VARIABLE INDEX FILM FOR TRANSPARENT HEAT MIRRORS

Abstract: A heat mirror film (12) for placement on a lamp envelope (10) in the form of an etalon or Fan-Bachner heat mirror. At least one of the layers of dielectric material of the heat mirror (12) is fabricated from a material having a non-constant index of refraction which index decreases significantly as infrared radiation frequencies are approached, thus increasing reflection at infrared frequencies as compared to a dielectric having a constant index of refraction. Such dielectrics may include semiconductors, such as indium tin oxide and heavily doped lanthanum hexaboride.

Description: I VARIABLE INDEX FILM FOR TRANSPARENT HEAT MIRRORS BACKGROUND OF THE INVENTION 1. The Field of the Invention This invention pertains to heat mirrors , and more particularly, to electric lamps in which energy at a first predetermined range of wavelengths such as infrared, is returned to the site of lamp energy emission, and energy of a second predetermined range of wavelengths, such as visible radiation, is transmitted out of the lamp, by means of a heat mirror. 2. Description of the Prior Art Attempts have been made to increase the effici¬ ency of electric lamps through the utilization of a trans¬ parent heat mirror placed on the envelope of the lamp. These transparent heat mirrors are filters which have a high reflectivity for infrared radition and a low reflec¬ tivity for visible radiation. In such a lamp, the shape of the lamp envelope is selected so that emitted radiation is reflected back to the light emission source to raise its operating temperature, thereby reducing the power needed to cause the energy emission source to emit light and increasing the lamp's efficiency. As described in U.S. Patent No. 4,409,512 assigned to the assignee of the invention, a heat mirror may be an etalon which is of a discrete film of a dielectric mater- ial sandwiched between discrete films of a material, for example, silver or another highly electrically conductive metal. In such filters, constructive and destructive interference results in a substantial rejection and reflection of light having a wavelength shorter than a preselected wavelength and transmission of light having a wavelength greater than the preselected wavelength. The reflected light is then directed back towards the energy producing source, for example, a filament in an incandes¬ cent lamp, thereby increasing its temperature and reducing the energy required for it to reach incandescence. In U.S. Patent No. 4,160,929 assigned to the assignee of the present ^ invention, another t-ype of heat mirror film, such as silver, copper, gold or aluminum is described which is formed by a highly electrically conduc¬ tive metal sandwiched between transparent dielectric layers whose index of refraction of light energy in the visible range substantially matches the index of absorbtion (imaginary part of the refractive index) of the metal. The metal film is highly conductive and reflects the IR energy but it is thin enough to pass light energy in the visible range. The dielectric layers provide matching and anti-reflection functions. Transparent heat mirrors may also be used advantageously in gaseous discharge lamps such as low- pressure sodium vapor lamps. In such lamps there is no central filament to which the infrared radiation may be reflected. Instead the entire volume of low-pressure sodium vapor acts as the emission source. In lamps of this type, the heat mirror traps the infrared energy on the envelope wall to raise the operating temperature of the source and the energy is also reflected back into the volume containing the sodium vapor. Thus, it is not strictly necessary to shape the heat mirror so that the infrared energy is reflected back to a particular location, such as in the case of a filament in an incandescent lamp. In the three-layer heat mirrors thus far de¬ scribed, IR reflectivity arises from two sources. First, fURj i —2H2__ i * -3- the imaginary index of silver and other metals increases almost linearly with wavelength. Silver, for example is inherently more reflective as the wavelength increases, i.e. toward and into the infrared. Second, longer wave- lengths produce smaller phase shifts in the dielectric for a given overall thickness d of dielectric. In the infrared region, the decreasing dielectric constant helps move the filter from the overall phase matching condition to phase mismatch and reflection will occur as a result of the phase mismatch. The combined effect of higher silver reflectivity » and decreasing dielectric constants then produces an IR reflectivity that increases with increasing wavelength. It should be noted, however, that without the altered dielectric constant, a higher silver reflectivity will not produce as readily an increase in the filter reflectivity. The phase shift 0 occuring in a dielectric in the infrared may be related to both the wavelength and the index of refraction of the dielectric by the following formula: 0 = 27. nd/Λ (1) where n equals the index of refraction of the dielectric, d equals the thickness of the dielectric and A equals the wavelength of the light incident to the dielectric. In the preferred embodiment of the invention a three-film or layer coating is used which is formed of films of insulator/silver/ insulator (I/S/I) or silver/ insulator/silver (S/I/S) in which at least one layer of the dielectric has a variable index of refraction. These transparent heat mirror coatings have greatly increased efficiency in the reflection of IR energy and the trans¬ mission of visible light as compared, for example, to a simple titanium dioxide coating. In the present invention, the decreased dielec- "* trie constant and the consequently higher IR reflectivity is enhanced by utilizing a dielectric material for one or both of the films having a non-constant index of refrac¬ tion, which in the case of a heat mirror is selected to decrease significantly as the longer infrared radiation ^ wavelengths are approached. It has been found that semiconductors having relatively high band gap energies, and doped to produce plasma wavelengths below approximately 1.2 micron, are suitable as dielectrics having the required variable index 1 ^ of refraction. The term "plasma wavelength" is understood to mean that wavelength at which an abrupt change in the optical properties of the material occurs which is caused by the free electrons in the material. Specific examples of such materials include indium tin oxide (ITO) doped at 20 a level greater than ten percent atomic weight of tin, and lanthanum hexaboride (LaBo,). It. is, therefore, an object of this invention to provide an improved heat mirror. It is a further object of the present invention 25 to provide an improved electric lamp. „ A still further object is to provide an improved coating for an energy efficient lamp in which the dielec¬ tric material is comprised of a material having an index of refraction that decreases as the infrared radiation 0 range is approached from the visible range. Another object is to provide an improved heat mirror coating for a lamp in which a central film of a highly conductive material is sandwiched between two film of dielectric material, at least one of which has an index 5 of refraction which decreases significantly toward the infrared. Another object is to provide an improved three- film heat mirror coating for a lamp in which a central layer of a dielectric material having a variable index of refraction is sandwiched between two layers of a highly conductive material. An additional object is to provide an improved lamp utilizing a heat mirror envelope surface which is made highly reflective for infrared radiation by the utilization of indium tin oxide in a heat mirror. A still further object is to provide an energy efficient lamp utilizing a transparent heat mirror in which lanthanum hexaboride is used as a dielectric material, BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will become more apparent upon reference to the following specification and annexed drawings, in which: Fig. 1 is a view, shown partly broken away, of an incandescent lamp made in accordance with the subject invention; Fig. 2 is a fragmentary view in cross section of a preferred form of an etalon coating in accordance with the invention; Figs. 3, 4, 5, 6, 7, 8, 9 and 10 are graphs useful in the explanation of the invention; and Fig. 11 is a fragmentary view in cross section of a preferred form of an insulator/silver/insulator coating in accordance with the invention. Fig. 12 is a graph useful in the explanation of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, an incandescent lamp 10 is shown which has the usual base 13 with threaded O 86/02775 -6- contact 14 and button contact 16. A stem 17 is attached to the interior of the base through which the sealing takes place. A pair of lead in wires 18 and 20 pass through the stem and one end of each of these wires makes contact with the base contacts 14 and 16. A filament 22 is mounted on lead in wires 18 and 20. The filament 22 shown in Fig. 1 is of tungsten wire which can be doped, if desired. The filament 22 is, preferably designed to have a shape conforming to the 1 0 geometry of the envelope. That is, the filament is shaped with respect to the lamp envelope, which serves as a reflector surface, so that there will be an optimization of. the possibility of interception by the filament of that portion of energy reflected by the envelope. The ^5 filament 22 is shown vertically mounted by supports which are connected to the lead in wires 18 and 20. As will be clear, other filament configurations may be preferable depending upon the. shape of the envelope. For example, if envelope 11 is elliptical and lamp 10 is of the discharge 0 type, reflected radiation will be directed mainly between the foci of the envelope 11 and these foci may be located at the lamp's electrodes so that the return radiation illuminates the discharge volume. If, on the another hand, if _the envelope 11 were elliptical and the lamp is 5 an incandescent type, the foci should be located just inside the ends of the filament. As shown in Fig. 1 , a generally spherical envelope 11 is provided, the envelope being non-spherical at its bottom end where the stem 17 is located. A spheri- 0 cally shaped base reflector 25 whose center of curvature is located at the filament is used to redirect filament radiation, emitted toward the base, back to the filament. In its spherical portion, the envelope is made as optically perfect as reasonably possible. That is, it is made 5 smooth and with a constant radius of curvature so that if the filament is located at the optical center of the envelope, there will be substan ially total reflection of energy from the envelope wall back to the filament, assuming the envelope is capable of reflecting the energy. Preferably, the filament is optically centered as close as possible within the spherical part of the envelope. The fill gas for the envelope can be selected in accordance with standard design criteria for filament life, decrease in energy consumption, etc. Thus, a conventional argon fill gas, krypton fill gas, or vacuum can be utilized. Other conventional fill gases or mixtures thereof also can be used. A heat mirror coating 12 is placed on the envelope 11. In the preferred embodiment of the invention, coating 12 is formed of films of a highly conductive metal and dielectrics. Non-constant index of refraction dielec¬ trics which have an index of refraction that decreases • significantly toward the infrared are used in place of conventional constant index of refraction dielectrics to enhance IR reflectivity. Examples of such materials are semiconductors having relatively high bandgaps, doped to produce a plasma wavelength below approximately 1.2 microns. The use of such semiconductors has the following advantages: 1. The refractive index of highly doped semicon¬ ductors can be designed, by varying the level of doping, to decrease rapidly in the near IR. This increases the IR reflectivity of the heat mirror films in the near IR and can give the heat mirror films a sharp transition from visible energy transmission to IR energy reflection. Indium tin oxide (ITO), for example, exhibits this behavior, This, when combined with a metal film, provides improved heat mirror charateriεtics since the dielectric may be selected to have an index of refraction substantially matching the metal's index of absorption in the visible range. This provides a minimium of reflection of visible /02775 ' . . ' -8- light. As the dielectric's index of refration shifts to a mismatch with the metal's index of absorption in the infrared range, reflection will occur due to the phase mismatch. 2. The IR reflectivity of very highly doped semi-conductors can be itself high and sharply increasing in the near IR. This behavior mimics the reflectivity desired of a heat mirror film and can enhance the overall characteristics of a given heat mirror film. Lanthanum hexaboride (LaB,) displays this property. It is preferred that all of the layers of the coating 12 be located on the interior of the envelope since this gives them the greatest degree of protection from handling and environment. A properly designed layered coating may, however, be located on the exterior of the exterior of the envelope in addition to or in place of a coating on the interior of the envelope. Generally, the films are formed in discrete layers. The general requirements of a transparent heat mirror coating is that it pass, or transmit, as large an amount of energy in the .visible range produced by the incandescent lamp filament as possible and that it reflect as much of the generated IR energy as possible back to the filament. As described in U.S. Patent 4,160,929, reflec¬ tion of IR energy back to the filament will increase its temperature at a given power level or maintain its temper¬ ature at a reduced power level thereby increasing the efficiency of the filament and improving the lumens per watt efficiency of the lamp. It will be clear to those skilled in the art that the novel heat mirrors of the present invention are not limited to use with lamps but may be also be used in any situation where it is desireable to transmit visible radiation and block passage or reflect incident infrared radiation. Such uses, may include, for example window glass, which would permit transmission of light from the summer sun, but block the heat; such window glass in OMPI _ . „_,_,_. O 86/02775 -9- winter would function to prevent loss of heat from a heated structure. Similarly, heat mirrors of this type may be used in both home, commercial and industrial ovens, where it is desirable to be able to observe the progress of the heating operations within the oven, without large loss of heat through the window. .The present invention may be used by providing a mutlifilm coating design following the teachings of the etalon principle (metal/insulator/metal), or alterna- 0 tively of U.S. Patent 4,160,929 (insulator/metal/insulator) coatings which will be discussed in detail below. An etalon coating utilizes a layer of insulating material, for example, an air dielectric, between two metal reflec¬ tive layers, for example, silver. In conventional etalon • -* coatings, the thickness of the layer of insulating material is chosen to produce a 180 * phase shift of energy of certain wavelengths passing through it in a two-way trip, i.e., traveling from the source through the insulator and being reflected by the metal film remote from the source 0 back toward the source. The resulting interference permits transmission and reflection of visible light frequencies. A device known in the art utilizing this prin¬ ciple is the Fabry-Perot etalon. So-called interference 5 filters also have been disclosed utilizing this principle, one such f-ilter shown in U.S. Patent 3,682,528 in which the etalon coating is sandwiched between two pieces of glass. Fig. 2 shows a fragment of a substrate 20, for example, of lime glass or Pyrex, on which an etalon 0 coating according to the invention is deposited. The etalon coating has three discrete film layers. The first of these is a film layer 31 of a highly electrically conductive reflecting material, such as silver, which is deposited on one surface of the substrate 30, a film layer 5 of a non-constant index of refraction dielectric material 32, which is deposited on the metal film layer 31, and an _OM?I _ outer film layer 33 of a highly electrically conductive reflecting metal, which may also be silver, and which is deposited on the dielectric. Any conventional and suitable technique can be used for depositing the three layers, some of these being, for example, chemical deposition, vapor deposition, vacuum sputtering, etc. The three film layers are preferably made separate and discrete from each other, i.e., it is preferred that there be no interdiffusion of the layered materials. The film layers, however, co- operate to produce the desired transmission and reflection characteristics and they are designed as a composite. Incident radiation, assumed to have components in the visible spectrum as well as components in the infrared spectrum, are shown by arrows R as impinging upon the layer 33 most remote from substrate 30. In accordance with the invention, the etalon coating is designed to have characteristics such that it will transmit a maximum amount of energy in the visible wavelength range and will reflect a maximum amount of energy in the longer wavelength range, including the infrared band. Fig. 3 shows a typical response curve for an etalon coating. The ordinate shows the transmission characteristics of the coating and the abscissa shows wave¬ length. Characteristically, there are a number of energy transmission passbands at different wavelengths, these wavelengths being integral multiples of one another. The etalon coating has a last transmission passband at the longest wavelength, shown as the third from the left. The number of transmission passbands depends upon the coating design. In the present invention, the coating is designed to use the last passband to transmit visible energy and to reflect IR energy. Also, the etalon is designed so that the last transmission peak of Fig. 3 (located at the long¬ est wavelength), falls at the peak of the lumens output of the energy source used for the lamp, in this case the filament. In the design of an etalon type coating, the nature of the insulating film layer, i.e., its index of refraction and thickness, controls the width and shape of the passband characteristics and, in conjunction with the metal layers, the slope of the passband cut-off, i.e., the sharpness at which the mirror makes the transition from transparent to reflective at the desired wavelength. The metal layers provide the infrared energy reflectance. An optimum design, insofar as an incandescent electric lamp is concerned, has a high transmission in the visible range with little absorption and a high reflection in the IR range. The design of the optimum etalon filter balances several considerations. Low visible absorption requires a thin metal film while high IR reflection requires a thick metal film. In addition, the location of the transmission peak in the visible range and a rapid rise in reflection as the IR is approached demands that the dielectric film 32, in thickness and index, must be properly designed in conjunction with the metal films 31 and 33. The characteristics of an ideal heat mirror are that all energy in the visible range be transmitted and that all energy in the IR range be reflected. Theoreti¬ cally, the break point between transmitted energy and reflected energy (IR) should occur at about 700 nanometers. That is, energy below 700 nanometers should be transmitted through the envelope and energy above 700 nanometers should be reflected. In practice, break points up to 850 nanometers and even somewhat higher can be tolerated. A graph showing the transmission characteristics of a preferred coating for one type of incandescent lamp is shown in Fig. 4. On this graph the light spectrum 41 produced by an incandescent filament operating at about 2900" and transmitted through a typical heat mirror is shown superimposed on the ideal heat mirror. Dotted line curve 40 in Fig. 4 represents an idealized wavelength reflectivity curve for the transparent heat mirror. Such a curve as 40 with a vertical cut-off line is not practically realizable. Referring to Fig. 5, curves 50 and 51 represent obtainable bandpass transmission characteristics utilizing an etalon type coating in which the dielectric film 32 has an index of refraction that remains constant with the wavelength of light incident thereupon. As explained previously, the thickness of the ^ metal layers controls the bandwidth and the thickness of the dielectric film, in cooperation with the metal films, controls the wavelength of its peak. The thickness of insulator 32 is chosen so that the phase angle due to two-way travel in the insulator " 5 between the metal films 31 and 33 plus two reflections off the metal film, is zero at the visible wavelength chosen for maximum transmission. The relative metal film, thicknesses are chosen to give the same individual reflec¬ tivities. With this arrangement, constructive interfer- 0 ence occurs and the overall transmission of the combina¬ tion is ideally 100 percent at the chosen visible wave¬ length, neglecting absorption in the metal films. Refer¬ ring again to Fig. 5, in an incandescent lamp according to the invention, operating on the last etalon peak, the 5 wavelength at which the transmission falls to one-half that of the peak is to be set at about 800 nanometers. It is at this wavelength that the eye has lost visual response to the energy and that IR energy is present. By designing the high wavelength wall of the transmission bandpass 0 properly, the IR reflectivity can approach the range of about 90 percent or better at 1000 nanometers, where the IR energy is effective, and continue to increase with increasing wavelength. At all wavelengths of the infrared, phase differ- 5 ences in the insulator decrease toward zero, while the phase shift on each reflection decreases toward 180 * , the CMPI conventional value taken in etalon design according to quarter wave theory. At some near IR wavelength the overall phase angle decreases from zero to -180, (on its way to -180 * at very large wavelengths) and destructive etalon interference occurs, giving an overall reflectivity 2 of 4R/(1+R„) . This overall reflectivity is very M close to unity for R u π > .5, where RM u is the IR reflec- tivity of one silver film. Thus, a particular εilver- insulator-silver combination can be designed to give a high IR reflectivity, at, for example, a wavelength of one micron. As wavelength increases further, the reflectivity increases uniformly toward unity. Referring again to Fig. 5, the transmission and reflection of an etalon according to the present invention containing a dielectric material of variable index of refraction is shown by curves 52 and 53, respectively. In this figure a constant index was taken as N « 1.936 for c • curves 50 and 51, while the variable index was chosen as n = 2.00(1-0.382 ( -0.3) 2) where is in microns. Referring to Fig. 6, curve 56 shows the variation of the index with wavelength of a variable index film; curve 55 shows a constant index for reference. Both indices are the same at .585 micron, which is near the peak of the visible luminous efficiency of a typical filament. The etalon filters were both designed for peak transmission at .585 microns and both have dimensions as follows: 90 angstrom/982 angstrom/90 angstrom for the silver/ITO/silver layers, respectively. An ideal heat mirror film has an IR reflectivity of unity, the departure from ideal in the IR region being 1-R where R is the reflectivity. Values of 1-R for the constant and variable index etalons are compared in Fig. 7 wherein curve 60 is for an etalon utilizing a dielectric having, a constant index of refraction and curve 61 is for an etalon having a variable index of refraction. The etalon having the variable index of refraction dielectric deviates from the ideal by a substant ially smaller amount than does the etalon having the dielectric with constant index, indicat ing the advantage in adjusting the index to optimize the three-layer etalon. The median of the IR energy falls near 1 .5 to 1 .6 microns . At 1 .5 micron the ratio ( 1-R) cons tant / ( 1-R) vari . a .bl , e is 0 .067/0.023 = 2.9. The variable index etalon is therefore almost three times closer to the ideal reflectivity at 1 .5 microns than an etalon using a constant ^ index of refraction dielectric. The average improvement in the near infrared is not as great since in the vis ible range both coatings are des igned to behave s imilarly. The variation in the vis ible transmission region of .5 to .7 microns between the two etalon coating types is less than 5 1 percent . It should be understood that conventional quarter wave theory cannot be used in the design of an etalon operating as disclosed in the subject invention. Conventional quarter wave theory considers phase changes 0 induced by the -metallic film as those due to a very thick film. For example, the phase change upon reflection from one metal layer of the etalon is taken in conventional quarter wave theory as -180 * . In the thin metal films used in this invention, reflection and transmission phase 5 changes depart from conventional quarter wave practice. Design according to that practice gives composite coatings which are far inferior to the coatings of this invention. The rapid rise in IR reflectivity displayed by thin film etalon coating is not predicted by conventional quarter 0 wave theory. In addition, conventional quarter wave theory demands a thickness of dielectric layer which can, when employed in practice, place the peak in visible light energy transmission far away from the portion of the visible wavelength region desired. 5 The characteristics of suitable non-constant dielectrics may be understood as explained below. The complex relative dielectric constant of a metal or semi-metal is given by the Drude free electron theory ( see for example P . B . Johnson and R. W . Christy , Phys. Rev . B , 6 , 4370 , 1972 , §V ) : <~r » 2 ? ■ " n o " 0 (*>+ C / χ ) (2) where * collision time, and n = high frequency index of refraction. The relationship between plasma wavelength p and free electron density J N is g J iven by J : „p "A e,, ~ ' , the free electron density being expressed in 10 21/cc, wavelength in microns and material constant A=(27Λ c/e) (£Me) 1 / 2, where c is the velocity of light, e is the elementary charge, the high frequency dielectric constant of the material and Me the effective mass of the charge carrier. Such a material, exhibiting selective reflection caused by the free electron plasma, may also be termed a plasma filter or plasma edge filter as described in British Patent No. 1,463,939. In a doped semiconductor such as may be utilized in the present invention doping content controls the plasma radian frequency. (3) / J)NN((11/. CC)1 = (c/Λ p )hz (4) The density of free electrons is N , the free electron mass is m and the absolute dielectric constant of free space is . The plasma frequency corresponding to λ is -F . Equating real and imaginary parts of to t= n - K + 2nKri yields: 1 where r * C p*- anα ^ i « 2nk « λ 1 1 The real index is n and the extinction coefficient is K. K can be eliminated from equation (5) by using 2 equation (7). This gives a quadratic equation in n which The reflection coefficient for a thick film of the semi-conductor in air is R = (n-1) 2 + K 2 (8) 20 (n+1)* + K" The values of n, and K depend on n and r and the scale wavelength divided by at a given period. -_ At the given wavelength £• and are determined and 25 3 3 r l equation 7 gives n, K. Reflectivity is given by equation (8). Doping indium tin oxide (ITO) with 7 to 20% atomic weight tin varies N from 1 x 10 21/cc to 3 x 21 ^ J ,U Λ 10 /cc. Thus, Ap varies from 1.4 microns at 7 percent atomic weight tin to .8 microns at 20 percent atomic weight tin. The value of 1/r =λ may be estimated from the surface resistance as measured by V. Hochmann (Optical Spectra, November 1978, page 68). The lowest resistance film (500 angstrom, 17 minutes deposition 35 1 time at 150 watts, with 260 * post heat for 15 minutes in air) was 50 -Λ/Q with: R ε " _1_ (9) s This film thickness is t, with G * = — * 4 mho/ 10 m then, r = *> T = < -> = 50 P _ _ P Large r values correspond to lower resistance. 1 5 Although n changes with r, the changes are relatively small and thus the exact value of r is not critical to the calculation. Referring now to ' Fig-. 8, the change in the index * of refraction with wavelength is shown for indium tin 2o oxide at 7 percent atomic weight tin and 20 percent atomic weight tin. Lanthanum boride (LaB-)is a narrow band semi¬ conductor with a band gap near point .08eV (R.N. Tsirev and S.V. Illarionov, Porosh, Metal, No. 6 (12). pp. 25 85-88, 1962). The reflectivity of LaB fi is given by Tsarev and Illarionov and by Kaner, U.S. Patent No. 3,288,625. The reflectivity for lanthanum hexaboride is shown in Fig. 9. As shown in Fig. 9 lanthanum hexaboride will have a region of absorption in the red. The absorp- -.. tion region can, however, be shifted into the near infrared by altering the doping content as shown in Fig. 10 which shows reflectivity curve for-^= .3 microns. Optimum values for would be within the' range .22-.30 microns. P Another type of heat mirror coating has a layered coating of insulator/ metal/ insulator as discussed in U.S. Patent 4,169,929 assigned to the assignee of the present invention. These coatings are also preferably deposited on the interior of the envelope 11 of the lamp 10. The general principles of a layered coating of this type are described in an article entitled "Transparent Heat Mirrors for Solar Energy Application" by John CC. Fan and Frank J. Bachner, at pp. 1012-1017 of Applied Optics, vol. 15, No. 4, April 1976. In that article a Ti0 2 /Ag/Ti0 2 coating is used on the under-surface of a glass flat plate reflector which is located above a solar absorber. The incident solar energy passes through the glass and the coating to the absorber. The IR from the heat absorber is reflected back to the absorber. A variable index material that is useful in insulator/metal/insulator coatings is highly doped indium tin oxide (ITO). In addition to having a variable index, indium tin oxide is more easily deposited than TiO_. ITO may be used to replace either one or both of the insulator layers in an insulator/metal/insulator coating. in accordance with the subject invention and as shown in Fig. 11, the envelope 11 is preferably of conven¬ tional glass used for lamp envelopes. The layers of the coating are designated 72A for the dielectric layer closest to the filament, 72B for the layer of metal, 72C for the dielectric layer most remote from the filament and are deposited sequentially on the interior of the glass. These layers may be deposited in the same manner used for an etalon type filter. In all cases, adequate control of the thickness of each of the layers should be maintained, In a preferred three-layer mirror of this type, the middle layer of metal 72B, which can be of silver, provides transparency to the visible energy and reflects IR energy. A thin layer of silver of about 20 namometers absorbs only about 10 percent or less of incident energy in the visible wavelength range. The dielectric layers 72A and 72C likewise transmit visible light energy and also serve as anti-reflection and phase matching layers. Inner layer 72A closest to the filament, matches the phase of the visible energy to the layer of metal 72B which acts to reflect IR energy but transmit visible light. The outer layer 72C then matches the phase to the glass envelope for final transmission from the envelope with little visible reflection. The thickness of the layers of coating 72A, 72B and 72C are selected to optimize the transmission of the visible energy and the reflection of the IR energy produced by the incandescent filament at its desired operating temperature. The filament operating temperature is ordinarily in the range of approximately 2600 * K to about 2900 * K. The operating temperature of the lamp filament is generally selected for lamp life and other considerations. For short lamp life, one that has a rated life of about 750 hours, the filament operating temperature is about 2900 * K. For an extended life lamp, one which operates • in excess of 2500 hours, the operating temperature is about 2750 * K. ' The color temperature is generally about 50 * K lower. In one embodiment, in which ITO is used for both insulator layers 72A and 72C, these layers are 41 nm in thickness, and the layer 72B of metal, is silver of 18.5 nm thickness. The material constants for an r.f. sputtered ITO film are given (Fan and Bachner, J. Electro- chem. Society 122, 1719, 1975, Appl . Opt. 15, 1012, 1976) as p = 1.6 microns and r = 103. Such a coating, if coated onto a good optical enclosure 11 (i.e. one which returns 90% of all incident radiation to a filament 22 if coated with a perfect reflector) produces a net energy savings of over 49% in comparison to a conventional 100 watt lamp, where both lamps have a 7 watt gas loss and a filament temperature of approximately 2800 * K. Thus, less than 51 watts would be required to produce the same lumen output, in the described ITO/Ag/ITO lamp, as the conven¬ tional 100 watt lamp. In another embodiment of the insulator/metal/ insulator coating, ITO is used for only one layer, for example 72C, and has a thickness of 44 nm. Ti0 2 is used for the other layer 72B, and has a thickness of 32 nm, while layer 72B is of silver and has a thickness of 21.3 nm. The spectral characteristics of such a lamp are shown in Fig. 12, in which curves 80 and 81 show the transmittance and reflectance of the coating, respectively at wavelenths between 300 and 3000 nm. Such a coating has an energy savings of over 52% compared to a conventional lamp. Such a lamp will produce 1224 lumens with only 37.5 watts, once again assuming a 7 watt gas loss and filament temperature of 2800 * K. The main criterion used for the selection of the components of the layers for an insulator-metal-insulator heat mirror is that the index of absorption of light energy of the dielectric layer (n) matches that of the metal (K) near the range of wavelengths ( Λ-) being considered. Other characteristics also must be considered, the principle ones being the transmissivity to visible light of the metal layer. Insulator-metal-insulator heat mirror coatings according to the present invention can have either two layers of a variable index dielectric or one layer of a variable index dielectric and one layer of a constant index dielectric. Preferably, the variable index dielec¬ tric is deposited on the glass envelope of the lamp. As disclosed above, the variation in index displayed by semiconductors such as 20 percent ITO and Lathanu hexaboride will be useful in improving either an I/S/I or S/I/S film by producing a sharper rise in reflec¬ tivity in the near IR and therefore provide for more complete reflection of IR energy. Bu-_ : .7 >

Claims: WHAT IS CLAIMED IS: 1. An energy efficient lamp comprising: an envelope; light emission means within said envelope for producing energy in the visible and infrared range upon the application of electrical current thereto, said light emission means being located with respect to the interior of the envelope and the major portion of said envelope being shaped with a curved surface such that energy produced by said " light emission means reaching said envelope may be reflected back toward said * light emission means; and a transparent heat mirror coating on said envelope curved surface, said heat mirror formed by at least one layer of a highly electrically conductive material thick enough to reflect infrared radiation and thin enough to transmit visible energy and at least one layer of a dielectric having a variable index of refraction, said dielectric having a non-constant index of refraction decreasing in the infrared range, whereby said dielectric causes substantial phase matching in the visible range and phase mismatching in the infrared range. 2. The lamp according to claim 1 wherein said heat mirror comprises an etalon. 3. The lamp according to claim 2 wherein said etalon comprises a layer of said variable index dielectric between first and second layers of said high conductivity material. 4. The lamp according to claim 3 wherein said variable index dielectric comprises indium tin oxide. 5. The lamp according to claim 4 wherein said indium tin oxide comprises 7 to 20% atomic weight tin. 6. The lamp according to claim 3 wherein said variable index dielectric comprises lanthanum hexaboride. 7. The lamp according to claim 6 wherein said lanthanum hexaboride is doped to have a plasma wavelength in the range of .22 through .3 microns. 8. The lamp according to claim 1 wherein said heat mirror comprises first and second layers of dielectric having a layer of highly electrically conductive material therebetween wherein at least one of said layers of dielectric is comprised of said variable index of refraction dielectric. 9. The lamp according to claim 8 wherein said first and second dielectric layers comprise said variable index dielectrics. 10. The lamp according to claim 8 wherein the index of refraction of said non constant dielectric substantially matches the index of absorption of said metal in said visible range. 11. The lamp according to claim 8 wherein said variable index dielectric, comprises indium tin oxide. 12. The lamp according to claim 9 wherein said variable index dielectric comprises indium tin oxide. 13. The lamp apparatus according to claim 12 wherein said indium tin oxide includes 7 to 20% atomic weight tin. 14. The lamp according to claim 8 wherein said variable index dielectric comprises lanthanum hexaboride. 15. The lamp according to claim 9 wherein said variable index dielectric comprises lanthanum hexaboride. 16. The lamp according to claim 14 wherein said lanthanum hexaboride has a plasma wavelength in the range of .22 to .30 microns. 17. The lamp according to claim 15 wherein said lanthanum hexaboride has a plasma wavelength in the range of .22 to .3 micron. 18. The lamp according to claim 1 wherein said high conductivity material is selected from the group consisting of silver, gold, copper, and aluminum. 19. A heat mirror comprising: at least one layer of a highly electrically conductive material thin enough to transmit visible electro¬ magnetic radiation and thick enough to substantially reflect infrared radiation, and at least one layer of a dielectric having a variable index of refraction, said dielectric having a non-constant index of refraction increasing in the infrared range, whereby said dielectric causes substantial phase match¬ ing in the visible range and substantial phase mismatch in the infrared range. 20. Th-e heat mirror according to claim 19 wherein . said heat mirror comprises an etalon. 21. The heat mirror according to claim 20 wherein said etalon comprises a layer of said variable index dielectric disposed between first and second layers of said highly conduc¬ tive material. 22. The heat mirror according to claim 21 wherein said variable index dielectric comprises indium tin oxide. 23. The heat mirror according to claim 22 wherein said indium tin oxide comprises 7 to 20% atomic weight tin. 24. The heat mirror according to claim 21 wherein said variable index dielectric comprises lanthanum hexaboride. 25. The heat mirror according to claim 24 wherein said lanthanum hexaboride is doped to have plasma wavelength in the range of .22 through .3 microns. 26. The heat mirror according to claim 19 wherein said heat mirror comprises first and second layers of dielec¬ tric having a layer of highly electrically conductive material therebetween wherein at least one of said layers of dielectric is comprised of said variable index of refraction dielectric. 27. The heat mirror according to claim 26 wherein said first and second layers comprise variable index of refrac¬ tion dielectrics. 28. The heat mirror according to claim 26 wherein said non constant dielectric has an index of refraction substantially matching the index of absorption of said metal in the visible light range. 29. The heat mirror according to claim 26 wherein said variable dielectric comprises indium tin oxide. 30. The heat mirror according to claim 28 wherein said indium tin oxide comprises 7 to 70% atomic weight tin. 31. The heat mirror according to claim 26 wherein said variable index dielectric comprises lanthanum hexaboride. 32. The heat mirror according to claim 31 wherein said lanthanum hexaboride is doped to have a plasma wavelength in the range of .22 to .3 microns. 33. The device according to claim 19 wherein said metal is selected from the group of silver, aluminum, copper or gold.

Inventors: WALSH PETER J, WALSH, PETER, J.
Applicants: DURO TEST CORP, DURO-TEST CORPORATION
Application year: 1984
Main classification: H01K1
IPC codes: G02B5, H01K1