabrication of EL lamps. Aclar's moisture-resistant properties can be
attributed to its density and intricate interwoven molecular structure. As
many as 70% to 80% of the applications for EL lamps use a fusion-sealed,
Aclar-encapsulated, coated foil lamp. However, the difficulties of
registering lamp nestings from one piece of equipment to the next with
Aclar has made EL lamp fabrication more costly as many of the operations
have been one up (see Fig. 3b). The lamp is dried to lower the moisture
content, and the lamination is fusion sealed at 450 degreesF. Attempts
have been made to use the advantages of Aclar in printed lamps with
moderate success. Because of its low surface energy, Aclar does not accept
printing inks, and the typical printed lamp will not withstand the
lamination temperatures required of fusion sealing. The higher
temperatures typically attack the printed layers, causing them to break
up. Manufacturers have made printed lamps on adhesive-coated Aclar, which
results in a lamp that performs about 50% to 70% as well as the typical
“one-up” foil lamp. The adhesive system allows lower lamination
temperatures but results in a lamp that has a weak link, allowing moisture
ingress through the adhesive in the lamp's edge seal. A relatively new
process for EL manufacture, implemented by many EL lamp manufacturers,
takes advantage of the best of both of the above methodologies. The
process allows many of the manufacturing steps to be completed “multi-up,”
dramatically reducing the lamp's completion time. ELtech Corp.'s
implementation, which it calls the 100/50 process, uses assembly
techniques that hold critical registration of the screen-printed foil
until the final die cut. This technique had previously been regarded as
impossible and was completed in only one part per machine cycle even in
the best circumstances. In the lamination process, the foil floats in a
pool of molten Aclar. In previous attempts at mass production, registration
had been lost because of the inherent expansion and contraction caused by
the process temperatures and conditions. The 100/50 process stabilizes the
dimensional characteristics of the Aclar throughout the lamp-manufacturing
process. Foil registration is completed and held from step to step within
tolerances acceptable for most applications. A manufacturing process
used by BKL, King of Prussia, PA, results in the patented Kard-O-Lite EL
material. The process involves no silk screening, but uses a highly
customized, roll-to-roll production process to reduce cost. Also, a
proprietary binder system produces what the company claims to be a 20%
increase in light output. Microencapsulated phosphors One of the most
significant product developments in the EL industry is the introduction of
microencapsulated phosphors, which offer their own moisture-resistant
coating with the idea of eliminating the need for Aclar packaging. The
elimination of these layers reduces the lamp's thickness and also
simplifies the manufacturing process. The microencapsulants being used
include silica and hydrolyzed alkylaluminum. Microencapsulated phosphors
have helped fuel a resurgence in the use of EL lamps. Technical problems
still remain with the performance of the microencapsulated lamps because
of the omission of the Aclar layer. Without this layer, the lamp's
dielectric is exposed to the elements, causing breakdown. Under typical
test conditions of 40 degreesC with 95% RH (the typical standard test used
in the EL industry) dielectric breakdown occurs in about 500 to 800
hours–depending on the lamp being tested. Also, lamp brightness is
affected. Overall, the benefits of microencapsulated phosphor technology
are real and offer the benefit of lower processing costs. Durel Corp.,
Tempe, AZ, denies that dielectric breakdown occurs readily and claims a
lifespan of up to three times that of conventional EL lamps for its line
of Durel 3 microencapsulated EL lamps. Measuring 8 to 10 mils thick, the
lamps have recently been incorporated into a new line of Timex watches
with lighted dials. Called the IndiGlo night-light system by Timex, the
system includes an EL lamp with its associated inverter inside the case
(see Fig. 4). Durel 3 lamps are available from $1 each, depending on lamp
size and quantity.
Inverters In the best circumstances, an EL lamp should be powered by a
compensating inverter with a variable dc input (potentiometer) so the user
can adjust the brightness of the lamp over time. Typically the inverter
can operate between -40% and +25% of the rated dc voltage. The
potentiometer should operate between these limits. In the past, inverters
were regarded as a limitation when considering EL because of their
size–typically about 1 in
include ELtech's recently introduced low voltage EL lamp. This device
emits usable light energy starting at a supplied power of 40 V/400 Hz.
Previously, the inverter was required to provide 120 V/400 Hz. This
reduction in the lamp's operating threshold has allowed the inverter's coil
size to be reduced. Today, inverter manufacturers have made significant
gains in size reductions with inverters as small as 5/8 x 5/8 x 3/8 in.
Applying EL technology In general, applications that require long life
or high brightness should consider other lighting technologies (see the
table for a comparison of EL to these technologies). High brightness can
be achieved with EL, but only for a few thousand hours. In a
high-brightness application, if EL is still a desirable option because of
power or space requirements, then the application engineer should allow
for re-lamping. Re-lamping, frequently done with LCDs, only requires that
the lamp be accessible (not be mounted to the LCD with adhesives) and have
a plug-in type connector. The lamp should easily slide out of the assembly
and be designed with rounded edges so there is no potential to disrupt the
elastomeric connectors of the LCD's glass. EL has developed a
reputation for poor life, largely due either to misapplication of the
technology or to the overselling of its capabilities by the industry. Some
manufacturers make claims that EL lamps will last for up to 100,000 hours.
This claim is essentially true, but the lamp must be used in low-humidity
and low-temperature environments. The lamp must also be used in an
application that can use brightness levels of 0.5 fL or less. The typical
usable life for the standard fusion-sealed foil lamp is about 15,000
hours, when powered with a compensating inverter. Longer-life lamps–of up
to 25,000 hours–are also available. Usable life is projected assuming
that the usable light energy for an LCD is about 3 to 4 fL, enough light
to provide display visibility in a darkened room. CAPTIONS
Lead Photo:
Despite the problems encountered during the initial commercialization of
EL technology, the qualities peculiar to the technology have guaranteed
its place in myriad applications.
Fig. 1. The high contrast levels achieved by using EL lamps as
backlights has fuelled the EL market as it rides the shirt-tails of the
almost ubiquitous LCD.
Fig. 2. The smart panels from E-L Products have a miniature circuit built
in to monitor the lamp on its life curve and adjust power to maintain a
desired light output level.
Fig. 3. While the screen-printed lamp sufficed for applications with
intermittent duty cycles, as many as 70% to 80% of the applications for EL
lamps have been fusion sealed because of that technique's much-improved
moisture resistance.
Fig. 4. The IndiGlo night-light system by Timex includes an EL lamp with
its associated inverter inside the case.
Products from the following companies are mentioned in this article:
BKL Corp. King of Prussia, PA Donald Kardon 215-277-2910
Durel Corp. Tempe, AZ Barbara Buss 602-731-6200
E-L Products Co. East Aurora, NY Tim Zeigler 716-655-0800
Electroluminescent Technologies Corp. Horsham, PA Brad Lizotte
215-441-0404
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