Abstract:
A serpentine avionics fluorescent tube with enhanced uniformity of luminance and chromaticity where the serpentine tube has a phosphor coating deposited therein after the fluorescent tube has been bent into a serpentine shape by pulling a spay nozzle emitting phosphor through the previously bent fluorescent tube.

Description:
This Application is a File Wrapper Continuation of application Ser. No. 08/021,366 file Feb. 23, 1993, now abandoned. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to a co-pending application entitled “Method and Apparatus for Manufacturing Serpentine Avionics Fluorescent Tubes With Enhanced Uniformity Of Luminance and Chromaticity”, filed by the same inventor on the same date herewith, and assigned to the same assignee, which application is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to fluorescent tubes and more particularly to serpentine fluorescent tubes for use in avionics equipment. 
     In today&#39;s aviation industry, avionics engineers are involved in a continuing quest to improve the optical performance of avionics displays. One particular area of concern is fluorescent lamps for back-lighting liquid crystal displays. 
     Typically, fluorescent lamps utilized in the avionics industry are serpentine and are constructed by creating a linear transparent glass tube and coating the interior of the tube with a fluorescent phosphor substance. The linear coated tubes are then fashioned into a serpentine shape by heating the glass tube to its working temperature and then bending the tube. 
     Another method has been to bend uncoated tubes into a “U” shape and then apply the phosphors via the typical phosphor slurry flush coat method used for linear tubes. Success has been claimed for uniform application of phosphors to “U” shapes using the flush coat method, but “S” shaped or “M” shaped tubes have not been uniformly phosphor coated with the typical slurry deposition method. In order to make “S” or “M” shaped tubes, it has been attempted to weld together 2 or 3 “U” shaped phosphor slurry coated tubes to create “S” and “M” shaped lamps respectively. 
     While these methods have been used widely in the past all existing methods of fabricating serpentine tubular lamps have several serious draw backs. First of all, when the tubes are bent after coating, the efficiency of the phosphors is diminished as a result of exposure to the high temperature required to allow bending of the tube. Secondly, the bending of the tube results in lacerations or cracks in the phosphor coating. This results in a diminution in luminance uniformity and chromaticity uniformity, as well as the absolute luminance per unit area. 
     In the method involving welding several slurry coated “U” shaped tubes together, the areas where the “welding” occurs are exposed to high temperatures and the phosphors therein are degraded as a result. 
     Consequently, there exists a need for improved manufacture of fluorescent tubes for use in the avionics industry, in which phosphor efficiency and uniformity of luminance and chromaticity are not degraded as a result of the fabrication process. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a fluorescent tube with enhanced phosphor efficiency. 
     It is a feature of the present invention to include a phosphor coating on the tube that is not reheated to a uniform temperature sufficient to bend the tube. 
     It is an advantage of the present invention to eliminate the adverse effects on the phosphors which occurs when they are heated to a temperature sufficient to bend the tube. 
     It is another object of the present invention to improve optical performance of fluorescent tubes. 
     It is another feature of the present invention to have a serpentine tube which is not bent after coating of the phosphor. 
     It is additional advantage of the present invention to eliminate the cracks and lacerations due to the bending of the tube and thereby reduce some of the adverse effects upon the uniformity of luminance and chromaticity. 
     The present invention provides an improved fluorescent tube having a non-reheated and non-bent phosphor coating disposed therein, which is to designed to satisfy the aforementioned needs, provide the previously propounded objects, include the above described features, and achieve the already articulated advantages. The invention is carried out in a “laceration-less” phosphor coating within the fluorescent tube in the sense that the lacerations typically associated with bending a phosphor coated tube into a serpentine shape have been eliminated. Additionally, the invention is carried out in an “excessive heat exposure-less” method in the sense that the excessive and phosphor damaging heat exposure associated with bending or welding a pre-phosphor coated tube is eliminated. Instead, the fluorescent tube contains a non-reheated and non-bent phosphor coating disposed therein after the fluorescent tube has been bent to a serpentine shape. 
     Accordingly, the present invention provides for a fluorescent tube having a uniform phosphor coating disposed therein which has not been heated to a temperature sufficient to permit bending of the fluorescent tube and which has not been cracked or lacerated to bending of the tube after coating of the phosphors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the following description of the preferred embodiment of the invention in conjunction with the appended drawings wherein: 
     FIG. 1 is a cross-sectional view of a fluorescent tube, of the prior art, showing non-uniform phosphor distribution thereon. 
     FIG. 2 is a cross-sectional view of the fluorescent tube, of the present invention showing the apparatus used to deposit the uniform phosphor coating. 
    
    
     DETAILED DESCRIPTION 
     Now referring to FIG. 1, there is shown a fluorescent tube, of the prior art, generally designated  100 , having a phosphor coating  102  disposed therein. Fluorescent tube  100  is shown having a first bend  104 , a second bend  106 , and a third bend  108 . First bend  104  is shown having a central side  110  and outer side  112 . The dimension D of the outer side  112  is clearly shown to be greater than the linear dimension d of the central bend  110 . The outer bend  112  is shown having several lacerations, cracks, or gaps  114  disposed therein. Typically, these lacerations, cracks, or gaps will in the phosphor coating appear when the tube  102  is bent. During the bending process the outer side  112  is stretched over a larger dimension than the innerside  110 . Since the phosphor coating was deposited before the tube was bent, the coating becomes damaged upon excessively stretching the outer side  112 . Bends  106  and  108  would have similar phosphor coating damage disposed therein. 
     Also due to the bending or welding process, the tube and phosphors deposited therein are heated typically to temperatures in excess of 700° C. These high temperatures cause the metal atoms in the phosphors to oxidize, which results in a degradation in the efficiency of the light output by such phosphors. It is believed that phosphors which emit blue light may be more adversely affected by this high temperature exposure and the concomitant metal atom oxidation. Additionally, subtle temperature induced changes in the phosphor lattice structure, such as amorphization, that perturb the crystal field, and consequently the energy levels, of the emitting phosphor atom, also contribute to a reduced phosphor quantum efficiency. 
     Now referring to FIG. 2, there is shown a cross-sectional view of a serpentine avionics fluorescent tube of the present invention, generally designated  200 , which shows a portion of the tube having already been coated with a phosphor layer  202  and a portion  204  of the tube in the process of having the phosphor layer being deposited and a portion  206  of the tube yet to be coated. Additionally, FIG. 2 shows the apparatus to use to coat the serpentine fluorescent tube of the present invention, which includes a spray nozzle  210  coupled to a hose  216  which is coupled to a pump or other device (not shown) for propelling the phophors through the hose  216  and out the nozzle  210  on to the tube. Since there is no bending of the tube after the coating is uniformly applied, the lacerations, cracks or gaps,  114  of FIG. 1 are not present in the present invention. Also shown are spacers  214  which help to center hose  216  and maintain a uniform spray  212 . 
     Since no heating to temperatures sufficient for tube bending or welding is required during the manufacture of the present invention, the layer  202  contains fluorescent phosphor particles whose quantum efficiency has not been degraded by thermally induced oxidation-reduction chemical processes. In particular, the blue light emitting phosphors will exhibit a relatively high quantum efficiency since relatively few metal atoms per unit coating weight become oxidized after phosphor deposition. Similarly, the lattice of relatively few blue emitting phosphor particles per unit coating weight have been thermally damaged by the fabrication method of the present invention. Thus, total efficiency is higher. In comparison to the prior art of fabrication serpentine lamps, relatively few phosphors are destroyed by amorphization or oxidation that occurs when phosphors are exposed to temperatures sufficient to permit bending of the tube. 
     The term serpentine when used herein shall mean having at least two curved portions therein such as “S” or “M” shaped, but any shape with more than a single bend is contemplated. 
     It is thought that the fluorescent tube of the present invention and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes may be made in the form, the construction, and the arrangement of the parts, without departing from the spirit and the scope of the invention, or sacrificing all of their material advantages, the form herein being merely preferred or exemplary embodiments thereof.