Abstract:
An optical attenuator varying in optical transmission in a systematic fashion, compensates for non-uniform illumination of vacuum fluorescent characters due to filament voltage drop.

Description:
BACKGROUND OF THE INVENTION 
     Vacuum fluroescent devices are those in which phosphor coated anode segments are bombarded with low velocity electrons from a thermionically heated filament all contained within an evacuated envelope. The electrons are accelerated from the filament to the anodes by a small electric potential applied therebetween. Electric potentials on the order of a few volts to a few tens of volts are typically employed. 
     For reasons of economy, it is desired to use directly heated filaments rather than indirectly heated filaments. The voltage drop from one end of the filament to the other causes a difference in accelerating potential between the filament and the anodes from end to end of the filament. In multidigit vacuum fluorescent devices, a perceptible variation in digit brightness is seen due to the difference in potential applied between the filament and the anodes from end to end due to the filament drop. 
     U.S. Pat. No. 4,045,704 attempts to solve the problem of varying brightness by interposing control grids at varying spacings between the filament and the anodes. By placing the grids closer to the filament at the high-potential end of the filament and progressively further away towards the low-potential end, substantially uniform brightness of the phosphor is achieved. 
     In U.S. Pat. No. 4,049,993 a similar benefit is achieved by the installation of a filament which slopes downward from one end to the other thus being closer to the grids and/or anode at the positively charged end of the filament than at the negatively charged end. This achieves a uniform electric field between the filament and the respective anode segments along the length of the filament to achieve substantially uniform electron bombardment velocity. This patent also proposes combining varying grid heights with the sloping filament to further improve the brightness uniformity. 
     Each of the structures recited in the preceding requires special precision treatment of the internal structure of the vacuum fluorescent display device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross sectional view of a vacuum fluorescent device according to the present invention. 
     FIG. 2 shows a simplified drawing including an optical filter according to the present invention. 
     FIG. 3 shows an optical filter having variable thickness. 
     FIG. 4 shows an optical filter having a stepped thickness. 
     FIG. 5 shows optical filtering being accomplished by varied openness of grids within the enclosure between the filament and the anode segments. 
     FIG. 6 shows optical filtering being accomplished by varied openness of grids within the enclosure between the cover plate and the filament. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown generally at 10 a vacuum fluorescent display device according to the present invention. An insulating substrate 12 has affixed thereon a plurality of conductive anode segments 14 having a coating 16 thereon phosphor material capable of being excited into optical emission by the impingement thereon of electrons. 
     Each anode 14 shown is part of a pattern of anodes which may be selectively energized to form an illuminated character such as letters or numerals. All anode segments 14 of a particular digit are aligned below a foraminous grid 18 which controls the illumination of all energized anode segments in its aligned character. 
     A filament 20 is suspended between filament supports 22a, 22b spanning the entire assembly of anodes 14 and grids 18. The filament 20 may be made up of one or more parallel resistance wires preferably of a material especially adapted to the emission of electrons at low temperature. For example, the filament 20 may be made of thoriated tungsten which is capable of emitting electrons at filament temperatures as cool as dull red. Besides providing support, the filament supports 22a, 22b may also provide electrical connection to the filament 20. 
     Electrical connection to all of the elements is accomplished by means well known in the art and are thus not shown. 
     A cover plate 24, suitably of transparent material such as glass is hermetically sealed to the substrate 12 at a sealing flange 26 using a low temperature frit 28.The volume 30 between the cover plate 24 and the substrate 12 is evacuated and gettered by means well known in the art. 
     An optical filter 32 is positioned in the line of sight between the viewing location, above the display device 10 as shown in FIG. 1, and the anodes 14. The optical filter 32 may have contrast enhancement properties as disclosed in U.S. Pat. No. 3,682,531. The present invention is not limited to placement of the optical filter external to the cover plate 24. An optical filter 32 within the enclosure is equally within the scope of this invention. 
     For the discussion which follows, it is assumed that the vacuum display device 10 has 3 sets of characters 34, 36 and 38, viewable along lines of sight 40, 42 and 44 respectively. In addition, it is assumed that character 38 is the brightest and that characters 36 and 34 are progressively less bright than character 36 due to the voltage drop along the filament 20 from filament support 22b to filament support 22a. Assigning an arbitrary relative brightness of 1 to the brightest character 38, relative brightness of the other two characters 36 and 34 can then be determined. For example, the relative brightness of character 36 might be 0.75 and that of character 34 might be 0.5. This means that character 36 is three quarters as bright as character 38 and that character 34 is half as bright as character 38. 
     Referring now to FIG. 2 the optical transmission of the optical filter 32 is made to vary from one end to the other by means well known in the art, being least transmissive along the line of sight 44 to the brightest character 38 and most transmissive along the line of sight 40 to the least bright character 34. If the transmission of the filter is related to substantially the inverse of the relative brightness of the characters, the light transmitted to the viewer along lines of sight 40-44 will be relative uniform. 
     The optical transmission is preferably governed by the relationship: ##EQU1## Where: T = optical filter transmission along line of sight 
     B = segment relative brightness in line of sight 
     α = a constant less than 1. 
     For example, the following tabulation shows relative brightnesses of 1, 0.75 and 0.5 and filter transmissions along lines of sight 44, 42 and 40 respectively of 0.4, 0.53 and 0.8 also respectively. The light transmitted through an optical filter such as optical filter 32 is equal to the brightness of the source multiplied by the filter transmission. Thus, for the values given in the table, the brightness of all characters is approximately 0.4 times the relative brightness of the unfiltered relative brightness of brightest character 38. This relatively minor reduction in the brightness of the brightest character is relatively insignificant in vacuum fluorescent devices in which the potential for very bright characters is readily realized. 
     
                       TABLE______________________________________    Relative   Filter       LightCharacter    Brightness Transmission Transmitted______________________________________38       1          0.4          0.436       0.75       0.53         0.434       0.5        0.8          0.4______________________________________ 
    
     It is also possible to purposefully cause nonuniform apparent brightness of the characters 34-38. It may be desirable, for example, to have character 34 very bright compared to characters 36 and 38. By reducing the filter transmission over characters 36 and 38, and/or increasing the filter transmission over character 34, a wide range of relative apparent brightness can be achieved. 
     The optical filter 32 shown in FIG. 3 accomplishes substantially the same result as the variable density filter of FIG. 2 by using a variable thickness of a material having constant optical density. The fraction of light transmitted through an optical filter is equal to 
     
         s = e.sup.-rx                                              (1) 
    
     Where: 
     s = fractional transmission 
     e = base of natural logarithms 
     r = index of transmission 
     x = thickness of filter Note that as thickness increases, the fractional transmission decreases. The wedge-shaped optical filter 32 in FIG. 3 may be made to exhibit the same transmission characteristics along its length as shown in the table. In which case, the apparent brightness of the characters 34, 36 and 38 viewed along lines of sight 40, 42 and 44 respectively will be approximately uniform. 
     FIG. 4 shows a third embodiment of the optical filter 32. This embodiment also employs a constant density filter material but varies the filter thickness in steps. The thickness of the steps produces optical transmission required along lines of sight 40, 42 and 44 to achieve uniform character brightness. 
     FIG. 5 shows the use of grids 18 having variable ratios of openings located between the filament 20 and the characters 34, 36 and 38. The grid 18 above least bright character 34 has very fine metal strands defining relatively large openings. The grid 18 or foraminous screen over the brightest character 38 has relatively small openings within relatively thick strands thereby reducing the apparent brightness of the character 38. The grid 18 over the intermediate character 36 has an intermediate openness. Openness is defined as the fraction of the grid 18 area which is occupied by openings. 
     The openness of the grids also interacts with the stream of electrons from the filament 20 toward the characters 34, 36 and 38. The reduced openness of the grid 18 over brightest character 38 may reduce the number and/or velocity of electrons striking the phosphor and thus also reduce the brightness with which the phosphor on character 38 glows. Consequently, the openness of the grids 18 are not the sole determinant of optical brightness seen along lines of sight 40, 42 and 44. Instead, both the occlusion of the glowing phosphor 16 by the grids 18 as well as the modification of phosphor 16 brightness by the grids 18 must be accounted for in determining the openness of the grids 18 to achieve uniform apparent character brightness. 
     The grids 18 may be individually or commonly connected to a voltage source for the purpose of controlling the flow of electrons to the anode segments, for multiplexing, for electrostatic shielding or for combinations thereof. When the grids 18 are in the electron path between the filament 20 and the anodes 14, the presence of the voltage on them interacts with the electron flow to modify either the electron density or electron energy striking the anode and thus vary the brightness of the glow. The grid 18 openness is adjusted to compensate for this effect to yield uniform character brightness along lines of sight 40, 42 and 44. This relationship is readily determined without experimentation by one skilled in the art. 
     Referring now to FIG. 6, the grid 18 is located between the cover 24 and the filament 20. The grid 18 may be in a single piece having varying openness from end to end, or alternatively, may be in discrete parts. The grid 18 may be connected to a voltage source (not shown) by well known means. The grid or grids 18 may be used as an electrostatic shield, electrostatic lens or other useful function in the device as well as performing optical filtering. The size of the openings in a grid 18 positioned above the filament 20 as shown in FIG. 6 will have little or no interference with the electron density or electron energy striking the characters 34, 36 and 38. Consequently, no adjustment in openness is required to counteract such interference. The grid 18 need not be located inside the enclosure 24, but instead may be located outside the enclosure occupying the position of the optical filter 32 shown in FIG. 1. 
     It will be understood that the claims are intended to cover all changes and modifications of the preferred embodiments to the invention, herein chosen for the purpose of illustration which do not constitute departures from the spirit and scope of the invention.