Abstract:
A flat form gas discharge lamp for illuminating a defined area includes a plurality of adjacent spaced apart substantially parallel channels extending across the defined area. The first channels are substantially co-planar with one another, and a plurality of second channels interconnecting end portions of respective pairs of the first channels to form a single confined region such that the first channels confine an ionizable medium for producing light under influence of an electric discharge. The second channels are substantially co-planar with one another while not being co-planar with the first channels.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a planar fluorescent lamp with features to provide uniform luminescence. 
     Thin, planar, durable, easily manufacturable and relatively large area light sources having a range of light intensity are useful in many applications. Such light sources are particularly useful as backlight sources for Liquid Crystal Displays (LCD) to provide readability in a wide range of ambient lighting conditions, from night time to direct sunlight. Such backlight sources are commonly used in avionics, industrial, mobile, and medical applications. Such sources must be very uniform so that the display is not adversely effected by bright or dim spots or patterns of light. Uniform light sources may also be tiled together to form even larger uniform light sources for back illuminating large LCD or transparency images such as X-ray films. 
     To construct uniform backlight sources the actual light source is placed behind a translucent material that diffuses the underlying light source. The diffuser absorbs light and reduces the amount that reaches the back of the display to be backlighted. The more transparent the diffuser is, the more efficient the backlight system is. However, the more transparent the diffuser is, the less diffusing it does. So, an efficient diffuse backlight system needs to have an inherently uniform original light source so the diffuser can be highly transparent and still be sufficiently diffuse. 
     Other forms of lighting devices may be used to create uniform light sources. Incandescent lamps and light emitting diodes may be used, however they have poor color qualities and poor luminous efficiency, resulting in high power consumption and heat generation. 
     An alternative to incandescent lights and light emitting diode arrays is fluorescent technology. Tubular fluorescent lamps have the advantage of being relatively efficient, generating relatively bright light, and having well-established manufacturing capability. In particular, serpentine tubes are especially efficient because they use a long channel length which minimizes parasitic electrode losses. However, tubular fluorescent lamps suffer from fragility requirements when used as optical elements to reflect and diffuse light, are not very durable for harsh environments, and have limited capability to operate effectively and effectively in low light applications. 
     There are types of flat planar fluorescent lamps that represent improvements in light output uniformity because they incorporate closely spaced patterns of serpentine light. Several planar fluorescent lamps are known in the art such as U.S. Pat. Nos. 3,508,103, 3,646,383 and 3,047,763. Typically such planar lamps are made with two plates of glass spaced apart with a separation material to form channels inside. The channels are coated with phosphorous material, filled with a selected gas and mercury vapor, electrodes are placed at channel ends and a discharge is caused by a high voltage and current flow. In these cases the light is more uniform than separate tubes would create, but there is still non-uniformity between the lighted channels. These kind of planar serpentine lamps do have the added efficiency of long channel lengths described for tubular serpentine lamps. 
     Lynn et al., U.S. Pat. No. 5,233,262, disclose a planar fluorescent lamp that defines a serpentine channel therein. The light emitted across the planar lamp is non-uniform because of the non-light emitting regions between the channels. To provide increased brightness, increased uniformity, and increased efficiency, an optical reflector is placed in the areas between the lighted channels which directs reflected light toward the front of the lamp. However, in a typical planar lamp the channels are formed into a serpentine pattern. At the turning ends there is an increased unlighted area where the lamp radius diverge. This results in less light per overall area being generated in the serpentine area. As a result, when the entire lamp is covered with a diffuser to homogenize the light, the serpentine bend areas appear less bright and uniformity suffers. It is known in the art that various optical devices, such as Fresnel lens sheets, can be used to concentrate light and to cause linear double imaging, which improves uniformity at the diffuser level, which allows the use of a more transparent diffuser, which makes the backlight system more efficient. However, such films are linear and if they are place in front of a serpentine pattern light source with their elements parallel to the main direction of the light sources, then when the light sources bend in their turns, the film has an undesirable optical result and there is nonuniform light radiated at the diffuser, resulting in non uniform light out of the diffuser. So planar fluorescent lamps that include serpentine lighted patterns in the lighted area of interest require more diffusing to make them uniform and as a result suffer losses in optical efficiency. In applications which require uniform light output across the entire lamp, the curved end portions are masked to prevent the light originating from the curved end portions from reaching the viewer. Unfortunately, the resulting masked display has a reduced useful area, decreased efficiency by having a masked region, a perimeter that is significantly greater than the nonmasked illuminated region, and a manufacturing expense greater than a display having a visible region the same as the nonmasked region. 
     What is desired, therefore, is a planar fluorescent lamp to create diffused uniform light that has uniform light output, increased efficiency, minimal size, no masking, and reduced manufacturing expense. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the aforementioned drawbacks of the prior art by providing a flat form gas discharge lamp for illuminating a defined area that includes a plurality of adjacent spaced apart substantially parallel channels extending across the defined area. The first channels are substantially co-planar with one another, and a plurality of second channels interconnecting end portions of respective pairs of the first channels to form a single confined region such that the first channels confine an ionizable medium for producing light under influence of an electric discharge. The second channels are substantially coplanar with one another while not being co-planar with the first channels. 
     In the preferred embodiment, the lamp therefore may include a set of straight channels without curved portions at the ends thereof. The resulting lamp has uniform light output without variation in the direction or spacing of the lighted channel. Such a configuration of lighted channels allows the use of optical films without anomalies caused by misdirection of channel and film optics and diffusers that are relatively transmissive but are sufficiently diffuse which produce a highly efficient diffuse lighting system. In addition, the entire surface of the lamp is useful for applications that require uniform light output, such as reading photo negatives. Also, such a lamp is useful for applications that have packaging limitations while still requiring uniform light output to the perimeter of the lamp. Moreover, increased light uniformity is achieved while using a single continuous arc stream in the advantageous manner of a long channel serpentine lamp with a single pair of electrodes one situated at each end, which minimizes power losses. The addition of additional electrodes would greatly reduce the lamp&#39;s efficiency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is plan view of an exemplary embodiment of a flat form lamp. 
     FIG. 2 is a sectional view along line  2 — 2  of the lamp of FIG.  1 . 
     FIG. 3 is a rear view of the lamp of FIG.  1 . 
     FIG. 4 is an alternative embodiment of the lamp of FIG. 1 shown in section. 
     FIG. 5 is an alternative embodiment of the holes of the lamp of FIG.  1 . 
     FIG. 6 is a plan view of another alternative embodiment of a flat form lamp. 
     FIG. 7 is a rear view of the lamp of FIG.  6 . 
     FIG. 8 is a sectional view along line  8 — 8  of the lamp of FIG.  6 . 
     FIG. 9 is a sectional view along line  9 — 9  of the lamp of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, a flat form lamp  12  for providing a uniform light output is shown. The lamp  12  is comprised of a first plate  14  molded to form the desired pattern and which is mounted above a second plate  16 . The first and second plates  14  and  16  are formed of a suitable transparent or translucent material such as clear glass or other vitreous materials. 
     The first plate  14  is molded with planar base portions  18  and shaped portions  20  which project forward from the plane of the base portions  18 . The shaped portions  20  are molded to form a set of straight, or substantially straight, channel patterns  22  across the display  12 . 
     The inner surfaces  30  of the shaped portions  20  and the upper surface  36  of the second plate  16  are spaced apart to define channels or cavities  24  therebetween. The planar base portion  18  of the first plate  14  is mounted by suitable techniques, such as a glass frit seal or glass welding, across the facing upper surface of the second plate  16  about the perimeter of the shaped portions  20  to hermetically seal the cavities  24 . The first plate  14  is also formed with a pair of raised pockets  26  and  28  at opposite ends of the shaped portions  20  for placement of electrodes  31  and  33 . The pockets  26  and  28  communicate with the cavities  24  and are also hermetically sealed. The cavities  24  are exhausted to a partial vacuum by a suitable exhaust tube, not shown, or other suitable mechanisms. 
     Referring to FIGS. 2 and 3, a third glass plate  54  is mounted against and hermetically sealed with the rear surface of the second plate  16 . The third plate  54  is molded with a plurality of shaped portions  56 . The shaped portions  56  of the third plate are spaced rearwardly from the second plate to define channels  62  therebetween. Holes  68  are formed in second plate  16  for communicating gas between channels  62  and channels  22 . The channels  62  formed by the third plate  54 , together with the holes  68  formed in the second plate  16  create an interconnect path that provides a continuous gas discharge path between the electrodes  31  and  33  along the length of the shaped portions  20  of the first plate  14 . 
     As shown in FIGS. 1 and 3, the cavities  24  and  62  are preferably of uniform depth but could be varied in accordance with the particular application. Wall thickness of the first, second, and third plates  14 ,  16  and  54  can be in the range of 0.02 inches to 0.06 inches. The total thickness of the combined front plate  14 , cavity  24 , and rear plate  16  can be in the range of 0.15 inches to 0.50 inches. While it is shown that the first, econd, and third plates  14 ,  16  and  54  are molded with the channels  22  and  62 , the invention contemplates that the second plate  16  could be molded with the shaped portion while the front plate  14  is flat, or both first plate  14  and second plate  16  could be formed with shaped portions. In any such manner, the straight or substantially straight channels can be achieved. 
     During fabrication, the inner surfaces  30  of the first plate  14  are coated with a layer of phosphors  32  of the type that absorb ultraviolet radiation and irradiate at wave lengths visible to the human eye. Another layer of phosphors  34  can be coated on the upper surface  36  of the second plate  16 . An activated powered phosphor such as magnesium tungstate or calcium Fluorochlorophosphate:Antimony:Manganese is suitable for this purpose. As desired, a suitable reflector layer, not shown, may be provided under phosphor layer  32  and over the inner surface  36  of the second plate  16  to increase brightness. The phosphors  32  and  34  and reflector material can be deposited by spraying, screen printing or other suitable techniques. The phosphors  32  and  34  can be selected in accordance with the light which they emit, and a single color phosphor can be used as well as a combination of different phosphors to provide multiple colors. 
     After evacuation, the cavities  24  and  62  are filled with a low pressure ionizable medium which carries electrical current. The ionizable medium can comprise an inert gas such as Argon which is charged with a small percentage of Mercury vapor to provide a fluorescent gas mixture. The ionizable medium could also comprise Neon gas or a Penning mixture, such as a mixture of Neon and Argon gasses or a mixture of Neon and Xenon gases. Gas pressure within the cavities  24  and  62  are preferably within the range of three to  33  torr. 
     The ionizable gaseous medium within the cavities  24  and  62  is excited into an electric discharge y a suitable control circuit, not shown, applying a voltage potential across electrodes  31  and  33 . The control circuit can apply either a direct current or alternating current to the electrodes  31  and  33 . The excited gas gives off photons of energy, and the partial pressure of the Mercury vapor is particularly rich in radiating UV photons. The phosphor coatings absorb the UV radiation and reradiate visible light which is emitted through the transparent shaped portions  20  to produce the lighted pattern. When the cavities  24  and  62  are filled with an ionizable medium comprising Neon or a mixture of Neon and other inert gas and mercury vapor, the current flow causes the Neon to itself emit a blue light. 
     A significant amount of light is not emitted from the cavities  62  in the third plate  54  because no phosphor is included therein. The cavities  62  do not include any phosphors therein so when an electrical control voltage is applied thereto it does not illuminate. It is to be noted that the electrical voltage does result in a faint light discharge from the gas within the cavities  62  but this does not significantly affect the uniform light output. 
     The third plate with its channels forming the interconnecting paths allows the front channels to be straight and free from curved portions. Accordingly, the lamp  12  has a set of straight channels  22  without the curved portions at the ends thereof. The resulting lamp  12  has uniform light output, and in particular at the ends of the channels  22 . In addition, the entire surface of the lamp  12  is useful for applications that require uniform light output, such as reading photo negatives. Also, such a lamp is useful for applications that have packaging limitations while still requiring uniform light output to the perimeter of the lamp. Moreover, increased light uniformity is achieved while using a single continuous arc stream with one pair of electrodes, one at each end which minimizes power losses at the electrodes thereof. The addition of additional electrodes would greatly reduce the lamp&#39;s efficiency. 
     The present inventor observed that the regions around the openings  68  have slightly decreased luminance. Referring to FIG. 4, in an alternative embodiment, the inner surface  70  of the shaped portions  56  and the rear surface  72  of the second plate  16  may be coated with phosphors  74  so that light shines through the holes  68  to provide increased uniform luminance. If the channels  22  are closely spaced together then adjacent channel walls will reflect the light emitting from each other. If the channels ( 22 ) are spaced apart then a reflector device such as tape or a molded shaped reflector can be placed between channels to enhance foreward reflected light and thus the uniformity. 
     The arch path within the channels  22 ,  62  tends to select the shortest path between the electrodes  31  and  33 . This results in the end portions of the channels  22  not being fully illuminated because the arc path enters the inner curved portions of the openings  68 . Referring to FIG. 5, to increase the luminescence near the end of the channels  22 , the holes  80  are preferably asymmetrical in nature, such as half a circle. The arch path is then forced to pass closer to the end of the channels  22  which increases the luminance at the end portions of the cavities  24 . 
     Referring to FIGS. 6-9, in an alternative embodiment, a set of two formed plates can be used to form the cavities. In this manner, the second plate can be eliminated. In particular, FIG. 6 illustrates an upper plate  100  with shaped portions  102  defined therein. FIG. 7 illustrates the lower plate  110  with the shaped portions  112  defined therein. The shaped portions  112  are sized and oriented to interconnect the ends of the shaped portions  102  together to form a continuous channel. FIG. 8 illustrates the combination of the upper plate  100  joined with the lower plate  110  and the arc path created between a pair of adjoining shaped portions  102 . FIG. 9 illustrates the combination of the upper plate  100  joined with the lower plate  110  and the separation of the shaped portions  102  by the lower plate  110 . 
     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.