Patent Application
20060226777
This invention relates generally to incandescent lamps and, more particularly, to incandescent lamps having transparent envelopes that carry infrared (IR)-reflective coatings. The invention also relates to lighting fixtures incorporating such incandescent lamps.
Incandescent lamps having transparent envelopes that carry IR-reflective coatings, typically in the form of multi-layer stacks of dielectric materials, are well known in the lighting industry. Such dielectric coatings include alternating layers of high-refractive index materials, e.g., niobia (Nb2O5), tantala (Ta2O5), and titania (TiO2), and low-refractive index materials, e.g., silica (SiO2), wherein the layer thicknesses are controlled to be substantially one quarter the wavelength of the light to be reflected by constructive interference. The successive layers of such coatings are typically created using physical vapor deposition (PVD), reactive sputtering, low-pressure chemical vapor deposition (LP-CVD), or plasma-enhanced chemical vapor deposition (PE-CVD) to deposit various oxides onto a substrate, such as glass.
Multi-layer dielectric coatings can be designed to be highly reflective in a range of wavelengths and highly transmissive in other wavelengths. For example, a dielectric coating that reflects IR light, usually in the range of 750 to 1600 nanometers (nm), but transmits other wavelengths of light, is commonly called a “hot mirror,” a “low-wavelength pass edge-filter,” or an “IR coating.” The transition from reflecting wavelengths to transmitting wavelengths can be made very narrow, typically around 50 nm or less.
IR coatings were first combined with quartz-halogen lamps in the late-1980s to increase an incandescent lamp's luminous efficacy. Incandescent light sources typically produce about 10-15% visible light and about 85-90% IR light. An IR coating on an incandescent lamp's transparent envelope reflects a substantial portion of the IR light emitted by the lamp filament back onto the filament. The filament absorbs a portion of that IR light, thereby reducing the amount of electrical power required to heat the filament to a given temperature and consequently increasing the lamp's luminous efficacy. Lamps incorporating linear filaments, e.g., GE's FCM/HIR lamp, exhibit improved luminous efficacy as high as 40% (from 28 lumens per watt (LPW) for an uncoated lamp to 39 LPW for a coated lamp). The IR coatings currently employed in the lighting industry by companies such as GE, Osram Sylvania, Philips Lighting, and Deposition Sciences have a spectral transmittance similar to the graph shown in
IR-coated quartz halogen lamps generally are available in two form factors: “linear lamps” and “elliptical lamps.” Linear lamps generally have a long, single-coiled filament and a concentric tubular envelope. Most of the IR light reflected by the coating is redirected back to the filament, because the filament is a cylindrical object concentric with the cylindrical IR-coated envelope. A typical linear IR lamp is GE's FCM/HIR, shown in
Transparent conductive coatings (TCCs), formed of materials such as indium tin oxide (ITO), have been widely used in products where it is desirable to make a non-conducting substrate, such as glass, electrically conductive yet highly transmissive to visible light. By appropriately varying the doping and thickness of the TCC and by controlling the deposition process, a coating can be made to have a visible light transmissivity greater than 85% and to be electrically conductive (e.g., ˜20 Ω/square). Such a coating also has the property of having a reflectivity to IR light that increases gradually at longer wavelengths. In one example, depicted in
As shown in
Another prior art IR coating design, which is disclosed in U.S. Pat. No. 6,476,556 to E. Cottaar, includes an interference film having a transmittance that averages at least 90% in the visible wavelength range of 400 to 760 nm and having a reflectance that averages at least 75% in the infrared wavelength range of 800 to 2200 nm. Preferably, the interference film has a reflectance that averages at least 85% in the infrared wavelength range of 800 to 2500 nm.
In general, prior art IR coatings for quartz halogen lamps are designed to reflect the maximum integrated IR power generated by the light source. In other words, the coatings have been designed to maximize the integrated sum of reflection at each wavelength above 700 nm multiplied by the radiated power of the filament at the same wavelength. Designers of such prior art IR coatings also have sought to maintain maximum visible transmission, usually at values greater than about 90%.
The IR coating designs described briefly above have proven to be effective in improving the luminous efficacies of incandescent lamps. However, there remains a continuing need for an improved incandescent lamp, and for a lighting fixture incorporating such a lamp, exhibiting yet a higher luminous efficacy. The present invention fulfills this need and provides further related advantages.
The present invention is embodied in an incandescent lamp incorporating an improved IR coating, as well as a lighting fixture incorporating such an IR coating, either in the lamp itself or elsewhere, providing a higher luminous efficacy than that which had previously been achieved.
In one form of the invention, an incandescent lamp includes a filament, a transparent envelope defining an enclosed space in which the filament is located, and an optical coating disposed on a surface of the envelope, for transmitting light emitted by the filament in a prescribed visible wavelength band, while reflecting back toward the filament light emitted by the filament in other wavelength bands, whereupon a portion of such reflected light is absorbed by the filament. The optical coating includes a plurality of dielectric layers having prescribed refractive indices and prescribed thicknesses, which are selected such that the optical coating provides a prescribed transmittance/reflectance spectrum having an average reflectance greater than 90% across an infrared wavelength range of 740 to 2000 nm and further having an average transmittance of less than 90% across a visible wavelength range of 400 to 700 nm. The optical coating cooperates with the filament such that the lamp provides a higher luminous efficacy than would a corresponding lamp lacking such an optical coating.
In other, more detailed features of this form of the invention, the optical coating is located on the outer surface of the transparent envelope. In addition, the refractive indices and thicknesses of the dielectric layers of the optical coating are selected such that the optical coating provides a transmittance/reflectance spectrum having an average reflectance greater than 95% across an infrared wavelength range of 740 to 2000 nm. The optical coating can include a stack of alternating layers of high- and low-refractive index materials, with the high-refractive index layers all incorporating a material selected from the group consisting of TiO2, Ta2O5, NbO2, and mixtures thereof, and with the low-refractive index layers all incorporating a material selected from the group consisting of SiO2, Al2O3, and mixtures thereof.
In still other, more detailed features of the invention, the optical coating further includes one or more transparent conductive layers, which can be contiguous with the plurality of dielectric layers. These one or more transparent conductive layers are configured to have an average reflectance greater than 70% across an infrared wavelength range of 2000 to 4000 nm. The incandescent lamp can further include an electrical connector to which the transparent envelope is secured, and a reflective coating disposed on a portion of the transparent envelope adjacent to the electrical connector, for reflecting visible and infrared light back toward the filament. The optical coating is configured such that the lamp has a luminous efficacy of preferably at least 40 lumens per watt, more preferably at least 60 lumens per watt, and most preferably at least 80 lumens per watt.
In a separate and independent form of the invention, an incandescent lamp includes a filament, a transparent envelope defining an enclosed space in which the filament is located, and an optical coating system disposed on a surface of the envelope that includes two distinct coatings: (1) a first coating including a plurality of dielectric layers having prescribed refractive indices and prescribed thicknesses, which are selected such that the first coating provides a prescribed transmittance/reflectance spectrum, and (2) a second coating including a transparent conductive material having a thickness selected such that the second coating provides a prescribed transmittance/reflectance spectrum. The first and second coatings cooperate with each other and with the filament such that the lamp provides a higher luminous efficacy than would a corresponding lamp lacking such an optical coating system on its envelope.
In other, more detailed features of the invention, the optical coating's first and second coatings are contiguous with each other and located on the outer surface of the transparent envelope. In one form the second coating is located on the side of the first coating opposite the filament, while in an alternative form, the second coating is located at an intermediate location within the plurality of layers of the first coating, closer to the side of the first coating opposite the filament than to the side of the first coating facing the filament.
In yet other, more detailed features of the invention, the first coating includes a stack of alternating layers of high- and low-refractive index materials, with the high-refractive index layers all incorporating a material selected from the group consisting of TiO2, Ta205, NbO2, and mixtures thereof, and with the low-refractive index layers all incorporating a material selected from the group consisting of SiO2, Al2O3, and mixtures thereof. The first coating is configured such that it has an average reflectance greater than 90% across an infrared wavelength range of 740 to 2000 nm and such that it has an average transmittance less than 90% across a visible wavelength range of 400 to 700 nm.
In addition, the second coating can include a transparent conductive material selected from the group consisting of indium tin oxide, aluminum-doped zinc oxide, titanium-doped indium oxide, cadmium stannate, tin oxide-zinc stannate, gallium-doped zinc oxide, gold, silver, and mixtures thereof. The second coating is configured such that it has an average reflectance across an infrared wavelength range of 2000 to 4000 nm of preferably greater than 70%, more preferably greater than 80%, and most preferably greater than 90%. The second coating also is configured such that it has an average absorptance in the visible wavelength range of 400 to 700 nm of preferably less than 20%, more preferably less than 10%, and most preferably less than 5%.
The incandescent lamp can further include an electrical connector to which the transparent envelope is secured, and a reflective coating disposed on a portion of the transparent envelope adjacent to the electrical connector, for reflecting visible and infrared light back toward the filament. The optical coating is configured such that the lamp has a luminous efficacy of preferably at least 40 lumens per watt, more preferably at least 60 lumens per watt, and most preferably at least 80 lumens per watt.
The present invention also is embodied in a lighting fixture incorporating a housing and a lamp socket carried by the housing, and further incorporating an incandescent lamp having a form like one of those described above. Further, the present invention can be embodied in a lighting fixture incorporating an optical coating as described above, located either on the envelope of the incandescent lamp, itself, or on another substrate of the fixture, separate and apart from the lamp, e.g., a fixed transparent envelope surrounding the incandescent lamp.
Other features and advantages of the present invention should become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.
With reference now to the illustrative drawings, and particularly to
With particular reference to
One of the traditional important advantages of forming lamp filaments of tungsten is that this material functions as a spectrally selective emitter, i.e., it has a higher emissivity at visible wavelengths than it does at IR wavelengths. This phenomenon is depicted in
Consequently, a substantial portion of the IR light that is reflected by the IR coating back toward the tungsten filament is, in turn, reflected by the filament back toward the coating. When this twice-reflected IR light strikes the coating a second time, a portion of it is reflected again back toward the filament, but another portion of it is transmitted through, or absorbed by, the coating. The successive reflections continue, essentially constituting an oscillation between the filament and the coating, until the IR light either is absorbed by the filament, is absorbed by the IR coating, or is transmitted from the lamp. This oscillation effect is illustrated in
In a dielectric IR coating, which exhibits negligible absorption, summing together the IR light transmitted through the coating both from its initial emission from the filament and from all of the subsequent reflections from the filament yields a value corresponding to the effective transmittance of the incandescent lamp as a system, or its system transmittance.
It should, therefore, be appreciated that the optimum dielectric IR coating on an incandescent lamp incorporating a tungsten filament is determined by reference to the lamp's system transmittance or reflectance, not its coating transmittance or reflectance. In particular, the optimum dielectric IR coating will provide a minimum integrated sum of the lamp's system transmittance at each wavelength above 700 nm multiplied by the radiated power of the filament at the same wavelength. A tungsten filament incandescent lamp incorporating a dielectric IR coating design optimized by reference to its system transmittance will exhibit a substantially higher luminous efficacy than will a lamp incorporating a coating design optimized by reference only to coating transmittance. In general, according to this new optimization method, the coating's transmittance should be less than 10% (and its reflectance greater than 90%) for all IR wavelengths ranging from 740 nm to the highest possible wavelength. Preferably, the coating transmittance at such wavelengths is as close to 0% as possible, and its reflectance as close to 100% as possible.
An IR lamp having a higher efficiency than that of the prior art coating represented in
Another exemplary coating in accordance with the invention (Coating B) includes a 54-layer stack of alternating layers of high-index niobia (Nb2O5) and low-index silica (SiO2). As shown in
Yet another coating in accordance with the invention (Coating C) includes a 55-layer stack of alternating layers of high-index tantala (Ta2O5) and low-index silica (SiO2). As shown in
It should be noted that the Coatings A, B, and C all provide transmittances in the visible wavelength range of 400 to 700 nm that are less than the “greater than 90% transmittance” values for the coating disclosed in the above-identified U.S. Pat. No. 6,476,556 to Cottaar; however, this reduced transmittance is not considered significant, for the reasons set forth below. This lower transmittance results from a higher reflectance in the visible wavelength range. But a substantial portion of this reflected visible light is reflected back toward the filament, where it is either absorbed as heat or is reflected by the filament back toward the envelope. A lower visible transmittance, therefore, is only slightly detrimental to the lamp's luminous efficacy, because the reflected visible light still contributes either to reheating the filament or to generating luminous flux, both of which increase the lamp's efficacy.
It should be noted further that Coatings A, B, and C were optimized for use in combination with a transparent conductive coating, as described below, and that the layer thicknesses and performance of these coatings would be modified somewhat if they were to be used as dielectric-only coatings. In addition, what appears to be undesirable variations in the visible transmittance of Coatings A, B, and C are in fact integrated in reality into much smoother transmittance curves in a typical fixture.
It will be noted in
A TCC can advantageously be substituted for one or more of the high-index refractive layers in the dielectric coating, because conductive coatings, likewise, have relatively high indices of refraction. Alternatively, the TCC can be a final layer overlaying the dielectric coating, which is then sealed with an anti-diffusion layer, such as silica (SiO2), to prevent further oxidation of the conductive coating at elevated temperatures. In general, a TCC should have low average absorption in the visible wavelength range of 400 to 700 nm, preferably less than 20%, more preferably less than 10%, and most preferably less than 5%. In addition, a TCC should have a high average infrared reflectance in the wavelength range of 2000 to 4000 nm, preferably greater than 70%, more preferably greater than 80%, and most preferably greater than 90%.
A TCC preferably exhibits a uniform, uninterrupted electrical conductance in order for it to provide high IR reflectance. Cracking or crazing in the TCC will reduce its electrical conductance and will substantially reduce its IR reflectance. If surface defects (e.g., pitting, cracking, or crazing) are present in the underlying dielectric coating, they are generally propagated into the TCC. The dielectric coating, therefore, must be deposited using a coating process that will yield an outermost surface substantially free of defects, in order for the TCC to provide the desired optical performance. The TCC itself also must be deposited using a process that does not create cracking or crazing in the TCC. Suitable deposition processes for dielectric coatings and TCCs are known to those skilled in the coating art.
As shown in
TCCs such as ITO coatings, of course, also can be combined with other IR coatings, e.g., the IR coating represented by
Although Coatings A, B, and C incorporate only two materials (a high refractive index material and a low refractive index material), dielectric coatings incorporating high, medium, and low refractive index materials (three material systems) alternatively can be used according to the invention. These alternative dielectric coatings will ordinarily exhibit the desirable characteristic of producing fewer interference effects in the visible portion of the spectrum. Suitable low-index materials include SiO2 and Al2O3, suitable high-index materials include TiO2, Ta2O5, and Nb2O5, and suitable medium-index materials include Y2O3, HfO2, and ZrO2. Low-, medium-, and high-index layers alternatively can incorporate combinations of any of the materials listed above. For example, a high-index layer can incorporate a combination of Nb2O5 and Ta2O5.
According to the invention, the two- and three-material coating systems, when illuminated at normal incidence, will transmit less than 90% average over the visible spectrum of 400 to 700 nm and will reflect more than 90% average over the IR spectrum of 740 to 2000 nm. The various coatings described above may be formed using any of a number of known deposition processes. These processes include atomic layer deposition, physical vapor deposition (PVD), reactive sputtering, low-pressure chemical vapor deposition (LP-CVD), and plasma-enhanced chemical vapor deposition (PE-CVD).
The TCC layer(s) included in the dielectric/TCC combination coating system is preferably located on the side of the coating opposite the filament. This minimizes the amount of IR light incident on the TCC and thereby minimizes the adverse effects of the layer's relatively high absorptance in the transition region.
In addition, in the case of a TCC layer(s) formed of ITO or similar material, the effectiveness of the layer in reducing the lamp's system IR transmittance (and increasing its IR system reflectance) can be optimized by varying its thickness and its carrier concentration. Carrier concentration is a function of doping level and various deposition parameters. The carrier concentration and the ITO layer thickness are varied until maximum efficacy is obtained for the coating in combination with a given dielectric stack. Maximum efficacy is obtained by centering the plasma frequency (i.e., the frequency at which transmittance equals reflectance), and the associated high absorption spectral region of the TCC, in the spectral region where high IR reflectivity is provided by the dielectric coating. The plasma frequency of the conductive coating is moved to a wavelength as low as possible, until the high absorption region of the coating terminates just above the visible wavelengths, i.e., 700 nm. The most desirable plasma frequency for an ITO coating produced by sputtering is approximately about 1400 nm or less. This differs substantially from ITO coatings used in most commercial applications, which typically have plasma frequencies between 1800 and 2500 nm, and which have not been used in combination with a dielectric coating on an incandescent lamp.
Since TCCs absorb rather than transmit non-reflected IR light, a combination dielectric and TCC will tend to operate at much higher temperatures than a dielectric-only coating. In order to keep an IR coated lamp relatively compact and effective at redirecting the IR light onto the filament, it is desirable to make the outer emissivity of the coating system as high as possible. This can be accomplished by adding an IR-emissive (IRE) coating on top of the TCC layer. An example of such an IRE material is ITO having a plasma frequency equal to the peak emission frequency produced by the lamp envelope. For example, at the maximum operating temperature of fused silica, 1000° C., an ITO layer having a plasma frequency of 2.3μ is most desirable. A diffusion barrier preferably is provided between the TCC and the IRE, in order to prevent degradation of both materials' optical properties at high temperatures.
An example of a dielectric/ITO/IRE coating system is shown in
It should be noted that the silica layer adjacent to either the 1.3μ ITO layer or the IRE layer is a dielectric layer, and it may also include additional dielectric layers (including both high- and low-refractive index materials) to improve the optical performance of a given coating.
The luminous efficacy of an incandescent lamp (η) is defined as the total luminous flux generated by a lamp (Φ) divided by the electrical power consumed by the lamp (P). The following table summarizes the approximate luminous efficacies of linear incandescent lamps incorporating envelopes coated with the various coatings which have been discussed above. It will be noted that a lamp incorporating the prior art IR coating represented in
The invention is embodied in an incandescent lamp having an extended high-reflectivity IR coating on its envelope, with the coating being configured such that the lamp provides an improved system transmittance as compared to a lamp incorporating an envelope that is uncoated or coated with a prior art IR coating like that characterized in
In an alternative embodiment of the invention, the lamp can be configured not to emit white light, but rather a narrower band of colored light. Specifically, the coating placed on the lamp's envelope can be configured to transmit the desired band of colored light, and to reflect not only IR light but also visible light outside the desired band. In the case of a lamp configured to emit blue light, this can be accomplished simply by configuring the coating to have a cutoff wavelength that is lower than it is in the case of the IR coating discussed above, i.e., a wavelength of about 540 nm rather than 740 nm. Alternatively, in the case of a lamp configured to emit some other visible color band, e.g., a red color band, this can be accomplished by configuring the lamp coating to transmit the desired red color band but reflect all wavelengths both above and below that band. If the conductive coating feature of the invention is to be used in a lamp that generates such a color band, such coating's thickness, doping, and carrier concentration should be optimized for that particular configuration. For example, for blue light, the conductive coating's plasma frequency could be reduced to increase the lamp's efficacy, because the increased ITO absorption between 540 and 740 nm would not affect the lamp's desired blue spectral output.
It should be appreciated from the foregoing description that the present invention provides an incandescent lamp incorporating a special optical coating system that enables the lamp to provide an improved luminous efficacy. In one form, the optical coating system includes a plurality of dielectric layers having prescribed refractive indices and prescribed thicknesses, which are selected such that the optical coating provides a prescribed transmittance/reflectance spectrum having an average reflectance greater than 90% across an infrared wavelength range of 740 to 2000 nm and further having an average transmittance of less than 90% across a visible wavelength range of 400 to 700 nm. In another form, the optical coating system includes two distinct coatings: (1) a first coating including a plurality of dielectric layers having prescribed refractive indices and prescribed thicknesses, which are selected such that the first coating provides a prescribed transmittance/reflectance spectrum, and (2) a second coating including a transparent conductive material configured such that the second coating provides a prescribed transmittance/reflectance spectrum. The invention also is embodied in a lighting fixture incorporating an optical coating as described above, located either on the envelope of the incandescent lamp, itself, or on another substrate of the fixture, separate and apart from the lamp, e.g., a fixed transparent envelope surrounding the incandescent lamp.
Although the invention has been described with reference only to the preferred embodiments, those skilled in the art will appreciate that various modifications can be made without departing from the invention. Accordingly, the invention is defined only by the following claims.
Priority is claimed under 35 U.S.C. § 119(e) to the U.S. Provisional Patent Application No. 60/669,597, filed on Apr. 7, 2005, entitled “Incandescent Lamp Incorporating Extended High-Reflectivity IR Coating,” by David W. Cunningham, which application is incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 60669597 | Apr 2005 | US |
