Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01K—ELECTRIC INCANDESCENT LAMPS
  • H01K1/00—Details
  • H01K1/28—Envelopes; Vessels
  • H01K1/32—Envelopes; Vessels provided with coatings on the walls; Vessels or coatings thereon characterised by the material thereof
  • H01K1/325—Reflecting coating
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B5/00—Optical elements other than lenses
  • G02B5/20—Filters
  • G02B5/28—Interference filters
  • G02B5/281—Interference filters designed for the infrared light
  • G02B5/282—Interference filters designed for the infrared light reflecting for infrared and transparent for visible light, e.g. heat reflectors, laser protection

Definitions

• 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.
• a heat mirror may be an etalon which is of a discrete film of a dielectric material sandwiched between discrete films of a material, for example, silver or another highly electrically conductive metal.
• a material for example, silver or another highly electrically conductive metal.
• 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 incandescent lamp, thereby increasing its temperature and reducing the energy required for it to reach incandescence.
• the energy producing source for example, a filament in an incandescent lamp
• heat mirror film such as silver, copper, gold or aluminum
• a highly electrically conductive 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 described, IR reflectivity arises from two sources.
• 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.
• longer wavelengths 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.
• phase shift ⁇ 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:
• n the index of refraction of the dielectric
• d the thickness of the dielectric
• ⁇ the wavelength of the light incident to the dielectric
• 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.
• I/S/I insulator/silver/ insulator
• S/I/S silver/ insulator/silver
• These transparent heat mirror coatings have greatly increased efficiency in the reflection of IR energy and the transmission of visible light as compared, for example, to a simple titanium dioxide coating.
• the decreased dielectrie 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 refraction, which in the case of a heat mirror is selected to decrease significantly as the longer infrared radiation wavelengths are approached.
• 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.
• ITO indium tin oxide
• LaB 6 lanthanum hexaboride
• a still further object of the present invention is to provide an improved electric lamp.
• a still further object is to provide an improved coating for an energy efficient lamp in which the dielectric material is comprised of a material having an index of refraction that decreases as the infrared radiation 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 of refraction which decreases significantly toward the infrared.
• Another object is to provide an improved threefilm 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
• 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.
• 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
• an incandescent lamp 10 which has the usual base 13 with threaded 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 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 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.
• envelope 11 is elliptical and lamp 10 is of the discharge 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.
• the foci should be located just inside the ends of the filament.
• a generally spherical envelope 11 is provided, the envelope being non-spherical at its bottom end where the stem 17 is located.
• a spherically 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.
• the envelope is made as optically perfect as reasonably possible. That is, it is made 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 substantially total reflection of energy from the envelope wall back to the filament, assuming the envelope is capable of reflecting the energy.
• 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.
• 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.
• coating 12 is formed of films of a highly conductive metal and dielectrics.
• Non-constant index of refraction dielectrics 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 refractive index of highly doped semiconductors 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.
• ITO Indium tin oxide
• ITO Indium tin oxide
• 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 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.
• 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 6 ) displays this property.
• 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.
• the films are formed in discrete layers.
• a transparent heat mirror coating 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, reflection of IR energy back to the filament will increase its temperature at a given power level or maintain its temperature 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 winter would function to prevent loss of heat from a heated structure.
• 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 alternatively 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 reflective layers, for example, silver.
• 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 back toward the source. The resulting interference permits transmission and reflection of visible light frequencies.
• Fabry-Perot etalon A device known in the art utilizing this principle is the Fabry-Perot etalon. So-called interference filters also have been disclosed utilizing this principle, one such filter 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 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 of a non-constant index of refraction dielectric material 32, which is deposited on the metal film layer 31, and an outer film layer 33 of a highly electrically conductive reflecting metal, which may also be silver, and which is deposited on the dielectric.
• a highly electrically conductive reflecting material such as silver
• a film layer of a non-constant index of refraction dielectric material 32 which is deposited on the metal film layer 31, and an 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 inter
• 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.
• 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 wavelength. 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 longest wavelength), falls at the peak of the lumens output of the energy source used for the lamp, in this case the filament.
• 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.
• 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.
• 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.
• 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.
• 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 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 reflectivities. With this arrangement, constructive interference occurs and the overall transmission of the combination is ideally 100 percent at the chosen visible wavelength, neglecting absorption in the metal films.
• the 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.
• 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.
• phase differences in the insulator decrease toward zero, while the phase shift on each reflection decreases toward 180o, the conventional value taken in etalon design according to quarter wave theory.
• the overall phase angle decreases from zero to -180, (on its way to -180o at very large wavelengths) and destructive etalon interference occurs, giving an overall reflectivity of 4R/(1+R M ) 2 .
• This overall reflectivity is very close to unity for R M > .5, where R M is the IR reflectivity of one silver film.
• a particular silverinsulator-silver combination can be designed to give a high IR reflectivity, at, for example, a wavelength of one micron. As wavelength in ase urther, the reflectivity increases uniformly towar nit
• 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
• 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 1 percent .
• 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 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 -180o. In the thin metal films used in this invention, reflection and transmission phase changes depart from conventional quarter wave practice.
• A (2 ⁇ c/e) ( ⁇ M e ) 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.
• 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.
• doping content controls the plasma radian frequency.
• the density of free electrons is N , the free electron mass is m and the absolute dielectric constant of free space is ⁇ .
• n, and K depend on n o and r and the scale wavelength divided by ⁇ p at a given period. At the given wavelength ⁇ r and ⁇ i are determined and equation 7 gives n, K. Reflectivity is given by equation (8).
• ITO indium tin oxide
• ⁇ p varies from 1.4 microns at 7 percent atomic weight tin to .8 microns at 20 percent atomic weight tin.
• Fig. 8 the change in the index of refraction with wavelength is shown for indium tin oxide at 7 percent atomic weight tin and 20 percent atomic weight tin.
• Lanthanum boride (LaB 6 )is a narrow band semiconductor with a band gap near point .08eV (R.N. Tsirev and S.V. Illarionov, Porosh, Metal, No. 6 (12). pp. 85-88, 1962).
• the reflectivity of LaB 6 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 absorption region can, however, be shifted into the near infrared by altering the doping content as shown in Fig.
• 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 C.C. Fan and Frank J. Bachner, at pp. 1012-1017 of Applied Optics, vol. 15, No. 4, April 1976. In that article a
• TiO 2 /Ag/TiO 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).
• ITO indium tin oxide
• the envelope 11 is preferably of conventional 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.
• 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 2600oK to about 2900oK.
• 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 2900oK. For an extended life lamp, one which operates in excess of 2500 hours, the operating temperature is about 2750oK.
• the color temperature is generally about 50oK lower.
• ITO 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.
• Such a coating if coated onto a good optical enclosure 11 (i.e.
• ITO is used for only one layer, for example 72C, and has a thickness of 44 nm.
• TiO 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.
• Fig. 12 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 2800oK.
• 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 ( ⁇ o ) 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 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.
• the variable index dielectric is deposited on the glass envelope of the lamp.
• the variation in index displayed by semiconductors such as 20 percent ITO and Lathanum hexaboride will be useful in improving either an I/S/I or S/I/S film by producing a sharper rise in reflectivity in the near IR and therefore provide for more complete reflection of IR energy.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Optical Elements Other Than Lenses (AREA)
• Optical Filters (AREA)
• Mirrors, Picture Frames, Photograph Stands, And Related Fastening Devices (AREA)

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• 1984
  • 1984-10-25 JP JP50411984A patent/JPS62501109A/ja active Pending
  • 1984-10-25 AU AU36158/84A patent/AU3615884A/en not_active Abandoned
  • 1984-10-25 WO PCT/US1984/001734 patent/WO1986002775A1/en not_active Application Discontinuation
  • 1984-10-25 EP EP19840904237 patent/EP0197931A4/de not_active Withdrawn

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