14.4 / C. J. Joshi
14.4: Development of Long Life, Full Spectrum Light Source for Projection
Display
Chandrashekhar J. Joshi
LUXIM Corporation, 1171 Borregas Avenue, Sunnyvale CA 94089 USA
Abstract
A new high performance light source has been developed utilizing
RF drive plasma technology called LIFI™. This new light source
is capable of delivering a light intensity suitable for projection
HDTV’s up to 70” and with further development is applicable to
front projectors. With start times averaging 6 seconds or less and
an estimated life time of 100,000 hours, LIFI™ meets the demands
of next generation TVs. This is the first RF drive light source to
be produced in high volume for the projection display market.
This paper describes the technology and reports the performance
of the LIFI™ light source.
1.
Introduction
It has long been recognized that the presence of metal electrodes
in conventional high intensity discharge (HID) lamps limits both
the performance and lifetime of such lamps[1]. Removing
electrodes from the design of the light source, in principle,
removes these limitations and in addition expands the design
space for useable emitters and offers a better luminous efficacy
[2,3]. The electrodeless light sources operate on the same basic
principle as HID lamps, converting electrical power into visible
radiation, except that the electrical power is supplied in the form
of radio frequency waves (RF). The RF is coupled to these lamps
in one of several ways: a capacitive RF discharge, an inductive RF
discharge or a surface wave sustained RF discharge [4-6]. Using
these techniques several products have been commercialized for
general lighting that function in a wide range of frequency from
256 KHz to 2.4 GHz [7-9]. The advent of high efficiency, high
power semiconductor amplifiers in the 400 MHz – 4 GHz range
has given a new impetus to this development especially in the
projection display space where extremely bright, point-like and
compact light sources are needed. In this paper we discuss the
basic operating principles and describe the performance
characteristics of a new addition to the RF drive lamp family:
LUXIM Corporation’s LIFI™ lamp [10,11]. See Figure 1.
HDTV up to 70” and with further refinements will be applicable
to front projectors. Furthermore, because of its manufacturability
it is the first RF drive light source to be commercialized in high
volume for the projection display market.
The LIFI™ Lamp
2.
™
The LIFI lamp is broadly speaking in the category of a surface
wave sustained RF discharge, but there are some critical
differences between LIFI™ and other such discharges. The quartz
bulb as shown in Figure 2 is embedded inside a metal-coated
dielectric waveguide made out of alumina or some other high
dielectric constant material. This helps to reduce the physical
dimensions of the device and minimizes the RF leakage through
the bulb opening. Figure 2 shows a schematic of the LIFI™ lamp
(a), a photograph of the lamp without the RF drive circuit (b), and
the RF drive circuit (c).
(a)
(b)
(c)
Figure 2.
(a) Schematic of the LIFI™ lamp including the solid state
amplifier,
(b) the ceramic puck with a quartz bulb embedded in it and
Figure 1. Photograph of LIFI™ 4000 lamp
(c) the R.F. drive board
LIFI™ is shown to produce over 76 lumens/watt with a color
rendering index of 88 and over 4000 lumens in an etendue of 27
mm2sr. Start times averaging 6 seconds or less when cold and
even shorter when hot, LIFI™ is particularly suited for projection
LIFI™ can be operated in a broad frequency range from hundreds
of MHz to several GHz where relatively efficient high power RF
amplifiers are commercially available. In Figure 2, the cylindrical
14.4 / C.J. Joshi
waveguide with two co-axial probes is typically called the “puck”.
The quartz bulb is filled with an inert gas, a trace amount of
mercury and a combination of metal halide salts to deliver a high
luminous efficacy and the desired spectrum. The feedback probe
samples the characteristic resonant frequency of the puck and
feeds it to the RF amplifier which in turn feeds the amplified
power to the puck via the drive probe. The lamp operates in three
distinct modes during start-up: ignition, warm-up and operation.
In the OFF mode, the puck acts as a high Q resonant cavity, or a
dielectric resonator storing the RF power until the circulating
electric field inside the puck is 30-50 times the value supplied by
the amplifier. The electric field leaks (evanescent field) into the
lamp cavity where it is able to ionize the low-pressure noble gas
and sustain a plasma. This plasma in turn evaporates the mercury
and metal halides setting up a metal-halide convection cell
forming a multi-atmosphere pressure, partially ionized plasma.
Simultaneously the Q of the cavity drops after which the puck acts
simply as a waveguide, transporting the RF from the amplifier to
the lamp. In the plasma-off mode, the RF leakage from the lamp
opening in the puck is small because the size of this opening is
much smaller than the RF wavelength inside the puck. However
when the plasma forms, the RF wavelength in the plasma can
become on the order of the plasma size and the RF leakage can
increase. However this is offset by extremely efficient RF
absorption.
The net spectrum from the bulb is the result of several
contributions. First both the plasma and the neutral vapor is
optically thick and any radiation that is emitted in the body of the
plasma column is absorbed and re-emitted several times until it is
finally emitted from the “skin” of the plasma,. In addition to the
characteristic metal atom ion emission there is significant
molecular emission.
The luminous efficacy of the LIFI™ is shown to be as high as 76
lumen/watt and 31% of the total lumens are collected into an
etendue of 27 mm2sr. This luminous efficacy result is because
once the steady state is reached, conduction and convection
losses, 40-50 watts and 10-15 watts, respectively, remain
approximately constant and further increase in RF power
contributes towards radiation.
3.
The breakdown of the noble gas leads to vaporization and
ionization of mercury followed by vaporization of the metal
halide salts. The inner diameter of the bulb determines the
functional form of the radial temperature profile, which for bulb
diameters smaller than 5 mm, tends to be parabolic with a
temperature maximum in the center and a wall temperature of
about 650oC.
The formation of the plasma shifts the resonant frequency of the
puck, however the RF amplifier feedback loop is continuously
able to track this frequency shift and feed the power to the plasma.
Once the metal halides have completely vaporized, the plasma
density rapidly increases in the center region to about 1015 cm-3.
It is this plasma that emits most of the visible radiation. The
transition from weak emitting noble gas plasma to full brightness
is fairly rapid (see later) and is referred to as the warm-up. Even
with this high density the plasma is only partially ionized with
ionization fraction on the order 10-5. A distinct plasma column
appears at the center of the bulb because this is where the
temperature and therefore the ionization rate is the largest. In the
steady state operation the peak temperature at the center is
estimated to be around 0.5 eV because of electron-ion and
electron-neutral thermalizing collisions. The diffusion of the
plasma column is hampered by collisions with the neutral atoms,
i.e.; the electrons are born and recombine within the column
without hitting the wall of the bulb.
uW/nm
3. Plasma Physics of the LIFI™ Lamp
Although a detailed discussion of the physics of LIFI™ is beyond
the scope of this paper, it is instructive to highlight the salient
points. With RF power of about 20 watts, the cavity Q is large
enough for the RF to ionize the noble gas raising the electron
density to approximately 1013 cm-3. The electron temperature at
this point can be estimated by balancing the collisional ionization
rate and the radiative and the three body recombination rates to be
about 1.1 eV.
Performance Data
In Figure 3 a typical LIFI™ 4000 spectrum is shown. The total
lumen output is typically 13,000 lumens at an RF power of 170
watts. Compared to a high pressure mercury lamp operating at
200 atmospheres, the LIFI™ spectrum is more continuous with a
substantial amount of light in both the red (600-700 nm) and the
blue (400-500nm) parts of the spectrum. The color temperature
can be made to be between 6500K to more than 10,000K using
different lamp fills.
90000
LIFI 4000
80000
mercury HID lamp
70000
60000
50000
40000
30000
20000
10000
0
380 430 480 530 580 630 680 730 780
nm
Figure 3. The spectrum emitted by LIFI™ 4000. Also shown
for comparison is the spectrum of a mercury HID lamp at a
pressure of 200 atmospheres
Figure 4 shows the expected change in the total lumen output
versus time for LIFI™ 4000 lamps. This curve was obtained as
follows: Five lamps were tested following a two hour ON and
half-hour OFF cycle. After 4000 hours of such ON-OFF cycling
the average lumen output was reduced to about 87% of the initial
output but the rate of change shows a trend toward stabilization.
A fit to the experimental data shows that at this rate the light
output should be reduced to about 80% of its initial level after
25,000 hours of operation.
Careful diagnostics of lamps
undergoing life testing has revealed that the lumen decline can
mostly be attributed to the degradation of the power electronics
and not the bulb. With further improvements in the drive circuitry
the lumen maintenance performance is expected to improve
significantly. We note that even at this rate of lumen drop-off
with time, the lamp has the potential to reach 100,000 hours of
operation but this awaits laboratory validation.
Another one of the major advantages of LIFI™ is its short time to
achieve full brightness. Figure 5 (blue curve) shows relative
14.4 / C. J. Joshi
brightness as a function of time after power on. The time to 95%
of full brightness is about six seconds. The red curve in the same
figure shows the RF power coupled to the bulb. Here one clearly
sees the ignition and warm-up phases of the lamp. The noble gas
breakdown occurs in less than one second after the RF is turned
on, but there is no significant light emission until about 2.5
seconds when mercury first vaporizes and begins to emit
radiation. The warm-up phase occurs at about 3 seconds when the
metal halides have evaporated and begin to emit visible radiation
strongly. One of the drawbacks of conventional HID lamps is the
long re-strike time when the lamp is hot. This is because the
pressure inside the lamp needs to
Summary
The full color spectrum, long life, fast start time and design
simplicity have enabled LIFI™ to be the first light source of its
kind manufactured in large quantities for commercial projection
display use. These applications include projection television up to
70 inches and multi-segmented video walls. Improvements
currently underway should lead to a 100% improvement in output
due to better bulb design and collection optics.
These
advancements will expand the applicability of LIFI™ to front
projection and other general lighting applications.
5.
Acknowledgements
The author acknowledges valuable contributions of R. Gilliard,
M. DeVincentes, G. Hollingsworth, A. Hafidi, T. McGettigan, P.
Lamond, M. Duelli, A. Pradhan, (Late) Y. Chang and M. Espiau
towards the development of LIFI™.
100%
Initial Lumens Maintained
4.
90%
80%
70%
60%
50%
6.
40%
[1] J. F. Waymouth, Electric Discharge Lamps, MIT Press
30%
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