White point control in backlights

• US 8,378,958 B2
• Filed: 03/24/2009
• Issued: 02/19/2013
• Est. Priority Date: 03/24/2009
• Status: Expired due to Fees

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Chen, Cheng, Qi, Jun, Zhong, John Z., Yin, Victor H., Chen, Wei
• Primary Examiner(s): Cheng, Joe H
• Application Number:
  US12/410,183
• Publication Number:
  US 20100245227A1
• Time in Patent Office: 1,428 Days
• Field of Search: 345/102, 345/204, 345/690, 345/691, 345 87- 91
• US Class Current: 345/102
• CPC Class Codes: G09G 2320/0242 Compensation of deficiencies in the appearance of colours
• CPC Class Codes: G09G 2320/041 Temperature compensation
• CPC Class Codes: G09G 2320/043 Preventing or counteracting the effects of ageing
• CPC Class Codes: G09G 2320/0606 Manual adjustment
• CPC Class Codes: G09G 2320/0633 by amplitude modulation of the brightness of the illumination source
• CPC Class Codes: G09G 2320/064 by time modulation of the brightness of the illumination source
• CPC Class Codes: G09G 2320/0666 for control of colour parameters, e.g. colour temperature
• CPC Class Codes: G09G 2360/145 the light originating from the display screen
• CPC Class Codes: G09G 3/3426 the different display panel areas being distributed in two dimensions, e.g. matrix
• CPC Class Codes: G09G 5/02 characterised by the way in which colour is displayed details of colour display specific for CRTs G09G1/28; specific for flat matrix panels other than liquid crystal displays G09G3/2003; specific for liquid crystal displays G09G3/3607
• CPC Class Codes: H05B 45/22 using optical feedback
• CPC Class Codes: H05B 45/28 using temperature feedback

Abstract

Systems, methods, and devices are provided for maintaining a target white point on a light emitting diode based backlight. In one embodiment, the backlight may include two or more strings of light emitting diodes, each driven at a respective driving strength. Each string may include light emitting diodes from a different color bin, and the respective driving strengths may be adjusted, for example, through pulse width modulation or amplitude modulation, to maintain the target white point. In certain embodiments, the driving strengths may be adjusted to compensate for shifts in the white point that may occur due to temperature or aging. A controller may adjust the driving strengths based on feedback from a temperature sensor, from an optical sensor, from a user input, or from calibration data included within the backlight or system.

Claims

1. A backlight, comprising:

a first string of first substantially white light emitting diodes each selected from a first bin and configured to be driven at a first driving strength, wherein the first bin comprises substantially white chromaticities within a first range of chromaticity coordinates;

a second string of second substantially white light emitting diodes each selected from a second bin and configured to be driven at a second driving strength, wherein the second bin comprises substantially white chromaticities within a second range of chromaticity coordinates, and the first and second ranges of chromaticity coordinates are approximately equidistant from a target white point in opposite directions on a chromaticity diagram;

one or more drivers configured to independently drive the first string at the first driving strength and the second string at the second driving strength; and

a controller configured to dynamically adjust at least one of the first or second driving strengths via the one or more drivers to produce a white point substantially similar to the target white point.

2. The backlight of claim 1, comprising a white point light emitting diode section, wherein the white point section comprises a third string of substantially white light emitting diodes each selected from a third bin, wherein the third bin comprises white chromaticities within a third range of chromaticity coordinates encompassing the target white point and such that each substantially white light emitting diode is substantially similar to the target white point.

3. The backlight of claim 1, wherein the first and second substantially white light emitting diodes comprise phosphorous light emitting diodes.

4. The backlight of claim 1, wherein the one or more drivers comprise a multi-channel driver with a first channel configured to drive the first string of first substantially white light emitting diodes and a second channel configured to drive the second string of second substantially white light emitting diodes.

5. The backlight of claim 1, wherein the first and second strings are mounted within an array tray of the backlight.

6. The backlight of claim 1, wherein the first and second strings are mounted within one or more light strips of the backlight.

7. A backlight, comprising:

a first string of substantially white light emitting diodes comprising a first group of substantially white light emitting diodes selected from a first bin, wherein the first bin comprises substantially white chromaticities within a first range of chromaticity coordinates;

a second string of substantially white light emitting diodes comprising a second group of substantially white light emitting diodes selected from a second bin, wherein the second bin comprises substantially white chromaticities within a second range of chromaticity coordinates, and the first and second ranges of chromaticity coordinates are approximately equidistant from a target white point in opposite directions on a chromaticity diagram;

one or more drivers configured to independently drive the first string at the first driving strength and the second string at the second driving strength to cause the backlight to produce an emitted white point that substantially matches the target white point;

a memory configured to store values defining the target white point; and

a controller configured to detect a shift in the emitted white point and to adjust the first or second driving strengths to compensate for the shift.

8. The backlight of claim 7, comprising a third string of substantially white light emitting diodes selected from a third bin, wherein the third bin encompasses the target white point.

9. The backlight of claim 7, comprising:

a first color compensating string of first color compensating light emitting diodes configured to be driven by the one or more drivers at a first color compensating driving strength;

a second color compensating string of second color compensating light emitting diodes configured to be driven by the one or more drivers at a second color compensating driving strength, wherein the controller is configured to adjust the first or second color compensating driving strengths via the one or more drivers to compensate for the shift.

10. The backlight of claim 7, wherein the one or more drivers are configured to receive control signals from the controller to adjust the first and second driving strengths.

11. The backlight of claim 7, wherein the values comprise x and y chromaticity coordinates of the target white point.

12. A backlight, comprising:

a first string of substantially white light emitting diodes each selected from a first bin, wherein the first bin comprises substantially white chromaticities within a first range of chromaticity coordinates;

a second string of substantially white light emitting diodes each selected from a second bin, wherein the second bin comprises substantially white chromaticities within a second range of chromaticity coordinates, and the first and second ranges of chromaticity coordinates are approximately equidistant from a target white point in opposite directions on a chromaticity diagram;

one or more drivers configured to independently drive the first and second strings to cause the backlight to produce an emitted white point that substantially matches the target white point;

a first color compensating string of first color compensating light emitting diodes configured to be driven independently from the plurality of light emitting diode strings at a first color compensating driving strength;

a second color compensating string of second color compensating light emitting diodes configured to be driven independently from the plurality of light emitting diode strings at a second color compensating driving strength; and

a controller configured to detect a shift in the emitted white point and to vary the first driving strength, the second driving strength, the first color compensating driving strength, or the second color compensating driving strength via the one or more drivers to counteract the shift in the emitted white point.

13. The backlight of claim 12, wherein the first string, the second string, the first color compensating string, and the second color compensating string are intermixed within the backlight.

14. The backlight of claim 12, wherein the first and second color compensating light emitting diodes are configured to emit a color compensating light outside of the first and second ranges.

15. The backlight of claim 12, wherein the first or second strings are each configured to be driven at a constant rate.

16. A method of operating a backlight, the method comprising:

driving a first string of first substantially white light emitting diodes selected from a first bin at a first driving strength, wherein the first bin comprises substantially white chromaticities within a first range of chromaticity coordinates;

driving a second string of second substantially white light emitting diodes selected from a second bin at a second driving strength, wherein the second bin comprises substantially white chromaticities within a second range of chromaticity coordinates, and the first and second ranges of chromaticity coordinates are approximately equidistant from a target white point in opposite directions on a chromaticity diagram, and wherein the first and second driving strengths are configured to produce an emitted color that corresponds to a target white point;

receiving feedback indicating a shift in the emitted color; and

dynamically adjusting at least one of the first or second driving strengths to return the emitted color to the target white point.

17. The method of claim 16, wherein receiving feedback comprises receiving at least one of a user input, a sensed temperature, or a sensed light color.

18. The method of claim 16, wherein dynamically adjusting at least one of the first or second driving strengths comprises modulating a duty cycle of a current driven through at least one of the first or second strings using pulse width modulation.

19. The method of claim 16, wherein adjusting at least one of the first or second driving strengths comprises varying an amount of a current driven through at least one of the first or second strings using amplitude modulation.

20. The method of claim 16, wherein adjusting at least one of the first or second driving strengths comprises receiving at least one of a color or brightness value from a sensor and determining a driving strength adjustment based on the value.

21. A system comprising:

a backlight comprising;

a first string of substantially light emitting diodes comprising a first group of substantially white light emitting diodes selected from a first bin, wherein the first bin comprises substantially white chromaticities within a first range of chromaticity coordinates;

a second string of substantially white light emitting diodes comprising a second group of substantially white light emitting diodes selected from a second bin, wherein the second bin comprises substantially white chromaticities within a second range of chromaticity coordinates, and the first and second ranges of chromaticity coordinates are approximately equidistant from a target white point in opposite directions on a chromaticity diagram;

one or more drivers configured to independently drive the first string at the first driving strength and the second string at the second driving strength to cause the backlight to produce an emitted white point that substantially matches the target white point;

a controller configured to dynamically and independently adjust the first or second driving strengths via the one or more drivers to maintain the emitted white point; and

a liquid crystal display panel configured to be illuminated by the backlight.

22. The system of claim 21, comprising a processor configured to generate images to display on the liquid crystal display panel.

23. The system of claim 21, wherein the system comprises a laptop computer, a desktop computer, or a computer monitor.

24. The system of claim 21, comprising a user interface configured to receive a user input, wherein the controller is configured to adjust the driving strengths based on the user input.