LED selection for white point control in backlights

• US 8,558,782 B2
• Filed: 07/13/2010
• Issued: 10/15/2013
• Est. Priority Date: 03/24/2009
• Status: Expired due to Fees

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: You, Chenhua, Drolet, Jean-Jacques Philippe, Chen, Wei
• Primary Examiner(s): Cheng, Joe H
• Application Number:
  US12/835,439
• Publication Number:
  US 20100277410A1
• Time in Patent Office: 1,190 Days
• Field of Search: 345/82, 345/83, 345 87- 91, 345/102, 345/204, 345/690, 315/309
• 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 2320/048 using evaluation of the usage time
• CPC Class Codes: G09G 2320/0606 Manual adjustment
• 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 3/3413 Details of control of colour illumination sources
• CPC Class Codes: G09G 3/3426 the different display panel areas being distributed in two dimensions, e.g. matrix
• CPC Class Codes: H05B 45/20 Controlling the colour of the light
• 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 (LED) based backlight. In one embodiment, the backlight may include two or more groups of LEDs, each driven at a respective driving strength. Each group may include LEDs of a different chromaticity, and the respective driving strengths may be adjusted, for example, by varying the duty cycles, to maintain the target white point. To ensure that the white point may be maintained over an operational temperature range of the backlight, the LEDs may be selected so that the chromaticities of each group of LEDs are separated by at least a minimum chromaticity difference. Further, the LEDs may be selected so that at the equilibrium temperature of the backlight, the LEDs may produce the target white point when driven at substantially equal driving strengths.

Claims

1. A display, comprising:

a backlight configured to operate over a temperature range;

a first string of first light emitting diodes arranged within the backlight, wherein the first light emitting diodes have a first chromaticity at an equilibrium temperature of the backlight;

a second string of second light emitting diodes arranged within the backlight, wherein the second light emitting diodes have a second chromaticity at the equilibrium temperature of the backlight and wherein the second chromaticity is separated from the first chromaticity by a chromaticity difference greater than a maximum chromaticity shift of the first light emitting diodes over the temperature range;

one or more drivers configured to independently drive the first string and the second string at respective driving strengths to produce an emitted white point that corresponds to a target white point; and

a controller configured to detect temperature changes within the display and to adjust a ratio of the respective driving strengths to maintain correspondence to the target white point over the temperature range.

2. The display of claim 1, wherein the first light emitting diodes are selected from a first bin and wherein the second light emitting diodes are selected from a second bin.

3. The display of claim 1, wherein the first chromaticity, the second chromaticity, and the target white point lie on a line within the CIE 1976 uniform chromaticity scale diagram.

4. The display of claim 1, wherein the chromaticity difference and the maximum chromaticity shift are measured as Δ

u′

v′

on a CIE 1976 uniform chromaticity scale diagram.

5. The display of claim 1, wherein the respective driving strengths are substantially equal at the equilibrium temperature of the backlight.

6. The display of claim 1, wherein the controller is configured to adjust a duty cycle ratio of the respective driving strengths to maintain correspondence to the target white point.

7. The display of claim 1, wherein the controller is configured to maintain a substantially constant luminosity over the temperature range.

8. The display of claim 1, comprising one or more sensors disposed in the backlight and configured to detect the temperature changes.

9. A display, comprising:

a backlight configured to operate over a temperature range;

a first string of first light emitting diodes arranged within the backlight, wherein the first light emitting diodes have a first range of chromaticities over the temperature range;

a second string of second light emitting diodes arranged within the backlight, wherein the second light emitting diodes have a second range of chromaticities over the temperature range;

a third string of third light emitting diodes arranged within the backlight, wherein the third light emitting diodes have a third range of chromaticities over the temperature range, and wherein the first range of chromaticities, the second range of chromaticities, and the third range of chromaticities are set apart from one another;

one or more drivers configured to independently drive the first string, the second string, and the third string at respective driving strengths to produce an emitted white point that corresponds to a target white point; and

a controller configured to detect temperature changes within the display and to adjust ratios of the respective driving strengths to maintain correspondence to the target white point over the temperature range.

10. The display of claim 9, wherein the first light emitting diodes are configured to emit red light, the second light emitting diodes are configured to emit blue light, and the third light emitting diodes are configured to emit green light.

11. The display of claim 9, wherein the first light emitting diodes, the second light emitting diodes, and the third light emitting diodes comprise white light emitting diodes.

12. The display of claim 9, wherein chromaticity differences between the first light emitting diodes, the second light emitting diodes, and the third light emitting diodes at an equilibrium temperature of the backlight each exceed maximum chromaticity shifts for each of the first light emitting diodes, the second light emitting diodes, and the third light emitting diodes.

13. The display of claim 12, wherein the chromaticity differences and the maximum chromaticity shifts are measured as Δ

u′

v′

on a CIE 1976 uniform chromaticity scale diagram.

14. The display of claim 9, wherein the ratios between the respective driving strengths comprise approximately 1:

1 ratios at an equilibrium temperature of the backlight.

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

independently driving a first string of first light emitting diodes and a second string of second light emitting diodes at respective driving strengths to produce an emitted white point that corresponds to a target white point; and

adjusting a ratio of the respective driving strengths in response to temperature changes to maintain correspondence to the target white point over an operational temperature range of the backlight;

wherein a chromaticity difference between the first light emitting diodes and the second light emitting diodes at an equilibrium temperature of the backlight is greater than a maximum chromaticity shift of the first light emitting diodes over the operational temperature range.

16. The method of claim 15, wherein adjusting a ratio comprises adjusting a duty cycle ratio of the respective driving strengths.

17. The method of claim 15, wherein adjusting a ratio comprises maintaining a relatively constant luminosity of the backlight.

18. The method of claim 15, wherein the chromaticity difference is greater than a second maximum chromaticity shift of the second light emitting diodes over the operational temperature range.

19. The method of claim 15, comprising detecting temperature changes using one or more temperature sensors disposed within the backlight.

20. A method of manufacturing a backlight, the method comprising:

arranging a first string of first light emitting diodes within a backlight, wherein the first light emitting diodes have a first chromaticity at an equilibrium temperature of the backlight;

arranging a second string of second light emitting diodes with respect to the first string of first light emitting diodes to produce a target white point over an operational temperature range of the backlight, wherein the second light emitting diodes have a second chromaticity at the equilibrium temperature of the backlight, and wherein the second chromaticity is separated from the first chromaticity by a chromaticity difference greater than a maximum chromaticity shift of the first light emitting diodes over the operational temperature range of the backlight;

configuring one or more drivers configured to independently drive the first string and the second string at respective driving strengths to produce an emitted white point that corresponds to the target white point; and

configuring a controller to adjust a ratio of the respective driving strengths in response to temperature changes to maintain correspondence to the target white point over the operational temperature range.

21. The method of claim 20, comprising configuring the controller to scale the respective driving strengths to maintain a constant luminosity of the backlight over the operational temperature range.

22. The method of claim 20, comprising selecting the first light emitting diodes and the second light emitting diodes so that light from the first light emitting diodes and the second light emitting diodes mixes to produce the target white point when the first light emitting diodes and the second light emitting diodes are driven at substantially equal driving strengths at an equilibrium temperature of the backlight.