LED Backlight for LCD with Color Uniformity Recalibration Over Lifetime

• US 20070285378A1
• Filed: 06/09/2006
• Published: 12/13/2007
• Est. Priority Date: 06/09/2006
• Status: Active Grant

• Current Assignee: Philips Lighting North America Corporation (Koninklijke Philips N.V.)
• Original Assignee: Philips Lumileds Lighting Company LLC (Apollo Global Management, Inc.)
• Inventors: Lankhorst, Martijn H. R., Grootes, Pieter, Hendriks, Robert
• Granted Patent: US 7,696,964 B2
• Time in Patent Office: Days
• US Class Current: 345/102
• CPC Class Codes: G09G 2320/0233 Improving the luminance or brightness uniformity across the screen
• CPC Class Codes: G09G 2320/0633 by amplitude 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 2320/0693 Calibration of display systems
• CPC Class Codes: G09G 2360/147 the originated light output being determined for each pixel
• CPC Class Codes: G09G 3/342 using several illumination sources separately controlled corresponding to different display panel areas, e.g. along one dimension such as lines
• CPC Class Codes: H05B 45/22 using optical feedback
• CPC Class Codes: H05B 45/3725 Switched mode power supply [SMPS]
• CPC Class Codes: H05B 45/46 having LEDs disposed in parallel lines
• CPC Class Codes: Y10S 362/80 Light emitting diode

Abstract

An LED light source for LCD backlighting is described that recalibrates itself over time so that color and brightness uniformity across the backlight is maintained over the life of the backlight. The backlight contains clusters of red, green, and blue LEDs, each cluster generating a white point. In one embodiment, each color in a cluster has its own controllable driver so that the brightness of each color is a cluster is separately controllable. One or more optical sensors are arranged in the backlight, and the sensor signals are detected by processing circuitry to sense the light output of any LEDs that are energized in a single cluster. The measured white point and flux are compared to a stored target white point value and flux for that cluster. The currents to the RGB LEDs are then automatically adjusted to achieve the target level for each cluster. This process is applied to each cluster in sequence until the recalibration is complete. The recalibration takes place at various times over the lifetime of the backlight to offset the effects of LED degradation over time. Variations of this technique are also described.

Claims

1. A light emitting diode (LED) light source system comprising:

a support structure;

clusters of LEDs mounted on the support structure in an array, the LEDs in each cluster including at least a first LED for emitting a first color, a second LED for emitting a second color, and a third LED emitting a third color,

the first color, the second color, and the third color, when combined, generating light having a white point;

a plurality of current sources, a first current source being connected to at least one LED emitting the first color, a second current source being connected to at least one LED emitting the second color, a third current source being connected to at least one LED emitting the third color, the current sources for controlling a brightness level of each color;

at least one optical sensor connected to the support structure, the at least one optical sensor detecting a light output of a cluster when at least one LED in a cluster is energized; and

a controller having a memory, the memory storing values for controlling an output current magnitude of each of the current sources so that a light output of the light source system has characteristics set by the values stored in the memory,

the controller for calibrating a white point for all clusters by energizing LEDs within selected clusters and adjusting currents through LEDs in the selected clusters to cause a light output of each cluster to more closely match a target white point for that cluster based on values stored in the memory,

wherein the controller is configured to perform the calibrating at various times over a lifetime of the light source system to offset degradation in the LEDs.

2. The system of claim 1 wherein the values stored in the memory represent current magnitudes for different colors of LEDs in each cluster.

3. The system of claim 1 wherein the values stored in the memory represent target brightness levels.

4. The system of claim 3 wherein a target brightness level is identified for each color in each cluster.

5. The system of claim 1 wherein the values stored in the memory identify a target white point for each cluster.

6. The system of claim 1 wherein the memory comprises a look up table containing a target white point for each cluster.

7. The system of claim 6 wherein a target white point identifies a brightness level for each color in a cluster.

8. The system of claim 1 wherein the values stored in the memory represent current magnitudes for different colors of LEDs in each cluster and target white points for the clusters.

9. The system of claim 1 wherein, in at least some of the clusters, there are a plurality of LEDs emitting the same color.

10. The system of claim 1 wherein the first color is red, the second color is green, and the third color is blue.

11. The system of claim 1 wherein the values stored in the memory represent target brightness levels, and wherein the target brightness levels are obtained by operating the system and controlling currents to each LED in each cluster to obtain target brightness levels to be detected by the at least one optical sensor during a subsequent recalibration of the system.

12. The system of claim 1 further comprising a reflective box at least partially surrounding the clusters of LEDs to mix the first color, second color, and third color.

13. The system of claim 12 wherein a cluster proximate to an edge of the box is controlled to have a brightness level lower than a brightness level of a cluster further from the edge of the box.

14. The system of claim 1 further comprising a current level controller for receiving digital values from the memory and converting the digital values to signals for controlling the plurality of current sources.

15. The system of claim 1 wherein the controller is configured to perform the calibrating upon a user initiating the calibration.

16. The system of claim 1 wherein the controller is configured to perform the calibrating automatically at predetermined intervals.

17. The system of claim 16 wherein the intervals comprise periods of use of the system.

18. The system of claim 1 further comprising a liquid crystal layer for selectively passing light from the clusters.

19. The system of claim 18 wherein the system is a liquid crystal display television.

20. A calibration method performed by a liquid crystal display (LCD) system comprising:

energizing different color light emitting diodes (LEDs) in a plurality of clusters of LEDs, the plurality of clusters forming a backlight for the LCD;

optically sensing a white point of each cluster as LEDs in each cluster are energized and generating signals corresponding to a sensed white point;

addressing a memory to obtain a previously stored target white point for each cluster being sensed;

adjusting currents to energized LEDs in each cluster to cause the white point of each cluster to substantially match the target white point for that cluster stored in the memory; and

storing values corresponding to currents used to cause the white point of each cluster to substantially match the target white point for that cluster.

21. The method of claim 20 wherein optically sensing the white point of each cluster comprises sensing a brightness level of each color in a single cluster as LEDs for each color in the cluster are energized.

22. The method of claim 20 wherein energizing different color LEDs in a plurality of clusters of LEDs comprises only energizing LEDs of a single color in a single cluster at a time.

23. The method of claim 20 further comprising a user initiating the method and the method then being performed automatically.

24. The method of claim 20 further comprising automatically performing the method at predetermined intervals.

25. The method of claim 20 further comprising obtaining and storing the target white points in the memory while the LCD system is at least partially assembled, the step of obtaining comprising energizing LEDs within clusters and detecting target white points by optical sensors forming part of the LCD system, the optical sensors being also used for the step of optically sensing a white point of each cluster during calibrating the LCD system.

26. The method of claim 25 wherein detecting target white points comprises measuring a light output of LEDs in an energized cluster and comparing the light output to a desired light output for the LCD system.

27. A light emitting diode (LED) light source system comprising:

a support structure;

clusters of LEDs mounted on the support structure in an array, the LEDs in each cluster including at least a first LED for emitting a first color, a second LED for emitting a second color, and a third LED emitting a third color;

each cluster having at least two LEDs of the first color;

a first LED of the first color in a first cluster being connected in series with a second LED of the first color in a second cluster so that an open-circuit failure of either the first LED or the second LED will not result in an energizing current being removed from one or more other LEDs of the first color in the first cluster or the second cluster.

28. The system of claim 27 wherein no LEDs of the same color in a single cluster are connected in series with each other.

29. The system of claim 27 further comprising a liquid crystal layer for selectively passing light emitted by the clusters, the clusters forming a backlight for the liquid crystal layer.

30. The system of claim 29 wherein the system is a liquid crystal display television.