Method and system of achieving accurate white point setting of a CRT display

• US 5,512,961 A
• Filed: 03/24/1993
• Issued: 04/30/1996
• Est. Priority Date: 03/24/1993
• Status: Expired due to Term

• Current Assignee: Apple Computer Incorporated (Apple Inc.)
• Original Assignee: Apple Computer Incorporated (Apple Inc.)
• Inventors: Cappels, Richard D. Sr.
• Primary Examiner(s): Powell, Mark R.
• Application Number:
  US08/036,349
• Time in Patent Office: 1,133 Days
• Field of Search: 348/191, 348/175, 348/176, 348/189, 348/655, 348/656, 348/657, 348/658, 348/675, 348/679, 348/173, 345/151, 395/162, 395/161
• US Class Current: 348/658
• CPC Class Codes: G09G 2320/043 Preventing or counteracting the effects of ageing
• CPC Class Codes: H04N 17/04 for receivers
• CPC Class Codes: H04N 9/645 Beam current control means
• CPC Class Codes: H04N 9/73 Colour balance circuits, e.g. white balance circuits or colour temperature control camera processing pipelines for colour balance H04N23/88

Abstract

A system and method achieve and maintain an accurate white point setting of a CRT display in a computing system. The CRT is initially calibrated by individually driving the individual color cathodes and by measuring tristimulus values and cathode beam current for each of the three primary colors. The tristimulus values are normalized by dividing each value by the beam current producing it, and the normalized values are then stored in a calibration memory contained in the display unit. A table of gamma values representing beam current as a function of video drive voltage is measured and also added to the calibration memory. Calibration of the display is implemented by driving the display controller with a white point value and calculating the cathode beam currents from the stored tristimulus values, required to produce an accurate CRT representation of the signal sent to the display controller. The beam current is sampled through a digital feedback loop, and adjustments are made to the color drive circuitry of the display controller to produce the required beam currents for a given white point. Alternative images can be also used instead of white point images for making periodic recalibrations, by derating the beam currents for a given image according to values stored in the gamma table.

Claims

1. A calibration system for accurately displaying color images on a computer display, comprising:

processor means external to the computer display for generating color digital video signals;

control means external to the computer display and connected to the processor means for receiving digital video signals and responsively generating analog video signals;

display means, including amplifier means and electrode means, coupled to the control means for receiving said analog video signals and responsively amplifying said analog video signals to produce analog voltages and displaying video images;

current sensing means coupled between the amplifier means and the electrode means, for measuring beam currents resulting from the analog voltages and generating beam current measurements;

color measurement means for generating tristimulus value measurements of the displayed images, and for transmitting the tristimulus value measurements and the beam current measurements to the processor means which divides the tristimulus value measurements by the beam current measurements to obtain normalized tristimulus value measurements; and

memory means connected to the processor means for storing the normalized tristimulus value measurements and the current measurements transmitted by the color measurement means.

2. The system for accurately displaying color images on a computer display means according to claim 1, wherein the analog video signals comprise Blue, Red and Green primary color components.

3. The system for accurately displaying color images on a computer display according to claim 2, wherein the analog voltages drive corresponding electrodes in the display means.

4. The system for accurately display color images on a computer display according to claim 2, wherein the current sensing means generates a current measurement for each of the analog primary color components.

5. The system for accurately displaying color images on a computer display according to claim 1, wherein the color measurement means is a spectraradiometer for generating the tristimulus value measurements of the images.

6. The system for accurately displaying color images on a computer display according to claim 1, wherein gamma values are computed based on the current measurements of the display means, and are stored in the memory means.

8. The system for accurately displaying color images on a computer display according to claim 1, wherein the normalized tristimulus value measurements are stored in the memory means prior to transmission to the processor means.

7. A calibration system for accurately displaying color images on a computer display, comprising:

processor means external to the computer display for generating color digital video signals;

display control means external to the computer display and connected to the processor means for receiving the digital video signals and responsively generating analog video signals;

analog control means connected to the display control means for receiving and amplifying the analog video signals and therefrom generating analog voltages, and connected to said processor means for receiving a gain control signal and responsively controlling the gain of amplification by the analog control means;

display means connected to the analog control means for receiving said analog voltages and responsively displaying video images;

current sensing means including

current sampling means connected between said analog control means and said display means, for measuring beam currents resulting from the analog voltages and generating beam current measurements, and

analog to digital conversion means connected between the current sampling means and the processor means, for receiving the beam current measurements from the current sampling means for converting the beam current measurements to digital current measurement signals, and for communicating the digital current measurement signals to the processor means which responsively adjusts the gain control signal for calibrating the control means; and

color measurement means for generating tristimulus value measurements of the displayed images and for transmitting the tristimulus value measurements to the processor means which normalizes the tristimulus value measurements with respect to the beam current measurements.

9. A method for accurately displaying color on a computer display having individual display electrodes, comprising the steps of:

a) determining first white point beam current values for the computer display during use, including the steps of

i) measuring a first beam current applied to one of the individual electrodes to produce a first beam current measurement;

ii) measuring tristimulus values of light produced by the computer display;

iii) calculating normalized tristimulus values by dividing the tristimulus values by the first beam current measurement;

iv) repeating steps (i) through (iii) for each of the remaining individual electrodes; and

v) calculating the first white-point beam current values from the normalized tristimulus values;

b) applying computer-generated white point video signals to the computer display for producing second beam currents at the individual display electrodes, said video signals generated by a host computer system external to the computer display;

c) measuring the second beam currents to produce second white point beam current measurements;

d) comparing the second white point beam current measurements to the determined first white point beam current values; and

e) adjusting the computer-generated video signals to produce new second white point beam current measurements approximately equivalent to the determined first white point beam current values.

10. The method for accurately displaying color on a computer display according to claim 9, wherein the method for calculating the first white point beam current values from the tristimulus values further comprises solving the following set of equations:

The defining formulae;

space="preserve" listing-type="equation">Xc=(R Xr)+(G Xg)+(B Xb)

space="preserve" listing-type="equation">Yc=(R Yr)+(G Yg)+(B Yb)

space="preserve" listing-type="equation">Zc=(R Zr)+(G Zg)+(B Zb)

Solutions for R, G and B;

##EQU2## where Xc, Xr, Xg, Xb, Yc, Yr, Yg, Yb, Zc, Zr, Zg, and Zb are the tristimulus values of r, g, and b primaries, normalized to the first beam currents; and

R, G, and B are the first beam currents for each primary.

11. The method for accurately, displaying color on a computer display according to claim 9, further comprising the step of storing the calculated first white beam current values in a calibration memory

12. A method for accurately displaying color on a computer display, comprising the steps:

a) determining first beam current values and gamma values for the computer display, said first beam current values calculated using normalized tristimulus values corresponding to the computer display;

b) applying computer-generated video signals having a known color content to the display for producing second beam current values at individual display electrodes, said video signals generated by a host computer system external to the computer display;

c) measuring the second beam current values produced by the video signals;

d) comparing the measured second beam current values to the determined first beam current values and gamma values; and

e) adjusting the computer-generated video signals to produce new measured second beam current values approximately equivalent to the determined first beam current values.

13. The method for accurately displaying color on a computer display according to claim 12, wherein the step a) of determining the gamma values further comprises measuring the second beam currents for various values of video signals at each of the individual electrodes.

14. The method for accurately displaying color on a computer display according to claim 12, further comprising the step of storing the determined first beam current values and the gamma values in a calibration memory.