Abstract:
A light having a shielded fixture housing which comprises a tapered reflector a halogen lamp at the narrowed end, a tempered glass shield at the light-emitting end at a selected distance from the halogen bulb, and vents disposed about the fixture to provide an airflow therethrough. Embodiments according to the present invention pass the Underwriters Laboratory “Cheesecloth Test” and provide a light with significantly reduced fire hazard. Further improvements include specifically disposed temperature sensors to interrupt the power to the lamp if the light is accidentally or intentionally misused.

Description:
FIELD OF THE INVENTION 
     The present invention relates to light assemblies shielded to protect the surrounding material, in particular, to torchiere lights having a thermal shield to reduce the surface temperature accessible by surrounding materials. 
     BACKGROUND OF THE INVENTION 
     Within the last decade, open-top, upward facing “torchiere” style floor lights have become an increasingly popular source of lighting. However, the typical 300 watt torchiere light of this era incorporates a halogen lamp having a bulb external surface temperature of 794 K which presents heat energy sufficient to ignite many materials commonly found in the light operating environment. In fact, the lights are reported to be the source of many fires, leading institutions, e.g. colleges, to ban the lights outright. 
     Prior attempts to limit the problem have lead some manufacturers to install a wire mesh or glass covers in the region just above the bulb in models sometime advertised as “state of the art.” Such wire mesh or glass covers still provide an access to the hot bulb or themselves are above a temperature which can enflame some materials. In order to objectively evaluate the fire hazard problem, Underwriters Laboratory (UL) has proposed a “Cheesecloth” test, wherein an acceptable light must complete seven hours of continuous operation without burning or igniting a piece of cheesecloth placed on top (in the direction of the lights emission) of the light. The wire mesh or glass covers do not significantly reduce the fire hazard, and apparently offer protection to the bulb more than to the surrounding. 
     SUMMARY OF THE INVENTION 
     The present invention provides a light having a high temperature bulb in a shielded fixture housing which provides significantly reduced surface, radiation and convection temperatures to remove the fire hazard presented to the surrounding material which may be near or in contact with a surface of the light. The fixture housing includes the halogen bulb within a ventilated, tapered reflector. The bulb is placed at the narrowed end of the tapered reflector and a planar glass shield is placed at the wide end, covering substantially all of the opening thereof. The fixture housing typically includes an annular screen about the periphery of the glass shield and a vent below the bulb to provide an airflow through the fixture and reduced internal and surface temperatures. 
     Further improvements include electrical temperature sensors disposed at the glass shield to interrupt the power to the lamp if temperatures at the shield exceeded a safe temperature, thus offering added protection in the event of intentional or accidental misuse causing blockage of the flow of air through the vents and/or screen. 
     Thus, embodiments according to the present invention are expected to pass the “cheesecloth” test and to offer significantly reduced surface operating temperatures to provide a safe, reliable halogen light. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and further features of the present invention will be better understood by reading the following Detailed Description together with the Drawing, wherein 
     FIG. 1 is a elevational cutaway view of one embodiment of the present invention; 
     FIG. 2 is an electrical schematic diagram of the embodiment of FIG. 1; 
     FIG. 3 is an elevational, partial cutaway view of an alternate embodiment of the present invention; and 
     FIG. 4 is an elevational, partial cutaway view of a further alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiment  50  of FIG. 1 provides a tapered reflector  52  having an opening at the narrower end of the reflector  52  to receive a lamp  54  and lamp mounting assembly  56  with peripheral spacing  58  to permit airflow thereabout. A cylindrical extension  60  is joined to the narrower end of the reflector  52  and includes a lamp reflector  62  also disposed with peripheral spacing within the cylindrical extension  60  to permit airflow thereabout. The open end of the cylindrical extension  60  receives a screened or perforated metal vents  64  which permits airflow therethrough, but inhibits flammable materials from inadvertently contacting the lamp reflector  62  or the lamp  54  bulb surface. 
     A tempered glass shield  70  is spaced above the lamp  54  bulb for a 300 watt lamp, in the present embodiment to permit the exterior surface temperature to be no greater than a selected safe temperature. The larger (upper) opening of the tapered reflector  52  is typically circular in shape, as is the glass shield  70 . In the embodiment  50  of FIG. 1, the glass shield  70  is generally centered about the opening and disposed above the lamp  54  bulb, and extends substantially, but not entirely to cover the opening of the tapered reflector. The remaining area between the larger opening of the tapered reflector  52  and the shield  70  is covered by a wire screen  72  or equivalent to prevent flammable material from being introduced into the interior region of the light fixture, yet still allow airflow therethrough. 
     The distance  74  between the lamp  54  bulb exterior surface and the shield  70  is important, and is selected according to the wattage of the lamp  54 . Generally, the larger wattage ratings, the greater distance  74 . More specifically in the setting of the torchiere light, it has been determined that as distances decrease from 3 cm, the temperature rises approximately exponentially until substantially equal to the bulb surface temperature (about 794 K for 300 watt halogen). For greater distances greater than 3 cm (and a substantially constant lamp  54  bulb diameter), an approximation of a linear temperature falloff may be made. Therefore, according to one embodiment of the present invention, the above temperature/distance relationship is set in a form to provide the preferred (minimum) bulb-to-shield distance  72  (D) according to the following relationship. 
     
       
         D≅{[(Watts)(1.65 K/W)+Room Temp](0.54)−Safe Temp}/(17° K/cm)+3 cm 
       
     
     where the Watts is the rated lamp wattage, the Room Temp is the ambient room temperature generally taken to be 300 K, the 0.54 term being empirically determined, and the Safe Temp being a temperature selected to provide the maximum permitted for the desired flammability safety margin, taken here to be 373 K. Accordingly, for a 300 watt lamp, a minimum distance D is about 6.3 cm. 
     A prototype according to the embodiment  50  of FIG.  1 . provides a distance  74  of 8 cm with a 300 watt lamp  54  in a reflector  52  having a larger opening diameter of 16.5 inches with a 13 inch diameter, 0.125 inch thick tempered glass shield  70  mounted slightly below (0.125 inch) the upper edge of the reflector  52 , and a metal screen mesh in the remaining 1.75 inch region between the shield  70  and the reflector  52  opening. Additionally, the narrower opening of the prototype reflector  52  is 7.5 inches with the bulb mounting assembly  56  and lamp reflector  62  having about a 0.5 inch air gap within the cylindrical extension  60 . Similarly, the lamp  54  bulb surface is spaced about 0.5 inch from the lamp reflector  62 , and the cylindrical extension is about 2.25 inches in length beyond its union with the reflector  52 . The vents  64 , mounted on the lower (distal to the reflector  52 ) end of the cylindrical extension  60  comprise a metal sheet having about 16-0.125 inch holes per inch. The cylindrical extension is longer when the fixture  50  is inverted (pointed downward) to accommodate greater influx of heated air. Additionally, the extension may comprise different (non-cylindrical) configurations to accommodate esthetics considerations as long as adequate internal air spacing are maintained. 
     The light fixture embodiment  50  is mounted to a hollow tubular pole  76  through which electrical wiring (not shown) is routed, and is held in a vertical floor position with the aid of a base  78  weighted with ballast to maintain stable vertical orientation of the assembled light. 
     Also according to the present invention, one or more temperature sensors  80  are disposed substantially at the interior surface of the shield  70 . The temperature sensors typically comprise normally-closed bi-metallic switches which open-circuit at a temperature (e.g. the Safe Temp in one embodiment) selected to result in an undesirable temperature on the exterior surface of the shield. The temperature sensors are preferably disposed on shield mounting members, typically metal brackets  82  which mount the shield  70  to the reflector  52  at three equally spaced places according to the embodiment shown. The brackets  82  may also comprise other means to captivate the shield in a spaced relationship to the reflector  72  as taught. The temperature sensors thus placed most advantageously monitor the ambient temperature at the shield near the air flow through the screen  72  while minimally obscuring the light output. 
     One or more temperature sensors may be mounted as shown in the embodiment  50  FIG.  1  and serially connected  80 A as shown in FIG. 2 with the lamp  54 A and the power switch  84  so that a temperature exceeding the activation temperature of the bimetallic elements (or equivalent) of the temperature sensors will cause the power to the lamp  54 A to be interrupted. 
     An alternate embodiment  90  of the present invention is shown in FIG. 3, wherein a tapered reflector  52 A comprises a larger tubular end portion  53  having a plurality of louvers  73  or other apertures which provide the openings for airflow. In this embodiment, the shield  70  extends to the tubular end portion. 
     A further alternate embodiment  100  is shown in FIG. 4 showing a variety of different constituent elements which are used together in the embodiment  100  of FIG. 4, or may be individually in place of corresponding elements in the other embodiments according to one skilled in the art. The reflector  52 B exists without extension  60 , having instead integrally formed vents  102  which provide the openings into which air flows. The lamp  54  and mounting assembly  56 A is retained to the reflector  52 B on the lamp reflector  62 A which has apertures therein to permit the desired airflow. An ultraviolet light shield  104  is included between the lamp  54  and the shield  70 A. The shield  70 A itself includes apertures therein to provide an exit for the airflow. 
     The present invention may be scaled, combined or modified to accommodate differing bulb dimensions and lamp wattage ratings according to the teaching herein. Also, the reflector  52  need not be a highly reflective material. Further modifications and substitutions according to one skilled in the art are within the scope of the present invention which is not limited except by the claims which follow.