A novel cathode ray tube (CRT), using a single electron gun and a movable screen, has been developed that now enables miniaturization of a full-color CRT with the same excellent viewing quality customarily found in larger-screen CRTs. In addition to the benefits of wide viewing angle, high resolution, high brightness, color purity, and full gray-scale features that are characteristic of CRTs, the need for only one electron gun is expected to also result in reduced power consumption and lower cost. The movable-screen design, a significant improvement over the earlier moving-shadow-mask version (1), is considered feasible for CRTs ranging in size from less than 1 in. (2.5 cm) to greater than 5 in. (12.7 cm). A unique and highly advantageous feature of the improved design, which is described in greater detail below, is the elimination of spatial offset of the color pixels. One obvious application with great commercial potential is in helmet-or head-mounted displays for a wide variety of virtual-reality systems. Other applications include portable or hand-held devices where compactness, low power, high resolution, and high brightness are desirable or advantageous, such as TVs, monitors for VCRs, and viewfinders for camcorders, especially for outdoor use.
The conventional CRT uses three electron guns, one for each primary color (red, green, blue), plus a stationary slotted or otherwise perforated shadow mask aligned with the color phosphors on the glass screen. The geometrical relationship between the mask and the guns is designed so that the electron beam from each gun impinges on only the phosphor dots of the desired primary color. Accurate alignment of the guns, shadow masks, and phosphors is critical to the purity of the primary colors and resolution of the display. Achieving the beam convergence and registration required for high resolution becomes extremely difficult for a miniature full-color CRT with three electron guns and is, therefore, commercially impracticable. Other single-electron-gun designs, such as the beam index tube and color shutter tube, lack either the high resolution or high brightness desirable for most miniature-display applications. By default, the miniature color display market is presently dominated by flat-panel displays (FPDs), the most common of which is theactive matrix liquid crystal display (AMLCD). This and all other miniature FPDs, either presently on the market or under development, have one or more of the following drawbacks: poor resolution (graininess), low brightness, narrow viewing angle, high cost, or high power consumption. The miniature full-color CRT described here has none of these drawbacks.
A simplified representation of the electron beam -- slotted shadow mask -- movable screen region is shown in the figure. In contrast to the earlier version (1), the shadow mask remains stationary and the moving part is a thin inner glass sheet that contains the parallel red-green-blue phosphor stripes and is mounted on piezoelectric actuators for precisely controlled movement. To write a given primary color, the electron gun is activated at the beam intensity needed to obtain the desired brightness. At the same time, the piezoelectric actuators are energized to align the phosphor stripes of that color with the slots in the shadow mask and also mask the other two colors with the solid portion. The entire color field is written before the screen is moved to uncover the next color. Full color is achieved by overlaying the three-color fields in time. In the improved movable screen version, the moving element is much lighter; thus shortening the hold-off time between color changes to less than 140 ms. If the mask is not perfectly aligned with the phosphor stripes during assembly of the CRT, it can be accomplished electronically during monitor calibration by applying dc-offset voltages to the piezoelectric actuators. The capability for electronic alignment is an important feature that offers not only the possibility of greater color purity and brightness, but also lower manufacturing cost by reducing the elaborate jigging required for alignment during assembly of the conventional shadow mask CRT. Having the light-emitting element--the screen--moving is a great advantage because the color pixels have no spatial offset as seen by the viewer. Up close, the viewer sees one composite color dot, not three primary color dots that the eye must then attempt to integrate. Except for the color shutter tube, which is seriously lacking in brightness, this feature is not found on other displays including conventional CRT's, AMLCDs, and FEDs (fuel-emission displays). It is particularly important where close-in viewing is necessary, such as in helmet-mounted displays.
A further improvement contributing to lower cost is assembly of the mask and actuator components on a laser-cut, multilayer ceramic stack with printed-circuit elements to power the piezoelectric actuators. The ceramic stack is part of the vacuum envelope wall and leads to lower part count, less complexity, and better alignment and rigidity.
By providing the means to rapidly move objects in vacuum with amplitudes up to 0.015 in. (0.38 mm) also the capability to withstand temperatures up to 450 °C, this work advances the state of piezoelectric technology. It is expected to find application in other areas, such as sensors and MEM (micro electromechanical) devices.
(1) B.K. Vancil and E.G. Wintucky, Ultra-high Resolution Miniature Color CRT for Virtual-Reality Applications, Proceedings of 5th National Technology Transfer Conference (Technology 2004), Vol. 2, NASA Conference Publication 3313, 1994.
This work was done by Bernard K. Vancil of FDE Associates for Lewis Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
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Refer to LEW-16187.