Display panels based largely on the principles of proximity-focused image-intensifier tubes have been proposed as alternatives to cathode-ray tubes (CRTs) and other conventional devices for wide displays. A panel of the proposed type would afford the high brightness and wide viewing angle (almost 180°) of a CRT, but it could readily be made much wider than the maximum dimension [50 in. (127 cm) diagonal] of currently available CRTs. The thickness of the panel [<4 in. (<10.2 cm)] would be much less than the depth of a typical CRT. Moreover, unlike a CRT, the panel would not introduce any geometric distortion into the displayed image because the image geometry would be established by a pixel structure within the panel.

This Display Panel would include a proximity-focused channel-plate image intensifier with a segmented control grid and a segmented photocathode illuminated by an electroluminescent panel.

Partly ignoring the two-dimensional aspect of the display for the moment, some basic physical aspects of the panel can be explained by reference to Figure 1. An electroluminescent panel on the back side would supply photons for excitation of a segmented photocathode. The photons would travel through an input window and through a transparent gate metal layer into the photocathode segment. The gate of the photocathode segment would be biased at about -20 V, relative to a channel plate, to encourage photoemission of electrons and force the emitted electrons toward the channel plate. A control grid based on the same principle as that of a control grid in a triode vacuum tube would be variably biased (probably to a potential between -10 and -30 V) to control the local brightness of the display by allowing more or fewer electrons to pass to the channel plate.

The channel plate would be about 2 cm thick, and the pores in the channel plate would be about 0.5 mm wide. The output side of the channel plate would be biased to a maximum potential of about 1 kV relative to the input side, so that the number of electrons striking the input pores of the channel plate would be multiplied to a large magnitude. The resulting cloud of electrons emerging from each pore in the channel plate would encounter an electric field that would accelerate the electrons toward a phosphor. The electric field would be provided by biasing a phosphor electrode at about 22 kV relative to the output side of the channel plate.

The phosphor electrode would be about 1,000 Å thick - thick enough to be opaque to light coming from behind but thin enough to pass electrons with kinetic energy >2 keV. Thus, the electrons would lose about 2 keV of kinetic energy traversing the phosphor electrode and would deposit the remainder of their kinetic energy in the phosphor, causing the phosphor to glow. The local intensity of the glow would depend on the bias applied to the local control grid. A multicolor display could be implemented by placing groups of red, green, and blue phosphors in registration with groups of three control grids in red/ green/blue sequence. Thus, a multicolor display would contain three times the number of control grids of a monochrome display.

Figure 2. Pixels Would Be Defined by intersections of photocathode and control-grid segments. The photocathode segments would be activated sequentially to turn on rows of pixels sequentially. The brightness of each pixel in a row during its "on" period would be controlled via the voltage applied to the corresponding control-grid segment.

The two-dimensional aspect of the display can be explained by reference to Figure 2. A pixel would be defined by an intersection between one of the vertical control-grid segments and one of the horizontal photocathode segments. The minimum pixel size would be of the order of 1 mm; although this size is too large for the desired resolution in a small display that would ordinarily be implemented in a liquid-crystal display unit, it is an appropriate size for a wide-screen television or similar display. One of the advantages of larger pixels is greater ease of fabrication.

During operation, one photocathode segment (defining a row of pixels) would be biased to promote photoemission while the other photocathode segments would be biased to inhibit photoemission. During a frame period, rows of pixels would thus be turned on sequentially, the sequence repeating for each subsequent frame period. During the "on" period for each row, each control-grid segment would be biased to the potential needed to obtain the desired brightness in the pixel lying at the intersection of the control-grid segment and the activated photocathode segment. An alternate version of this display panel replaces the channel plate with a uniform grid mesh. This mesh is biased to the same level as the channel-plate input side. Although this version of the display will not be as bright as the channel-plate version, existing photocathode materials support current densities that will allow output light levels near that of conventional CRT's without using a channel-plate electron multiplier. Also, this alternate version is substantially easier to fabricate.

This work was done by Leslie James Payne of Goddard Space Flight Center.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

the Patent Counsel
Goddard Space Flight Center ; (301) 286-7351

Refer to GSC-13708.

Photonics Tech Briefs Magazine

This article first appeared in the January, 2000 issue of Photonics Tech Briefs Magazine.

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