WO2007027181A1

WO2007027181A1 - Cathode ray tube having improved light output gradient and visual perception

Info

Publication number: WO2007027181A1
Application number: PCT/US2005/031166
Authority: WO
Inventors: Robert Lloyd Barbin; Istvan Gorog
Original assignee: Thomson Licensing
Priority date: 2005-08-31
Filing date: 2005-08-31
Publication date: 2007-03-08
Also published as: TW200731795A

Abstract

A cathode ray tube (CRT) having a glass envelope is disclosed. The glass envelope is formed of a rectangular faceplate panel having a viewing screen and a tubular neck connected thereto by a funnel. An electron gun is positioned in the neck for directing electron beams toward the faceplate panel. The electron beams emitted from individual cathodes of the electron gun form individual sub-images of a corresponding particular color when scanned on the screen according to a video input signal. A video signal processing system having luminance processing is employed to correct the light output at a plurality of points along the viewing screen by adjusting the drive voltage of the video signal according to a voltage correction factor calculated to achieve desired light output at each point.

Description

Cathode Ray Tube Having Improved Light Output Gradient and Visual Perception

Field of the Invention

The invention is related to Cathode Ray Tubes (CRT) and more particularly to one having improved light output gradient and visual perception.

Background of the Invention In a conventional CRT design, it is desirable to have a uniform or at least smooth transition light output across the screen as viewed by the eye. However, CRTs typically have significantly lower light output near the edges of the screen. Light output at the edges is sacrificed for a number of reasons, principally because of thicker panel glass at the edges and reduced system transmission in the comers. Maintaining adequate purity and white uniformity is also a consideration in the CRT design which causes register tolerance to be increased significantly in the corners relative to the center, thus reducing the mask and matrix transmission and consequently, the light output. The human eye is relatively insensitive to a smooth transition in light output from the center to the corners for normal television viewing because of a very low spatial frequency. Some degree of light output center-to-edge grading has been found to be commercially acceptable. For example corner light output as low as 50 percent of the center, while not desirable, has in the past been acceptable for consumer use.

With the advent of flat CRTs, managing the light output gradient, which is defined as the light output change across the screen, has become more challenging. In order to achieve a flat-panel CRT, the glass on the viewing side of the faceplate panel is relatively flat, while the glass on the inside of the CRT has a curvature to accommodate the electron beam paths emanating from the electron guns. These characteristics result in an increased thickness of glass near the edges of the screen, which is known as the panel wedge. The wedge is also larger on flat-panel CRTs for implosion protection requirements. The thicker glass in the panel wedge absorbs more light, further reducing the edge to center light output gradient. Typical designs using normal center and edge tolerances, may have only 40 percent light output gradient which may not be commercially unacceptable. Although the light output gradient from center to the edge of the screen may take different forms, it is often characterized by a parabolic function wherein the light output reduction is proportional to the square of the distance from the center. Depending upon the thickness characteristics of the faceplate panel glass, and other factors, light output gradient may be characterized by other higher order functions. What is needed is a system and method for improving the light output gradient across the screen of the CRT.

Summary

The invention provides a CRT having a glass envelope. The glass envelope is formed of a rectangular faceplate panel having a viewing screen and a tubular neck connected thereto by a funnel. An electron gun is positioned in the neck for directing electron beams toward the faceplate panel. The electron beams emitted from individual cathodes of the electron gun form individual sub-images of a corresponding particular color when scanned on the screen according to a video input signal. A video signal processing system having luminance processing is employed to correct the light output at a plurality of points along the viewing screen by adjusting the drive voltage of the video signal according to a voltage correction factor calculated to achieve desired light output at each point.

Brief Description of the Drawings

The invention will now be described by way of example with reference to the accompanying figures of which: Figure 1 is a cross-sectional view a CRT according to the present invention;

Figure 2 is a graphical representation of an exemplary light output gradient and correction factors;

Figure 3 is a graphical representation of a peak drive voltage limiting transformation according to a peak current limiting algorithm

Figure 4 is a block diagram of a color receiver for driving the CRT of Figure 1; and

Figure 5 is a block diagram of a method according to the invention for correcting light output gradient in the CRT of Figure 1.

Detailed Description of the Preferred Embodiments

Figure 1 shows a cathode ray tube (CRT) 1 , for example a wide screen tube having a glass envelope 2 comprising a rectangular faceplate panel 3 and a tubular neck 4 connected by a funnel 5. The funnel 5 has an internal conductive coating (not shown) that extends from an anode button 6 toward the faceplate panel 3 and to the neck 4. The faceplate panel 3 comprises a viewing faceplate 8 and a peripheral flange or sidewall 9, which is sealed to the funnel 5 by a glass frit 7. The faceplate panel thickness is generally smaller in the center and increases approaching the sidewall 9 to define a panel wedge. A three-color phosphor screen 12 having a plurality of alternating phosphor stripes is carried by the inner surface of the faceplate panel 3. The screen 12 is a line screen with the phosphor lines arranged in triads, each of the triads including a phosphor line of each of the three colors. A mask assembly 10 is removably mounted in predetermined spaced relation to the screen 12. An electron gun 13, shown schematically by dashed lines in Figure 1, is centrally mounted within the neck 4 to generate and direct three inline electron beams, a center beam and two side or outer beams, along convergent paths through the tension mask frame assembly 10 to the screen 12. A- The electron gun 13 can consist of three guns being vertically oriented, which direct an electron beam for each of three colors, red, green and blue. The red, green and blue guns are arranged in a linear array extending parallel to a minor axis of the screen 12. The phosphor lines of the screen 12 are accordingly arranged in triads extending generally parallel to the major axis of the screen 12. Likewise, the mask of the mask assembly 10 has a multiplicity of elongated slits extending generally parallel to the major axis of the screen 12. It should be understood by those reasonably skilled in the art that various types of tension or formed shadow mask assemblies which are well known in the art may be utilized. Further, the invention also has applicability for electron guns systems where the electron guns are oriented horizontally and the phosphor lines are arranged along the minor axis of the screen 12. The CRT 1 is designed to be used with an external magnetic deflection system having yoke 14 shown in the neighborhood of the funnel-to-neck junction. When activated, the yoke 14 subjects the three beams to magnetic fields which cause the beams to scan vertically and horizontally in a rectangular raster over the screen 12. In design of the CRT 1, light output across the screen 12 may be measured using a constant beam current at each location to determine a light output gradient of the screen 12. Although the following description will be directed to correcting a typical light output gradient which has been measured in typical flat panel CRT, the light output gradient correction method and system described here may be applied to correcting undesirable light output at any point on the screen. Such undesirable light output, while typically caused by the profile of the faceplate panel 3, may be caused by any number of factors, such as, but not limited to, variations in mask etching, matrix printing, filming, and aluminizing. The shape of the light output gradient from center to the screen edge may take on different forms, but it is often approximated by a parabolic function. That is, the reduction in light output is proportional to the square of the distance from the center of the screen 12. For the purpose of this description and for simplicity of the explanation, assume that the light output gradient to be corrected is characterized by a parabolic function. It should be understood, however, that in practice the light output gradient may have a different characteristic curve such as a higher order curve, i.e. fourth-order, or higher order characteristic curve. It should also be understood that the light output gradient may have different characteristics in the horizontal direction than in its vertical direction. It should also be understood that the principles of the invention to correct the particular light output gradient described may also be applied to other light output gradients or localized undesirable light outputs on a point by point basis. Having first measured the light output gradient of the CRT 1 , by way of an illustrative example, Figure 2 shows the light output distribution or light output gradient, LO old, for a parabolic light output gradient of 0.4. Theoretically, the light output gradient LO old can be corrected to any desired level. In this example, light output gradient is corrected to a parabolic light output gradient with a LOG of 0.8 as shown by the LO new curve. The equations of the curves shown in Figure 2 are:

LO old = l-0.6r²

LO new = l-0.2r²

Where LO old represents the light output before correction relative to the light output at the center, r represents the distance from the center normalized to the distance to the edge of the screen, and LO new represents the light output after correction relative to the light output at the center.

Correction of the light output gradient to move from the LO old curve to the LO new curve is achieved by increasing the video drive as a function of screen position by the amount needed to increase the beam current by the ratio of LO new/ LO old. This is shown as current new/old in Figure 2. In this particular example, the current must be doubled at the screen edge to increase the light output grade from 0.4 to 0.8.

For CRT electron guns, the beam current is proportional to the drive voltage raised to the gamma power. The relationship can be described by: I= iζ_*V^gamma

Where I represents beam current, V represents drive voltage, and K is a constant relating to the cutoff and bias setting of the electron gun. Gamma represents the exponent of the exponential transfer characteristic from voltage to current for the electron gun Consequently, to determine the drive voltage needed, the desired current ratio is reduced by the 1/gamma power. For CRTs, the gamma typically has a value of 2.7. The correction factors for beam current and drive voltage for this example may be specified as follows:

Current new/old = (1 -0.2I*)/(1 -0.6I*) and

Voltage new/old = [(l-0.2i*)/(l-0.6i²)]^{I/Eaιinm) = [(1-O.2r²)/(1-O.6r²)]⁰³⁷

The voltage correction factor is then applied as a multiplying factor to the drive voltage of each of the three electron guns as a function of the distance from the center of the screen 12. Light output gradient is therefore improved by moving to the LO new curve. Applying the voltage correction factor to the drive voltage, however, sometimes may have a deleterious effect because the increased beam current may have undesirable effects on spot size and resolution of the image. As drive voltage is increased, beam current is increased, and spot size is also increased thus reducing resolution of the image. In the example described, the beam current in the extreme corners doubles with the application of the light output gradient correction. If the input video signal has high drive signals in this area, the increasing beam current with the light output gradient correction could cause unacceptable resolution performance. One must also be careful that the increased video drive does not drive the gun beyond zero bias. However, if the video signal in the corners is a low current level flat field, the visual benefit of the improved light output gradient could be applied without significant loss of resolution. The human eye response is logarithmic with brightness stimuli. Consequently, one can more easily recognize small differences in low level light signals than corresponding differences in high level or high brightness signals. The result is that light output gradient is more visible on low brightness fairly uniform fields than with high peak brightness images.

A point-by-point peak current limiter which applies an algorithm that limits the peak drive voltage and, therefore, the peak beam current to some predetermined value, but allows low level signals to be increased, gives significant improvement in the visual effects of the light output gradient without significantly degrading the resolution, resulting in an improved overall image. Such an exemplary peak limiter is shown graphically in Figure 3. The x-axis of Figure 3 shows the drive voltage from the light output gradient correction calculations, normalized to a predetermined peak drive voltage Vpk at an arbitrary point in the X-Y map. The Y-axis is the normalized drive voltage after the peak limiter. The voltage Vpk corresponds to the peak current not to be exceeded for the resolution reasons explained above. Vpk may be selected as a constant at all X, Y screen locations, or it may be predetermined variable with X, Y based on the CRT spot size performance. Figure 3 shows the application of both a sharp peak limiter shown by the solid line or a smoother transition peak limiter shown by the dotted line. Other algorithms could be applied to achieve different graphical representations of the peak limiter while maintaining the underlying principle of correcting the light output gradient of the low current large area signals where the light output gradient is most visible and limiting the small area high peak current where the spot resolution is more visible than the light output gradient. This invention is equally applicable to a video frame store system or to a system without video frame store by determining the location of the video on the screen from the horizontal and vertical scan timing signals with the proper video drive correction being applied thereto to obtain the light output gradient correction described above. A system for implementing the described light output gradient correction is shown in

Figure 4. Figure 4 represents a block diagram of a color receiver incorporating the light output gradient correction methods described above. Generally, the color receiver employs an antenna which receives a television signal which is passed through a tuner to an intermediate frequency demodulator (IF). The IF separates sound, video and sync signals. The sync signals are sent to a sync processor which drives a vertical deflection driver and horizontal deflection driver as is well known in the art for controlling the yoke 14. Although not shown here, dynamic focus voltages may also be applied to the CRT electron guns. The video drive signal is fed to a color decoder and a luminance processing portion. The luminance processing includes the normal brightness and contrast controls as well as video processing functions such as average beam limiter and automatic electron gun cutoff control.. The color decoder and luminance processing drive a color matrixing circuit which outputs uncorrected red green and blue signals (R, G, B) to the video correction controller which ultimately drive the electron gun 13 of the CRT 1. The electron beams emitted from individual cathodes of the electron gun form individual sub-images of a corresponding particular color when scanned on the screen according to a video input signal.

Figure 5 shows a block diagram of the light output grading signal processing methods described above and incorporated within the video correction controller. First, by measuring the CRT 1 for light output at multiple screen locations an uncorrected light output gradient X, Y map 22 is developed. The process involves predetermining uncorrected light output gradient over the screen area and a desired light output gradient over the screen area which are input to the video processing system.

This may be determined from design or typical values or measured and stored for each specific CRT. A desired light output gradient X, Y map 24 is also determined. These maps 22, 24 are used to calculate the beam current correction factor at step 20 as described above. The beam current correction factor is then converted to a voltage correction factor at step 30. It should be understood that steps 20 and 30 are achieved as static calculations as a function of CRT design or measured on individual CRTs. The voltage correction factor is applied to the existing light output gradient X, Y map at step 40 to develop a modified frame store containing point by point drive voltages. The peak current limiter algorithm is then applied at step 50 to the R, G, B video drive signals which are then output to the picture tube drivers as R', G', B'. It should be understood that steps 40 and 50 are achieved through dynamic calculations as a function of video input.

The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting.

Claims

What is claimed is:

1. A cathode ray tube (CRT) comprising: a glass envelope having a rectangular faceplate panel with a viewing screen and a tubular neck connected thereto by a runnel; an electron gun positioned in the neck for directing electron beams toward the faceplate panel, said electron beams emitted from individual cathodes of said electron gun form individual sub-images of a corresponding particular color when scanned on the screen; and a video signal processing system having luminance processing which corrects the light output at a plurality of points along the viewing screen by adjusting the drive voltage of the video signal according to a voltage correction factor calculated to achieve desired light output at each point.

2. The CRT of claim 1 further comprising a predetermined uncorrected light output gradient over the screen area and a desired light output gradient over the screen area which are input to the video processing system.

3. The CRT of claim 2 wherein data representing the light output gradients is utilized to convert light output to an electron beam current and then to a drive voltage at each point.

4. The CRT of claim 3 further comprising a peak current limiter which is applied to the drive voltage at each point to limit drive voltage that would result in an undesirably high drive current at each point.

5. A method for correcting a light output gradient of a cathode ray tube (CRT) comprising the steps of: predetermining and storing an uncorrected light output at a plurality of points along a screen of the CRT; determining a desired light output for each point along the screen; calculating current ratios of the desired light output to the existing light output for each point along the screen; converting the current ratios to voltage ratios; applying the voltage ratios to an existing drive voltage to develop a modified o drive voltage for each point along the screen; and applying a peak current limiting algorithm to the modified drive voltage at each point.