Patent Publication Number: US-6218776-B1

Title: Enhanced brightness of flat fluorescent lamp

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention generally relates to flat fluorescent lamps and, more particularly, enhancing brightness of flat fluorescent lamps. 
     2. Background 
     It is often desirable to have a flat fluorescent lamp where brightness can be enhanced by tuning light into a smaller, yet more intense, viewable cone. Such a lamp is useful in many applications, such as displays in avionics applications and for automatic teller machine displays. 
     Typical flat fluorescent lamps, for example, U.S. Pat. No. 5,343,116, issued Aug. 30, 1994, to Winsor, are constructed having a substrate frit to a transparent cover lid, forming an enclosure. Diffuse channels are milled into the substrate such that they are located in the interior of the enclosure. Standard phosphors are added to the interior of the enclosure just before it is flushed with a material for emitting energy, such as argon or mercury. Energy is emitted in the form of visible light when an electric potential is introduced to the lamp. 
     In the past, attempts have been made to enhance flat fluorescent lamp brightness through use of reflective materials for redirecting light. U.S. Pat. No. 5,818,164, issued Oct. 6, 1998, to Winsor, discloses application of a reflective coating to the substrate exterior to reflect light back into the lamp to increase output. U.S. Pat. No. 5,479,069, issued Dec. 26, 1996, to Winsor, discloses application of reflective materials to the interior portion of the lamp to redirect light that would have been lost through rear emission. The result of applying reflective materials in this manner is to effect the forward emission of substantially all light produced by the lamp. While the addition of reflective material has enhanced the lamp efficacy, in that more light is emitted from the lamp without requiring an increase in power, these designs result in wasted light because the light is emitted in a viewable cone that exceeds the range of viewing angles required by the particular application. Therefore, even though use of reflective materials can enhance flat fluorescent lamp efficacy, a significant portion of the emitted light is wasted because it falls outside the desired viewing range. Further, the addition of reflective materials can add complexity and cost to the manufacturing process. 
     Attempts have also been made to enhance flat fluorescent lamp brightness by altering the shape of the diffuse channel to redirect light. For example, the flat fluorescent lamp described in U.S. Pat. No. 5,479,069 has a diffuse channel that has a symmetrical cross-section with the upper portion of the channel walls tapering outward from the diffuse channel cavity. While this type of configuration acts to enhance efficacy by directing light forward, it exits the cover lid in a viewable cone of light that is greater than the desired range of viewing angles for the application for which the lamp is designed. Therefore, even though more light is exiting the lamp, light that falls outside the desired viewing range is wasted because the outward-tapered, symmetrically-shaped diffuse channel of the prior art lamps does not enable the intensification of forward-emitted light into a focused viewing cone. 
     Other prior art designs have similarly utilized channel shape to force the reflected light to exit the lamp in a forward direction. Referring to FIGS. 2A-E, as light is emitted from the flat fluorescent lamp, it strikes channel wall  112  and is reflected at roughly the same angle as it approached the wall. This causes in increase in efficacy, as more light is exiting the lamp over the desired viewing angle without requiring a corresponding increase in power input. Prior art designs shown in FIGS. 2A-E are exemplary of diffuse channel  116  cross-sections that have been configured to improve flat fluorescent lamp efficacy. These figures show a diffuse channel cross-section that is symmetrical through the vertical plane and is milled so that the upper portion of channel wall  112  tapers outward away from the diffuse channel cavity  122 . This outward tapering can be slight, such as FIG. 2E, or it can be extreme, as seen in FIGS. 2A-B. These symmetrical, outward-tapered diffuse channel designs assist light in being forward-emitted by having a configuration that permits light striking the channel wall to exit through cover lid  114  (as seen in FIG.  1 B). This increases the efficacy of the lamp, in that overall lamp output is now greater without requiring a corresponding increase in power. However, light emitted through cover lid  114  is emitted as a cone of light that covers the full range of angles. Often, however, only a small range of the visible cone of light is needed for a particular application. Therefore, all light that falls outside that desired range is wasted because the resulting forward-emitted viewing cone of the prior art design is too large compared to the desired viewing cone for many applications. 
     SUMMARY OF THE INVENTION 
     A flat fluorescent lamp according to various aspects of the present invention exhibits brightness enhancement through selective angular tuning of the emitted cone of light. A cover lid is attached to a substrate to form an enclosure that has interior and exterior portions. A fluorescent material and a material for emitting energy in response to an electric potential are placed suitably in the interior portion of the enclosure. A diffuse channel is formed such that the shape of the channel selectively tunes light exiting the cover lid into an intensified cone of light that corresponds to the requirements of the lamp&#39;s intended application. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter of the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, may best be understood by reference to the following description taken in conjunction with the claims and the accompanying drawings, in which like parts may be referred to by like numerals: 
     FIG. 1A is a plan view of a flat fluorescent lamp; 
     FIG. 1B is a cross-sectional view of the device of  1 A; 
     FIGS. 2A-E are cross-sectional views of prior art lamp diffuse channel shapes; 
     FIGS. 3A-E are cross-sectional views of exemplary embodiments of diffuse channel shapes according to the various aspects of the present invention; 
     FIG. 4 is a cut-away view of a flat fluorescent lamp with a reflective coating on the exterior of the substrate; and 
     FIG. 5 is a cut-away view of a flat fluorescent lamp with a reflective coating in the interior portion of the enclosure. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EXEMPLARY EMBODIMENTS 
     The ensuing descriptions are preferred exemplary embodiments only, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the ensuing descriptions provide a convenient description for implementing a preferred embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in the preferred embodiments without departing from the spirit and scope of the invention as set forth in the appended claims. 
     Referring to FIGS. 1A-B, a flat fluorescent lamp  100  includes a substrate  102  having two sidewalls  104  and  106  and two end walls  108  and  110  forming a rectangular perimeter. Substrate  102  is formed of any suitable material that is preferably rigid and self-supporting, such as glass or ceramic. A diffuse channel  116  is formed by at least one channel wall  112  extending from substrate  102  to cover lid  114 . Channel walls  112  extend in one direction and then alternate to the other direction so that a continuous channel is formed within substrate  102  from one corner to another. A transparent cover lid  114  is suitably attached to substrate  102  such that the lid  114  and the top portion of sidewalls  104  and  106 , end walls  108  and  110 , and channel walls  112  form an enclosure within lamp  100 . The transparent cover lid  114  is preferably constructed of a material having a coefficient of thermal expansion that substantially matches that of substrate  102 . 
     At least a portion of the enclosure interior is coated with a material that fluoresces in the visible spectrum when bombarded with ultraviolet radiation. The fluorescent material may be any material that produces light upon bombardment of ultraviolet radiation, such as phosphors, and more particularly, rare earth phosphors. Additionally, a plasma or other ultraviolet emissive material, such as mercury and argon, is placed in the enclosure. 
     The plasma or other emissive material is ignited through sparking caused by the electric potential between two electrodes contained in filament housings  118  and  120 . For example, filament wires (not shown) are suitably contained in filament housings  118  and  120  and extend into lamp  100  for exciting the plasma or other ultraviolet emissive material. Filament housings  118  and  120  are suitably located on the bottom exterior of substrate  102 . To create filament housings  118  and  120 , small glass bodies contain the filaments which are with a glass frit, with the glass frit having a lower melting point than that of the filament housing. The attachment of the filament housing  118  and  120 , which are suitably soldered to the bottom exterior of substrate  102  with the filament wires in place, can have a variety of configurations as to their location and attachment. Filament housings  118  and  120  may be located on the same side corners or opposite corners of the bottom exterior of substrate  102  or in any other suitable configuration to facilitate excitation of the plasma or other emissive material in the channel  116 , such as at opposite ends of the arc created by diffuse channel  116 . 
     Diffuse channel  116  may be formed in many shapes to provide the desired tuning. A lamp according to various aspects of the present invention harnesses the light that would have been otherwise wasted due to light emission outside of the desired viewable range. This results in the selective angular tuning of emitted light into a more intense cone of viewable light without requiring a greater power input. Referring to FIGS. 3A-C, diffuse channel  116  has a cross-section that has a symmetrical shape and channel walls that are formed in substrate  102  so that the top portion of channel wall  112  tapers inward toward the diffuse channel cavity  122 . When light hits channel wall  112 , it is reflected at approximately the same angle as it struck the wall, forcing it toward cover lid  114 . However, the tapering inward toward diffuse channel cavity  122  forces the light to exit cover lid  114  in a focused cone of light that is the desired range for the particular application. This enables the lamp to emit the same amount of light over a smaller area in a more concentrated range of viewing angles, resulting in a substantially brighter cone of emitted light. 
     In FIGS. 3D-E, the diffuse channel  116  of a display according to various aspects of the present invention has a cross-section that is asymmetrical and a channel wall  112  formed such that the top portion tapers inward toward the diffuse channel cavity  122 . This type of design, which functions similarly to the symmetrical design, is useful in applications where light is viewed from only one direction, such as at an automatic teller machine display where the individual stands over the display unit. The resulting selective angular tuning of emitted light from this design also yields a concentrated cone that is designed to correspond to the desired range of viewable angles for the particular application. 
     The amount of light tuning that is desired for a particular application can be adjusted based on the amount of channel wall  112  tapering. Preferably, the angle of inward tapering of the upper portion of channel wall  112  is between 0° and 45°, and more preferably, is between 1° and 30°. Nonetheless, the brightness of the flat fluorescent lamp tends to be enhanced even where the degree of channel wall  112  tapering exceeds the preferred range; however, some light may be reflected back into the channel, yielding a light-recycling effect that may inhibit the lamp from achieving optimal brightness enhancement through tuning, depending on the light absorption properties of the lamp channel. Therefore, even though there is an optimum range of tapering, brightness may be enhanced even where the degree of tapering exceeds the preferred range. 
     In alternate embodiments, additional materials may be included to enhance lamp performance. For example, a semi-transparent layer may also be applied to at least some portion of the interior of the enclosure to prevent ultraviolet emissive material migration into the fluorescent material or into the matrix of substrate  102  or cover lid  114 . The semi-transparent layer can be any suitable material, such as an aluminum oxide, which tends to extend the useful life of the lamp. 
     In other embodiments reflective material  124 , such as aluminum or ceramics, may be used to further enhance the flat fluorescent lamp&#39;s brightness. Reflective material  124  may be placed in any suitable configuration, such as the exterior bottom of substrate  102  (shown in FIG. 1) or in the enclosure interior of the lamp. For example, referring to FIG. 4, the application of reflective material  124  to the bottom exterior of substrate  102  tends to redirect light that would have been rear-emitted. Consequently, light that would have escaped through the bottom of substrate  102  may be directed forward and emitted as viewable light through cover lid  114  (as seen in FIG.  1 B). Reflective material  124  applied to the exterior of the substrate for capturing rear-emitted light in combination with the diffuse channel  116  shaped to tune the light, provides further brightness enhancement to the flat fluorescent lamp. 
     Reflective material  124  may also be applied to all or part of the interior of the lamp enclosure to further enhance brightness. Reflective material  124  may be applied to the entire surface to redirect light that would have been rear-emitted toward the cover lid  114 . Also, reflective materials may be applied to only a portion of the enclosure interior, for example to diffuse the bottom of channel  116  as shown in FIG. 5, yielding a masking effect which can be configured to direct light to strike the tapered sides at selected angles enhancing the selective angular tuning effect of the flat fluorescent lamp. Thus, use of channel tuning maximizes the usefulness of the light bar concentrating it into a smaller viewing range, without requiring the addition of more materials. Reflective material  124  in the lamp interior in combination with the selective angular tuning of emitted light by the diffuse channel  116  of the present invention will further increases the brightness enhancement provided by diffuse channel tuning. 
     From the foregoing, it can be seen that a flat fluorescent lamp with an enhanced brightness may be provided for by selective angular tuning of emitted light by altering the cross-sectional shape of the diffuse channels which focuses, and subsequently intensifies, the emitted cone of visible light. However, the applications of this invention are vast and many variations are available. In addition, it will be understood that the above descriptions are preferred exemplary embodiments only, and are not intended to be limiting in any way. Various modifications, substitutions, and other applications of the embodiments discussed herein may be made without departing from the spirit and the scope of the invention as set forth in the appended claims.