Note: Descriptions are shown in the official language in which they were submitted.
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HIGH INTENSITY DISCHARGE LAMP WITH
SINGLE CRYSTAL SAPPHIRE ENVELOPE
Related Applications
[0001] This application claims the benefit of U.S. Patent Application Serial
No.
09/969,903 filed October 2, 2001 and entitled "Sapphire High Ifatensity
Discharge Projector
Lamp" which is a continuation of U.S. Patent Application Serial No. 09/241,011
filed on
Febniary l, 1999 and entitled "Sapphire High Isatensity Discharge
Pf°ojector La~ap". Both
applications are expressly incorporated herein, in their entireties, by
reference.
Field of Invention
[0002] The present invention relates to a high intensity discharge lamp that
produces a
radiation spectrum suitable for various applications, such as image
projection, automotive,
medical, communications (optical fibers) and general lighting applications.
Background Information
[0003] Image projection is one of the major fields of application for visible
light
generated by a high intensity discharge ("HID") lamp. The conventional HID
lamp optimized for
visible light has major attributes that render it particularly suitable for
use in image projection.
Such HID lamp typically emits light from a plasma arc formed inside an
envelope between two
electrodes which are spaced a particular distance apart. The radiation
spectrum of the light
emitted from the HID lamp depends on the gases and other materials contained
within the lamp
(the "B11"). In a conventional projection system, the light from the lamp is
collected via a series
of optical elements and projected tlmough an image gate onto a screen to form
a projected image.
The element which forms the image at the image gate can be film or any type of
a light
modulator, e.g., liquid crystal displays ("LCD"), digital micro-mirror devices
("DMD'~ or liquid
crystal on silicon displays ("LCoS"). In image projection applications, the
utility of the HID
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lamp may be defined by its optical efficiency, power efficiency, color
rendition, arc stability
(absence of "flicker"), arc gap, physical size, initial cost, operating cost,
and overall system cost.
HID lamps can also be designed to produce ultraviolet ("LTV") or infra-red
("IR") radiation for
applications with similar performance requirements.
[0004] A conventional HID lamp presently has light transmissive envelopes made
from
quartz or polycrystalline alumina ("PCA", also known as "ceramic" envelopes).
In general,
image projection applications require the HID lamp with a clear envelope,
small arc sizes and
narrow light beams. The HID lamp with quartz envelopes generally meets these
requirements,
however, PCA envelopes are translucent and generally not suitable for image
projection and
similar applications. The PCA envelope lamp is usually constructed with
relatively large gaps as
necessary for large tight source applications. More recently, the HID lamp
envelope has been
made from poly crystalline sapphire ("PCS'~ which is produced by conve~ion in
place of PCA
envelopes. Although PCS envelopes improve light transmissivity and other
characteristics of the
envelope compared to PCA envelopes, PCS envelopes still have microscopic
surface undulations
that render them not suitable for most image display projection and related
applications.
Therefore, the conventional HID lamp continues to rely primarily on quartz
envelopes.
[0005] The use of a quartz envelope places substantial limits on the
conventional HID
lamp in terms of meeting the above listed desired features for image
projection. For example, the
quartz envelope has a relatively low melting temperature, power load factor,
thermal
conductivity and tensile strength. Such considerations effect the lamp optical
efficiency,
efficacy, power capacity, size, life and the ability to control flicker.
Furthermore, the quartz
envelope is permeable to a number of additives, such as sodium or hydrogen,
which are
important in the spectral tailoring of the emitted light.
[0006] The Image Projection Industryhas established that a correlated color
temperature
("CCT") of 6,500° I~ ("D65 standard") is the light source specti~~n
most desirable for image
projection because it has a high color rendition index and is close to
daylight quality The
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conventional quartz envelope HID lamp is generally designed to operate at
pressures from about
120 up to a maximwn around 200 atmospheres utilizing a fill of pure mercury.
However, a high
pressure mercury lamp has CGT about 7,000° K to 9,000° K. The
light from such HID lamp
must be filtered in order to achieve a more compatible CCT however filtering
can reduce lamp
efficiency by about 30 to 40%. Metal halide additives have typically been
added to mercLUy
lamps for the purpose of tailoring the light spectrum to a more desirable CCT
("metal halide"
lamps). However, the effectiveness of metal halides is reduced as operating
pressure increases to
the point of minimal contribution at the maximum current operating pressures
for the quartz
envelope lamp. A conventional Image projection system uses light sources with
a wide range of
CCT from a typical 3,000° to 3,300° K t<mgsten halogen lamps, to
4,000° to 5,000° K for metal
halide HID lamps, 5,500° to 6,500° K for short arc Xenon lamps,
and over 7,000° K for a
mercury lamp.
[0007] In the image projection field, the industry has moved steadily in
recent years
toward utilizing smaller light modulators based upon foundry fabricated
silicon wafers, e.g.,
DMD and LCoS, with diagonals of 0.9 down to 0.5 inches. Such small apertures
require that the
HID lamp used have arc gaps in the range between 0.8 mm --1.3 mm in order to
obtain an
efficient optical match between the light emitted by the HID lamp and the
aperture optics. As
lamp gaps become smaller the efficacy of the HID lamp is reduced and the power
that can be
supplied to the plasma arc is limited by the envelope material thermal
characteristics. In order to
increase the efficacy of smaller arc gap lamps, the operating pressure must be
increased.
However, quartz envelope properties limit the pressure and power load factor
that one can use in
such HID lamps to about 200 at~n and about 20 watts/cm2. Also, in applications
such as image
projection, lamps must be essentially flicker free. Flicker in an arc lamp is
associated
parametricallyto the lamp bulb size and the hll pressure. Using conventional
quartz envelopes,
one needs to remain below 200 atm in Lamp pressure in order to achieve flicker
free operation.
Summary of Invention
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[0008] The object of the present invention is to improve the efficacy,
lifetime and
spectral stability of a high intensity discharge ("HID") lamp. The present
invention utilizes
single crystal sapphire ("SCS") in an envelope of the lamp to replace
conventional envelope
materials. The SGS envelope lamp according to the present invention may be
physically smaller,
generate light more efficiently, and produce a plasma with greater luminance
and stability than a
conventional HID lamp. The SCS envelope lamp may be utilized, e.g., in
applications that
require a small, powerful light source with a narrow beam width such as image
projection,
automobile headlamps, fiber optic light sources, and the like.
[0009] SCS has substantially superior properties compared to conventional
materials
(e.g., quartz or polycrystalline ahunina) that are utilized in the envelopes
ofthe conventional HID
lamp. These properties include higher tensile strength, greater burst pressure
resistance, higher
softening and melting points, greater thermal conductivity, and a higher power
load factor. These
advantages allow the SGS envelope lamp according to the present invention to
operate at higher
pressures and temperatures and produce more usable light per watt of power
input. In addition,
the superior chemical resistance of SCS pernits the use of a broader range of
fill gases and
additives to produce light in a specific spectnim for the application. For
example, for visible
light radiation in the 400 nm to 700 urn spectrum, this versatility should
allow correlated color
temperatures to be set and consistently held in a narrow range between
4,000° K to 9,000° K. In
addition to visible light radiation, the present invention may also be
'utilized to produce radiation
emissions in the ultraviolet (200-400 nm) and near infra-red (700 nm to about
2,500 nm) spectra
with similar benefits.
[0010] The SCS envelope lamp may have an effective life four to five times
longer than a
conventional quartz envelope lamp, even when operating at significantly higher
temperatures and
pressures. This is accomplished by matching the thermal expansion
characteristics of the seal
materials and other components to those of the envelope, thereby minimizing
the stress on the
seals. In addition, the SCS envelope lamp may be manufactured to tighter
tolerances with greater
consistency than quartz or polycrystalline alumina, and, by using automated
manufacturing
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techniques, at the same or lower cost.
[0011] The plasma in the SCS envelope lamp maybe produced in a continuous non-
flash
mode by providing a constant voltage across two end electrodes in waveforms
suitable for high
pressure operations. The SCS envelope lamp may ufiilize direct or alternating
current. In another
embodiment, the SCS envelope lamp may be without electrodes and powered by
microwaves or
radio frequency radiation. Alternatively, the SCS envelope lamp may be
operated as a hybrid
using both electrodes and microwave power.
Brief Description of the Drawinus
[0012] Figure lA is a top view of an envelope of a lamp according to the
present
invention;
Figure 1B is a side view of the envelope illustrated in Figure lA;
Figure 1C is an end view of the envelope illustrated in Figure lA;
Figure 2A is a side view of an LCD projector system using a SCS envelope lamp;
Figure 2B is a cross-sectional view of a first exemplary embodiment according
to the present
invention of the envelope which utilizes electrodes;
Figure 3 is a chart comparing heat effect on quartz walls and SCS walls;
Figure 4 is a chart showing stress on a bulb as a function of tensile
strength;
Figure 5 is a cross-sectional view of a second exemplary embodiment according
to the present
invention of the envelope which utilizes electrodes;
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Figure 6 is a cross-sectional view of a third exemplary embodiment according
to the present
invention of the envelope which does not utilize electrodes;
Figure 7 is a side view cross-section of a SCS envelope electrodeless lamp;
Figure 8A shows an exemplary embodiment of end plugs of the SCS envelope lamp.
Figure 8B shows another exemplary embodiment of the end plugs of the SGS
envelope lamp.
Table 1 is a comparison of sapphire to quartz;
Table 2 is a comparison of tensile strength at various temperatures of quartz
and sapphire; and
Table 3 is a comparison of thermal conductivity between quartz and sapphire.
Detailed Description of the Invention
[0013] Embodiments of the present invention will be described in detail with
reference to
the accompanying drawings.
[0014] The present invention describes a HID lamp with a SCS envelope and a
method
for manufacturing the envelope. Such SCS envelope lamp may be optilW zed for
applications in
the visual light range as well as in the UV or IR range of the radiation
spectrum.
[0015] Structural integrity of the SCS envelope lasnp depends upon the
physical
characteristics of the envelope and end plug materials and the effectiveness
of the seals. The
envelope and end plugs of the present invention may be manufactured to close
tolerances for a
consistent fit. The necessary holes in the end plugs for the electrode leads
may be produced by
conventional or laser drilling or by utilization of small diameter SCS tubing.
The SCS envelope
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lamp according to the present invention may preferentially be assembled using
seal materials
with similar thermal expansion characteristics to the SCS components, such as
nanostructured
ahunina silicate, in order to minimize stress related failure that results
from the lamp heating and
cooling cycle. These seals may operate at temperatL~res above 1,000° K
as compared to seal
temperatures of about 500° K for quartz. The abrasion resistance and
strength of the SCS
components, and consistently close component tolerances, makes possible low
cost, automated
lamp assembly techniques, not possible with quartz or PSA envelope lamps.
[0016] Figure lA shows a top view of a SCS hollow tube envelope 100. An inner
diameter d of the envelope 100 may range from 1 mm to more than 20 mm, while
an outside
diameter D of the envelope 100 may range fmm 2 mm to snore than 23 mm. The
length L of the
envelope 100 may range from 3 mm to more than 400 mm.
[0017] SGS properties are compared with quartz and polycrystalline alumina in
Table 1.
The tensile strength of SCS is compared with quartz as a function of
temperature in Table 2. The
thermal conductivity of SCS is compared with quartz as a function of
temperatl~re in Table 3.
[0018] SCS is an anisotropic monoaxial crystal that may be produced in tubular
form
from the crystallization of pure alumimun oxide using the edge defined film
growth technique
("EFG") or similar crystal growing methods. SCS is one of the hardest and
strongest known
materials, chemically inert, with excellent optical and dialectical
characteristics and thermal
stability up to 1,600" Celsius. Its wide optical transmission range of 0.17 to
5.5 mkm makes it
ideal for production of envelopes for transmission of ultraviolet ("UV"),
visible, and infra-red
("NIR") light. SCS is also insoluble in hydrofluoric, sulphuric and
hydrochloric acid, and most
important for HID lamp applications, it does not outgas or divitrify. The
operating temperaW re
of SCS higher than quartz and SCS has significantly higher thermal
conductivity. Raw SCS
W bing is presently available from a number of vendors, such as Saphikon and
Kyocera.
Commercial and SGS tubing, as delivered, has problems with holding circular
cross-section
tolerances. This can be taken care of by appropriate machining of the
appropriate surfaces, i.e.,
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reaming the interior and polishing the exterior using diamond tooling to
obtain a uniform and
specified wall thickness. The SCS envelope may tolerate a higher outer surface
temperature than
quartz and may handle conduction heat flux of greater than 150 watts/cm3
compared to the 20
watts/cm3 of quartz in the HID lamp applications.
[0019] Figure 2A shows an optical projection system having the SCS envelope
lamp 10
with a reflector 11. The light of the SCS envelope lamp 10 is focused on an
entry face 13 of a
hollow light pipe 15, preferably of the type described in U.S. Patent
5,829,858 which is
incorporated by reference. The bean is focused by lens 18 and 19 onto a
Fresnel plate 20 and a
LCD plate 21 which forms an image. The image is focused on the screen by
projector lens 23.
[0020] Figure 2B is a side view cross-section of the SCS envelope lamp 10. One
exemplary method of sealing the plugs 200 to the tubing is to use techniques
for sealing PCA
plugs to PCA tubing as described, e.g., in U.S. Patent 5,424,608. In Figure
2B, the envelope 100
is used. The plugs 200, which preferably are made ofPCA or SCS, close off the
ends of the
envelope 100. The plugs 200 are sealed to the envelope 100 with a halide
resistant seal material
to form a pressure and chemical resistant seal and contain the gases inside
the region bounded by
the inside diameter d and the surface facing the discharge of the plugs 200.
The halide resistant
seal material may be composed from materials, e.g., including ahuninunr,
titanium or tungsten
oxides as available from vendors, such as Ferro Inc. of Cleveland. The melting
point of such
materials may be about 800° C to 1,500° C, and most preferably
about 1,200° C to 1,400° C.
[0021] Electrode bases 202, 203 may be fitted into the electrode base
receptacles 204,
205 with sufficient clearance for wetting by the fill glass via capillary
action. The electrode
bases 202, 203 may be composed of niobium or tantalum and have coefficients of
expansion
close to that of sapphire (8 x 10'6 K-') . An electrode stem 206 may be
attached to the electrode
base 202 by welding. An electrode stem clearance hole 208 is sufficiently
large to allow
emplacement of the electrode stern 206, 210 with clearance too small to allow
wetting of the
clearance hole 208 by the glass sealing material through capillary action.
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[0022] The filling of the discharge vohune takes place prior to insertion of
the electrode
stems 206, 210. Spherical electrode tips 207, 209 may be formed after assembly
by heating with
lasers or by drawing high current through the discharge. After assembly, the
glass seal is applied
by melting glass into the space between the electrode base receptacle 204 and
the electrode base
202.
[0023] Another exemplary filling method for feeding the mercury, noble gases
and other
potential fills may be used to manufacture the electrode bases 202, 203 as
hollow W bes with an
exit opening into the space between the electrode stem 206 and the plugs 200.
Upon filling, the
exit opening rnay be sealed with a high melting point solder. The solder may
be melted with a
laser beam projecting through the hollow tube.
[0024] Polycrystalline alumina plugs contain multiple small crystals which
present a
variety of different crystal faces with respect to the surface of the seal
boundary. The coefficient
of thermal expansion of each crystal with respect to its boundaries is a
function of the crystal
orientation. Thus, the expansion and contraction due to thermal cycling of the
lamp when it is
W rned on and off is different for each crystal orientation with respect to
the seal boundary. These
different rates of expansion and contraction lead to degradation of the seals
with thermal
recycling.
[0025] SCS plugs are preferable to polycrystalline alumina plugs. In
particular, if the
long axis (the C axis) of the plugs 200 is oriented parallel to the long axis
(the C axis) of the
envelope 100, then there is no relative change in dimensions of the seal which
is beneficial for
long life with thermal cycling. The plugs 200 maybe shaped as shown in Figures
8A and 8B. A
cylindrical opening 800 may be machined to be approximately 0.02 mm larger
than the electrode
bases 202, 203. A hole 801 may be sized to be approximately 0.3 mm in diameter
greater than
the electrode stems 206, 210. In particular, the electrode bases 202, 203 are
fitted into the larger
openings 800, 804 with sufficient clearance for wetting the fill glass via
capillary action. The
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electrode bases 202, 203 maybe composed of niobium or tantalum which may have
coefficients
of expansion close to that of sapphire (8 x 10-' K-'). The electrode stem 206
maybe attached to
the electrode base 202, e.g., by welding. The clearance holes 801, 803 are
sufficiently large to
allow emplacement of the electrode stems 206, 210 with clearance too small to
allow wetting of
the clearance hole 800 by the glass seal through capillary action.
[0026] An exemplary method according to the present invention of sealing the
plugs 200
to the envelope 100 is to machine and polish the two adjacent surfaces so that
a sealing region
805 which is siW ated therebetween is less than 0.02 mrn. This may be
accomplished with
grinding or laser shaping with a final polishing step. For example, the outer
surface of the plugs
200 may be coated with about 1-5 layers of nanostnxctured alumina silicate
with a 1% to S%
mixture of Titanium-dioxide (TiOz). These materials may be obtained from
Bailcowski
Corporation of New Jersey. The coating process maybe prefonned utilizing a
flame spraying or
electrostatic deposition. The sealing region 805 may be heated with a laser or
centered in an
oven to complete the sealing operation.
[0027] The opening 804 and the hole 803 may be machined with a high-speed
drill or be
shaped with a laser as shown in Figure 8B. For example, the laser that may
drill such a shaped
opening is a 157 nm F2 laser light. The space between the electrode base 202
and the openings
800, 804 may be filled with (a) a glass frit for a lower tempeiaW re operation
or (b) the
nanostructured alumina-silicate for a higher temperature operation. The final
sealing step is to
sinter the assembly in an oven or with a laser sintering system. Sintering
temperat«res may be,
for example, 1,700° C to 2,000° C. The seal made with
nanostructured alumina-silicate may be
especially useful for long life under thermal cycling because alumina oxide is
used the basic
material to grow SCS.
[0028] This SCS envelope lamp 10 may be filled with a greater variety of
halides and
background gases than those fills which can be used in quartz lamps. For
example, scandimn
and rare earth halides may be used, With their favorite spectrum in the
optical region. In quartz
to
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envelopes, such halides form reactions that lead to deposition of the silicon
on the thoriated
tungsten electrode and depletion of the scandium or rare earth fills. See, for
example,
Waymouth, J.F., "Electric Discharge Lamps," M1T Press, Cambridge, MA, 1971.
[0029] In addition, fills such as sulfur, sodium, hydrogen and chlorine can be
used.
Utilization ofthe envelopes, in combination with the various fills, may more
than double lamp
efficacy to about 120 L/w to 180 L/w for arc gaps in the range between 1 rrun
and 2 mm. This
improvement is due to increased plasma luminance. Lumen maintenance is
improved
dramatically and the life of the lamp is extended to four or five times that
of fused quartz
envelope lamps.
[0030] Figl.~re 2B illustrates another exemplary embodiment of the SCS
envelope lamp
according the present invention which has a short arc. This embodiment may be
particularly
useful for image projection systems where the arc gap must be optically
matched to the size of
the image generation device. The arc gap required for current projection
systems is generally less
than 2 rmn with gaps as small as 0.8 mm required for the latest generation of
reflective image
devices, 0.5" diagonal.
[0031] Short mercury arc HID lamps with quartz envelopes, which have been
optimized
to gap length s of 1.8 rmn and inside diameter d of 3.8 mm with fill densities
between 40 and 65
mg/cm3 operating at 70 to 150 watts are limited to about 70 I/w output and are
subject to
"flicker" and premature failure of the quartz envelope due to devitrification.
(See, for example,
U.S. Patent 5,239,230). Halide versions of such lamps are limited to about 70
I/w with
limitations due to the physical properties of the quartz envelope.
[0032] A mercury filled HID lamp is described, e.g., in U.S. Patent 5,497,049.
This
patent describes, for example, that with an inside diarneter d of less than
3.8 mm and a power
level of 70 to 150 watts, an outside diameter, D, of 9 mm and a pressure of 20
atm, the inside of
the quartz begins to liquefy and devitrify leading to premature failure in
less than 100 hours.
11
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[0033] Quantitative analysis of the above-optimized quartz lamps is as
follows:
[0034] The data for quartz from Table 2 and Table 3 are used to parameterize
the
temperature behavior of the thermal conductivity and the tensile strength of
the materials. The
geometry of the lamp and the input parameters of pressure, power and fill
amount of Mercury
(Hg) and Xenon (Xe) and other gases are taken from U.S. Patent 5,497,049. The
temperature
drop across the tube wall is calculated as follows:
T = qWT/k
where:
T = temperature drop between inner and outer wall,
q = heat flux in watts/square cm,
WT = wall thickness in cm, and
k = thermal conductivity in watts/cm-K.
[0035] The total mechanical stress on the tube wall is determined by summing
the
thermal stress due to the temperatlu~ gradient and the mechanical hoop stress.
The th~nal stress
on the low temperature surface on the tube is givenby:
(thernal) = E ( T/2(1- ))
where:
= coefficient of thermal expansion
E = Young's modulus
= Poisson's ratio.
[0036] The Hoop Stress is givenby:
(hoop) = pressure dl(2 WT)
where:
Pressure = fill pressure.
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[0037] When using the following values
WT = 2.6 mm
d = 3.8 mm
L=5rmn
Power = 70 watts
Pressure = 20 attn
= 0.5 x 10-~
E = 11 x 10-~ lb/ in2
and when the outside wall temperature of the bulb is 25° C, the inner
wall temperature would be
1,400° K which is consistent with their description of failure at that
small size of d at 3.8 mm.
Under those conditions the total stress on the bulb would be 53% of the
maximum stress of 7,000
lbs/in2 .
[0038] Comparison with SCS under the same conditions and with:
a = 8 x 10-''
E=11x10-6
and an outer wall temperature of 25° C gives an imier wall temperature
of 331° K with a total
stress on the bulb of 3.9% of the maximmn allowable stress.
[0039] The SCS envelope lamp is capable of being optimized with improved
performance compared to quartz envelope HID lamps. Figure 3 shows the inner
wall
temperature of quartz and SCS envelope lamps compared as a function of fl1e
outer wall
temperature. Note that up to 1,273° K the inner wall temperature stays
within safe limits for the
SCS envelope lamp, while the quartz lamp fails at room temperature. Figure 4
is the safety
factor defined as the actual total stress/maximum tensile strength. This
factor should be a
maximum of 0.3 to 0.4 for safe operation. Note that the quartz Iainp would
fail at room
temperature, but that the sapphire lamp stays within feasible operating limits
up to 1,273° K.
[0040] For example, with m inner diameter of 1.61nm and an outs diameter of
3.2 mm,
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the SCS envelope lamp, operating at 150 watts and a pressure of 200 atm, would
have an inner
wall temperature of 317° C when the outer wall temperature is
25° C and an inner wall
temperature of 880° C when operating at an outer wall temperature of
800° C. The safety factor
would be 0.064 at 25° C outer wall temperature and 0.363 at 800°
C outer wall temperature.
When operating at 600 atm, the safety factor would be 0.083 at 25° C
outer wall temperature and
0.412 at 800° C outer wall temperature.
[0041] Improved eIT'icacy of light output, with gap sizes between 1 mm and 2
mm is
desirable, especially in projector lamps. By allowing operation at higher fill
pressures, the
stronger SCS tubing allows higher power density and thus higher efficacy. For
example, the
mercury HID quartz lamp described in U.S. Patent 5,497,049 described an
increase in efficacy
from 17 L/w at pressures of about 20 atm to 70 L/w at pressures of 50 atm,
with roughly a square
root dependence on pressure. Basically, increased pressure resulted in
increased efficacy Lentil
the discharge went unstable.
[0042]. The pressure at which the discharge goes unstable is determined by the
Grashof
number:
Gr = c ~(d/2)3(pressure)2
where:
pressure = mercury content in mg/ cm2
c = 9.86
(Note that 1 mg/cc of mercury is equivalent to 1 atm at 25° C).
[0043] In quartz HID lamps in this range Gr must be less than 1,400 for stable
operation.
It can be seen from this relationship that a lamp with the imier diameter d
greater than 3.8 mm
would have a value of Gr greater than 1,400 and would be unstable at mercury
contents greater
than 60 mg/cc.
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[0044] The envelope, in the SC S envelope lamp 10 design shown in Figures 2A
and 2B,
may prevent "flicker" at smaller diameters and much higherpressu res. For
example, a SCS
envelope lamp with a value d of 2 mm and an arc gap s of 1.4 mm and a chamber
length S of 3
mm would have a value of GT less than 1,400 for pressures of 120 to 135 mg/cc.
This may result
in flicker-free operation in this pressure range.
[0045] For example, the SCS envelope lamp having the inner diameter d of 1.6
rnrn and
operating at 400 atm would have a Grashof number of about 800 which is within
the stability
limits.
[0046] The Grashof number defines a plasma arc stability condition. It is
based on the
ratio of a buoyancy force to a viscous force and defines the stability
boundary for the gas
dynamic forces set up by tile arc discharge plasma and its enviromnent. Other
factors can help
determine whether or not a specific plasma arc actually goes unstable and
"flickers". For
example, the electrode tip design can be modified to diminish "flicker" by
adjusting the supply of
electrons to the arc and by modifying the electric field structure at the base
of the arc.
[0047] The time dependence of the plasma arc temperature and electron number
density
profile can also influence the development of a plasma instability and thus
"flicker". The time
dependence of the applied voltage (waveform) detern pines the time dependence
of the plasma arc
temperature and niunber density profile. Suitable variations in these
waveforms can diminish
flicker.
[0048] The SCS envelope lamp according to the present invention, because of
the
relatively small ratio of an inner wall diameter to an arc length, may operate
in a "wall stabilized"
mode. In other words, "wall stabilization " may be used as a description of a
plasma arc
operating with a low Grashof munber, because the Grashof nLUnber is
proportional to the cube of
the diameter, making small values of diameter beneficial.
CA 02465786 2004-05-05
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[0049] The SCS envelope lamp according to the present invention may be broadly
described as operating in a "continuous non-flash" mode. Operating ranges,
that may be utilized
for the SCS envelope lamps according to the present invention, may include
applied voltages
between 0.1 volts and 600 volts and applied currents of between 2 amps and 150
amps. For
example, one mode of "continuous non-flash" operation is to apply a constant
voltage between
the electrodes. This is called a direct current ("DC") operation. In this
case, one electrode is an
anode and another one is a cathode.
[0050] A second exemplary mode of "continuous non-flash" operation is to apply
alternating current ("AC") in which the voltage reverses polarity on a
periodic time dependent
basis. The SCS envelope lamp according to the present invention may operate,
for example, with
time dependent reversal frequencies which can vary between 16 cycles per
second to over 1,000
cycles per second. Some of these alternating wavefonns can be "sinusoidal" and
others could be
"square waves".
[0051] Efficacy is also much improved for SCS envelopes. Based on the increase
in
efficacy with pressure described in U.S. Patent 5,497,049, the performance of
this HID lamp may
be extrapolated to be in the range of 70 Llw to 90 L/w. Thus, improvements in
efficacy into the
range of 90 L/w may be achieved with mercury fill lamps alone. Further
increases ofefficacy
may be expected by filling the bulb with alternative elements such as sodium,
sufi~r and
selenium. These elements all increase luminous efficiency and can be expected
to further
increase output in other versions of the SCS lamp.
[0052] A larger SCS envelope lamp which develops considerable pressure on the
end
plugs, maybe built with the design shown in Figure 5. In Figure 5, a second
metallic barrier is
built into the SCS envelope Lamp. This second barrier utilizes a new seal
geometry in which the
pressure from the SCS envelope lamp is taken in compression on the seal face
rather than in
tension, as in the design shown in Figures 2A and 2B. Figure 5 is a side cross-
section of the SCS
envelope lamp. In the case the design shown in Figure 5, the envelope 100 is
used and the two
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plugs 300, preferably are made of PCA or SCS, to close the ends of the
envelope 100 as a "first"
seal. The plugs 300 are sealed to the envelope 100 to form a pressure and
chemical resistant seal
and contain the gases inside the region bounded by the inside diameter d and
the surface facing
the discharge of the plugs 300. The plugs 300 are sealed to the envelope 100
with q halide
resistant glass 301 to form a pressure and chemical resistant seal and to
contain the gases. The
glass 301 maybe made from materials including aluminum, titanium or tungsten
oxides available
from vendors such as Feno Inc. of Cleveland. The melting point of such
materials may be about
1,300° C. As discussed above, for highertemperature operation an
alternative seal technology is
to use nanostructtrred alurnina-silicate ceramic doped with titanium or
tungsten. The
nanostrrrctured material may have dimensions of 50 nrrr to 1,000 nm.
[0053] A "second" seal is provided in this design to further improve the
lifetime of the
SCS envelope lamp. A "electrode disc" is inserted in a groove in the tubing in
such a way that
the pressure on the ends is taken in compression by the envelope 100, giving a
more stable and
pressure-resistant seal. The "first seal" takes the pressure in shear, and as
bulb diameter
increases the shear resistance of the seal does not scale with the diameter.
The "second" seal
being under compression can absorb much higher forces without flexing or
tearing. The pressure
from the plasma results in a compressive force on the second seal that is
taken up by the tensile
strength along the C axis of the envelope 100.
[0054] The second seal is preferably formed as follows. An electrode base 302
is welded
into the electrode disc 310. An electrode stem 306 is also welded into the
electrode disc 310 as
shown. The electrode base 302 maybe composed of nickel or molybdenum. The
electrode disc
310 may be composed of niobium or tantalum which have coeffrcients of
expansion close to that
of SCS (8 x 10-6 K-'). The subassembly consisting of the electrode base 302,
the electrode disc
310, and the electrode stern 306 is tapped into place. The electrode disc 310
is designed to be
flexible enough to slip into an electrode seal receptacle 311. Upon assembly
the SCS envelope
lamp is first filled appropriately and then an electrode disc seal 312 is made
with halide-resistant
glass doped with titanium and tungsten. Similarly, the electrode end comprises
an electrode base
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303 welded to an electrode disc 313 and an electrode stem 307.
[0055] Niobium is the preferred material for the second seal. Its coefficient
of thermal
expansion is 7.1 x 10-6 K-'. The coefficient of thermal expansion
perpendicular to the G axis of
SCS is 7.9 x 10-6 K-' . Over a 1,200° C change in temperaW re this
small difference results in less
than 1.2 x 10-3 mm differential expansion, which reduces temperatl~re cycling
problems in the
seal.
[0056] FigL~re 7 illustrates another exemplary embodiment of the SCS envelope
lamp
which does not utilize electrodes. Similarly to the SCS envelope lamp shown in
Figure 5, the
electrode disc 310 and the electrode disc 311 are retained, but the electrode
base 302, the
electrode stem 306 and the electrode stem 303 and the electrode stem 302 are
not present in the
SCS envelope lamp shown in Figl.~re 7. 'Ibis assembly may be fitted into an
electrodeless lamp
receptacle, and the receptacle can be designed to apply microwave or RF power
without the
creation of electrical arcs on the metallic components.
[0057) This type of electrodeless SCS envelope lamp has advantages over the
conventional quartz technology in typical commercial electrodeless lamp
applications. In
particular, the high temperature capability ofthe envelope allows operation of
the bulb at power
densities much greater than 50 watts/cm3 without rotation.
[0058] This design utilizes the disc seal concept as described above and shown
in Figure
5, but only as a sealing device. This allows construction of a robL~st
electrodeless lamp capable
of operation at pressures over 300 atln.
[0059] The electrodes may be adapted for A.C. operation. Their shape and size
would be
changed for D.C. or pulsed operation. The SCS envelope lamp of the present
invention may
maintain a CCT of between &,500° K and 7,000° K with continuous
non-flash operation.
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[0060] Preferably, the envelope 100 has a substantially cylindrical shape with
an inner
diameter d of between 1 mm and 25 mm and an outer diameter D of 2 rnm or more.
The fill
mercury density is between 10 mgJcm3 and 600 mg/cm3; and the operating
pressure ranges
between 20 atm and 600 atm. The efficacy of light output exceeds 60 L/w and
mostpreferably
75 L/w; the seals are capable of operating up to 1,400 °C; and the arc
plasma has a temperature
between 4,000 and 15,000 °C.
[0061] The high pressure (up to 600 atln) regime of operation with a mercury
fill is
primarily for emission of visible radiation at high efficiency.
[0062] For operation in the UV or IR range of the radiation spectrum, the bulb
fill
material, the discharge plasma temperature and the optimu2n operating pressure
are tailored for
the desired spear um.
[0063] For UV in the range of 200 nm to 400 nm, the mercury fill amount is
typically 10-
20 mg/cm3 and the xenon fill pressure is between 0.5 atm to 20 atm. Dopant
atoms and
molecules could be one or more of cadmium, iron chloride, iron bromide,
chromium chloride,
chromium boride or vanadium. These elements are rich in lines between 200 and
400 mn.
Operating temperatures of 6,000° K to 7,000° K are typical for
UV production. Alternatively, the
mercury can be left out entirely and the xenon fill pressure established in
the range from 0,5 atm
to 200 atm. This pure xenon fill can be operated up to 15,000° K for
generation of UV in the 200
nm to 400 nm region. Dopants can also be added to the mercury free xenon fill.
This single
crystal sapphire bulb can have many applications such as a spot source for UV
curing of coatings
and inks.
[0064] For IR in the range of 700 nm to 2,500 nm, the mercury fill amount is
typically
10-20 mg/cm' and the xenon fill presswe is between 0.5 atin to 20 atin. Dopant
atoms could be
one or more of cesium, potassiu2n or rubidium which are rich in infrared
lines. The arc operates
with typical temperatures between 4,000° K and 6,000° K.
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[0065] The SCS envelope lamp according present invention may have the
following
advantages over the conventional lamp:
(1) it has better optical efficiency (e.g., matching of the SCS envelope lamp
etendue to
that of the image gate element);
(2) it has better power efficiency (e.g, referred to as efficacy and measured
in L/w);
(3) it has better color rendition (e.g., a High Color Rendering Index);
(4) it may last longer (e.g., four to five times longer) with superior lumen
maintenance
than a conventional HID lamp;
(5) it has a smaller physical size;
(6) it reduces initial cost;
(7) it reduces operating cost and enhances manufacturing tolerance;
(8) it reduces system cost;
(9) it may allow a flicker free operation at pressures as high as, e.g., 600
atm, thus
achieving substantially higher efficacies than the conventional HID lamp with
quartz envelope
achieves; and
(11) it may be effectively tailored for specific applications; for example,
the SCS
envelope has a high chemical stability, this allows the use of a wide range of
X11 additives and
gases (e.g., sodium, hydrogen, neon, chlorine, sulfur, selenium, etc.) which
cannot be used with
conventional quartz envelope lamps, thus allowing the light spectnim to better
tailored for an
image projection or any other specific application. In addition, the wide
range of alternative fill
materials maypermit the elimination ofrnercuryfrom the lamp which is
particularly desirable in
consumer product applications.
[0066] Another advantage of the SCS envelope lamp according to the present
invention is
that it provides an opportunity to use a number of fill additives that cannot
be used with
conventional quartz envelope HID lamp, and thus allowing the flexibility to
tailor the light
spectrum to the desired CCT for pr ojection effectively increasing the lamp
useful efficacy.
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[0067] The SCS envelope lung according to the present invention may be
utilized in
various industries, for example, in image projectors, automobile headlamps,
fiber optic light
sources and other non-speciality applications, such as home lighting.
[0068] There are many modil:ications to the present invention which will be
apparent to
those skilled in the art without departing form the teaching of the present
invention. The
embodiments disclosed herein are for illustrative purposes only and are not
intended to describe
the bounds of the present invention which is to be limited only by the scope
of the claims
appended hereto.
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