US 3758185 A
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.
Description (OCR text may contain errors)
United States P.v T "5011, W
Gelber Sept. 11, 1973  THERMAL CONTROL FILTER 2,Sl9,722 8/1950 Turner 350/164 X I 2,559,389 7/1951 Beeber et al.. ll7/33.3 X Inventor. Robert M. GGIbEI', Healdsburg, Calif. Dreyfus et x 73 Assisnee; optic caning Laboratory, 1c 2,932,592 4/1960 Cameron 117/106 Santa Rosa Calif 3,235,397 2/1966 Mlllendorfer 117/333 (22] Filed: Mm FOREIGN PATENTS OR APPLICATIONS 966,705 10/1950 France 117/333  Appl. No.: 130,183
Primary Examiner-Willaim D. Martin 52 us. c1 350/1, 117/333. 117/71 R, Ass/slam Examiner-William Schmidt 1 17/71 M, 350/166 Attorney-Flehr, Hohbach, Test, Albritton & Herbert  Int. Cl. G021 13/16, G02b 5/28  Field of Search 117/333, 106 R,  ABSTRACT 7/691 R1 68; 350/11 164466 Ih ermal control filter having a substrate capable of transmitting visible'ene rgy and having a surface with a  References cued coating thereon with the coating being formed of a first UNITED STATES PATENTS layer of dielectric material on the substrate side of the 3,290,203 12/1966 Antonson et al. 350/164 x coating and a bilayer Consisting of a layer of 8 3,432,22s 3/1969 Rock 117/333 ie ial different from the substrate and a metal layer of 2,668,478 2/1954 either copper or gold. The thin layer serves as a nuc1e- 2,750,852 6/1956 ating layer for the metal layer and is on the substrate C l n t I 3 ,1 x side of the bilayer. The coating has a second dielectric o e e a 3,432,314 3/1969 Mazdiyasni 350/164 x layer formed on the atmosphere we of the metal 2,412,496 12/1946 Dimmick 350/!64 X 20 Claims, 5 Drawing Figures i l fa Pmmmsm i W LOW VISUAL REFLECTION Fig.2
COPPER DESIGN GOLD DESIGN zoiuudmm m 20622235 \0 INVENTOR.
BY Robert M Gelber 3 (344,
W, we 9W Attorneys THERMAL CONTROL FILTER BACKGROUND OF THE INVENTION l. 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. 7 l ,009, filed Sept. 10, I970 and since issued as US. Pat. No. 3,682,528 dated Aug. 8, I972, 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 elficiently 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. I 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. I 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. I there is shown an oven II 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 I3. A window I4 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 I6 and I7 of glass which are hennetically 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 I4 is provided with a thermal control filter incorporating the present invention.
As shown in FIG. 2, the thermal control filter consists of the pane I6 which serves as the substrate for the filter and a coating 18 which is carried by the substrate I6. The substrate I6 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 1.5l7. The substrate I6 is provided with two parallel surfaces 2] 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 l6 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 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 US. 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 250C. to 450C. 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 HO. 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 .t 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 to 200 Angstrorns.
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 l00 to 200 Angstroms, whereas to obtain the desired optical properties, the thickness can range to a i 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 thereonv 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 pro tective 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 350C in a normal atmosphere for I000 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 l6 and is exposed to the air in the space 31 between the panes l6 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. 7l,009, filed Sept. I0, i970, now US. 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 200C. 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 200C. for a cool black body is low in comparison to an incandescent source such as tungsten which has a black body temperature of approximately 3200K. In the case of a black body of this latter temperature, 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.l5. The emissivity as defined is one minus the reflectance.
When it is desired to utilize the filter in environments having temperatures substantially above 350C., 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 l6 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 thick ness of 270.48 millimicrons which can vary 1 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 nu cleation 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,O,) which has an index of refraction of L9. This layer of yttrium oxide has a physical thickness of I66 millimicrons and a quarterwave optical thickness of l260.60 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,) 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, the physical thickness and the quarterwave optical thickness can be varied within i 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. 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. lt 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 4% percent, and then increases until approximately 1000 millimicrons and then decreases until approximately l700 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 to 550C.
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 300C. 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 vention has been directed to the utilization of the filter in conjunction with black body sources having temperatures in excess of 200C., 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 millin-icrons and is generally characterized as the shading c0- efficient 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 inven tion would have excellent shading coefficients and U- factors.
1. In a thermal control filter, a substrate formed of a material capable ritfirzwnitflng isible energy, said substrate having a su ace, a coating formed on said surface, said coating comprising a first layer of a dielectric material on the substrate side of said coating, a bilaycr consisting of a thin nucleation layer of a material different from the substrate on the substrate side of said bilaycr 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 fonned on the atmosphere side of said coating, said coating providing a filter having a reflectance greater than percent for the wavelength region from 4 microns to 50 microns.
2. A filter as in claim 1 wherein said metal layer is fonned 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 l3.5 millimicrons.
5. A filter as in claim I wherein said metal layer is formed of gold.
6. I 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 [66 i 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 I wherein said thin nucleation layer is formed of nickel.
I]. 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 i 17 millimicrons and 37 i ll millimicrons respectively.
12. A filter as in claim I 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 sugface, said coating being formed of at least four layers counting from the substrate, the first of said layers being formed ofa 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 fonned of aluminum oxide and wherein said first layer has an optimum physical thickness of 58 i l7 millimicrons and said fourth layer has an optimum physical thickness of 37 i l l 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 i 9 millimicrons and said fifth layer has an optimum physical thickness of 80.l5 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 I66 1 34 millimicrons.
i l l