Patent Publication Number: US-6670763-B2

Title: Display lamp with reflector having IR-reflective coating

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to display lamps. More particularly, it relates to low voltage display lamps having a gold-coated reflector to reduce heat radiation and transmittance. 
     Low voltage display lamps are known in the art. Low voltage display lamps for use in standard lamp sockets having line-voltage, such as, e.g., the well known MR16 lamps, comprise a reflector assembly that works in conjunction with a voltage converter such as a solid state electronic ballast. The ballast is contained within a lamp housing together with, disposed in close proximity to and directly behind the reflector assembly. Consequently, it is important to minimize radiant heat from the reflector assembly to the ballast in order to ensure proper operation and a long service life. 
     Current display lamp designs employ a flat circular heat shield or plate which is disposed behind the elliptical reflector of the reflector assembly and in front of the ballast. This heat shield serves to protect the ballast by reflecting infrared radiation (IR) generated by the filament and transmitted through the reflector, thereby reducing the ballast&#39;s operating temperature. However, a significant portion of the reflected IR is directed at the interior surface of the lamp housing. Consequently, the lamp housing, which is already subject to direct IR energy from the filament, now absorbs roughly twice the IR compared to that radiated directly from the filament to the housing. 
     The result is that the housing is more susceptible to melting from absorbed IR, and also that the absorbed IR will be conducted as heat through the housing material to the ballast, thereby raising the ballast operating temperature and shortening its service life. 
     Existing means for solving the problem of ballast heating include multi-layer coatings applied to the concave reflector surface that are designed to reflect IR instead of transmit it through the reflector toward the ballast. 
     However, such coatings are difficult to apply correctly and often are very expensive. Most such coatings involve applying a discrete IR-reflective coating layer separately from and beneath a visible light-reflective coating layer, thereby contributing an additional coating process. It has been further suggested that a broad-band dichroic coating that would reflect in both the visible and IR spectra could be used. However, such coatings would be difficult to apply correctly, and could adversely affect the lumen efficiency of the lamp. 
     There is a need in the art for a low voltage display lamp for use in standard line-voltage electric lamp sockets, comprising an effective IR-reflective coating that can be applied to the reflector, without adversely affecting the lumen efficiency or light-reflective characteristics of the lamp. Such a coating would effectively reflect IR away from the ballast, and from the lamp housing. Such a coating will effectively reduce the ballast operating temperature. 
     SUMMARY OF THE INVENTION 
     A low voltage display lamp is provided having a lamp housing, a reflector assembly, and a solid state electronic ballast. The reflector assembly has a light source therein, and is located within the lamp housing, with the ballast located behind the reflector assembly. The reflector assembly also has a reflector with a concave inner surface and a convex outer surface, and an IR-reflective layer is disposed on the convex outer surface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic side view of a low voltage display lamp having a flat circular heat shield characteristic of the prior art. 
     FIG. 2 is a partially schematic side view of a low voltage display lamp having an IR-reflective coating layer according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     In the description that follows, when a preferred range, such as 5 to 25 (or 5-25) is given, this means preferably at least 5, and separately and independently, preferably not more than 25. 
     As used herein, “MR16” means a low voltage display lamp as is generally known in the art, having a nominal diameter of two inches. 
     With reference to FIG. 1, pictured is a characteristic or conventional low voltage display lamp  10 . The lamp  10  comprises a solid state ballast  30  and a reflector assembly  50 , both contained within a lamp housing  40 . Lamp  10  further comprises socket coupling means (preferably threaded) for electrically coupling the electronic ballast  30  to a lamp socket (not shown). The ballast  30  is disposed in the throat  42  of the housing  40  directly behind the reflector assembly  50 . The reflector assembly  50  preferably comprises a curved reflector  12 , preferably ranging from substantially elliptical to substantially parabolic in shape, a filament or light source  16 , and a transparent cover plate  18 . The reflector  12  has a concave inner surface  13  and a convex outer surface  15 , and is preferably substantially parabolic in shape. A light-reflective coating layer (not shown) is coated onto concave surface  13 . The reflector  12  typically comprises a borosilicate glass material. The light source  16  is disposed within the reflector  12 , facing concave surface  13 . During operation, light source  16  of reflector assembly  50  is electrically coupled to ballast  30  via metal pins, wires, or some other known means (not shown). The reflector  12  terminates in a rim  11  forming the entire perimeter of the open end of the reflector  12 . 
     The lamp  10  may optionally comprise a nose or boss  14  formed integrally with and extending outwardly from the outer surface of the base  17  of the reflector  12 . The boss  14  preferably has a rectangular cross-section, though cross-sections of other shapes are possible and can be used. Preferably, the reflector  12  and the boss  14  are integrally formed from glass, preferably borosilicate glass. The lamp of FIG. 2 is of this same general construction. 
     With reference to FIG. 1, a conventional lamp  10  comprises a conventional or known heat shield  20 . The heat shield  20  is positioned between base  17  of reflector  12  and ballast  30  in order that the heat shield reflects IR transmitted through the reflector  12  away from the ballast  30 . As can be seen in FIG. 1, a heat shield  20  as described above reflects incident radiation  2 , and directs it as reflected radiation  4  toward a point  8  along the interior surface of the lamp housing  40 . In addition to the reflected radiation  4 , point  8  also receives direct radiation  6  from light source  16 . Hence the reflected radiation  4  effectively doubles or increases the absorbed IR load at point  8 , thereby significantly increasing the localized housing temperature around point  8 . It will be understood that such double or enhanced absorption is not a discretized effect around a single point  8  as portrayed in FIG.  1 . Discrete point  8  is pictured merely for illustration. This double absorption phenomenon occurs along the interior surface of housing  40 , thereby significantly increasing its temperature. 
     Increased housing temperature increases the danger of housing meltdown, requiring that housing materials having high softening or melting points must be used. In addition, absorbed IR is conducted as heat through the housing back to the throat portion  42  which encloses the ballast  30 . The conducted energy is then transferred to the ballast via conduction through the physical pathways between the ballast  30  and the housing  40 , and via radiation from the housing  40  to the ballast  30 . Additionally, thermal currents transfer thermal energy to the ballast via convection as known in the art. Thermal energy transferred to the ballast  30  via the above mechanisms raises the ballast&#39;s operating temperature thereby reducing its service life. 
     Now referring to FIG. 2, convex surface  15  of reflector  12  is coated with an IR-reflective layer  35  effective to reflect transmitted IR back through reflector  12  to exit lamp  10  through clear cover  18 . IR-reflective layer  35  is made from a material capable of withstanding operating temperatures in excess of 200, preferably 250, preferably 300, preferably 350, preferably 400, ° C., without tarnishing, becoming oxidized, or otherwise being affected in a manner adverse to its IR-reflectivity. IR-reflective layer  35  is or comprises preferably a gold, less preferably silver, less preferably aluminum, less preferably nickel, less preferably titanium, less preferably chromium layer, less preferably some other metal layer, less preferably a metal alloy layer, less preferably some other material known in the art. Preferably, the reflective layer  35  is 50-200, preferably 60-180, preferably 75-160, preferably 90-140, preferably 100-130, preferably 110-125, preferably about 120, nm thick. 
     Gold is most preferred because it is highly impervious to adverse temperature effects, and does not tarnish, melt, oxidize, or otherwise deform under operating temperatures up to and in excess of 400° C. In addition, gold exhibits a substantially flat reflectivity profile throughout the relevant IR spectrum (about 0.7-4.0μ wavelength), at about 99% reflectivity. (The glass in reflector  12  is essentially fully absorbent of IR radiation beyond 4.0 μ, transmitting none through to the reflective layer  35 ). When gold is used in reflective layer  35 , a base layer  36  is preferably deposited on convex surface  15  between convex surface  15  and reflective layer  35 , preferably by vacuum vapor deposition. Base layer  36  is as thin as possible to effectively serve its adhesive purpose. Base layer  36  is preferably less than 20, more preferably 16, more preferably 12, more preferably 10, more preferably 8, more preferably 6, more preferably 5, more preferably 4, nm thick. Base layer  36  is most preferably pure titanium or titanium, less preferably chromium, less preferably any other material (preferably metallic) having good adhesion to both surface  15  and the gold reflective layer. 
     It should be noted that gold can be deposited directly onto a glass surface. However gold exhibits very poor adhesion to glass, and thus immediately flakes off upon even the slightest contact. Nevertheless, because the gold layer in the finished lamp  10  is totally enclosed, it is possible to provide a gold reflective layer according to the present invention without a base layer  36 , so long as the lamp is manufactured in such a way as to ensure no contact with the gold-deposited convex surface of reflector  12  once the gold has been deposited thereon. It is probable that such a manufacturing process would introduce excessive cost and would be quite cumbersome; accordingly it is preferable to provide the base layer  36  when a gold layer is used. 
     In a less preferred embodiment, use of some materials other than gold in reflective layer  35 , for example silver or aluminum, will obviate the need for base layer  36  because such materials are sufficiently adherent to glass (borosilicate glass) to effectively adhere directly to convex surface  15  of reflector  12 . Though silver has a substantially uniform reflectivity profile in the IR-spectrum, and similarly to gold is further about 99% reflective of IR radiation, silver suffers from the limitation that it tarnishes easily via oxidation at high temperature. Thus, when silver is used in reflective layer  35 , the silver layer should be sufficiently thick such that tarnish cannot penetrate through the silver layer to the silver surface immediately adjacent convex surface  15 . Alternatively, when silver is used in reflective layer  35 , a protective coating layer, e.g. silica, can be deposited over the silver reflective layer to prevent silver tarnishing or oxidation. Providing such a thick silver layer will yield a silver reflective surface adjacent convex surface  15  that is substantially unaffected by tarnish from the opposite side of the silver layer. Thus reflective layer  35  may be disposed on convex outer surface  15  with or without the presence of base layer  36 . 
     In addition to preventing direct IR radiation to ballast  30 , and to preventing reflected IR from being directed toward housing  40  (see reference numeral  4  in FIG.  1 ), the reflective layer  35  also substantially prevents direct radiation to housing  40  from light source  16  (see reference numeral  6  in FIG.  1 ). As can be seen in FIG. 2, incident radiation  2  is directed forward through reflector  12  as reflected radiation  9 , to exit the lamp. The transparent cover  18  transmits nearly 100% of the reflected IR, absorbing almost none. Consequently, the reflected IR substantially escapes the lamp, and therefore is not absorbed by the lamp housing  40  to raise its temperature. Optionally, a heat shield  20  can be disposed between reflector  12  and ballast  30  as shown in FIG.  1 . 
     It is believed that invented reflective layer  35  will decrease the ballast temperature by 5-10° C. Current MR16 display lamps operate in the range of 20-71 watts (W). The higher the wattage, the greater the light output of the lamp. Ballasts used in conjunction, and in close proximity, with 20 W MR16 lamps operate near threshold temperature due to the transfer of heat from the light source  16  to the ballast  30  via the various mechanisms described above. The invented reflective layer  35  allows a ballast to be incorporated into a housing in close proximity with a higher wattage MR16 lamp, (e.g. at least or about 35 W, 45 W, 55 W, 65 W, or 71 W), and to operate sufficiently below threshold temperature to ensure long life, preferably rated at more than 3000, preferably 3500, preferably 4000, preferably 4500, preferably 5000, hours. 
     Though the above-described preferred embodiment has been described with regard to an MR16 lamp, it will be understood that the invention could be applied to display lamps of different shapes and sizes without departing from the scope of the invention. For example, the invented reflective layer  35  can be utilized in MR8, MR11, MR20, MR30, MR38, PAR16, PAR20, PAR30, and PAR38 display lamps, as well as any other reflector lamp known in the art, and would be similarly provided and comprised as described above. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.