U.S. patent number 3,758,185 [Application Number 05/130,183] was granted by the patent office on 1973-09-11 for thermal control filter.
This patent grant is currently assigned to Optical Coating Laboratory, Inc.. Invention is credited to Robert M. Gelber.
United States Patent |
3,758,185 |
Gelber |
September 11, 1973 |
THERMAL CONTROL FILTER
Abstract
Thermal control filter having a substrate capable of
transmitting visible energy and having a surface with a coating
thereon with the coating being formed of a first layer of
dielectric material on the substrate side of the coating and a
bilayer consisting of a thin layer of a material different from the
substrate and a metal layer of either copper or gold. The thin
layer serves as a nucleating layer for the metal layer and is on
the substrate side of the bilayer. The coating has a second
dielectric layer formed on the atmosphere side of the metal
layer.
Inventors: |
Gelber; Robert M. (Healdsburg,
CA) |
Assignee: |
Optical Coating Laboratory,
Inc. (Santa Rosa, CA)
|
Family
ID: |
22443445 |
Appl.
No.: |
05/130,183 |
Filed: |
April 1, 1971 |
Current U.S.
Class: |
359/360; 427/160;
427/166; 427/404 |
Current CPC
Class: |
G02B
5/208 (20130101) |
Current International
Class: |
G02B
5/20 (20060101); G02b 013/16 (); G02b 005/28 () |
Field of
Search: |
;117/33.3,16R,69,71R,68
;350/1,164-166 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Martin; Willaim D.
Assistant Examiner: Schmidt; William H.
Claims
I claim:
1. In a thermal control filter, a substrate formed of a material
capable of transmitting visible energy, said substrate having a
surface, a coating formed on said surface, said coating comprising
a first layer of a dielectric material on the substrate side of
said coating, a bilayer consisting of a thin nucleation layer of a
material different from the substrate on the substrate side of said
bilayer and a metal layer in contact with said thin nucleation
layer, said thin nucleation layer serving as a nucleation layer for
said metal layer and being thin enough so that it is substantially
ineffective in changing the optical properties of the filter, and a
second layer of a dielectric material formed on the atmosphere side
of said coating, said coating providing a filter having a
reflectance greater than 80 percent for the wavelength region from
4 microns to 50 microns.
2. A filter as in claim 1 wherein said metal layer is formed of
copper.
3. A filter as in claim 2 wherein said copper layer has a thickness
ranging from 8 to 45 millimicrons.
4. A filter as in claim 2 wherein said copper layer has a thickness
of approximately 13.5 millimicrons.
5. A filter as in claim 1 wherein said metal layer is formed of
gold.
6. A filter as in claim 5 wherein said gold layer has a thickness
ranging from 2 to 30 millimicrons.
7. A filter as in claim 5 wherein said gold layer has an optimum
thickness of approximately 4.4 millimicrons.
8. A filter as in claim 5 together with an additional layer
disposed between said metal layer and said second layer of a
dielectric material and being formed of yttrium oxide having a
physical thickness of 166 .+-. 34 millimicrons.
9. A filter as in claim 8 wherein said first and second layers of a
dielectric material are formed of titanium dioxide and magnesium
fluoride, respectively.
10. A filter as in claim 1 wherein said thin nucleation layer is
formed of nickel.
11. A filter as in claim 1 wherein said first and second layers of
a dielectric material are formed of aluminum oxide and have optimum
thicknesses of 58 .+-. 17 millimicrons and 37 .+-. 11 millimicrons
respectively.
12. A filter as in claim 1 wherein said metal is formed of a metal
selected from the group consisting of copper and gold.
13. In a thermal control filter, a substrate formed of a material
capable of transmitting visible energy, said substrate having a
surface and a coating formed on said surface, said coating being
formed of at least four layers counting from the substrate, the
first of said layers being formed of a dielectric material, the
second of said layers being a thin nucleating layer formed of a
material different from the substrate and being thin enough so that
it is substantially ineffective in changing the optical properties
of the filter, third of said layers being formed of copper and
having a thickness ranging from 8 to 45 millimicrons, said thin
nucleating layer serving as a nucleating layer for said copper
layer, and said fourth layer being formed of a dielectric
material.
14. A filter as in claim 13 wherein said first and fourth layers
are formed of aluminum oxide and wherein said first layer has an
optimum physical thickness of 58 .+-. 17 millimicrons and said
fourth layer has an optimum physical thickness of 37 .+-. 11
millimicrons.
15. A filter as in claim 13 wherein said copper layer has an
optimum physical thickness of approximately 13.5 millimicrons.
16. In a thermal control filter, a substrate formed of a material
capable of transmitting visible energy, said substrate having a
surface and a coating formed on said surface, said coating
comprising at least five layers counting from the substrate, said
first layer being formed of a dielectric material, said second
layer being formed of a thin nucleating layer being thin enough so
that it is substantially ineffective in changing the optical
properties of the filter, said third layer being formed of gold
having a thickness ranging from 2 to 30 millimicrons, said thin
nucleating layer serving as a nucleating layer for said gold layer,
said fourth layer being formed of yttrium oxide, and said fifth
layer being formed of a dielectric material.
17. A filter as in claim 16 wherein said first layer is formed of
titanium dioxide and said fifth layer is formed of magnesium
fluoride.
18. A filter as in claim 17 wherein said first layer has an optimum
physical thickness of 29.4 .+-. 9 millimicrons and said fifth layer
has an optimum physical thickness of 80.15 .+-. 24
millimicrons.
19. A filter as in claim 16 wherein said gold layer has an optimum
thickness of approximately 4.4 millimicrons.
20. A filter as in claim 16 wherein said yttrium oxide layer has a
thickness of approximately 166 .+-. 34 millimicrons.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to thermal control filters of the type
having a transparent substrate with a coating thereon formed of
thin films.
2. Description of the Prior Art
In copending application Ser. No. 71,009, filed Sept. 10, 1970 and
since issued as U.S. Pat. No. 3,682,528 dated Aug. 8, 1972, there
is disclosed an infra-red interference filter which is of a type
intended primarily for the separation of heat from light emanating
from incandescent sources. Spectrally, the performance of this
filter provides a high transmission in the visual spectrum and high
reflection in the infra-red spectrum with a very rapid transition
between the visible and the infra-red. There are, however, other
applications where thermal control is desired where the source of
radiation is from a much lower temperature source than an
incandescent lamp. Examples of such sources are warm bodies such as
ovens and furnaces. In such lower temperature applications, there
is still a need for a filter which will transmit visible light and
which at the same time will reject infra-red energy so that heat
which is present on one side of the filter is confined to that side
of the filter and does not flow to the other side of the filter.
Thus, if the filter is used as a window in a building, the heat
from the outside would be excluded from the building during the
summertime and in the wintertime heat within the building would not
flow out of the building. In the case of an oven or furnace, the
heat would be contained within the oven or the furnace to make the
oven or furnace perform more efficiently and, in addition, the
outside surface of the filter would be appreciably cooler than a
simple uncoated window, and thus, less dangerous to human beings.
The specific filter which is disclosed in the copending application
has an additional disadvantage in that it is inadequate for use at
very high temperatures. Also, the filter has the disadvantage that
it is relatively expensive because it utilizes silver. There is,
therefore, a need for a thermal control filter which can be
utilized in conjunction with low temperature sources, which is
relatively inexpensive to produce, and has a high operating
temperature.
SUMMARY OF THE INVENTION AND OBJECTS
The thermal control filter consists of a substrate formed of a
material capable of transmitting visible energy. The substrate has
a surface and a coating is formed on the surface. The coating is
comprised of a bilayer which consists of a thin layer of material
which is different from the material of the substrate and a metal
layer. The coating also consists of a layer of a dielectric
material on each side of the bilayer which serves as a protection
for the bilayer and also enhances the optical properties of the
filter.
In general, it is an object of the present invention to provide a
thermal control filter which is capable of reflecting infra-red
energy while transmitting visible energy and which can be utilized
in connection with temperature sources which have a temperature
substantially lower than that of an incandescent lamp.
Another object of the invention is to provide a filter of the above
character which can be utilized in a relatively high temperature
environment.
Another object of the invention is to provide a filter of the above
character which is relatively inexpensive.
Another object of the invention is to provide a filter of the above
character which can be readily produced with a high yield.
Another object of the invention is to provide a filter of the above
character that utilizes a bilayer formed of relatively inexpensive
materials.
Another object of the invention is to provide a filter of the above
character in which the filter is protected by a layer of dielectric
material on each side of the bilayer.
Another object of the invention is to provide a filter of the above
character which is particularly useful for windows for ovens and
furnaces.
Another object of the invention is to provide a filter of the above
character which has low emissivity.
Another object of the invention is to provide a filter of the above
character which has a hard, durable coating which greatly reduces
heat flow and surface temperature.
Additional objects and features of the invention will appear from
the following description in which the preferred embodiments are
set forth in detail in conjunction with the accompanying
drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is an isometric view of a portion of an oven having a window
therein utilizing a thermal control filter incorporating the
present invention.
FIG. 2 is a cross-sectional view of the window shown in FIG. 1
taken along the line 2--2 of FIG. 1.
FIG. 3 is a graph showing the optical characteristics of a filter
incorporating the present invention.
FIG. 4 is a graph showing the reflectance and emissivity of a
filter incorporating the present invention.
FIG. 5 is an enlarged cross-sectional view similar to FIG. 2
showing another embodiment of a filter incorporating the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1 there is shown an oven 11 of a conventional type, for
example, one which is utilized in the home which is heated either
by gas or electricity. As is well known to those skilled in the
art, such an oven is comprised of a housing 12 which is provided
with an opening normally closed by a hinged door 13. A window 14 is
provided in the door to permit the user of the oven to view the
interior of the oven. Conventionally, such windows consist of two
spaced panes 16 and 17 of glass which are hermetically sealed so as
to provide an air pocket therebetween which serves as an insulator.
As is well known to those skilled in the art, more recent ovens are
provided with a self-cleaning feature in which the ovens are raised
to a high temperature. In order to prevent the window 14 from
reaching an excessively high temperature which could burn a person
touching the window, the window 14 is provided with a thermal
control filter incorporating the present invention.
As shown in FIG. 2, the thermal control filter consists of the pane
16 which serves as the substrate for the filter and a coating 18
which is carried by the substrate 16. The substrate 16 is formed of
any suitable material but it is preferable that it be formed of a
material which is highly transmitting in the visible region of the
spectrum. Thus, for example, the substrate 16 can be formed of a
glass having an index of refraction of approximately 1.517. The
substrate 16 is provided with two parallel surfaces 21 and 22 with
the surface 21 being exposed to the outside air or the medium and
the surface 22 having the coating 18 covering the same. The panes
16 and 17 can be referred to as the outer and inner panes if
desired.
The coating 18 consists of at least four layers identified as
first, second, third and fourth layers 26, 27, 28 and 29,
respectively, counting from the substrate with the layers 27 and 28
forming a bilayer and layers 26 and 29 being formed of layers of
dielectric material which serve to protect the bilayer and which
also add to the optical properties of the filter as hereinafter
described.
The bilayer consists of a relatively thin layer 27 which is formed
of a material different from the substrate and which serves as a
nucleating layer for the thicker metal layer 28. The thin layer 27
is preferably maintained as thin as possible so that it will have
very little, if any, effect upon the optical characteristics of the
filter. Thus, it has been found that the layer 27 can have a
thickness ranging from 5 to 29 Angstroms. This thin layer 27 can be
formed of any one of a number of materials. However, it has been
found that nickel and chromium are particularly satisfactory. The
other metals which also can be utilized are rhodium, palladium,
Nichrome, tungsten, etc. In addition, certain dielectric materials
such as titanium monoxide, silicon monoxide and aluminum oxide, and
metal oxide mixtures such as those disclosed in U.S. Letters Patent
No. 3,034,924 also can be utilized to provide a satisfactory
pre-coat or nucleating layer for the metal layer 28.
As pointed out above, the thickness of this pre-coat or thin layer
is not critical providing it exceeds a certain minimum thickness
which is believed to be approximately 5 Angstroms. The effect of
this pre-coat or thin layer appears to be that of a nucleation
layer which gives to the thin metal film 28 the optical properties
of the bulk metal. In other words, the pre-coat or thin layer 27 is
so thin as to be ineffective in changing the optical properties of
the filter but causes the subsequent thin metal film to behave as
if it were a homogeneous metal slab or, in other words, a slab of
bulk metal.
The thin metal layer 28 is formed of a metal which makes it
possible for the coating to withstand relatively high temperatures
as, for example, temperatures ranging from 250.degree.C. to
450.degree.C. To meet these high temperature requirements, it has
been found that it is necessary to select either copper or gold as
the metal for this layer. When copper is utilized for this layer as
shown in the embodiment disclosed FIG. 2, the copper layer has a
thickness of approximately 13.5 millimicrons. However, the
thickness can range from approximately 8 to 50 millimicrons. It has
been found that the thinner the copper layer, the greater the
visual transmission, the greater the emissivity so that the heat
flow through the window increases. Conversely, when the copper
layer is thicker, the visual transmission decreases and the
emissivity decreases so that the heat flow through the glass is
decreased. Thus, the coating performs more effectively as the
thickness of the copper is increased. However, this thickness
increase has the disadvantage in that as the copper thickness is
increased, the visual transmission decreases.
In order to make the bilayer consisting of the layers 27 and 28
into a coating which is hard, durable and stable, it is necessary
that the two additional layers 26 and 29 be provided on opposite
sides of the layer. The layers 26 and 29 are formed of a suitable
dielectric material such as aluminum oxide and magnesium fluoride
for low index materials, and titanium dioxide for a high index
material.
In one embodiment of the invention shown in FIG. 2, the dielectric
layer 26 was formed of aluminum oxide, or sapphire. The layer 26
had a physical thickness of 58 millimicrons and a quarter wave
optical thickness of 382.8 millimicrons. These thicknesses can be
varied .+-. 30 percent while still retaining satisfactory optical
characteristics. To obtain the desired mechanical properties from
the layer 26, it is only necessary that it have a thickness which
is greater than 100 to 200 Angstroms.
The layer 29 was formed to have a physical thickness of 37
millimicrons and a quarter wave optical thickness of 244.2
millimicrons. Again, as with the first dielectric layer 26, the
minimum thickness for the layer 29 to obtain the necessary
mechanical durability and stability should be from 100 to 200
Angstroms, whereas to obtain the desired optical properties, the
thickness can range to a .+-. 30 percent from the optimum 37
millimicron thickness.
The layers 26,27,28 and 29 forming the coating 18 can be deposited
in a conventional manner. The materials are evaporated sequentially
in a vacuum chamber carrying the substrates which are to be coated
to form the coating of the present invention thereon. Thus, the
coatings 26,27,28 and 29 can be deposited in that order in the
vacuum chamber.
The first dielectric layer 26 performs an adhesion function for the
bilayer as well as protecting the bilayer from air and from the
chemicals that are in the glass substrate. The second or outer
layer 29 serves as a protective layer for the bilayer and protects
it from the atmosphere. Thus, it can be seen that two dielectric
layers stabilize the coating in that they provide mechanical
durability as well as protection. In addition, they serve an
optical function by reducing the reflection from the coating from
the side viewed by the viewer as shown in FIG. 2.
When the coating is completed, the bilayer provides the principal
properties which are desired from the filter; however, the two
dielectric layers give the coating the desired hardness, durability
and stability. It has been found that the copper layer by itself
even with the nickel pre-coat layer 27 is unstable. It will oxidize
in the atmosphere. In addition, it will not pass humidity tests and
abrasion tests. By surrounding the bilayer with the dielectric
layers, the coating will readily pass conventional humidity,
hardness and abrasion tests for thin film coatings. However, the
most important characteristic is that the two dielectric layers
make the coating stable so that the coating will not oxidize and so
that it can be utilized in high temperature environments.
Coatings such as that provided in FIG. 2 have been capable of
withstanding a standard 20 rub abrasion test, a 24 hour humidity
test, and a tape test. They have also been able to withstand a high
temperature test which consisted of baking the filter at a
temperature of 350.degree.C in a normal atmosphere for 1000 hours.
Such a high temperature test had little or no effect upon the
emissivity of the filter.
After the coating 18 has been applied to the substrate 16, the
substrate 16 with the coating thereon and the pane 17 are sealed
around their outer peripheries and are positioned in such a manner
that there is provided a space 31 between the coating and one
surface of the pane 17 which is filled with air normally at
atmospheric pressure. Thus, it can be seen that the coating is
provided on the inside of the pane 16 and is exposed to the air in
the space 31 between the panes 16 and 17. The assembly can then be
incorporated in an oven door as shown in FIG. 1. The coating 18 has
a very low visual reflection when viewed from the outside as shown
in FIG. 2. This is a very desirable characteristic. In addition,
the reflection, if possible, should be without color, i.e.,
neutral.
The spectral performance of a filter such as that shown in FIG. 2
is shown in FIG. 3. As can be seen, the graph shown in FIG. 3
covers the wavelength region from approximately 400 millimicrons to
2500 millimicrons. Four curves are shown in FIG. 3 with two of the
curves being transmission curves and two of the curves being
reflection curves. Two of the curves deal with an embodiment of the
invention hereinafter described, whereas the other two curves 32
and 33 deal with the embodiment shown in FIG. 2. Curve 32 is the
curve which shows the reflection for one design incorporating the
present invention and shows that the reflection at 400 millimicrons
is 8 percent and that it is relatively neutral until approximately
600 millimicrons, after which the reflection begins to climb and
climbs steadily through 2500 millimicrons where the reflection is
approximately 69 percent. The transmission curve 33 shows that the
transmission starts off at 400 millimicrons at approximately 63
percent and then goes up to a peak transmission of approximately 75
percent at 600 millimicrons and then falls off through the
infra-red region until at 2500 millimicrons that transmission is
approximately 13 percent. Thus, it can be seen that the filter does
not have neutralness of transmission throughout the visible region.
In other words, the transmission is not flat across the visual
spectrum.
In comparing the characteristics of this filter with the filter
which is disclosed in copending application Ser. No. 71,009, filed
Sept. 10, 1970, now U.S. Pat. No. 3,682,528, it can be seen that
the present filter utilizing copper does not have the neutralness
of transmission in the visible spectrum but has an appearance which
is slightly coppery in transmission. In addition, the transmission
of the copper design does not fall off as rapidly as does the
transmission utilizing silver and for that reason the silver is
more desirable for use with high temperature sources such as a
tungsten light bulb. The reflection of the copper filter of the
present invention and the silver filter in the earlier filed
application are similar but it can be seen that the copper
reflection does not increase as rapidly as the reflection of the
silver filter. This is relatively unimportant in many applications
as, for example, where warm body sources are utilized. In such
applications, it is necessary that the high reflection be obtained
at wavelengths of 4 microns and greater, in which region the
reflection of the copper filter is just as great as the silver
filter. Aproximately 95 percent of the power spectrum of a black
body having a temperature of approximately 200.degree.C. is between
4 to 50 microns with the peak radiation being at approximately 6
microns. Thus, it can be seen for a filter to have a coating which
has good emissivity for applications such as for oven doors, it
must have low emissivity in the region of 4 to 50 microns. This
temperature of 200.degree.C. for a cool black body is low in
comparison to an incandescent source such as tungsten which has a
black body temperature of approximately 3200.degree.K. In the case
of a black body of this latter temperature, 85 percent of the
energy falls at wavelengths less than 2.5 microns. It is necessary
to have good infra-red reflection between 0.7 and 2.5 microns for
heat rejection. For applications such as oven doors, it is
unnecessary to have high rejection until approximately the 4 micron
wavelength region.
The filter of FIG. 2 has a distinct advantage in that the materials
from which it is formed are extremely inexpensive. In addition, the
materials are easy to evaporate and control so that the yield is
very high.
In FIG. 4, there is shown a graph which depicts the heat properties
of a filter made in accordance with FIG. 2. The visual properties
of the filter are shown in FIG. 3 in which the visual region is
represented by the 400 to 700 millimicron region. To determine the
heat reflecting properties of the filter, it is necessary to
examine the region from 4 to 50 microns. The curve 34 in FIG. 4
shows the heat reflectance and the emissivity for the region from 4
to 50 microns and shows that the reflectance is approximately 0.85
and the emissivity is 0.15. The emissivity as defined is one minus
the reflectance.
When it is desired to utilize the filter in environments having
temperatures substantially above 350.degree.C., it is desirable
that another filter incorporating the invention of the type shown
in FIG. 5 be utilized. The coating 36 shown therein is formed on
the surface 22 of the substrate 16 and consists of at least five
layers 37, 38, 39, 41 and 42. Since gold is utilized in place of
copper in this design as hereinafter described, and since the gold
is intrinsically very soft, it is necessary to add to the four
layers of the previous embodiment at least one additional layer to
obtain the necessary hardness and durability for the coating. Also,
because gold has different optical properties from that of copper,
it is necessary to utilize different materials and a different
design in order to obtain a low visual reflection through the glass
substrate 16.
The first layer 37 of the coating 36 counting from the substrate is
formed of titanium dioxide with an index of refraction of 2.3. This
layer 37 has a physical thickness of 29.4 millimicrons and a
quarterwave optical thickness of 270.48 millimicrons which can vary
.+-. 30 percent and still obtain satisfactory results. The second
or next layer counting from the substrate is layer 38 which is a
thin nucleation layer formed of a suitable material such as nickel
as hereinbefore explained. This thin nucleation layer makes it
possible to obtain bulk optical properties from a thin layer of
gold. The thin layer 38 can have the same thickness as described in
connection with the previous embodiment.
The next layer is layer 39 formed of gold having a physical
thickness of 4.4 millimicrons. This is the optimum thickness for
the gold layer. However, if desired, the thickness can vary from
2.0 millimicrons to approximately 30 millimicrons in thickness. In
the same manner as with the copper layer, the emissivity properties
of the coating can be improved by sacrificing visual
transmission.
The next layer 41 which is the layer next adjacent to the gold
layer is formed of yttrium oxide (Y.sub.2 O.sub.3) which has an
index of refraction of 1.9. This layer of yttrium oxide has a
physical thickness of 166 millimicrons and a quarterwave optical
thickness of 1260.60 millimicrons. This layer is provided for
hardness and gives the desired stability and hardness to the
coating. The optical thickness and physical thickness of this layer
can vary within .+-. 20 percent without too seriously impairing the
properties of the coating.
The last layer, layer 42, is a dielectric layer and is utilized for
optically matching the coating to air and consists of a dielectric
layer formed of magnesium fluoride (MgF.sub.2) which has an index
of refraction of 1.38. This magnesium fluoride layer has a physical
thickness of 80.15 millimicrons and a quarterwave optical thickness
of 442.428 millimicrons. For the MgF.sub.2 the physical thickness
and the quarterwave optical thickness can be varied within .+-. 30
percent while still obtaining acceptable results. The optimum
thicknesses set forth give a low visual reflection with the most
neutral coating. As soon as these optimum thicknesses are changed,
the visual reflectance is increased and there is a loss of
neutrality in color. In certain applications, it may be desirable
to obtain a specific color by reflection and this can be obtained
by changing the thicknesses of the dielectric layers.
The layers forming the coating 36 also can be evaporated in a
conventional manner in a vacuum chamber by sequentially evaporating
the materials onto the substrate.
The spectral performance of a filter of the type shown in FIG. 5 is
shown in FIG. 3 in which the curve 46 represents the reflection
obtained from the gold filter and the curve 47 represents the
transmission which is obtained from the gold filter. It can be seen
that the transmission by the gold filter is very similar to the
transmission by the copper filter. It starts out at 400
millimicrons with a transmission of approximately 60 percent. It
peaks at a transmission peak of 75 percent at 575 to 580
millimicrons and then falls off in the infra-red. Initially, it
falls off more rapidly than the copper but then it falls off less
rapidly so that at 2500 millimicrons, the transmission is
approximately 18 percent. The reflection for the gold starts at 400
millimicrons at 16 percent and decreases to a minimum of 550
millimicrons at approximately 41/2 percent, and then increases
until approximately 1000 millimicrons and then decreases until
approximately 1700 millimicrons and then increases to 2500
millimicrons, at which time the reflection is approximately 42
percent. The reflection continues to increase until when 4 microns
is reached, which is a critical region, the reflection of the gold
is just as good as the copper and both reflect at 85 percent as
represented by the curve 34 in FIG. 4. The emissivity is less than
0.2, i.e., 0.15.
Thus it can be seen that the gold also provides a very satisfactory
thermal control filter and that it can be utilized in the same
manner as the filter which is shown in FIG. 2. The only difference
is that the coating is comprised of more layers which makes it more
expensive and, in addition, it requires the use of gold which is
more expensive than copper. It, however, does have the advantage in
that it can be utilized in much higher temperature environments as,
for example, temperatures ranging from 250.degree. to
550.degree.C.
It is apparent from the foregoing that there has been provided a
new and improved thermal control filter which is particularly
adapted for energy emitted from black body sources having
temperatures substantially less than the temperatures of
incandescent lamps. The filter utilizes a coating which is
relatively inexpensive and which can be readily produced with a
high yield. It will withstand the conventional humidity and
abrasion tests and, in addition, can withstand high temperatures in
excess of 300.degree.C. for extended periods of time. The coating
has low emissivity which makes it possible to obtain a drastic
reduction in surface temperature of windows which are exposed to
heat. The optical characteristics are determined primarily by the
single metal layer that it utlized which consists of either copper
or gold.
Although the principal emphasis on the present invention has been
directed to the utilization of the filter in conjunction with black
body sources having temperatures in excess of 200.degree.C., the
coatings are also particularly adaptable for use with architectural
glass to control the heat input through a window due to solar
radiation which is in the region from 400 to 2500 millimicrons and
is generally characterized as the shading coefficient of the
architectural glass. The thermal through-put of heat through the
glass due to the temperature difference existing on opposite sides
of the glass is determined by the transfer of energy in the region
from 4 to 50 microns and is often characterized as the U-factor of
architectural glass. As can be seen from the curves shown in FIGS.
3 and 4, architectural glass with coatings thereon incorporating
the present invention would have excellent shading coefficients and
U-factors.
* * * * *