UHP Lamps for Projection Systems

UHP Lamps for Projection Systems
UHP Lamps for Projection Systems
Pavel Pekarski, Ulrich Hechtfischer, Gero Heusler, Achim Koerber, Holger Moench, Jens
Pollmann-Retsch, Ulrich Weichmann and Arnd Ritz
Philips Research Laboratories, Weisshausstr.2, D-52066 Aachen, Germany
Projection systems have reached a convincing performance relying on the outstanding properties of the
UHP lamp. In our presentation we discuss the UHP concept as well as the topics of lamp ignition and the
electrode stability during the lamp’s lifetime.
UHP lamp contains only mercury as radiating species.
INTRODUCTION
Short arc lamps are a key component for projection
systems to achieve highest efficiency for small display
sizes. Looking back, the introduction of the UHP lamp
(Ultra High Performance) concept by Philips [1,2] gave
the projection market a significant technological
breakthrough. UHP lamps today are the standard for
most commercially available front and rear projectors.
The combination of highest brightness with lifetimes
extending up to more than 10000hrs is ideal for the
projection application.
2
THE UHP LAMP CONCEPT
2.1
High Luminance and Continuous Spectrum
Intensity [W /nm ]
An average arc luminance >1 Gcd/m2 is necessary for
high efficiency projection with today's small size
displays. Such a high luminance can be produced with
rare gas discharges (but low efficacy) and with pure
mercury discharges. The maximum luminance that can
be reached in thermal equilibrium is physically linked to
the discharge temperature by Planck’s law. Metal halide
additives, which have been used in the past to improve
the lamp spectrum, lower the discharge temperature and
thus radiate at much lower luminance. Therefore the
0.40
0.30
A very high operating pressure of more than 200 bar is
used to reach a high burning voltage of the lamp,
despite the short arc gap. For mercury pressures above
200 bar more light is emitted in the continuum radiation
than in the atomic spectral lines (fig.1). Especially the
red light above 595 nm strongly depends on the lamp
pressure. The lamp with 160 bar (fig.1) gives 20% less
red light than the >200 bar UHP lamp. For good color
balancing in projection systems it is therefore essential
to realize ultra high lamp pressures.
2.2
Regenerative Chemical Cycle
UHP lamps can reach lifetimes of more than 10000
burning hours enabled by the regenerative chemical
cycle using a patented halogen filling [1,2]. Adding a
certain amount of oxygen and halogen to the lamp
atmosphere prevents the tungsten evaporated from the
lamp electrodes to condense on the wall, as in the colder
regions the tungsten atoms react chemically to form
oxyhalide molecules.
2.3
Stable Arc
The electrodes of short arc lamps change their shape
after some ten or hundred hours of operation. The arc
plasma will change its attachment on this rough surface
frequently (fig.2). The moving arc affects the light
distribution on the display and therefore causes
disturbing brightness variations on the screen. Philips
160bar
200bar
240bar
0.50
0.40
0.20
0.10
0.00
400
500
600
w avelength [nm ]
700
cum ulative probability
1
standard situation
0.30
stabilised
0.20
0.10
0.00
Figure 1: Measured UHP spectra of 1.3 mm lamps at
100 W depending on the mercury pressure.
0
50
100
150
jum p w idth [µm ]
200
250
has introduced the first solution to stabilize the arc
during the whole lifetime of the lamp. This has been
realized in the UHP-S electronics using a specially
designed lamp current [3]. The solution is closely
related to the interaction of driving mode and arc
attachment. Contrary to the standard "block" form
current shape, the application of the special current form
with extra current pulses at the end of each half-wave
results in a stable arc attachment on the electrode over
the whole lifetime.
3
LAMP IGNITION PHYSICS
To create a gas discharge, primary electrons are needed.
All charge carriers in a lamp which has been switched
off for a while are neutralized. Primary electrons can be
produced statistically by natural radioactivity, but for a
small volume of UHP lamp it is unlikely that this will
happen right at the time the lamp should ignite.
Traditionally the primary charges are created via field
emission of electrons from the electrodes. Due to the
modifications of the electrodes during their lifetime we
cannot rely on sharp edges and the corresponding high
field strengths but have to apply 20kV to the lamp for a
save ignition. Alternatively electrons can be also
produced via the photoeffect on the tungsten electrodes.
As the work function of tungsten is 4.54eV we need
photons of higher energy, i.e. UV radiation with
λ < 270nm . This UV radiation can be produced by an
ignition aid which is often called UV-enhancer and is
basically a small lamp that incorporated into the sealing
of the UHP burner [4,5]. Some millimeters in the
middle of one sealing are not closed leaving a small
cavity with the molybdenum foil going through it. An
external wire (“antenna”) creates an electrical field and
a first ignition takes place inside this small cavity.
Thanks to the sharp edges of the foil much lower
voltages can be used to extract electrons. A capacitive
discharge between the foil and the antenna is operated
in the UV-enhancer and a part of the produced photons
is conducted along the sealing by total internal
reflection towards the main burner cavity. There they
have a chance to create electrons and the discharge also
starts in the main cavity.
Once a primary breakdown has been achieved the lamp
requires voltages of only a few hundred volt in a glow
state and below hundred volts when the arc has
established (<1s).
Sometimes a “hot-restrike” of the lamp is necessary,
only a few seconds after switching it off. Although
plenty of charges still exist at this time high voltage is
required to create a breakthrough in the high-pressure
gas. A metal wire close to the burner as indicated in
fig.3 can modify the field distribution inside the burner
cavity and helps to lower the required restrike voltage.
The field forming wire can be combined with the
antenna around the UV-enhancer as indicated in fig.3.
The UV-enhancer guarantees low ignition voltages for
the larger off-times. Even after days in total darkness
the lamp ignites safely at voltages much below 5kV.
The implementation of the UV enhancer into the sealing
of the UHP burner and the combination of the antenna
for the UV enhancer and the field formation inside the
burner allows an extremely compact product.
The system is operated in combination with a lamp
driver which works with only 5kV ignition voltage. This
allows a smaller and less energy dissipating induction
coil. The choice for 5kV was made to guarantee hot
restrike in less than a minute even without cooling.
In addition to the smaller driver the lower ignition
voltage allows a more compact projector design around
the lamp. Insulation distances in air are required in the
order of 1mm/kV. The achieved reduction from 20kV to
5kV corresponds therefore to a size reduction of 15mm.
The design of a compact projector clearly profits from
this “invisible” size reduction.
3
1
2
Figure 3: Schematic drawing of an UHP burner and the
UV-enhancer cavity in the left sealing. The additional
antenna-wires (3) are electrically connected to the electrode
(2).
4
CONCLUSIONS
The demands of projection systems towards small
displays, miniaturized systems and finally bright
projected images on the screen are well matched by the
UHP lamp technology. The trend to increase the optical
requirements to the light source will continue, however,
demanding for an even shorter arc length and higher Hg
pressures. A significant reduction in arc size from 1.3
mm to 1.0 mm has been already realized for most UHP
types, a further step will follow.
5
REFERENCES
[1]
E.Fischer and H.Hoerster, High Pressure Mercury
Vapour Discharge Lamp, US-Patent 5109181
[2]
E.Fischer, Ultra High Performance Discharge
Lamps for Projection TV Systems, 8th Int. Symp. on Light
Sources, Greifswald, Germany, 1998
[3]
H.Moench, G.Derra, E.Fischer, X.Riederer, Arc
Stabilization for Short Arc Projection Lamps, SID 2000
[4]
H.Moench, G.Derra, E.Fischer, H. de Regt and
X.Riederer, New Development in Projection Light Sources –
Shorter Arcs and Miniaturisation, SID 2001
[5]
H.Moench, G.Derra, E.Fischer and X.Riederer, UHP
Lamps for Projection, Journal of the SID, vol 10, nr 1, 2002
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