Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~.'17~77~
A-36421/HCH/LCB
--1--
OPTICAL COATINGS FOR HIGH TEMPERATURE APPLICATIONS
This invention relates generally to optical coatings and
specifically to optical coatings for high temperature appli-
cations. More specifically, this invention relates to hightemperature, energy saving lamps with an optical coating
thereon to improve energy efficiency.
Thin film optical coatings of the interference filter type
which utilize two materials of different indices of refrac-
tion have not generally been applied in high temperature en-
vironments in which the coatings are exposed to the air at
temperatures in excess of 500C for many hours. Typically
thin film optical coatings do not survive these operating
environments, failure being due to one of the following:
loss of adhesion of the optical coating to the substrate,
interdiffusion of the materials of the high and low refrac-
tive index layers of the coating, decrease in the index
ratio of the two materials, evaporation of the thin film
layers, or unacceptable increases in the absorption of the
coating.
One application in which thin film optical coatings are use-
ful is to improve the illumination efficiency of incandescent
~5 lamps. It is well-known that applying a hot mirror type of
optical coating to the envelope of an incandescent lamp in-
creases its energy efficiency. The hot mirror reflects
infrared energy emitted by the filament back to the filament
while transmitting the visible light portion of the electro-
magnetic spectrum emitted by the filament. This lowers the
~ 7~
amount of electrical energy required to be supplied to thefilament to maintain its operating temperature. For example,
U. S. Patent 3,949,259, 4,017,758, 4,127,789, 4,160,929, and
4,227,113 disclose the use of various types of hot mirrors
on all or portions of an incandescent lamp envelope. How-
ever, none of these references discloses specific applica-
tions in which the optical coating is formed on a lamp
envelope surface which operates in air at a temperature sub-
stantially in excess of 500C.
U. S. Patent 4,017,758 teaches the use of a hot mirror opti-
cal coating consisting of a composite of a heavily doped
metal oxide filter formed nearest the filament body of the
lamp and a multilayer interference filter disposed either
adjacent to the heavily doped metal oxide filter or on a dif-
ferent surface of the lamp envelope. For example, the '758
patent suggests that both filters may be disposed on the in-
side wall of the lamp envelope or both on the outside wall or
one component on the inside and the other on the outside wall
surface, respectively. The '758 patent also discloses a
special lamp embodiment utilizing a double wall lamp envelope
and suggests various combinations which may be employed for
disposing the interference filter and the doped metal oxide
filter on lamp envelope walls in such an embodiment. Al-
though the '758 patent makes specific reference to use of thecomposite filters disclosed therein in halogen lamps, the
reference does not disclose any example of interference
filter materials which could survive the operating tempera-
tures of the surface of the lamp envelope of a halogen lamp.
The only high refractive index materials referred to in the
'758 patent are zinc selenide, zinc sulfide, and titanium di-
oxide. Thus, while the '758 patent refers to the use of
silicon dioxide as the low refractive index layer in an
interference coating (and it is well-known that silicon di-
oxide will survive in a high temperature environment) thehigh refractive index materials referred to in the '758
patent will not survive the high temperature environment of
~7~7704
--3--
about 800C on the outside surface of the envelope of a
halogen lamp.
Accordingly, it is the principal object of this invention to
provide an optical coating comprising layers of low and high
refractive index materials which will withstand a high temp-
erature environment in excess of 500C.
It is another object of this invention to provide a multi-
layer optical interference filter which is capable of with-
standing a high temperature environment.
It is another object of this invention to provide a hot mir-
ror optical coating which may be utilized in a high tempera-
ture environment.
It is a further object of this invention to provide a halogencycle lamp envelope with an optical interference filter form-
ed on the outer surface thereof which will survive the oper-
ating temperature of the lamp envelope.
It is a further object of this invention to provide a halogenlamp with an energy saving optical interference filter formed
on an outer surface of the lamp envelope.
This invention is based on the discovery that an optical
coating which comprises a first set of layers consisting at
least primarily of silicon dioxide and a second set of layers
consisting at least primarily of tantalum pentoxide will sur-
vive a high temperature environment even where the opticalcoating is operated at the high temperature environment in
air for a substantial period of time. Many other optical
coating combinations with silicon dioxide as the low
refractive index material and other refractory-type high
refractive index materials such as titanium dioxide will not
survive similar high temperature operating environments.
It has also been discovered that optical coatings in accor-
11777C~4
--4--dance with this invention will survive the high temperature
environment of the outside surface of a halogen lamp envelope
having a small radius of curvature, since a small curvature
accentuates problems of coating stresses due to thermal mis-
matches.
Accordingly, one aspect of this invention features a coatedarticle useful in high temperature environments substantially
in excess of 500C where the article comprises a substantial-
ly transparent substrate formed of a material adapted towithstand a high temperature environment and an optical coat-
ing formed on one surface of the substrate and comprising a
first set of layers consisting at least primarily of silicon
dioxide and a second set of layers consisting at least pri-
marily of tantalum pentoxide. The optical coating may, forexample, comprise an interference filter formed of alternat-
ing layers of these first and second sets thereof. The
interference filter may comprise a bandpass filter designed
to transmit radiation in a preselected first wavelength band
and to reflect radiation in adjacent wavelength region. A
specific example of such a bandpass filter is a hot mirror
having high transmittance for visible light and high infrared
reflectance. Alternatively, the bandpass filter may be a
color filter having a high transmittance for a preselected
portion of the visible light spectrum and high reflectance
for adjacent spectral regions. The substrate on which the
optical coating is formed may comprise a fused quartz lamp
envelope adapted to be utilized in a halogen cycle incandes-
cent lamp operating at an outer envelope surface temperature
of at least about 800C with the interference filter formed
on the outer surface of the lamp envelope.
In accordance with another aspect of this invention, a coated
article is provided which is useful in high temperature envi-
ronments substantially in excess of 500C and comprises asubstantially transparent substrate formed of a material
adapted to withstand the high temperature environment and an
optical coating formed on one surface of the substrate and
comprising a multilayer interference filter having high re-
flectance of infrared radiation and high scattering of vis-
ible light. This interference filter is formed by deposit-
ing on the substrate a multilayer dielectric stack composedof alternate layers consisting at least primarily of silicon
dioxide and tantalum pentoxide and then baking the coated
substrate in air at a temperature of at least about 1100C.
In accordance with another aspect of this invention an im-
proved energy efficient halogen lamp is provided. The halo-
gen lamp comprises a lamp envelope having a geometry which
has an internal focal point, line or plane and formed of a
substantially transparent material capable of withstanding
operating temperatures of at least 800C. A high melting
point metal filament is mounted within the lamp envelope sub-
stantially at the focal point, line or plane and a halogen
gas is provided to fill the envelope. An interference
filter is formed on an outer surface of the lamp envelope and
is comprised of alternate layers consisting at least primari-
ly of silicon dioxide and tantalum pentoxide, respectively.
The interference filter formed on the halogen lamp may be a
bandpass filter having high transmittance for visible light
and high reflectance of infrared radiation. Alternatively,
the interference filter may comprise a bandpass filter having
high transmittance radiation in a preselected portion of the
visible light spectrum and high reflectance of radiation in
adjacent wavelength regimes to produce a lamp which has a
light output of a preselected color.
The interference filter formed on the halogen lamp may also
comprise a visible light scattering, infrared reflecting
filter formed by depositing on the outer surface of the lamp
envelope a multilayer dielectric stack of the primarily sili-
con dioxide and tantalum pentoxide layers having a high
transmittance of visible light and high infrared reflectance
1~L77~7~9~
--6--
and then baking the envelope and filter in air at a tempera-
ture of at least about 1100C to convert the filter from a
visible light transmitting filter to a substantially visible
light scattering filter.
The halogen lamp in accordance with this invention may also
utilize a multilayer interference filter formed on substrates
utilized as end reflectors in the lamp envelope.
This invention enables for the first time improvements in
energy efficiency to be applied in an optimal fashion to
halogen cycle lamps by enabling the formation of an optical
interference coating directly on the outside surface of the
halogen lamp envelope which generally operates at a tempera-
ture of about 800C. Improvements in performance in therange of about twenty-five to thirty percent have been mea-
sured in 1500 watt halogen cycle lamps to which the invention
has been applied. This level of improvement would not be
practicably achieved if the IR reflecting coating were placed
on a separate surface surrounding and spaced from the outer
surface of the lamp envelope to reduce the operating tempera-
ture of the coating.
The optical coatings of this invention may also find useful
application in a wide variety of other high temperature envi-
ronments such as heat reflecting windows for furnaces, laser
pump lamps, and discharge lamps such as arc lamps utilized in
theater projection equipment and the like. Generally the
invention is applicable to providing optical coatings for use
in any high temperature environment in which optical inter-
ference filter type of optical coating performance will pro-
vide an improvement in operating efficiency or other operat-
ing aspects of the apparatus on which the coating is
employed.
Other objects, features, and advantages of this invention
will be apparent from a consideration of the following de-
tailed description taken in conjunction with the accompanyingdrawings.
Fig. 1 is a partly sectioned elevational view of a halogen
lamp incorporating an optical coating in accordance with this
invention.
Fig. 2 is a fragmented elevational view of a hot mirror coat-
ing design utilizing the principles of this invention.
Fig. 3 is a graph illustrating the spectral emission of a
black body.
Fig. 4 is a graph showiny the visible transmittance and in-
frared reflectance characteristics of an exemplary optical
coating in accordance with this invention.
Fig. 5 is a graph of the spectral reflectance of a shortwave
pass dielectric stack component of the overall optical coat-
ing illustrated in Fig. 2.
Fig. 6 is a graph of the spectral reflectance of a 2:1 di-
electric stack employed as one component of the optical coat-
ing depicted in Fig. 2.
Fig. 7 is a graph of the spectral reflectance of another
shortwave pass dielectric stack used as one component of the
optical coating depicted in Fig. 2.
Fig. ~ is a graph of the spectral transmittance, reflectance,
and scatter response of a visible light scattering, infrared
reflecting optical coating in accordance with this invention.
Referring now to Fig. 1, the principles of this invention
will be set forth in their application to a halogen cycle
tungsten lamp 10. It should be understood, however, that
the principles of the invention are applicable to any high
1~7t7704
--8--
temperature environment in which an optical coating may find
utility. The halogen cycle lamp 10 comprises a lamp envelope
11 which includes a fused quartz tube 12 and a pair of end
sealing and mounting structures 13. Along the central axis
of the quartz tube 12 a coiled tungsten filament 15 is sup-
ported by a plurality of support structures 16. End reflec-
tors 17 may be provided at the ends of the tungsten filament
15. In the manufacturing process, the halogen cycle lamp is
formed by sealing the tube 12 using the sealing end sections
13 and then evacuating the tube 12 and refilling it with an
appropriate reactive halogen atmosphere.
During operation of the halogen lamp 10, the halogen gas re-
acts with tungsten which has evaporated from the filament.
The resulting gas is chemically decomposed at the hot surface
of the tungsten filament so that the tungsten atoms therein
are deposited on the filament and the halogen is freed to
scavenge additional liberated tungsten atoms. In order for
the halogen cycle lamp to operate properly, the quartz tube
12 must be maintained at a high temperature in the vicinity
of about 8~0C and generally this is accomplished by keeping
the diameter of the quartz tube 12 relatively small. For
example, a typical lamp may be about ten inches long and
about three-eights inch in diameter.
In accordance with this invention an optical coating 14 is
deposited on the outer surface of the quartz tube 12. This
optical coating comprises a first set of layers consisting at
least primarily of silicon dioxide and a second set of layers
consisting at least primarily of tantalum pentoxide. The
design of optical coating 14 may take one of a number of
forms depending on the spectral performance which is desired
for the coating. Generally, the optical coating 14 will com-
prise one or more dielectric stacks in which alternating
layers of silicon dioxide and tantalum pentoxide are formed
to produce an interference filter.
~1'7'~704
For convenience the optical filter layers will be referred
to as layers of silicon dioxide and tantalum pentoxide,
but it should be understood that the silicon dioxide layers
may not consist solely of silicon dioxide and the tantalum
pentoxide layers may not consist solely of tantalum pen-
toxide. In each instance some amounts of other dielectric
film constituents may be present~ For example, the tantalum
pentoxide may also contain a small percentage of another
refractory oxide such as titanium dioxide. It should also
be understood that the optical filter 14 may take one of
several forms, each of which embodies the general principle
that it is a selectively reflecting coating, i.e. it is sub-
stantially transparent to radiation in spectral regions in
which it is desirable that the lamp 10 emit radiation and is
substantially reflecting over the remainder of the spectrum
of substantial emission of electromagnetic radiation by the
hot filament. By reflecting back to the hot filament, radia-
tion which is not desired to be emitted from the lamp con-
serves the energy otherwise required to maintain the filament
at operating temperature and thus reduces overall energy re-
quirements for operating the lamp.
One of the alternative forms which optical coating 14 may
take is the coating design 14A depicted in Fig. 2 and having
the design parameters set forth in Table 1 below. The over-
all performance of the coating is depicted in Fig. 4. As
shown by the dashed curve 22, the coating 14A has high trans-
mittance in the visible region of ~he electromagnetic radia-
tion spectrum ~etween 400 nanometers and 700 nanometers and
has a high reflectance throughout the remainder of the spec-
trum, principally the near infrared where there is substan-
tial emission of electromagnetic radiation by the hot fila-
ment of the lamp, as shown by the curve 21 in Fig. 4.
Fig. 3 illustrates the radiant power spectrum from a 3,000
~elvin black body and shows that only a small percentage of
77U~
--10--
TABLE 1
LAYER INDEX OFTHICKNESS (nm) QWOT*
5 Air 1.000
1 1.45894.28 550
2 2.130129.11 1100
3 1.458188.56 1100
4 2.130129.11 1100
10 5 1.458188.56 1100
6 2.130129.11 1100
7 1.458188.56 1100
8 2.130129.11 1100
9 1.458188.56 1100
1510 2.130129.11 1100
11 1.45894.28 550
11' 1.45894.28 550
12 1.458180.00 1050
13 2.130123.24 1050
2014 1.458360.00 2100
2.130123.24 1050
16 1.458360.00 2100
17 2.130123.24 1050
18 1.458360.00 2100
2519 2.130123.24 1050
1.458180.00 550
20' 1.45894.28 550
21 1.~5877.14 450
22 2.130105.63 900
3023 1.458154.28 900
24 2.130105.63 900
1.458154.28 900
26 2.130105.63 900
27 1.45877.14 450
Substrate 1.460
*Quarter Wave Optical Thickness (i.e. reference wavelength at
which layer has a quarter wave optical thickness)
1177704
the total radiation from the filament of a halogen cycle lamp
is in the visible light region between 40n and 700 nano-
meters. The majority of the emitted radiation is in the in-
frared region above the visible light region of the spectrum.
Unless the lamp is to be used for both heating and lighting,
the emission of the infrared radiation from the lamp is
wasteful of energy and in some applications produces an un-
desirable heating of the surrounding environment. For ex-
ample, in theater and stage lighting where high intensity il-
lumination is required, the heating effect from the high in-
tensity lamps is unwelcome since it overheats the area which
is being illuminated. By employing a visible light transmit-
ting, infrared reflecting optical coating 14 on the lamp 10,
the emitted radiation in the infrared region is reflected
back to the filament 15 where it serves a useful purpose in
keeping the filament heated and yet the major portion of the
visible light emitted by the filament escapes the lamp and
perform useful work in illuminating the surrounding environ-
ment.
Referring specifically to Fig. 2 and Table 1, it is seen that
the performance of the overall filter depicted in Fig. 4 is
attained in this instance by combining three types of dielec-
tric stacks into an overall interference filter 14A. As
25 shown in Table 1, the layers labeled 21-27 form a first di-
electric stack I which has a dielectric stack design general-
ly expressed as (L/2 H L/2)3 and comprises a shortwave pass
interference filter at a design wavelength of 900 nanometers.
The spectral reflectance of this shortwave pass stack is de-
picted in Fig. 5. This dielectric stack is considered a
shortwave pass stack since it has very low reflectance at
wavelengths less than the design wavelength of 900 nanometers
and then a region of substantial reflectance at wavelengths
greater than 900 nanometers. The second dielectric stack II
35 is a 2:1 dielectric stack at a design wavelength of 1050
nanometers and having a stack design generally expressed as
(LHL)4. The spectral reflectance of this 2:1 stack is de-
picted as the curve 24 in Fig. 6.
~ ~7 ;~ ~ O 4
-12-
The third dielectric stack III utilized in the coating 14A
is a shortwave pass filter at a design wavelength of 1100
nanometers and having a design generally expressed as
(L/2 H L/2)5. In each of the above design expressions for
the various dielectric stacks I, II, and III, the "L" desig-
nates a layer of low refractive index material (i.e. silicon
dioxide in this case) which has a quarterwave optical thick-
ness at the design wavelength. Similarly, the designation
"H" refers to a layer of higher refractive index material
(i.e. tantalum pentoxide in this case) which has a quarter-
wave optical thickness at the design wavelength. Referring
to the shortwave pass stack I for which the design specifica-
tion is (L/2 H L/2)3, it is thus seen that each of the L/2
layers in the formula are layers which have an optical thick-
ness equal to an eighth wave at the design wavelength. Inthe physical filter embodiment,the first and last layers in
the stack I, i.e. layers 21 and 27 in Table 1 are actual
eighth wave layers of the low index silicon dioxide material.
On the other hand, layers 23 and 25 turn out to be quarter-
wave layers since they consist of two eighth wave layersformed at the same time. This same analysis holds for the
shortwave pass stack III which utilizes five components of a
(L/2 H L/2) design. The layers 1 and 11 are eighth wave
layers, whereas the layers 3, 5, 7, and 9 turn out physically
to be quarterwave layers, being the sum of two eighth wave
layers. Furthermore, in actually building the filter, the
layers 11, 11', and 12 become one physical layer and the
layers 20, 20', and 21 become a single physical layer of the
low index silicon dioxide material.
The designations for the respective layers on the righthand
side of Fig. 2 should be interpreted as follows: the H and
L designations again refer to a quarterwave layer of low and
high index material respectively and the subscripts A, B, and
C refer to the three different design wavelengths where A
signifies design wavelength of 900 nanometers, B designates a
i~7~;'704
-13-
design wavelength of 1050 nanometers, and C designates a de-
sign wavelength of 1100 nanometers.
Other types of optical coatings may also be useful on the
halogen cycle lamp 10 depicted in Fig. 1. For example, an
optical coating 18 may be formed on the end reflectors
17 of the lamp. In this case, the optical coating 18 may be
designed to reflect all components of the radiation emitted
by the filament 15 since this will tend to maintain the
energy emitted in the directions of the end reflectors with-
in the cavity of the envelope 12 where it can do useful work
in heating the filament and otherwise maintaining the inter-
nal temperature of the lamp.
Other designs for the optical filter 14 may also be desirable
in certain applications. For example, in certain applica-
tions a colored light output is desired from the lamp. One
way of achieving a colored light output is to filter the vis-
ible light emitted from the lamp through an absorbing-type
color filter which transmits only the desired component of
the visible light spectrum. However, such an absorbing fil-
ter wastes the energy emitted from the lamp and dissipates
it in the filter itself. In accordance with this invention,
the optical coating 14 may be designed to have a passband
which encompasses only a selected portion of the visible
spectrum such that only that portion of the radiation emit-
ted by the lamp exits the lamp and all radiation at adjacent
wavelengths including portions of the visible and the infra-
red are reflected back into the lamp and onto the filament
to increase the energy efficiency of the overall lamp. The
design of a narrow bandpass filter having high transmittance
only in a portion of the visible light spectrum corresponding
to the color desired to be emitted from the lamp is well
within the skill of the art, for example, by following the
general teachings in Chapter 7 of H. A. Macleod's, Thin Film
Optical Filters, American Elsevier Publishing Company,
New York (1969). Such filters could also be designed uti-
770~
-14-
lizing the concept set forth in Chapter 20 of MIL HBK. 144
published in October, 1962 by The Department of Defense.
Chapter 20 is entitled "~pplication of Thin Film Coatings"
and is authored by Philip Baumeister. Each of these refer-
ence works is incorporated by reference into this applicationas teaching all dielectric optical filter designs and design
concepts which could employ the principles of this invention.
In other words, it should be unders~ood that this invention
is generally applicable to all types of optical filters and
in particular optical interference filters of the bandpass or
edge filter type.
Generally the optical coating 14 shown in Fig. 1 would be
formed on the lamp envelope 11 in a vacuum deposition chamber
utilizing standard vacuum coating technology. For example,
deposition of the optical coating on a small diameter lamp
envelope may be accomplished in a standard planetary type de-
position chamber by adding another degree of rotation which
rotates each quartz lamp tube along its axis so that all por-
tions of the outer surface thereof are uniformly exposed tothe deposition source within the chamber. Generally, both
the silicon dioxide and the tantalum pentoxide layers of the
coating will be deposited in a reactive gas mode, onto a sub-
strate which is maintained at a temperature of at least about
275C. Either electron beam evaporation sources or resis-
tance heated sources may be utilized. Reactive gas deposi-
tion involves bleeding oxygen into the chamber during the
deposition process. To obtain a good yield of optical coat-
ings on lamp envelopes having a small radius of curvature, it
has been found preferable to arrange the deposition source
with respect to the quartz tube substrate such that the aver-
age angle of arrival of the deposited material at the sub-
strate will not exceed about thirty-five degrees.
Optical coatings employing the principles and materials of
this invention have been built and tested at temperatures up
to 1100C. At temperatures below 1100C, the optical perfor-
1~7~t~4
-15-
mance of the filter remains substantially constant with no
evidence of loss of adhesion of the coating, increase in
absorption of the coating or interdiffusion of the layers of
the coating. It has also been found that by baking the coat-
ing at 1100C in air for a number of hours, the coating canbe transformed from a visible light transmit~ing, infrared
reflecting filter to a substantially visible light scatter-
ing, infrared reflecting filter. The spectral performance
of such a filter is depicted in Fig. 8. When the optical
coating is exposed to this level of temperature in air for a
significant period of time, the coating breaks up into many
small islands which are very scattering for light in the
visible portion of the spectrum but appear to radiation in
the infrared region as a continuous reflecting film. The
spectral performance depicted in Fig. 8 is for an optical
coating of the design set forth in Table 1 above. Other
coating designs could be fashioned which would optimize the
scattering in the visible region and otherwise change the
spectral transmittance, reflectance and scattering response
of the filter.
Actual halogen cycle lamps employing the optical coating de-
sign depicted in Fig. 2 and set forth in Table 1 above have
been fabricated and tested to demonstrate the improvement in
energy efficiency of the lamp with the optical coating ap-
plied. 1500 watt lamps have been tested and have shown per-
formance improvements in the range of twenty-five to thirty
percent. These proven performance improvements correlate
well with theoretical percentage improvements values which
have been calculated to be in the thirty to thirty-five per-
cent region.
As previously indicated, the principles of this invention
could be applied in other types of lamp environments such as
arc discharge lamps in which an excited plasma emits light
of various wavelengths. Due to the large number of free
electrons in the plasma, plasma is a good absorber as well
77Q4
-16-
as a good emitter. Consequently, the concept of reflecting
unwanted components of the light emit~ed from the plasma back
into the plasma should also improve the energy efficiency of
arc lamps. The principles of this invention may also be
applied to laser pump lamps which utilize either a plurality
of flash lamps or continuously operated incandescent lamps
surrounding a ruby rod within a cavity. Since the ruby laser
rod only absorbs light in certain portions of the spectrum,
improvements in energy efficiency can be achieved by placing
on the pumping lamps an optical coating which only transmits
useful light to the laser rod. The unwanted light is
reflected back into the pumping lamp to improve the lamp's
efficiency.
While the principles of this invention have been discussed
above in connection with several alternative embodiments, it
should be understood that numerous other applications of the
principles may be found by those of ordinary skill in this
art. Accordingly, the invention is not limited to the speci-
fic exemplary applications described above but may be employ-
ed in any high temperature coating environment where optical
coating may be employed to improve some aspect of the per-
formance of the device to which the coating is applied.
