Electric lamp and discharge devices – Cathode ray tube – Plural beam generating or control
Reexamination Certificate
1999-11-03
2002-04-09
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Plural beam generating or control
C313S412000, C313S421000, C313S409000, C313S441000, C313S396000, C315S369000, C315S364000, C315S382000, C315S386000, C315S003000
Reexamination Certificate
active
06369498
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to cathode ray tubes (CRTs) and more specifically to a CRT in which a pair of electron guns are used to write and erase an image on a target via secondary electron emission.
2. Description of the Related Art
Electron guns are used to write and erase a charge pattern onto a beam-addressing surface of a light valve target. The charge pattern imparts a modulation onto a light beam in proportion to the pixel intensities and directs the modulated light beam through projection optics to form a video display. Such beam-addressed light valve targets have been demonstrated using transmissive and reflective liquid crystals, reflective membranes, deformable mirror layers and pixelated micromirror arrays.
Most of these targets utilize the secondary electron emission characteristics of the addressing surface to write and erase the charge pattern. The addressing surface is characterized by a secondary electron emission curve that plots the emission coefficients &dgr; i.e. the ratio of emitted secondaries to incident primaries, against the landing energy of the primary electrons. At landing energies between first and second crossover points (&dgr;1), the surface exhibits a coefficient greater than one. Outside that region, the surface exhibits a coefficient less than one. In general, clean conductors have coefficients less than one and insulators have coefficients greater than one for useful beam energies.
In known systems, the write gun emits primary electrons that strike the target's addressing surface with a landing energy above the first crossover causing more secondary electrons to be ejected than incident primary electrons. The secondaries are collected by a collector electrode (grid or plate), that is held at a relatively positive potential with respect to the addressing surface. This produces a charge pattern that has a positive net charge, which increases the pixel potentials and in turn actuates the liquid crystal, membrane, reflective layer or micromirror to modulate the light. The degree of modulation is controlled by changing the beam current.
In video applications, each charge pattern or frame must be erased prior to the next pass of the write gun. It is well known that the brightness of the light modulator is closely tied to the optical throughput of the target. In large part, optical throughput is determined by the frame time utilization of each pixel, i.e. how long the pixel is held in its modulated position before it is erased. Ideally, each pixel would be held at its intended modulated position until that pixel was to be rewritten and then instantaneously erased. This would maximize the amount of light passed through the projection optics while maintaining video performance.
A common erasure technique is RC decay, in which the deposited charge is bled off over the frame time. The device's RC time constant must be short enough that the pixel intensity is erased prior to writing the next value in order to maintain video performance. The main drawback, however, is the fact that approximately two-thirds of the available light is lost due to RC decay. This greatly limits the display's brightness and contrast capabilities.
In the 1950s, U.S. Pat. No. 2,682,010 to Orthuber and entitled “Cathode-Ray Projection Tube” introduced scanning an electron beam over a transparent dielectric element suspended above an array of reflective “flaps”. The deposited charge pattern exerts electrostatic forces on the flaps causing them to deflect and form a projected image.
Orthuber suggests two possible ways to erase the charge pattern, the traditional RC decay as shown in his FIG.
3
and the use of a separate erase gun as shown in his FIG.
4
. The erase gun operates between the first and second crossovers and leads the write beam by a short interval, such as one or two periods of the horizontal sweep frequency, so that each pixel is restored to reference potential shortly before being subjected to the write beam.
As shown in his
FIG. 4
, Orthuber suggests placing “two complete beam generating or deflecting systems”, i.e. the write and erase guns in a single off-axis neck. Orthuber's double bi-potential guns each have an emitter (cathode); a wehnelt suppressor electrode (biasing electrode), a focusing electrode, a set of vertical electro-static deflection plates and a set of horizontal electro-static deflection plates. Based upon the drawing, the Orthuber guns do not have standard triodes. Ordinarily, the triode is comprised of an emitter, a Wehnelt suppressor electrode and a first accelerator.
The Orthuber guns use the focusing electrode to function as both the focusing electrode and the first accelerator. This is extremely bad practice. In fact, this gun would not function properly. Since Orthuber shows a target that is tilted by 45° the device would require dynamic focus voltage correction. If a dynamic voltage were applied to the focusing electrode, which functions as a first accelerator, then the cathode emission would modulate uncontrollably.
Orthuber also shows horizontal deflection plates directly in front of the focusing electrode. The volume between the focusing electrode and the horizontal deflection plates forms the main lens of a bi-potential type electron gun. The main lens in Orthuber's gun is not rotationally symmetric. This configuration would cause uncontrollable astigmatism and cause the electron gun to not function properly.
By placing two complete beam generating systems in a single neck in the manner depicted in
FIG. 4
, Orthuber's CRT would require a specially designed and manufacturer stem to bring the external potentials inside the single neck. Orthuber's design would require a 22-pin rotation stem with two isolated high voltage pins (one for each focusing potential), four open pins (two on each side of the high voltage pins) and sixteen low voltage pins. This would be very difficult to fit inside the neck glass without having arcing between the pins and would be very expensive to manufacture.
In the early 1970s, Westinghouse Electric Corporation developed an electron gun addressed cantilever beam deformable mirror device, which is described in R. Thomas et al., “The Mirror-Matrix Tube: A Novel Light Valve for Projection Displays,” ED-22 IEEE Tran. Elec. Dev. 765 (1975) and U.S. Pat. Nos. 3,746,310, 3,886,310 and 3,896,338. A low energy scanning electron beam deposits a charge pattern directly onto cloverleaf shaped mirrors causing them to be deformed toward a reference grid electrode on the substrate by electrostatic actuation. Erasure is achieved by raising the target voltage to equal the field mesh potential while flooding the tube with low energy electrons to simultaneously erase all of the mirrors, i.e. the whole frame. This approach improves the modulator's FTU but produces “flicker”, which is unacceptable in video applications.
At the same time IBM was developing the DSDT as described by James Ross and Eugene Kozol's paper entitled “Performance Characteristics of the Deformographic Storage Display Tube (DSDT)” IEEE Intercon Technical Papers, Session 7, pp. 1-8, 1973. The DSDT is a dielectric membrane (target), which consists of an electronically controllable storage substrate, a deformable material layer, and a reflective layer. The target is mounted in the tube envelope so the storage substrate faces the electron gun. The deformable material with its conformal reflective layer is isolated in the separate front chamber of the tube. Deformations are created in the deformable material as the result of negative electrostatic charges deposited by the on-axis write gun, which is operated above the second crossover. These deformation are converted into a visual image by the off-axis schlieren optical systems. The charge pattern is erased by an off-axis flood erase gun that operates between the first and second crossovers. Electronic control of these guns provides for storage mode, variable persistence mode and selective erase modes of control.
In
Patel Nimeshkumar D.
Roy Sikha
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