PLASMA LIGHTING
O
ver the last several years, LEDs
have been the “it” technology
in the lighting world. LIGHTFAIR
is seemingly dominated by LED
companies; the federal government under
the auspices of the DOE has an extensive
program in place to support LED applications; and even the mainstream media
has devoted major coverage to LEDs as
a potential replacement technology for
incandescent.
LEDs, however, are not the only emerging light source. Another technology—
light emitting plasma (LEP) —is gaining
Photos courtesy of Luxim
The Plasma
Paradigm
Light emitting plasma is on the rise in high-illuminance
applications
traction as a viable system for high-illuminance applications, specifically as a
replacement for 400-W-plus metal halide
and high-pressure sodium systems, by offering the same delivered lumens using
about 40 percent less energy. It’s not only
the lighting industry that’s taking notice.
Forbes magazine included an article last
year on LEP, while the Wall Street Journal
recognized Luxim, a LEP developer based
in Silicon Valley, as one of its “Top 10
Clean Tech Companies” in the U.S.
Aside from the energy savings, LEP has
a number of other important attributes,
ranging from dimmability, to expected
lifetime, to color. LEP can be dimmed to
20 percent output allowing further energy
savings. In addition, LEP has a projected
50,000-hour life at 70 percent lumen maintenance. This compares favorably to 18,000
hours for metal halide. LEP lamps are available today at a color temperature of 5,200K
with warmer color temperatures expected
BY RANDY REID AND APURBA PRADHAN
shortly (Luxim, for example, demonstrated
4,800K systems at LIGHTFAIR). Finally, LEP
delivers a continuous spectrum ensuring
high color quality; systems are available
today at 75, 80 and 95 CRI.
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PLASMA LIGHTING
Figure 1
HOW LEP WORKS
The term plasma is used in the industry
which are typically the weakest link of
an HID lamp
A number of fi xture companies have
designed reflectors that capitalize on the
to describe sources with a continuous
• Elimination of quartz wall darkening, a
compactness of the LEP source. These
spectrum. The LEP system (Figure 1) com-
source of lumen depreciation and fail-
reflectors direct light to the work plane
prises three components: emitter, driver
ure, as electrode material evaporates
and offer much better uniformity and a
and power supply. The emitter consists of
and deposits on the transparent sur-
more even distribution than traditional
a quartz capsule embedded in a ceramic
face of lamp.
HID. These improvements allow street-
puck. Inside the capsule is a blend of gas-
• Elimination of molybdenum foils allow-
light pole spacing, for example, to be ex-
ses and halides designed to emit a certain
ing faster warm-up and restrike times
panded from 100 ft (typical of HPS) to 150
spectrum. A highly reflective material
As a result, over the years, there has
ft while maintaining IES Recommended
placed between the capsule and the puck,
been a great deal of interest in plasma
Practices. On new construction, this re-
causes the light to emit in a forward pattern.
light sources. The wireless revolution has
duction of fi xtures and poles allows LEP
The driver, which is essentially a solid-
provided cost effective, efficient and reli-
to have lower capital costs and lower
state RF amplifier, creates electrical en-
able solid-state amplifiers that make LEP
operating costs compared to legacy tech-
ergy that is fed into the puck by a coaxial
light sources possible today.
nologies (Figure 2).
cable. The ceramic puck then focuses
that energy onto the capsule, energizing
the mixture inside and causing the LEP
lamp to emit brilliant white light.
A quartz capsule is at the heart of the
“Real-world” lighting efficiency is an-
DIRECTIONALITY AND POINT SOURCE
other important consideration with any
On the applications front, a single LEP
emerging light source. As these new tech-
source, only a few millimeters in size, can
nologies become available, lighting design-
produce the light needed for a complete
ers and end users learn how to assess the
LEP system. This simple construction has
luminaire. Because the source is compact
technology’s benefits. A hard-learned les-
no electrodes, no glass-to-metal seals
and directional, light can be harvested
son from LED implementations is that it is
and no alien materials inside the capsule.
from it more effectively than from induc-
not the light on the datasheet that matters
This simplicity and purity of construction
tion, LED or HID. The directional source
but the light on the ground. For many rea-
gives the LEP emitter its efficiency and
prevents light from being trapped and
sons light on the ground can be less than
ruggedness. This simple design has the
wasted in the luminaire. The point source
the datasheet might lead one to believe
following benefi ts:
optics effectively and uniformly map the
(operating temperature, fixture efficiency,
• Elimination of energy wasted as heat
source to an illuminated area prevent-
measurement discrepancy, binning varia-
ing unwanted light spill which can cause
tion). There are also reasons why light on
glare and light pollution.
the ground may be higher than quoted on
in the electrodes themselves
• Elimination of glass-to-metal seals
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LD+A | October 2011
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PLASMA LIGHTING
the datasheet but these cases seldom intrude on real-life experience.
Today LEP fi xtures deliver system efficiencies above 70 lumens per watt with
excellent backlight, uplight and glare control. Furthermore, LEP fi xtures in development can deliver circa 90 lumens per watt.
These figures compare favorably with
best-in-class LED fi xtures on the market
and on the drawing board today.
Significantly, for outdoor applications,
LEP has excellent mesopic and scotopic
lumens. In fact, LEP had the highest scotopic/photopic ratio of all sources tested
in preparation for the new IES Lighting
Handbook (10 th Edition). For the first time,
this new edition includes guidelines to account for the improved nighttime visibility
that comes with full spectrum sources.
Figure 2
This allows further energy savings using
LEP in outdoor applications.
COMPLEMENT NOT COMPETITOR
LED and LEP is not an either/or proposition. In high-illuminance applications in
particular, LEP can serve as high-output
complement to LED. Both technologies
offer efficiency, life and digital control.
The basic building block of LED systems
is the LED chip with an output of circa
100 lumens. The basic building block of
LEP is the quartz emitter with an output
of 20,000 lumens. Given this starting point,
one can envisage a family of fixtures that
uses LED for low and medium illuminance
and expands to LEP for high illuminance.
Inside this broad positioning, LEP has
Roadway lighting is among the suitable applications for LEP.
unique advantages where small form factor is important. These include high-mast
systems (compactness reduces wind
load), portable systems (compactness
simplifies transportation and setup) and
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A single LEP source, only a few millimeters in size, can produce the light needed for a complete luminaire. The directional source prevents light from being
trapped and wasted in the luminaire.
beam systems (compactness simplifies
costs are falling rapidly and are projected
optical design).
to match the costs of mainstream HID lu-
rays of LEDs, bigger heat sinks and a larger
minaires by FY 2013.
fixture. In short, LED scales down well. LEP
Finally, there is the question of cost.
Because its architecture is based on a
LED does have an advantage over LEP
single emitter and a single driver, LEP has
in its ability to scale down. If a job calls for
lower cost than LED in high-illuminance
a 3,000 lumens, the LEP system would still
applications. While costs vary by vendor,
require an emitter, driver and power supply
quantity and terms broadly speaking we
and would therefore have approximately
see LED and LEP systems having similar
the same cost as a 20,000-lumen LEP sys-
costs at about a 5,000-lumen output level.
tem. In this particular case, LED would be
As output increases, however, LED costs
a more cost-effective solution because it
scale proportionately while LEP costs
scales down well. On the other hand, if the
remain relatively flat. This provides LEP
job called for a 45,000 lumens, LEP still has
with a significant cost advantage at the
the same three components and similar
20,000-lumen level. Furthermore, LEP
cost. By contrast, for LED to scale up, the
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manufacturer would have to add more ar-
scales up well. Q
THE AUTHORS
Randy Reid, Member IES (1995), is
the vice president of marketing for
Luxim, a developer of light emitting
plasma. Mr. Reid is a past-president of the IES (2002-03) and was
chairman of the 2010 IES Annual
Conference held in Toronto.
Apurba Pradhan, Member IES
(2009), is the director of product
marketing at Luxim.
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