WO2004039060A1

WO2004039060A1 - Cathode ray tube with sensor feedback

Info

Publication number: WO2004039060A1
Application number: PCT/IL2003/000788
Authority: WO
Inventors: Shlomo Guterman; Ron Shem-Tov
Original assignee: Elop Electro-Optical Industries Ltd.
Priority date: 2002-10-22
Filing date: 2003-10-01
Publication date: 2004-05-06
Also published as: AU2003272047A1

Abstract

As part of the present invention, the position of an electron beam on an inner surface of a CRT display screen may be sensed by a sensor and a feedback signal correlated with a characteristic of the sensed electron beam may be produced. A processing or computing unit may estimate a characteristic of the CRT based on the feedback signal and may adjust parameters of geometric correction, brightness level and focus potential in accordance with the estimated characteristic.

Description

CATHODE RAY TUBE WITH SENSOR FEEDBACK

FIELD OF THE INVENTION

The present invention relates generally to the field of cathode ray tubes

("CRT"). More specifically, the present invention relates to a CRT with sensor

feedback from sensors on a display screen, allowing for dynamic adjustment of

circuits driving the CRT.

BACKGROUND OF THE INVENTION

CRT's have been used for display of information continually over the past

few decades. The general principles by which a CRT operates are well known.

However, many enhancements and innovations (e.g. high-resolution color screens)

have been made since the CRT's first introduction. Nevertheless, adjustment of

various CRT parameters (e.g. offset, gain and brightness) which may affect the

quality of an image displayed are for the most part still performed manually.

Each CRT has unique and changing physical characteristics which need to

be taken into consideration when generating control signals (e.g., cathode drive,

x-axis deflection and y-axis deflection) to drive the specific CRT. A CRT's

characteristics depend on a variety of factors including its geometry, materials from

which its components are constructed, internal temperature and external influences

such as background electromagnetic radiation. According to the prior art, a CRT

based display system may be driven by deflection circuitry, which adjusts incoming video signals to conform to predefined positioning characteristics of the CRT. The

deflection circuitry ma3' be tuned once at the factory where the display system is

produced and, aside for minor manual adjustments a user may make while viewing

a picture on the CRT, the deflection circuitry is not subsequently updated to take

into account the CRT's changing characteristics.

U.S. Pat. No. 4,456,853 ("the '853 patent") teaches a cathode-ray tube

including feedback means capable of producing a feedback signal indicative of the

position, in at least two dimensions, of a scanning electron beam. The feedback

means comprises a plurality of feedback elements disposed at selected locations

within the tube enclosure. With a shadow-mask type CRT, the elements are formed

on the gun-side surface of the shadow mask itself. With other kinds of CRT's, the

elements are formed either on an interior supporting member or on the interior

surface of the display medium. In one embodiment, the elements are formed of a

phosphorescent material. In other embodiments, the elements are formed of

materials capable of producing, upon excitation by a scanning electron beam,

signals of visual or electrical character. The signals thus produced may be employed

in a closed-loop correction system to accomplish automatic convergence and/or

geometric adjustment of a displayed image.

The system in the '853 patent, however, only works with CRTs operating in

raster scanning mode where the sensors are positioned in an active area of the raster

pattern. The sensor elements according to the teachings of '853 are placed in

positions over which the electron beam of the CRT passes at a regular interval. However, the system according to the '853 patent may not work if the electron

beam does not pass over the sensor elements, such as may occur if the CRT is

operated with a small raster pattern, or the CRT is operated in stroke mode.

CRTs operated in stroke mode do not have an electron beam scanning

across each point on the screen at a regular interval. Instead, a beam scans over a

limited area. Thus, in the feedback system according to the '853 patent, the sensors

would be inactive during stroke mode operation and the feedback system would

have limited data upon which to base its positioning adjustment of a video signal.

SUMMARY OF THE INVENTION

As part of the present invention, there may be one or more sensors fixed

substantially around a display screen on a CRT. The sensors may provide a

feedback signal to a computing or processing unit. The feedback signal may be

indicative of the position, focus and intensity of an electron beam engaging an inner

surface of the display screen. The computing unit may derive from the feedback

signal an estimate of the CRT's display characteristics and, based on the display

characteristics, may adjust parameters for controlling, e.g., geometric correction,

cathode amplification and/or focus potential. An internal excitation unit may

generate a test signal intended to direct the CRT's electron beam to an area on the

CRT screen where one or more of the sensors may reside. The excitation unit may

generate a test signal mtermittently. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference

to the accompanying drawings, in which like components are designated by lilce

reference numerals, wherein:

Fig. 1 is a schematic, isometric-view, illustration of a CRT having a set of

sensors according to some embodiment of the present invention;

Fig. 2 is a schematic front-view illustration of the screen of the CRT of Fig.

1 with photon and/or electromagnetic radiation sensors fixed around a viewing area

of the CRT screen, according to some embodiment of the present invention;

Fig. 3 is a schematic, cross-sectional view, illustration of a CRT according

to some embodiment of the present invention;

Fig. 4 is a schematic, block diagram, illustration of a CRT associated with

CRT driving circuitry according to some embodiment of the present invention;

Fig. 5 is a schematic flow chart showing the steps of a method for

initialization of deflection, brightness and focus parameters of a CRT driving circuit

in accordance with embodiments of the invention;

Fig. 6 is an illustration of the results that may be received when scanning of

the photon sensors;

Fig. 7 is a block diagram showing the steps of a method of deflection and

brightness real time adjustment (RTA) in accordance with embodiments of the

invention; and Fig. 8 is a block diagram showing the steps of brightness and focus

adjustments that may be included in the deflection and brightness RTA procedure of

Fig. 7.

It will be appreciated that for simplicity and clarity of illustration, elements

shown in the figures have not necessarily been drawn to scale. For example, the

dimensions of some of the elements may be exaggerated relative to other elements

for clarity. Further, where considered appropriate, reference numerals may be

repeated among the figures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, numerous specific details are set forth

in order to provide a thorough understanding of the invention. However, it will be

understood by those skilled in the art that the present invention may be practiced

without these specific details. In other instances, well-known methods, procedures,

components and circuits have not been described in detail so as not to obscure the

present invention.

Unless specifically stated otherwise, as apparent from the following

discussions, it is appreciated that throughout the specification discussions utilizing

terms such as "processing", "computing", "calculating", "detennining", or the like,

refer to the action and/or processes of a computer or computing system, or similar

electronic computing device, that manipulate and/or transform data represented as

physical, such as electronic, quantities within the computing system's registers and/or memories into other data similarly represented as physical quantities within

the computing system's memories, registers or other such information storage,

transmission or display devices.

Embodiments of the present invention may include apparatuses for

performing the operations herein. This apparatus may be specially constructed for

the desired purposes, or it may include a general purpose computer or digital signal

processor ("DSP") selectively activated or reconfigurable by a computer program.

Such a computer program may be stored in a computer readable storage medium,

for example, any type of disk, including floppy disks, optical disks, CD-ROMs,

magnetic-optical disks, read-only memories (ROMs), random access memories

(RAMs), electrically programmable read-only memories (EPROMs), electrically

erasable and programmable read only memories (EEPROMs), magnetic or optical

cards, or any other type of media suitable for storing electronic instructions, and

capable of being coupled to a computer system bus.

The processes and displays presented herein are not inherently related to

any particular computer or other apparatus. Various general purpose systems may be

used with programs in accordance with the teachings herein, or it may prove

convenient to construct a more specialized apparatus to perform the desired method.

The desired structure for a variety of these systems will appear from the description

below. In addition, embodiments of the present invention are not described with

reference to any particular programming language. It will be appreciated that a variety of programixiing languages may be used to implement the teachings of the

inventions as described herein.

As part of the present invention, there may be one or more sensors fixed

substantially around a display screen on a CRT. The sensor may provide a feedback

signal to a computing unit. The signal may be indicative of the position, focus and

intensity of an electron beam engaging an inner surface of the display screen. The

computing unit may derive from the feedback an estimate of the CRT's

characteristics and, based on the estimated characteristics, may adjust parameters

controlling a geometric correction unit, a cathode drive and/or a focus potential. An

excitation unit may generate test signals intended to direct the CRT's electron beam

to an area on the CRT screen where one or more of the sensors may reside. The test

excitation unit may generate test signals intermittently.

Turning now to Fig. 1, there is shown a schematic, isometric, view of a

CRT 100 including a set of photon sensors (122a to 122h) according to some

embodiments of the present invention. Reference is also made to Fig. 2, which

schematically illustrates a front view of the CRT 100 of Fig. 1. CRT 100 may have

at least three control lines relevant to the present invention, namely, a cathode driver

line 130, an x-axis deflection line 132, and a y-axis deflection line 134. CRT 100

may also have a focus voltage control line (not shown in Fig.l), as is known in the

art. Feedback signals from sensors 122, as described in detail below, may be carried

via a feedback line 140. The CRT 100 may have a display screen 110 with an active viewing area.

The active viewing area is a portion of the screen 110 on to which a picture or

image may be projected by an electron beam produced by a cathode gun inside the

CRT 100. In some embodiments of the present invention, as shown in Figs. 1 and

2, the sensors may be arranged in a substantially circular configuration within a

portion of the screen 120 outside the active viewing area. However, it will be

apparent to those skilled in the art that many other suitable configurations may also

\ be devised.

Turning now to Fig. 3, there is shown a cross section of a CRT according to

some embodiments of the present invention. An inner surface 150 of the screen 110

may be coated with emitting material, which may include any phosphorous

materials known in the art, for example, phosphors emitting visible and/or infrared

light in response to an impinging electron beam. Sensors 122 may detect photons

produced or emitted when an electron beam strikes emitting material regions 152 on

the inner surface 150 near the position of the sensors. The sensors 122 may include

photo diodes, photocells, infrared photo diodes, or any functionally equivalent

electronic component. Various phosphorescent materials may be used to coat area

150, for example, visible-light emitting phosphors, e.g., green phosphors or any

other color phosphors and/or infrared-emitting phosphors, as are known in the art.

In some embodiments of the present invention, the emitting material of

regions 152 opposite sensors 122 may be different from the emitting material

coating the rest of inner surface 150 opposite the viewing area of the screen. The different emitting material in regions 152, regardless of its exact composition, may

emit photons of a different wavelength and/or different intensity from those emitted

by the emitting material coating the inner surface 150 opposite the active viewing

area of the screen. In an embodiment of the present invention, for example, the

emitting material in regions 152 may emit infrared radiation when struck by an

electron beam, and the emitting material coating the inner surface 150 opposite the

active viewing area may emit visible light, for example, green light produced by

phosphors, as is known in the art. The sensors 122 may be selected to detect only

infrared radiation (e.g. photo diodes) and thus may produce a signal when an

electron beam strikes the different emitting material 152. The sensors 122 may be

selected to correspond to the different emitting material 152. That is, tlie sensors

may be selected to detect the presence and characteristics of photons emitted by the

different emitting material 152 when struck by an electron beam.

Turning now to Fig. 4, there is shown a CRT with a CRT driving circuit

200 according to some embodiment of the present invention. Video signals may be

amplified by amplifiers 202A (amplifies an X-axis deflection input signal), 202B

(amplifies a Y-axis deflection, input signal), and 202C (amplifies an intensity input

signal), and may pass through multiplexers ("MUX's") 206A, 206B and 206C,

respectively. MUX 206A and MUX 206B may pass the signals to a geometric

correction unit 208. The geometric correction unit 208 may apply a polynomial

function to one or more component of the video signals, thereby controlling x-axis

and/or y-axis deflection of the electron beam. The X and Y axis control signals, once adjusted and/or amplified, may be applied to deflection coils in the CRT yoke,

as shown in Fig. 3. The geometric correction unit 208 may adjust upper and lower

values of the control signals such that the electron beam is deflected to portions of

the inner surface of me CRT screen opposite the active viewing area of the screen,

thereby to center the picture. The geometric correction unit 208 may also adjust the

maximum and minimum values of the control signals such that the electron beam's

sweep is from one end to another end of the active viewing area, so that a full

picture is as wide as the active viewing area of the CRT screen may support. The

above two parameters may be referred to as "offset" and "gain" respectively.

"Offset" and "gain" are only a few of the parameters a geometric correction unit

208 may adjust. A geometric correction unit 208 may also adjust the video signal to

compensate for numerous possible distortions such as roll, none-perpendicularity,

none-linearity, pincushion, and others.

A cathode driver 209 may amplify and adjust a signal being applied directly

to the cathode gun C. By adjusting the voltage and/or current supplied to the

cathode gun C, it is possible to adjust the intensity of the electron beam during

operation of CRT 100. The greater the intensity of the beam, the brighter the spot

the beam generates on the screen 110.

A test excitation unit 204 may produce test signals which may pass through

the MUX's 206A, 206B and 206C and may be applied to the geometric correction

unit 208 (from MUX's 206A and 206B), to the cathode amplifier & driver 209

(from MUX 206C), and finally to the CRT 100. The test signals may be generated such that they cause the electron beam to sweep across an area of the inner surface

150 opposite one or more sensors 122. The testing signals may be triggered,

controlled and monitored by a computing or processing unit 212 (e.g. CPU). The

computing unit 212 may also receive feedback from the sensors 122 over line 140

which is sampled by an analog-to-digital (A/D) converter 210A. Each of the

sensors 122 may be located at a known point on the screen. Thus, when the

computing unit 212 receives an indication from a sensor 122 that the electron beam

is striking a point opposite the sensor 122, the computing unit may compare that

sensor's coordinates (X_se_so_r5Ysenso_r,) and values against the position or coordinates

(Xi_ni_bYi_ni_t) corresponding to the coordinates and the values that are set after

completing the calibration procedure and stored on memory device 219. By

comparing the desired deflection (Xi_nit,Y__t) relative to tlie actual deflection

(Xse_ns_orsYse_nso_r the computing unit may estimate parameter value adjustments for

geometric correction unit 208, thereby reducing possible errors between the desired

and actual deflection. Appropriate correction may be performed through a

digital-to-analog (D/A) converter 21 IB.

The sensors 122 may also provide feedback regarding the focus and

intensity of the electron beam. A signal produced by the sensors 122 may indicate a

brightness level of a spot produced by the electron beam striking the emitting

material corresponding to the sensor. For example, a change in the amplitude in a

sensor's signal may correlate to the intensity of the spot produced by the electron

beam. The gradient or slope of a signal produced by a sensor as the electron beam passes over the emitting material corresponding sensor may indicate the focus of the

beam. It should be appreciated that the sharper the slope of the signal, the more

focused the beam may be.

The computing unit 212 may calculate focus and brightness values. The

computing unit 212 may send a signal to adjust the voltage supplied by High

Voltage Power Supply unit 215 (HVPS) and the form of a Focus Grid "F" in the

CRT 100 in accordance with the brightness and focus feedback signals from the

sensors 122. The HVPS 215 and Focus Grid "F" may be adjusted by the computing

unit 212 through a D/A 211A and a D/A 211C, e.g., to maintain a predefined

brightness and focus level for the CRT. The CPU may be programmed to maintain a

predefined brightness and focus level defined during initial calibration of the CRT.

Brightness may be adjusted by increasing the magnitude of the voltage applied to

the CRT's electron gun. The computing unit 212 may produce a signal that may be

applied, directly or through D/A 211 A, to the cathode driver 209. The cathode

driver 209 may be adjusted to either increase or decrease the cathode gun voltage,

depending on whether an increase or decrease in the brightness is required. Focus

may be adjusted by adjusting the signals applied to the HVPS 215, which controls

Focus Grid "F". Adjustments of brightness and focus in a CRT are well known in

the art, and any suitable method currently known or to be devised in the future may

be used in conjunction with the present invention.

In some embodiments of the present invention, the CRT may operate in a

stroke mode. That is, the electron beam may be directed to trace a pattern across the inner surface 150 opposite the active area of the screen. Stroke mode differs from

raster mode in that the beam is only directed to those points which are part of the

picture to be presented. In stroke mode, a given pattern may be traced and retraced

several times per second. The duty cycle for tracing may be anywhere from 20% to

90%). During the off-cycle, test excitation unit 204 may produce a short duration test

signal. During the off-cycle, the MUX's 206A, 206B and 206C may couple the test

signals into the CRT. The test signals may direct the electron beam to an area

associated with one or more of the sensors in order to determine an error value

between the initialized and actual beam deflection. Numerous test algorithms may

be used to estimate CRT characteristics in conjunction with the present invention.

Reference is now made to Fig. 5, which is a schematic block diagram

illustration of an initialization of deflection, brightness and focus parameters. These

parameters may be used in test algorithms and may be calculated and determined

during an initialization process. The initialization process may usually be performed

after completion of the CRT's calibration procedure.

The initialization process may begin when photon sensors 122a, 122c, 122e

and 122g are scanned (step 300). The scanning results are used to pre-determine a

"Center of Gravity" of each photon sensor. Fig. 6 schematically illustrates a graphic

representation of exemplary results from scarining the photon sensors. The "Center

of Gravity" of each sensor is saved as an initialized value (step 310). For example,

an initialized value that may be used for photon sensor 122A may be stored as a

variable EXC_A(VGC)_INIT. Accordingly, initialized parameters that may be used for photon sensors 122e, 122c and 122g may be stored as variables

EXC_E(VGC)_IMT; EXC_C(HGC)_INIT and EXC_G(HGC)_INIT. In step 320,

the initialized values may be used to calculate and detennine positional distances,

for example, relative to sensor pairs 122a- 122e and 122c- 122g, which sensor pairs

may be placed at diametrically opposite positions along the circumference of the

CRT display screen, as shown in Figs. 1 and 2.

The positional distances relative to the sensors may be used to set the gain

and offset parameters. The positional distances may be calculated, for example, by

subtracting the aforesaid variable that represent the "Center of Gravity" of photon

sensor 122e from the variable that represent the "Center of Gravity" of photon

sensor 122a. The result is determined as the value that represents the positional

distance for the X-axis (Δ_HEXC_INIT). Similarly, the value that represents the

positional distance for the Y-axis (Δ_VEXC_INIT) may be determined by

subtracting the aforesaid variable that represents the "Center of Gravity" of photon

sensor 122g from the variable that represents the "Center of Gravity" of photon

sensor 122c). In step 330. An average brightness value, e.g.

ALL_BRG_AVR_ΓNIT, of the photon sensors may also be determined. Step 330

may be executed after step 320 or, in alternate embodiments, in parallel with step

310 described above.

Another value that may be determined for each of the scanned sensors is the

initial focus value (step 340). Any suitable method for determining focus values in

CRT's, as is known in the art, may be used in step 340. For example, the focus value of the CRT may be determined by finding the spatial location of the half

values of the variables that represent the "Center of Gravity" of the photon sensors

122a to 122g. Afterwards, the spatial locations may be used to detenxiine the cross

sectional area at half the height of the graphic representation of the results that may

be received when scanning the photon sensors (see Fig. 6). The value that represents

the area in that position may be used to determine the focus parameter for each

photon sensor. Step 340 may be executed after step 320 and step 330 or, in alternate

embodiments of the invention, in parallel with either or both of these steps. The

values that may be calculated in steps 300 to 340 may be stored in a computer

readable storage medium, such as, but not limited to, electrically erasable and

programmable read only memories (EEPROMs) (Step 350).

Once the initialization of deflection, brightness and focus parameters is

completed and the CRT is operational, real time adjustment (RTA) procedures may

be applied to periodically adjust the various parameters.

Reference is now made to the block diagram of Figs. 7 A and 7B, which

schematically illustrates a deflection and RTA procedure in accordance with

embodiments of the invention. This procedure may be performed during normal

operation with no display interruption.

Referring to Fig. 7A, the deflection and RTA procedure may begin when

photon sensors 122a and 122e (representing the Y-axis) are scanned and a "Center

of Gravity" for each photon sensor may be determined (step 400). The "Center of

Gravity" for each sensor is saved as its current value. For example, a current value for photon sensor 122a may be stored as a variable CURR_A(VGC)_INIT.

Accordingly, the current value for photon sensor 122e may be stored as a variable

CURR_E(VGC) NIT.

In step 410, the current "Center of Gravity" values may be used to calculate

and to determine a current positional distance relative to sensor pair 122a- 122e. The

positional distance relative to the sensors may be used to set the gain and the offset

parameters. The current positional distances may be calculated, for example, by

subtracting the variables that represent the current vertical "Center of Gravity" of

sensor 122e from the current vertical "Center of Gravity" of sensor 122a. The result

may be stored as a variable Δ_VEXC_CURR. In this embodunent of the invention,

the value of the current positional distance may be used, for example, first to adjust

the gain parameter and, afterwards, to adjust the offset parameter.

In step 420, a value (V_GAINJERR) of the erroneous gain rate may be

calculated by subtracting the initialized value for the Y-axis (Δ_VEXC_INIT) from

the current value (Δ_VEXC_CURR). If the result equals "0" (Step 422), the current

gain parameter is not erroneous and the erroneous vertical offset rate may be

determined (step 430). However, if the result is more than "0", the vertical gain

parameter may be decreased by a step down signal that may be received from the

CPU, e.g., through D/A 21 IB (step 424). If the result is less than "0", the vertical

gain parameter may be increased by a step up signal that may be received from the

CPU, e.g., through D/A 21 IB (step 426). In step 430, a value of the vertical error offset rate may be calculated and

detennined. The calculation may be performed, for example, by subtracting the

initialized values for the "Center of Gravity" of photon sensor 122a from its current

value. The result may be stored as a variable (V_OFFSET-ERR). If the result equals

"0" (Step 432), the current offset parameter is not erroneous and the gain and the

method may proceed to determine the offset parameters of the horizontal axis (step

440). However, if the result is more than "0", the vertical offset parameter may be

decreased by a step down signal that may be received from the CPU, e.g., through

D/A 21 IB (step 434). If the result is less than "0", the vertical offset parameter may

be increased by a step up signal that may be received from the CPU, e.g., through

D/A 21 IB (step 436).

Referring to Fig. 7B, it will be apparent to persons of ordinary skill in the

art that the gain and offset parameters for the horizontal (X-) axis may be calculated

based on the current scanned values of the sensors 122c and 122g, using steps

(440-476 in Fig. 7B) generally analogous to those described above for calculating

the vertical (Y-) axis gain and offset parameters.

Reference is now made to Fig. 8, which is a schematic block diagram

illustrating brightness and focus adjustments that may be included in a deflection

and brightness RTA procedure in accordance with embodiments of the invention. In

step 500, a current average brightness value (BRG_AVR_CURR) may be

detennined. In step 510, the erroneous brightness value (e.g. BRG_ERR) may be

calculated by subtracting the initial average brightness value of all photon sensors (ALL_BRG_AVR_INIT) from the current average brightness value

(BRG_AVR_CURR). If the result equals "0" (Step 512), the current average

brightness value is not erroneous and the focus parameter may be detenrined (steps

600-626). However, if the result is more than "0", the brightness gain parameter

may be decreased by a step down signal that may be received from the CPU

through, e.g., D/A 211A (step 514). If the result is less than "0", the vertical

brightness gain parameter may be increased by a step up signal that may be received

from the CPU through, e.g., D/A 211A (step 516).

In step 600 the half value of the current average brightness value may be

determined (e.g. HV_BRG_AVR_CURR) and the X-axis and Y-axis values that

correspond to the half value may be found. In step 610 a calculation of the current

cross section area corresponding to the half values may be performed. The result is

a value that represents the current focus value. A calculation of the erroneous cross

section area rate may be detennined in step 620 by subtracting the initial focus value

from the current focus value. If the result equals "0" (Step 622), the current focus

value is not erroneous and the deflection and brightness RTA procedure is

completed and may begin during normal operation with no display interruption.

However, if the result is more than "0", the focus voltage may be decreased by a

step down signal that may be received from the CPU, e.g., through D/A 211C (step

624). If the result is less than "0", the focus voltage may be increased by a step up

signal that may be received from the CPU, e.g., through D/A 211C (step 626). When the system is first initiated, the computing unit 212 may adjust

parameters in the geometric correction unit 208, the cathode driver 209, and the

focus voltage of HVPS 215. Thus, by virtue of the mechanism of this invention, all

the parameters mentioned above, e.g., geometric correction, brightness level and

focus potential, may be calibrated automatically during system first "turn on"

operation, without resorting to optical bench instrumentation, such as a theodolite

and/or a photometer, which are typically required by prior art devices. For example,

by knowing the exact position of each sensor, including sensors 122b, 122d, 122f,

122h, as well as the position of the sensors in relation to the center of the CRT,

correction of geometric distortion may be performed automatically using "center of

gravity" mathematical manipulation as described above. Therefore, in embodiments

of the present invention, there is no need for positional calibration, as is typically

perfoπned by a theodolite. Additionally, by knowing in advance the required values

of brightness and focus, calibration of brightness and focus may be performed

automatically, without the use of light measurement devices, such as a photometer.

While the invention has been described with respect to a limited number of

embodiments, it will be appreciated that many variations, modifications and other

applications of the invention may be made which are within the scope and spirit of

the invention.

Claims

1. A circuit for use with a cathode ray tube (CRT) comprising:

at least one sensor fixable to a portion of a display screen of the CRT,

said at least one sensor adapted to produce a feedback signal responsive to a

characteristic of an electron beam striking an inner surface of said portion of

the display screen;

an excitation unit adapted to generate a test signal intended to direct

said electron beam to said inner surface of said portion of the display screen;

and

a computing unit adapted to receive the feedback signal produced by

said at least one sensor and to estimate at least one characteristic of the CRT

based on said feedback signal, said computing unit further adapted to

produce at least one output useful for adjusting at least one, respective,

parameter of the CRT based on the at least one estimated CRT characteristic.

2. The circuit according to claim 1 wherein said at least one parameter

comprises at least one geometric correction parameter.

3. The circuit according to claim 1 wherein said at least one parameter

comprises brightness level.

4. The circuit according to claim 1 wherein said at least one parameter

comprises focus level.

5. The circuit according to claim 1, wherein said at least one sensor

comprises a photo diode.

6. The circuit according to claim 1, wherein said at least one sensor

comprise a photoelectric cell.

7. The circuit according to claim 1 wherein said computing unit

automatically produces an initial level of said at least one output to adjust

said at least one parameter upon activation of the circuit.

8. A method for adjusting a picture produced by a cathode ray tube (CRT)

comprising:

producing a test signal excitation intended to direct an electron beam

to strike an inner surface of a display of the CRT in proximity with a sensor

mounted on said display;

sensing a characteristic of said electron beam sfriking said inner

surface and producing a feedback signal responsive to the sensed electron

beam characteristic; estimating at least one characteristic of the CRT based on the feedback

signal; and

adjusting at least one parameter of the CRT in accordance with the at

least one estimated characteristic.

9. A method according to claim 8 wherein said at least one parameter

comprises at least one geometric correction parameter.

10. A method according to claim 8 wherein said at least one parameter

comprises brightness level.

11. A method according to claim 8 wherein said at least one parameter

comprises focus level.

12. The method according to claim 8 wherein estimating said at least one

CRT characteristic and adjusting said at least one parameter are perfoπned

automatically upon activation of the CRT.

13. A cathode ray tube (CRT) comprising:

an electron beam gun to emit an electron beam; a display screen having an inner surface and an outer surface, said

inner surface being coated with a material to emit photons when excited by

said electron beam; and

at least one sensor fixed to a portion of the outer surface of said

display screen outside an active viewing area, said at least one sensor

adapted to produce a feedback signal responsive to a characteristic of said

electron beam striking said inner surface of the display screen in proximity

with said sensor.

14. The cathode ray tube according to claim 13 wherein said at

least one sensor comprises a plurality of sensors arranged in a

substantially circular configuration around a portion of said

display screen outside said active viewing area.