Abstract:
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.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    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. 
         [0002]    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.  FIG. 1  shows the emission spectrum of a blackbody at ˜3000° K simulating that of a tungsten filament in a conventional light bulb. Human eyes are sensitive to light with wavelengths between ˜400 and 750 nm, and a large portion of the emitted light from the tungsten filament is at longer wavelengths than human eyes can detect. About 90% of the input electrical power is converted into these invisible infrared photons, many of which are absorbed in the bulb envelope and thereby heat the envelope. If these longer wavelengths can be reflected back towards the hot filament before reaching the bulb envelope, while allowing the visible wavelengths to pass through the envelope, the unseen heat energy will be re-absorbed by the filament, and less input electric power will be required to maintain visible light output, thus improving the efficiency of the bulb. In the ideal case where infrared reflection is perfect and there is no thermal conduction of heat from the filament to the bulb envelope, the infrared reflecting bulb will be an order-of-magnitude more efficient than a conventional light bulb. 
         [0003]    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.  FIG. 2  shows the spectral reflectance of a conventional single stack dichroic mirror. As depicted, the second passband may start at about 1100 nm with additional passbands occurring at even longer wavelengths, failing to reflect most of the IR radiation that extends up to ˜4 microns. Single stack dichroic hot mirrors typically reflect the wavelength range from ˜750-to-1250 nm while advanced multi-stack hot mirrors may reflect from ˜750-to-2000 nm. For a 3000° K black body, single and multi-stack hot mirrors usually reflect about 32% and 62%, respectively, of the total photon energy emitted by a filament. 
         [0004]    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 INVENTION 
       [0005]    In 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. 
         [0006]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  shows the emission spectrum of a blackbody at ˜3000° K; 
           [0008]      FIG. 2  shows the spectral reflectance of a conventional single stack dichroic mirror; 
           [0009]      FIG. 3  shows a micro/nanostructured film operating as a hot mirror in accordance with the present invention; 
           [0010]      FIG. 4  shows calculated transmission fractions of the micro/nanostructured film of  FIG. 3 ; 
           [0011]      FIG. 5A-5B  show the emission spectra of a blackbody, superimposed on the transmission curves of embodiments of the film in  FIG. 3 ; 
           [0012]      FIG. 6  is a schematic cross sectional view of an incandescent light bulb having a micro/nanostructured film in accordance with the present invention; 
           [0013]      FIG. 7A  is a schematic longitudinal cross sectional view of an elongated tubular incandescent light bulb having a micro/nanostructured film in accordance with the present invention; 
           [0014]      FIG. 7B  is a schematic transverse cross sectional view of the incandescent light bulb in  FIG. 7A , taken along the line VII-VII; 
           [0015]      FIG. 8  is a schematic longitudinal cross sectional view of another embodiment of an incandescent light bulb having a micro/nanostructured film in accordance with the present invention; 
           [0016]      FIG. 9  is a schematic cross sectional view still another embodiment of an incandescent light bulb having a micro/nanostructured film in accordance with the present invention; 
           [0017]      FIG. 10A  is a schematic side view of an exemplary linear filament of the type used in the light bulbs of  FIGS. 7A-8  and exploded partial segment thereof; 
           [0018]      FIG. 10B  is a schematic side view of a compact helical filament of a type that might be used in the light bulbs of  FIGS. 6 and 9 ; 
           [0019]      FIG. 11  shows calculated re-absorption fractions of light initially emitted by a filament as a function of film reflectivity and the average number of photon reflections from the film; 
           [0020]      FIG. 12A  is a schematic cross sectional view of yet another embodiment of a light bulb in accordance with the present invention; 
           [0021]      FIG. 12B  is a schematic cross sectional view of the light bulb shown in  FIG. 12A , taken along the line XII-XII; 
           [0022]      FIG. 13A  is a schematic cross sectional view of a further embodiment of a light bulb in accordance with the present invention; 
           [0023]      FIG. 13B  is a schematic cross sectional view of the light bulb shown in  FIG. 13A , taken along the line XIII-XIII; 
           [0024]      FIG. 14  is a schematic diagram of an exemplary planar filament of a type that might be used for the light bulbs of  FIGS. 12A-13B . 
           [0025]      FIGS. 15A-15E  show exemplary steps that might be followed in forming one embodiment of a micro/nanostructured film on a substrate in accordance with the present invention; 
           [0026]      FIGS. 16A-16C  show exemplary steps that might be followed informing another embodiment of a micro/nanostructured film on a substrate in accordance with the present invention; 
           [0027]      FIGS. 17A-17C  show exemplary steps that might be followed informing yet another embodiment of a micro/nanostructured film on a substrate in accordance with the present invention; 
           [0028]      FIG. 18  is a schematic diagram illustrating a thermo-photovoltaic power generator including selective emitter and a micro/nanostructured film in accordance with the present invention; and 
           [0029]      FIG. 19  is an emission spectrum for the thermo-photovoltaic power generator shown in  FIG. 18 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    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. 
         [0031]    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. 
         [0032]      FIG. 3  shows a reflective micro/nanostructured film  300  operating as a hot mirror for incoming radiation  304  that may have a wide spectral range. As depicted, the film  300  includes multiple holes or apertures or openings  302  that are sized to let the short wavelength portion  306  of the incoming radiation pass through and to reflect long wavelength portion  308 . In general, wavelengths that are longer than ˜2.5 times the aperture diameter are reflected while shorter wavelengths are transmitted, i.e., the film  300  operates as a low-pass filter. 
         [0033]      FIG. 4  shows calculated transmission fractions of circular apertures  302  as a function of aperture size and the thickness of the film  300 . The term transmission fraction refers to the effective cross section of the apertures divided by the actual aperture area. As depicted, wavelengths that are longer than ˜2.5 times the aperture diameter are reflected, regardless of the film thickness. As such, for transmission of visible light, typically ranging from 400 to 750 nm, the aperture diameter needs to be ˜300 nm. Smaller diameters can be used to filter out red and yellow light thereby creating predominantly blue or green light. Non-circular apertures or a distribution of aperture size can be used to adjust the overall perceived color or color temperature for white light applications. The film  300  may be a patterned metallic (e.g., aluminum, silver, gold, nickel, etc.) film with a thickness greater than 30 nm to produce reflectivity in excess of 90%. The film  300  may be freestanding or deposited on a transparent substrate such as glass, quartz, etc. For instance, the film  300  may be formed on the surface of a light bulb envelope such that the long wavelength portion  308  may be reflected back to the bulb&#39;s filament. 
         [0034]      FIG. 5A  shows the emission spectrum of a blackbody at ˜3000° K, superimposed on the transmission curve of a film of the type depicted in  FIG. 3 , wherein the film has circular apertures of diameter 0.25 microns. As depicted, the film reflects most of the infrared light and operates as a low-pass filter. The transmitted infrared energy can be less than the transmitted visible energy, which may yield a high operational efficiency of the light source. 
         [0035]      FIG. 5B  shows another emission spectrum of a blackbody at ˜3000° K, superimposed on the transmission curve of a film of the type described in  FIG. 3 , wherein the film has circular apertures of diameter 0.2 microns. As can be noticed, in contrast to the aperture size of  FIG. 5A , the decrease in aperture diameter will shift the transmission curve of the film toward blue, making the users perceive an increase in intensity of blue light. It will also be noticed that the film having smaller apertures will reflect a larger portion of the infrared light. 
         [0036]    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. 
         [0037]      FIG. 6  is a schematic cross sectional view of an incandescent light bulb shown at  600  and including an envelope  602  of glass, quartz, or other suitable material. The interior surfaces of the envelope side portion  603  are coated with a totally reflective film  604  while the interior surface of the envelope end portion or cap  605  is coated with a micro/nanostructured film  606  in accordance with the present invention. More specifically, in addition to the envelope  602 , the light bulb  600  includes a filament  608  for emitting radiant energy and a filament holder  618  through which power leads  612  pass. The cap  605  is a disk made of an optically transparent material, such as glass, quartz, etc., and coated on its interior surface with a micro/nanostructured film  606 . This cap can be flat or curved. The body portion of the bulb envelope  602 , may be joined with the cap  605  at the last step of manufacture. The filament  608  may have a linear, a planar spiral or a non-planar spiral shape. A linear shaped filament may be in the form of an elongated coil. 
         [0038]    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. 
         [0039]    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). 
         [0040]    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. 
         [0041]    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. 
         [0042]      FIG. 7A  is a schematic longitudinal cross sectional view of and/or type of incandescent light bulb shown at  700  and having a micro/nanostructured film in accordance with the present invention.  FIG. 7B  is a cross sectional diagram of the bulb shown in  FIG. 7A , taken along the line VII-VII. As depicted in  FIGS. 7A-7B , the bulb  700  has a generally cylindrical shape and includes: a bulb envelope  702 , preferably formed of, but not limited to, quartz or glass, and forms a cylindrical cavity  712 ; a linear filament  704  positioned along the longitudinal axis of the cylindrical cavity  712 ; and power leads  710 . A solid film  708  is coated on the base of the bulb envelope  702 , while the rest of the envelope  702  is coated with a micro/nanostructured film  706  of the type described above to reflect infrared energy back to the filament  704  while passing visible light. 
         [0043]      FIG. 8  is a schematic cross sectional view of still another embodiment of incandescent light bulb shown at  800  and having a micro/nanostructured film in accordance with the present invention. The bulb  800  has a generally cylindrical shape and includes: a bulb envelope  801  comprised of a cylindrical portion  802 , an end cap  806 , and a base disk  810 ; a filament  812  located along the longitudinal axis of the envelope  802 ; a mounting base  804  for holding the bulb envelope  801 ; and power leads  814  extending through the disk  810  and connected to the filament  812 . The bulb envelope  801  may be formed of optically transparent materials, such as quartz or glass. The mounting base  804  may include a screw or bayonet type connector and have an inner surface with the reflective characteristics of the solid film  708  in  FIG. 7A . The end cap  806  and the cylindrical portion  802  are coated with a micro/nanostructured film  808  of the type described above to reflect infrared energy back to the filament  812  while passing through visible light. As a variation, the cap  806  may be coated with a solid film so that the no light is passed out of the end of the bulb. 
         [0044]      FIG. 9  is a schematic cross sectional view of yet another type of incandescent light bulb shown at  900  and having a micro/nanostructured film in accordance with the present invention. As depicted, the bulb  900  includes: a bulb envelope  901  having a spherical portion  902  internally coated with a micro/nanostructured film  904 , and a cylindrical portion  914 ; a base cap  916 ; a threaded mounting base or connector  910 ; power leads  912 ; and a filament  906  located at the center of the spherical portion  902 . The filament  906  may be a freestanding coil or a patterned, sputter-deposited layer on a ceramic substrate. The interior surface of the cylindrical portion  914  and base cap  916  may be coated with a solid film  908 . As a variation, part of the spherical portion  902  on the base side may be coated with a solid film, making the light bulb more or less unidirectional. 
         [0045]    As pointed out above, the micro/nanostructured film of the light bulbs in  FIGS. 6-9  may be a metallic thin film directly formed on the bulb envelope substrate with ˜300 nm diameter holes. Alternatively, the micro/nanostructured film may be a metallic thin film with 500-to-800 nm diameter holes and formed on top of a dielectric hot mirror stack that is deposited on the bulb envelope. In both cases, the film thickness is not critical as long as it is thicker than about 30 nm. In general, the former approach may require finer photolithographic detail than the second approach. 
         [0046]    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. 
         [0047]    As discussed above, the filaments used in light bulbs of the types shown in  FIGS. 6-9  may have various structures and be made of different materials. For example,  FIG. 10A  is a side view of an exemplary linear filament  1000  of the type used in the light bulbs of  FIGS. 7-8  and an enlarged view of a segment  1002  thereof. As depicted, the linear filament  1000  may be formed of a 20-30 micron diameter wire coiled into an elongated rope or helix to shorten overall filament length. The bulb wattage and operating voltage may determine the wire dimension. For instance, a 100-watt, 115-volt filament may require use of a 24 micron diameter wire that is 110 cm long. The elongated linear filament  1000  may alternatively be bent to a desired shape and used in the light bulbs  600  ( FIG. 6) and 900  ( FIG. 9 ). 
         [0048]      FIG. 10B  is a schematic side view of a compact helical filament  1010  suitable for use in the light bulbs of the types shown in  FIGS. 6 and 9 . The filament  1010  may be fabricated by bending a length of linear helical coil, such as depicted at  1000  in  FIG. 10A , and the overall dimension of the filament  1010  may be 1-10 mm, for instance. A segment  1012  of the filament  1010  may be similar to the segment  1002  in  FIG. 10A  except that the segment  1012  would be curved. The filament  1010  may be configured to have a high optical density such that most of the reflected infrared energy is focused thereon and thus absorbed thereby the portion of the reflected light passing through the filament is minimized. 
         [0049]    The filaments in  FIGS. 10A-10B  may be formed of tungsten, for instance, and designed to operate at a temperature of ˜3000° K. As tungsten at ˜3000° K has an emissivity of ˜0.4 in the near infrared wavelength range, only ˜40% of the infrared radiation returning to the filament will be reabsorbed. The remaining 60% will be reflected back towards the bulb envelope for another back-and-forth reflection cycle with additional energy absorption at the filament. To get high bulb efficiency, the number of reflections between leaving and returning to the filament needs to be minimized, and the absorption fraction at the reflecting surfaces, which collectively refers to the micro/nanostructured film and the solid film, needs to be minimized. Ideally, all infrared light leaving the filament should be returned by a single reflection from the reflecting surfaces with a reflection factor of at least 90%. 
         [0050]      FIG. 11  shows calculated re-absorption fractions for long wavelength radiation initially emitted by a filament as a function of film reflectivity, and the average number of photon reflections from the film covered surfaces before returning to the filament. More than 70% of the emitted thermal radiation can be reabsorbed by the filament if the film is at least 90% reflective. Thus, the use of reflecting surfaces having a reflectivity greater than 90% will enable &gt;70% of the input electrical power to be converted into visible light. This is about 7 times more efficient than a conventional incandescent bulb and twice as efficient as a fluorescent bulb. Gold and silver offer &gt;95% reflectivity from 800-to-5000 nm and thus are candidates for the reflecting surfaces. Other metallic materials, such as copper and aluminum, may also be used for the reflecting surfaces. 
         [0051]    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.  FIG. 12A  is a cross sectional view of a light bulb, shown at  1200 , that may be used to replace a conventional LED light source.  FIG. 12B  is a schematic transverse cross sectional view of the bulb  1200 , taken along the line XII-XII. As depicted, the bulb includes: a substrate  1202  having elongated channel or elongated cavity  1214  formed therein to provide an inner surface that is coated with a solid reflective film  1204 , and two end walls  1216 ; a filament  1206 ; a pair of power leads  1212 ,  1213  through which power to the filament  1206  is supplied; a cover plate  1208  formed of optically transparent material, such as quartz or glass; and a micro/nanostructured film  1210  coated on the inner surface of the cover plate  1208 . The filament  1206  may be linear and located at or near the focus of the parabolic reflector cavity formed by the solid film  1204  coated on the surface of the cavity  1214  in the substrate  1202 . The radiation reflected from the solid film  1204  will eventually strike the micro/nanostructured film  1210  at near normal incidence and thus allows visible light to pass through the apertures and infrared radiation to be reflected back to the filament  1206 . The internal cavity  1214  may be under vacuum to minimize conductive losses. As a variation, the inner surfaces of the end walls  1216  may also be coated with solid films. 
         [0052]      FIG. 13A  is a schematic cross sectional view of another embodiment of a light bulb in accordance with the present invention and which may be used to replace a conventional LED light source.  FIG. 13B  is a schematic transverse cross sectional view of the light bulb  1300 , taken along the line XIII-XIII. As depicted, the bulb  1300  includes: a substrate  1302  having an elongated channel or cavity  1320  formed therein to provide an inner surface that is coated with a pair of solid reflective films  1304 ; two end walls  1305 ; a filament  1310 ; a pair of filament support/power leads  1308  through which power to the filament  1310  is supplied and to which the solid films  1304  are respectively connected; a pair of power pads  1306  formed on the outer surfaces of the substrate  1302  and respectively connected to the solid films  1304 ; a cover  1314  formed of optically transparent material, such as quartz or glass; and a micro/nanostructured film  1312  coated on the inner surface of the cover  1314 . The filament  1310  has a linear shape and located at or near the focus of the parabolic reflector cavity formed by the film  1312  coated on the cover  1314 . Infrared light reflected by the micro/nanostructured film  1312  will strike the solid films  1304  at near normal incidence, and retrace its path back to the filament  1310  to be absorbed thereby. The internal cavity  1320  of the light bulb  1300  is under vacuum to minimize conductive losses. It is noted that the solid films  1304  functions as power conductors to the filament  1310 . A narrow gap  1316  electrically isolates the two solid films  1304  from each other and respectively coupled to the two ends of the filament  1310 . As a variation, the inner surfaces of the side walls  1305  may be coated with solid reflective films. 
         [0053]    The filaments  1206  ( FIG. 12A) and 1310  ( FIG. 13A ) are formed of coiled tungsten wire of the type shown in  FIG. 10A . Alternatively, the filaments  1206 ,  1310  might be planar filaments that are deposited and patterned on ceramic materials by using conventional semiconductor or MEMS processing techniques, such as batch fabrication technique. 
         [0054]      FIG. 14  shows an exemplary planar filament  1400  that can be used in the light bulbs of  FIGS. 12A-13B . The length (when stretched), width, and thickness of the filament  1400  may be 1.4 cm, 2 microns, and 2 microns. 2 micron wide traces, separated by 1 micron gaps, may yield a 600 micron long (L) by 70 micron wide (W) filament  1400 . It should be apparent to those of ordinary skill that the length and width of the filament  1400  may be changed depending on the wattage and voltage of the filament. 
         [0055]    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. 
         [0056]    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.  FIGS. 15A-15E  show exemplary steps followed in forming a micro/nanostructured film on a substrate by use of a lift-off patterning technique. As depicted in  FIG. 15A , a photoresist layer  1502  is first formed on a substrate  1500 , such as the cap  605  in  FIG. 6 , for instance. Then, a mask  1504  is arranged above the photoresist layer  1502  and radiation is projected through the transparent openings of the mask so that the pattern in the mask is transferred onto the photoresist layer  1502 , as shown in  FIG. 15B  and the exposed portions of the resist layer are hardened. Subsequently, as depicted in  FIG. 15C , the unexposed portion of the layer  1502  is selectively removed to reveal the surface of the substrate  1500 . Next, as shown in  FIG. 15D , a reflective layer  1510  is deposited on the exposed surfaces of the substrate  1500  and the resist layer  1502 . Finally, the remaining portions of the resist layer  1502  are lifted off to leave a patterned film  1512  on the substrate  1500 , as shown in  FIG. 15E . 
         [0057]      FIGS. 16A-16C  show exemplary steps followed in forming a micro/nanostructured film on a substrate by use of a nano-imprinting technique in accordance with the present invention. As depicted in  FIG. 16A , a layer  1602  may be formed on a substrate  1600 , wherein the layer is made of photoresist or other indentable material such as a polymer. Then, a previously prepared nanopatterned indenter  1604  is brought into in engagement with the substrate  1600  to transfer an intended pattern onto the layer  1602 , as shown in  FIGS. 16B and 16C . Subsequently, the steps previously described with respect to  FIGS. 15D and 15E  are conducted to form a micro/nanostructured film on the substrate  1600 . 
         [0058]      FIGS. 17A-17C  show exemplary steps for forming a micro/nanostructured film on a substrate in accordance with yet another embodiment of the present invention. As depicted in  FIG. 17A , a photoresist layer  1702  is first formed on a substrate  1700 . Then, a mask  1704  is arranged below the substrate  1700  and radiation  1708 , such as X-rays, is projected through the transparent openings of the mask  1708  and the substrate to transfer a pattern in the mask  1704  onto the photoresist layer  1702 , as shown in  FIG. 17B . Subsequently, as depicted in  FIG. 17C , the unexposed portions of the layer  1702  are selectively removed to reveal the surface of the substrate  1700 . Next, the steps illustrated in  FIGS. 15D-15E  are conducted to form a micro/nanostructured film on the upper surface of the substrate  1700 . It is noted that the mask  1704  may be applied to outside light bulbs. 
         [0059]      FIG. 18  is a schematic diagram of a thermo-photovoltaic (TPV) power generator shown at  1800  and having a micro/nanostructured film  1812  in accordance with the present invention. As depicted, the thermo-photovoltaic (TPV) power generator  1800  includes: a reflective cavity  1802  including a transparent window  1807  through which radiation passes; a heat source  1804  for generating heat energy; a selective thermal emitter (or, selective emitter)  1806  having micropatterned structures for emitting thermal radiation with black body emissivity at particular wavelengths as disclosed in U.S. Pat. No. 6,583,350; a dichroic cold mirror  1808  for reflecting short wavelength light back to the selective emitter  1806 ; a micro/nanostructure film  1812  for reflecting long wavelength light back to the selective emitter  1806 ; and low-bandgap photocells  1810  for heat-to-electricity conversion. The selective emitter  1806  is formed of rare-earth ceramics. 
         [0060]    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. 
         [0061]      FIG. 19  shows the emission spectrum of the selective emitter  1806  at 1800° K with a suitable bandpass created by 700-nm diameter apertures in a film  1812  and a 1200-nm cutoff dichroic cold mirror  1808 . The dichroic cold mirror  1808  operates as 1200-nm cutoff high pass filter, while the Pbw represents the wavelength range converted into electricity by the photocell  1810 . The micro/nanopatterned film  1812  reflects infrared radiation from 1600-to-5000 nm (and longer) back to the selective emitter  1806 . With this approach, thermal-to-electric conversion efficiencies &gt;40% are possible in the TPV power generator  1800 . As a variation, the TPV power generator  1800  may include a micro/nanostructured film deposited on the surface of the dichroic cold mirror  1808 . 
         [0062]    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. 
         [0063]    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. &amp; Tech., B 23 (6), November/December 2005, pp. 3158-3163, wherein the cross-like openings operate as a narrow bandpass filter. 
         [0064]    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.