Micro-nanostructured films for high efficiency thermal light emitters
Devices for emitting light in intended frequency ranges. Each device includes a thermal light source for generating light and a reflective film including holes for transmitting a portion of the light shorter than a cutoff wavelength and reflecting the rest of the light back to the light source. The thermal light source reabsorbs the reflected light and thereby increases the operational efficiency thereof.
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The present invention generally relates to thermal light emitters and, more particularly, to light emitters including a thermal source of radiation and a film with micro/nanostructured openings formed therein for selectively passing predetermined wavelengths of radiation and reflecting other wavelengths of radiation.
A conventional incandescent light bulb is about 10% efficient in converting input energy into visible light in the wavelength range of 400-to-750 nm, where most of the input energy is radiated as infrared light with wavelengths longer than 750 nm.
A conventional approach to fabricating a selective long-wavelength reflector, or “hot mirror,” is to use one or more dielectric stacks composed of three layers with alternating indices of refraction. This type of hot mirror is also called a dielectric interference mirror or dichroic mirror. At least three depositions of materials, each with a well-defined thickness requirement to create the desired optical interference, may be needed to produce a conventional hot mirror. A typical single stack dichroic mirror may produce high transmission in the visible wavelength range, but the long wavelength reflection range is not wide enough to reflect most of the spectrum emitted by a 3000° K blackbody.
As the thickness of each layer of the dichroic mirrors determines the wavelength band of the reflected light, each layer needs to be deposited with high precision. Also, the dichroic mirror requires a number of layers to reflect most of the IR energy emitted by a filament. Moreover, each of the multiple layers needs to be uniformly coated on the light bulb surface, which may translate into high manufacturing cost. Thus, there is a strong need for a reflector that can operate as a low-pass filter and can be applied to conventional light bulb design in a cost-effective manner.
SUMMARY OF THE INVENTIONIn one embodiment, a device for emitting light includes: a light source for generating light and a reflective film including holes for transmitting a portion of the light shorter than a cutoff wavelength and reflecting the rest of the light back to the light source.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.
The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention because the scope of the invention is best defined by the appended claims.
As will be described below, various embodiments of the present invention provide thermal light emitters, each having a heat source and a micro/nanostructured or micro/nanopatterned film for selectively passing visible light but reflecting long-wavelength thermal radiations back to the heat source. Unlike existing approaches that use a dichroic mirror of multiple layers to limit the radiated wavelengths, the micro/nanostructured film of the present invention is reflective of thermal radiation outside the visible spectrum but has a plurality of holes or apertures or openings that pass visible light thus forming a low-pass filter. The shape and dimension of the holes are set to determine the cutoff wavelength. The reflected energy is returned and re-absorbed by the heat source, thus increasing the operational efficiency of the thermal light emitter.
The film 300 may be applied to the envelope of a thermal light emitter having an enclosed filament or other thermal emitter equivalent to the filament. More specifically, the film 300 may be positioned to surround the hot filament or be formed on a light emitting surface that reflects infrared radiation back towards the thermal emitter while allowing the short wavelength visible light to escape in a preferred direction. Solid or unapertured films may be used on those of areas of light reflecting envelope where total light reflection is intended, and films with apertures may be used where only the shorter wavelength light is intended to pass through. Applications may include, but are not limited to, high efficiency incandescent light bulbs, high efficiency micromachined light bulbs to replace light emitting diodes (especially white LEDs that are typically UV-pumped fuorescents), and photovoltaic thermal energy converters.
Almost all of the interior of the body 614, with the exception of the filament 608 may be coated with a solid film 604. The film 604 is preferably a totally reflective thin metallic film and may be applied using traditional thin film deposition techniques, such as evaporation or sputtering. The film 604 may be highly reflective to infrared radiation to provide a high visible light generation efficiency.
As suggested above, the micro/nanostructured film 606 applied to the cap 605 a thin metallic film with ˜300 nm diameter apertures formed on the interior of the cap to allow visible light to escape while keeping longer wavelengths within the reflective cavity for eventual re-absorption by the filament 608. Herein, the term reflective cavity refers to the interior space of the light bulb 600 surrounded by the solid film 604 and micro-nanostructured film 606. The film 606 can be applied using a number of techniques, such as lift-off patterning, masked reactive etching, shadow mask deposition, and direct-write laser deposition to create the metallic apertured thin film. Lift-off patterning discussed below is a preferred batch-fabrication technique that can pattern a variety of metals on many different substrates (bulb envelope materials).
The bulb envelope 602 and the solid film 604 are shaped to generally form a paraboloidal reflector, and the filament 608 is located in or near the focus of the paraboloid. The paraboloidal reflector is of the type used to generate floodlights or directed beam lights, for example. As a variation, the bulb envelope 602 and the solid film 604 could be in the form of a parabolic reflector with the filament 608 located in or near the focus of the parabola. The parabolic reflector might be of the type used to generate linear lights or fan beam lights, for example.
A typical 100-Watt incandescent bulb has an output of ˜17 lumens/Watt and a 23 Watt fluorescent bulb has an output of ˜65 lumens/Watt. Moreover, the most efficient white light LEDs have an output of ˜50 lumens/Watt. An incandescent light bulb of the type described and shown at 600 with integrated reflector and aperture array may achieve output levels of ˜90 lumens/Watt based on a 5× increase in efficiency compared to a standard incandescent design. For example, the light bulb 600 may be radiation-hard, operate over a broader temperature range, and provide a common technology for use in generating a variety of perceived colors.
As pointed out above, the micro/nanostructured film of the light bulbs in
Hole sizes in the film can be varied to alter the “color” of the bulb, e.g., smaller holes will produce “bluer light”. This enables use of lower operating temperatures for the filament to significantly prolong life. In this case, filament size (but not power) needs to be increased to provide the same visible light output. In addition, non-circular holes, e.g., square, hexagonal, or elliptical, can also be used to adjust the transmitted light spectrum. Furthermore, an incandescent light bulb having a micro/nanostructured film of the type described above can provide a direct replacement for conventional light bulbs, with visible light output efficiencies greater than fluorescent bulbs, while still allowing illumination variation and control using conventional dimmer circuits. In contrast, fluorescent bulbs will not work with mass-market dimmers.
As discussed above, the filaments used in light bulbs of the types shown in
The filaments in
Unlike existing LED light sources which use different phosphors or semiconductors to generate different colors, the coating of micro/nanostructured films with different aperture sizes on the inner surfaces of incandescent light bulbs can provide, in accordance with another embodiment of the present invention, incandescent bulbs suitable for replacing the LED sources.
The filaments 1206 (
Applications of the micro/nanostructured film may include efficient lighting in harsh environments (space, reactors, etc.) and common terrestrial environments. They may be also used as single lamps and arrays of lamps for alphanumeric displays, flat panel displays, and efficient backlighting for liquid crystal displays. A more efficient backlight may extend battery-powered laptop, PDA, cell phone, etc., operation without sacrificing image brightness.
As discussed above, conventional techniques, such as lift-off patterning, masked reactive ion etching, shadow mask deposition, and direct-write laser deposition, may be used to create a micro/nanostructured film on a substrate.
The photovoltaic cell 1810 may be made of gallium antimonide (GaSb), for instance. In that case, wavelengths longer than 1.59 microns will not produce power in the cell because the photon energy is lower than the cell bandgap energy of 0.78 eV. The most efficient energy production may occur at wavelengths slightly shorter than the bandgap energy because any photon energy in excess of 0.78 eV will be wasted as heat within the photovoltaic cell 1810. As such, the overall efficiency of the TPV power generator 1800 may be increased by using a combination of the dichroic cold mirror 1808 for reflecting short wavelength radiation and micro/nanostructured film 1812 for reflecting radiation longer that 1.59 micron, wherein the film 1812 combined with the mirror 1808 may form a band pass filter.
As discussed above, TPV power generator efficiency can be enhanced using micro/nanopatterned thin film reflectors. The enhanced heat-to-electrical conversion efficiency of the TPV power generator 1800 significantly reduces waste of thermal energy. Other applications of the micro/nanopatterned film may include terrestrial power generators using solar heat or fuel combustion, and space power reactors.
It is noted that the micro/nanostructured film for use in the embodiments of the present invention includes holes or openings. The openings have various shapes, such as circular, ellipsoidal, square, rectangular, rhomboidal, and polygonal. These openings provide near 100% transmission at short wavelengths and different from the cross-like openings described in the technical paper, “Rapid Prototyping of Infrared Bandpass Filters Using Aperture Array Lithography,” K. Han, M. Morgan, A. Ruiz, S. C. Vernula and P. Ruchhoeft, Jour. Vac. Sci. & Tech., B 23 (6), November/December 2005, pp. 3158-3163, wherein the cross-like openings operate as a narrow bandpass filter.
It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.
Claims
1. A device for emitting thermal radiation, comprising:
- a source for emitting thermal radiation; and
- a reflective film including a plurality of openings formed therein, the size and shape of said openings being determined to transmit radiation having wavelengths shorter than a predetermined threshold wavelength and to reflect radiation having wavelengths exceeding said threshold wavelength back to said source such that said source absorbs at least a portion of the reflected radiation.
2. A device as recited in claim 1, wherein said openings are holes having a generally circular shape and said threshold wavelength is about 2.5 times the diameter of said holes.
3. A device as recited in claim 1, wherein said openings have a shape selected from a group of shapes consisting of circular, ellipsoidal, square, rectangular, rhomboidal, and polygonal.
4. A device as recited in claim 1, wherein said device is an incandescent light bulb and said source is a filament, said filament being a helical wire or a coil formed of a helical wire.
5. A device as recited in claim 1, wherein said device is an incandescent light bulb and said source is a planar filament that is formed of a tortuous wire in a plane.
6. A device as recited in claim 1, wherein said device is an incandescent light bulb including a filament as said source and a bulb envelope that surrounds said filament and wherein said bulb envelope includes:
- a first portion having a first surface on which said reflective film is formed such that the radiation having wavelengths shorter than said predetermined threshold wavelength passes through said first portion, and
- a second portion having a second surface on which a solid reflection film is formed, said solid reflection film being totally reflective to prevent the thermal radiation from passing through said second portion;
- wherein at least one of said first and second portions includes a curved surface and said filament is located at a focal point of said curved surface.
7. A device as recited in claim 6, wherein said first portion is a flat or curved plate and has a shape selected from a group of shapes consisting of circular, ellipsoidal, square, rectangular, rhomboidal, and polygonal and wherein said second portion includes a curved surface having a paraboloidal curvature.
8. A device as recited in claim 6, wherein said first portion includes a curved surface having a circular cylindrical shape and said second portion has a flat disk shape.
9. A device as recited in claim 6, wherein said first portion includes a curved surface having a spherical curvature and said second surface has a circular cylindrical shape.
10. A device as recited in claim 6, wherein one of said first and second portions is a rectangular plate and the other includes a curved surface having a parabolic cylindrical shape.
11. A device as recited in claim 1, wherein said reflective film is a metallic coating.
12. A device as recited in claim 1, wherein the thickness of said reflective film is greater than 30 nm.
13. A device for generating electric current, comprising:
- a source for generating heat energy;
- a selective thermal emitter operative to receive said heat energy and to emit thermal radiation;
- a housing enclosing said source and selective thermal emitter and having a transparent window through which the thermal radiation from the selective thermal emitter passes;
- a photovoltaic cell located outside said cavity to receive the thermal radiation that has passed through the window and operative to convert the received thermal radiation into an electric current; and
- a reflective film interposed between said window and said photovoltaic cell and including a plurality of openings formed therein, the size and shape of said openings being determined to transmit radiation having wavelengths shorter than a first predetermined threshold wavelength and to reflect radiation having wavelengths exceeding said first threshold wavelength back to the source such that said source absorbs at least a portion of the radiation reflected by said film.
14. A device of claim 13, wherein said selective thermal emitter is made from a rare-earth ceramic.
15. A device of claim 13, further comprising:
- a dichroic cold mirror interposed between said window and said photovoltaic cell and operative to transmit radiation having wavelengths exceeding a second predetermined threshold and to reflect radiation having wavelengths shorter than said second predetermined threshold back to said source such that said source absorbs at least a portion of the radiation reflected by said dichroic mirror, said second threshold wavelength being shorter than said first threshold wavelength;
- wherein said first cutoff wavelength corresponds to the bandgap of said photovoltaic cell.
16. A device of claim 15, wherein said reflective film is formed on a surface of said dichroic mirror facing said transparent window.
17. A device of claim 15, wherein said reflective film is formed on a surface of said photovoltaic cell facing said transparent window.
Type: Application
Filed: Nov 20, 2006
Publication Date: May 22, 2008
Applicant: The Aerospace Corporation (El Segundo, CA)
Inventor: Siegfried Janson (Redondo Beach, CA)
Application Number: 11/602,828
International Classification: H01K 1/26 (20060101); G01J 5/00 (20060101);