Multi-digit luminescent display tube having wider grid frame members on both ends of a series of grids

#This invention relates to a so-called multi-digit luminescent display tube, such as a fluorescent or phosphorescent display tube, and more particularly to one comprising novel grids.

#In a luminescent display tube of this type, there is a multi-unit tube, generally called a multi-digit tube, for displaying a plurality of selected displays, namely, numerals, letters, and/or symbols. The multi-unit tube comprises a plurality of luminescent anode segment groups, a hot cathode or cathodes, a first and a second support for supporting the cathode over the anode segment groups, and a plurality of grid members between the cathode and the respective anode segment groups.

#The display tube of the multi-unit type is typically used in an electronic portable calculator in which a large scale integration circuit (an LSI) is frequently adapted for portability of the calculator. Since a circuit is generally operated at low electrical power and low voltage, the display tube which is directly driven by the large scale integration circuit should also luminesce at low voltage and low electrical power.

#FIG. 1 is a perspective view of a general luminescent display tube;

#FIG. 2 is an enlarged fragmentary plan of a conventional display tube;

#FIG. 3 shows an arrangement of grid members of a conventional display tube;

#Referring to FIG. 1, a luminescent display tube comprises a substrate 20 of an electrically insulating material, such as glass or ceramics, and a plurality of luminescent anode segment groups 21 comprising a plurality of segmented luminescent anodes 21a-21g on the substrate 20. The anode segment groups 21 are regularly spaced along a line segment 22, namely, a straight line of a finite length. In the example being illustrated, the segmented luminescent anodes, or anode segments, 21a-21g each anode segment group 21 are arranged in a substantially figure-of-eight configuration and display a selected one of the numerals 0 to 9. A plurality of grid members 23 are assigned to the respective anode segment groups 21. Each of the grid members 23 comprises a frame piece 24 of a substantially rectangular contour, a metal mesh 25 supported by the frame piece 24, and a spacer 26 connected to the frame piece 24 in order to supply grid voltage. The luminescent display tube further comprises a hot cathode 27 and a first support 28 and a second support (not shown) on both longitudinal sides of the line segment 22 for supporting the cathode 27 over the anode segment groups 21. Each of the frame piece 24 for each anode segment 21 comprises a first and a second metal members 29 and 30 intersecting the line segment 22 and a pair of metal members 31 and 32 parallel with the line segment 22. The first metal members 29 for the anode segment groups 21 are nearer to the first support 28 than the second metal members 30 for the anode segment groups 21.

#As is known in the art, anode currents bombarding a luminescent layer on the anode segments 21a-21g should be increased to display sufficient luminescing displays at a low rated voltage. That is, it is necessary to enlarge the perveance of a luminescent display tube. Therefore, the gap between the cathode 27 and the grid members 24 is narrowly designed in usual. But, in this type of the display tube, the distances from each of anode segments 21a-21g of the respective anode segment groups 21 to the cathode 27 are very unequal because of a narrow gap. Consequently, the displays of both end units nearest to supports, such as 28, are dark due to the lack of electron emission of the cathode 27 at end portions thereof and to the unevenness of the perveance.

#Referring to FIG. 2, description will be made of conventional grid members 23 for a better understanding of this invention. Each grid member 23 is supported between a plurality of anode segments 21a-21g of each anode segment group and a cathode 27. Further, an upper and a lower metal 31 and 32 of a frame piece 24 are parallel with the cathode 27 and anode segments 21a, 21d and 21g. That is, the distances between the cathode 27 and the anode segments 21a and 21d are substantially constant at each anode segment group and, moreover, the shield effect of space charges due to a current flow in the upper and lower members 31 and 32 to the anode segments 21a and 21d are substantially uniform thereat. Therefore, the luminescing displays of the anode segments 21a and 21d are comparatively uniform in each segment group.