Abstract:
A lighting fixture comprising for use in an environment which requires protection from EMI/RFI emissions. The fixture comprises a body forming a light-emitting opening, lighting components including at least one lamp in the body, a lens covering the opening, and a conductive grid across the opening, the grid being separate from the lens, electrically connected to the body, and positioned between the lighting components and the opening. The grid is a conductive screen which covers the entire opening between the component area of the fixture and the light-emitting opening area of the fixture, thereby encasing the lamps in metal which prevents EMI and RFI from escaping outside the fixture.

Description:
RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/786,804 filed on Mar. 28, 2006, the contents of which is incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention is related generally to interior luminaires and more particularly to lighting fixtures which prevent unwanted EMI/RFI emissions from radiating from the lighting fixtures, most particularly luminaires used in hospital operating rooms.  
       BACKGROUND OF THE INVENTION  
       [0003]     Many ceiling-mounted fluorescent luminaires used in locations such as hospital surgical suites or research laboratories require shielding to protect the location from electromagnetic interference and radio frequency interference (EMI/RFI). This is generally accomplished using a combination of metal housings and filters. When higher levels of protection are necessary, a metallic paint layer is silk-screened onto the smooth inside surface of the lens of the fixture.  
         [0004]     The metallic paint layer is then electrically connected to the metal fixture housing of the light. The goal of using a metallic paint on the lens of a metallic lighting fixture is to encase all of the electrical components of the lighting fixture in a metallic enclosure, thereby preventing EMI and RFI from escaping into the environment outside of the fixture. Such an enclosure is known as a Faraday cage. Since the primary use of lighting fixtures is to provide light, light-emitting openings which allow light to pass are necessary, and something other than a solid metallic surface is required. A very thin layer of metallic paint has been used to create the conductive enclosure. The present invention utilizes a metallic grid to create a more effective Faraday cage and a more durable and robust fixture.  
         [0005]     Electromagnetic waves do not penetrate very well through holes that are less than about a wavelength across. Therefore, it is possible to prevent the escape of the EMI/RFI radiation generated by the electrical components within a lighting fixture by ensuring that the openings (areas without a conducting surface) are sized less than some fraction of the shortest wavelength of being generated within the fixture—and the smaller the opening, the more effective it is at blocking the penetration.  
         [0006]     The basic physical relationship is frequency f=c/λ, where frequency is in cycles per second, c=speed of light, and λ=wavelength, all in a consistent set of units. The speed of light c is approximately 3×10 10  centimeters per second (cm/sec). Therefore, with a metallic grid which has openings on the order of one centimeter (cm) across, electromagnetic radiation having a frequency of 3×10 10  cycles per second (300 GHz) will be blocked to some degree, and electromagnetic radiation at a fraction of this frequency will be more effectively blocked from penetrating a metallic grid.  
         [0007]     The shielding effectiveness of a metallic grid also depends on the electrical properties of the metallic grid such as the conductivity of the grid material and the gauge of the grid elements. A grid made from heavier gauge material will be a better conductor than one made with thinner material and thus a more effective shield.  
         [0008]     Various lighting fixtures have been developed to include an enclosure around the lamps to prevent electrical interference. Examples of such prior art fixtures are those disclosed in the following United States patents: U.S. Pat. No. 3,564,234 (Phlieger), U.S. Pat. No. 5,195,822 (Takahashi, et al.), U.S. Pat. No. 6,297,583 (Kohne, et al.), U.S. Pat. No. 6,153,982 (Reiners), U.S. Pat. No. 5,702,179 (Sidwell et al.), U.S. Pat. No. 5,882,108 (Frazier), and U.S. Pat. No. 5,902,035 (Mui).  
         [0009]     Some lighting fixtures in the prior art having an EMI/RFI shield have a number of shortcomings. Lighting fixtures having an EMI/RFI shield that consists of a thin, silk-screened layer of conductive paint on the fixture lens lack the durability often required in various applications. The thin metallic layer is fragile and easily damaged, both during manufacturing as well as in service. The uniformity of layer thickness is also a problem, causing inconsistent resistance readings across the conductive layer and less effective shielding and uneven optical performance. Damage due to unwanted contact with the layer and inconsistent layer thickness during application result in diminished shielding performance and higher cost.  
         [0010]     The use of electronic dimming ballasts in such lighting fixtures introduces a more severe shielding requirement because of the frequencies of the EMI/RFI which are produced by such ballasts. However, the use of dimming ballasts is desirable in many applications, particularly in hospital operating room environments. The shielding achievable with silk-screened conductive paint applied to the fixture lens is inadequate to deal with such severe shielding demands.  
         [0011]     When using a lighting fixture in a medical setting, it is particularly important that the fixture be durable and able to be cleaned. Lighting fixtures with EMI/RFI shield are routinely used in hospital surgical suites or research laboratories, and given the sterile atmosphere that accompanies these locations, the lighting fixtures are routinely sanitized. Therefore, it would be desirable to have a lighting fixture which is both robust and easy to clean. Such fixtures must be strong enough to withstand numerous and frequent cleanings and also must allow easy access for cleaning. Further, in order to be easily cleaned, the outer surfaces of the fixtures should be configured to avoid the collection and trapping of dirt and permit the entire outer surface to be cleaned effectively. Thus, for these several reasons, it is desirable to eliminate the silk-screened shielding layer for lighting fixtures requiring EMI/RFI shielding.  
         [0012]     In EMI/RFI shielded lighting fixtures, it is desirable that the components of the fixture, other than the shield across the light-emitting opening, also complete an effective Faraday cage in order to shield the environment from EMI/RFI radiation. In applications such as the medical applications mentioned above, the remaining parts of the fixture must withstand the same frequent cleanings and not impede effective cleaning of the fixture. Thus, it would be desirable that such a fixture have smooth sealed outer elements to ensure ease and effectiveness of cleaning and to ensure that the conductive elements which comprise the Faraday cage are adequately connected electrically for shielding effectiveness. It is also desirable that the light emitted through the lens be a large percentage of the light produced by the lamps in the lighting fixture.  
         [0013]     In summary, there are a number of problems and shortcomings in prior lighting fixtures with an EMI/RFI shield.  
       OBJECTS OF THE INVENTION  
       [0014]     It is an object of this invention to provide a shield for lighting fixtures that includes increased EMI/RFI protection while overcoming some of the problems and shortcomings associated with the prior art.  
         [0015]     Another object is to provide an EMI/RFI shield for lighting fixtures which provides effective EMI/RFI shielding when an electronic dimming ballast is incorporated in the fixture.  
         [0016]     Another object is to provide an EMI/RFI shield for lighting fixtures which meets and exceeds the formal standards for radiated emissions provided by the U.S. military.  
         [0017]     Another object is to provide an EMI/RFI shield for lighting fixtures which eliminates the silk-screen process.  
         [0018]     Another object is to provide an EMI/RFI shield for lighting fixtures which is durable when handled or routinely cleaned.  
         [0019]     These and other objects of the invention will be apparent from the following descriptions and from the drawings.  
       SUMMARY OF THE INVENTION  
       [0020]     This invention is a lighting fixture which prevents unwanted EMI/RFI emissions from radiating from the lighting fixture. The lighting fixture comprises a body forming a light-emitting opening, lighting components including at least one lamp in the body, and a lens covering the opening. The lighting fixture also includes a conductive grid across the opening. The grid is separate from the lens, electrically connected to the body, and positioned between the lighting components and the opening.  
         [0021]     In certain desirable embodiments, the conductive grid is a substantially planar screen and is made of stainless steel. In preferred embodiments, the grid substantially covers the opening. In some embodiments, the grid is electrically connected to the body with a plurality of conductive hold-downs, and the body and grid are electrically grounded. Preferably, the lighting fixture includes an electronic dimming ballast disposed within the body.  
         [0022]     In preferred embodiments, the lens is translucent and includes two layers, a refractive inner layer and a transparent outer layer. Preferably, the grid is spaced from the lens sufficiently to diffuse the image of the grid through the lens.  
         [0023]     In the invention, it is highly desirable to have a seal between the lens and the body. It is also desirable that the body includes a lens frame which has the light-emitting opening, a frame seal between the lens frame and the body, and a lens seal between the lens and the lens frame. Preferably, the lens frame is electrically connected to the body.  
         [0024]     In certain preferred embodiments, the perimeter of the grid is on a grid shelf secured to the body. It is desirable that the grid include a conductive grid frame attached to the perimeter of the grid.  
         [0025]     In some preferred embodiments, the lighting components include at least one reflector and the at least one lamp is associated with the at least one reflector.  
         [0026]     In highly-preferred embodiments, the at least one reflector is a plurality of reflectors and at least some of the reflectors have associated lamps. Preferably, the reflectors are positioned to direct a first portion of light centered around a first direction and a second portion of light centered around a second direction. In highly preferred embodiments, the first direction is in a downward-outward direction.  
         [0027]     The term “translucent” as used herein refers to permitting light to pass through but diffusing or refracting the light such that objects on the opposite side are not clearly visible, thereby causing sufficient loss of image clarity to prevent the perception of distinct images.  
         [0028]     The term “opening” as used herein refers to the space in the lighting fixture through which the light travels from the lighting components to the room.  
         [0029]     The term “hold-downs” as used herein refers to a wide variety of fasteners, including but not limited to a clip or a swing-out tab. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]     The drawings illustrate a preferred embodiment including the above-noted characteristics and features of the invention. The invention will be readily understood from the descriptions and drawings. In the drawings:  
         [0031]      FIG. 1  is a perspective view of the lighting fixture with the lens in place.  
         [0032]      FIG. 2  is a perspective view of the lighting fixture of  FIG. 1 , with the lens removed to illustrate the grid within the fixture.  
         [0033]      FIG. 3  is cutaway view of the lighting fixture of  FIG. 1 .  
         [0034]      FIG. 4  is a cross-sectional view of the of the lighting fixture of  FIG. 1 , illustrating certain internal details.  
         [0035]      FIG. 5  is an enhanced view of a portion the cross-sectional view of  FIG. 4 .  
         [0036]      FIG. 6A  presents test results of radiated emissions from the lighting fixture of  FIG. 1 .  FIG. 6A  shows a comparison with the limit established by MIL-STD-461E RE102.  
         [0037]      FIG. 6B  presents test results of radiated emissions from a lighting fixture using a silk-screen shield of the prior art.  FIG. 6B  shows a comparison with the limit established by MIL-STD-461E RE102.  
         [0038]      FIG. 7  is a schematic cross-section illustrating the illumination pattern of the fixture of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0039]      FIGS. 1-5  illustrate an embodiment of a lighting fixture  10  having an EMI/RFI shield whereby electromagnetic interference is substantially prevented from radiating from lighting fixture  10 . As shown in  FIG. 1 , lighting fixture  10  has a body  12  which forms a light-emitting opening  14 . Body  10  also has a number of lighting components which include several reflectors  16  and  16   a  and several associated lamps  18  and  18   a  as illustrated in  FIG. 3 . In  FIG. 3 , four reflectors  16  and associated lamps  18  are included, and one reflector  16  and two associated lamps  18   a  are also installed in lighting fixture  10 . As shown in  FIGS. 1 and 4 , body  12  includes a lens  20  which covers light-emitting opening  14 . A conductive grid  22  is positioned across light-emitting opening  14 . Grid  22  is separate from lens  20  and is electrically connected to body  12  as illustrated in  FIG. 4 .  
         [0040]      FIG. 1  illustrates body  12  of lighting fixture  10  along with light-emitting opening  14  and lens  20  fixed in its corresponding lens frame  38 . Lens frame  38  is positioned around and encompasses the perimeter of lens  20  as shown in  FIG. 1 .  
         [0041]      FIG. 2  illustrates lighting fixture  10  in the same orientation as in  FIG. 1  except that lens  20  has been removed and grid  22  is visible. As shown in  FIG. 2 , grid  22  extends across light-emitting opening  14 . Reflectors  16  and  16   a  are positioned beneath grid  22  and are partially visible in  FIG. 2 .  FIGS. 1 and 2  show a number of brackets  32  which are attached to body  12  of lighting fixture  10  so that lighting fixture  10  can be mounted in a ceiling channel (not shown). Lighting fixture  10  can be mounted in various orientations in a ceiling channel.  
         [0042]     As illustrated in  FIG. 2 , conductive grid  22  is a substantially planar screen preferably made of stainless steel. Grid  22  is electrically and mechanically connected to body  12  with several swing-out tabs  36  (hold-downs) as shown in  FIG. 5 . Body  12  and grid  22  are electrically grounded through the ground connection of the electrical service (not shown) to fixture  10 .  
         [0043]      FIG. 3  is a cutaway view of the lighting fixture  10  of  FIG. 1 .  FIG. 3  illustrates how reflectors  16  and  16   a  and associated lamps  18  and  18   a  are positioned beneath grid  22 .  FIG. 3  shows four such reflectors  16  and one reflector  16   a  and their associated lamps  18  and  18   a.  As shown in  FIGS. 3 and 4 , each reflector  16  and  16   a  is positioned to direct light through grid  22  and through light-emitting opening  14  to create and an illumination pattern  42  as illustrated in  FIG. 7 .  
         [0044]     A wide variety of illumination patterns are possible depending on the shape and position of reflectors  16  and  16   a  and lamps  18  and  18   a.  As illustrated in  FIG. 7 , illumination pattern  42  is represented by the dual-lobe shape labeled with reference number  42 . The distance from fixture  10  to any point along pattern  42  generally represents the amount of light being emitted from lighting fixture  10  along the corresponding direction from lighting fixture  10  to the point on pattern  42  as shown in  FIG. 7 . In this embodiment, reflectors  16  and  16   a  are generally directing light in two directions  44   d  and  46   d,  a first portion  44   p  of light centered around a first direction  44   d  and a second portion  46   p  of light centered around a second direction  46   d  as illustrated in  FIG. 7 . As shown in  FIG. 7 , first direction  44   d  is generally downward, and second direction  46   d  is generally downward and outward. Light from lamps  18   a  and reflectors  16   a  primarily comprises the light in first portion  44   p,  and light from lamps  18  and reflectors  16  primarily comprises the light in second portion  44   p  as illustrated in  FIG. 7 . For example, such a dual-lobed illumination pattern is useful for illuminating a work area (not shown) and the neighboring or surrounding walls (not shown). Numerous other useful and practical illumination patterns are possible.  
         [0045]      FIG. 4  is a cross-section of lighting fixture  10 , providing an additional view of this embodiment of inventive lighting fixture  10 .  FIG. 4  illustrates reflectors  16  and  16   a  positioned beneath grid  22 . Grid  22  is positioned in between reflectors  16 - 16   a  and lens  20 .  FIG. 4  also shows that lens  20  includes two layers, a refractive inner layer  26  and a transparent outer layer  28 . As illustrated in  FIG. 4 , the transparent outer layer  28  is clear, providing a smooth outer surface to enable effective cleaning. Refractive inner layer  26  diffuses the light passing through opening  14  as well as diffuses the image of grid  22  as viewed from outside opening  14 ; grid  22  is spaced from lens  20  by a distance sufficient to diffuse the image of grid  22  as viewed through lens  20  as shown in  FIG. 4 . Lens  20  can also be a single layer with a refractive inner surface and a smooth transparent outer surface.  
         [0046]      FIG. 4  also illustrates a grid shelf  30  on which and to which grid  22  is secured. Shelf  30  can be made of the same conductive material as body  12  and is electrically connected through the fabrication process of body  12  such as by welding (not shown).  FIG. 4  also shows an electronic dimming ballast  34  which is housed in body  12  of lighting fixture  10 . Ballast  34  is used to control lamps  18  and  18   a  and is one of a variety of ballasts available for use as illustrated in  FIG. 4 .  
         [0047]     As shown in  FIG. 4 , body  12  includes a lens frame  38  into which light-emitting opening  14  is incorporated.  FIG. 4  also shows a frame seal  48  between lens frame  38  and body  12 , and a lens seal  50  between the lens  20  and lens frame  38 . Preferably, lens frame  38  is electrically connected to body  12 . Seals  48  and  50  serve to maintain the enclosure integrity of fixture  10 , thereby enabling it to withstand frequent cleanings, including exposure to liquids.  
         [0048]     As shown in  FIG. 4 , the body  12  has a plurality of adjustable brackets  32  adapted for mounting the body  12  into at least one ceiling channel. Adjustable brackets  32  are operative to swing into a position of engagement with the ceiling channel (not shown), thereby mounting fixture  10  as illustrated in  FIGS. 1-5 .  
         [0049]      FIG. 5  is an enhanced view of a portion of  FIG. 4  which more clearly illustrates several elements of fixture  10 . Grid  22  includes a grid frame  22   f  around the perimeter of grid  22 .  FIG. 5  also shows a set of hold-downs  36  which are used to secure grid  22  to body  12  and to provide a good electrical connection between grid  22  and body  12 . Hold-downs  36  can be selected from a variety of fasteners, including but not limited to clips or swing-out tabs. Hold-downs  36  shown in  FIG. 5  are swing-out tabs secured to body  12  with threaded fasteners  36   f.    
         [0050]      FIGS. 6A and 6B  illustrate the shielding performance of the embodiment of fixture  10  ( FIG. 6A ) compared to limits set by MIL-STD-461E RE102 and the shielding performance of a prior art fixture utilizing a silk-screened conductive paint EMI/RFI shield ( FIG. 6B ). In fixture,  10 , grid  22  is constructed of  304  stainless steel wire 0.022″ diameter and welded in a square pattern 0.478″ on centers. Grid  22  includes grid frame  22   f  and is configured to be is 19″ by 48″. In the fixture for which  FIG. 6B  shows radiated emissions, the silk-screened layer has a thickness of about 0.002″, and the overall fixture is otherwise of similar size and the general configuration of fixture  10 . The frequency data of the two graphs in  FIGS. 6A and 6B  range from 2 to 30 MHz. Although radiated emissions testing is done over a much larger range (up to 1 GHz),  FIGS. 6A and 6B  illustrate the test results for the frequencies of greatest interest and importance for such a fixture, ranging between 2 and 30 MHZ due to the source frequencies from electronic dimming ballast  34  incorporated into fixture  10 .  
         [0051]     In  FIG. 6A , the radiated emission data  60  is generally well below 30 dB μV/m (microvolts per meter) with only one small region above this level around a frequency of 24 MHz. All of the data in this frequency range is below the approximately 44 dB μV/m limit illustrated by reference number  62  established by MIL-STD-461E RE102. In  FIG. 6B , the radiated emission data  64  is generally higher across the frequency range when compared to plot  60  of  FIG. 6A , and the data in the region of about 25 MHz and above is above standard  62 .  
         [0052]     A wide variety of materials are available for the various parts discussed and illustrated herein. While the principles of this invention have been described in connection with specific embodiments, it should be understood clearly that these descriptions are made only by way of example and are not intended to limit the scope of the invention.