Solid-state, color-balanced backlight with wide illumination range
A color balanced solid-state backlight provides feedback control of each color using a single photodetector by imposing a modulation pattern on the solid-state lamps revealing individual colors to the photodetector. The photodetector signal provides feedback controlling color balance over a small range of instantaneous brightness less than larger range of average brightness of the display to provide for accurate color balance throughout a large range of average brightnesses.
1. A backlight system comprising:
a set of groups of solid-state lamps, the lamps of each group providing a different color of light;
a photodetector positioned to receive light from all of the groups to produce a measurement signal; and
a modulator communicating with each group to modulate a brightness of light from each group during:
(i) a first period wherein the groups are jointly energized to provide a multispectral backlight source of predetermined color and brightness and
(ii) a second period wherein the groups are independently modulated to provide measurement signals revealing relative intensities of each color;
wherein the modulator provides pulse-width control of the lamps during the first period using currents whose non-zero magnitudes are independently varied for each color as functions of the measurement signals obtained from the second period.
2. The backlight system of claim 1 wherein the first period is not less than nine times longer than the second period.
3. The backlight system of claim 1 wherein, during the second period, the lamps of each group are sequentially energized while lamps of remaining groups are not energized.
4. The backlight system of claim 1 wherein multiple groups of lamps are energized simultaneously during the second period.
5. The backlight system of claim 1 including a sample circuit sampling the measurement signal at a subset of a time during which the lamps are energized during the second period.
6. The backlight system of claim 1 wherein the modulator controls relative intensities of each color to provide the predetermined color during the first period.
7. The backlight system of claim 6 wherein the modulator provides feedback control of the groups using the measurement signal and provides separate feedback paths for each group.
8. The backlight system of claim 1 further including a memory circuit storing the measurement signal for each color acquired from the second period for use during the first period.
9. The backlight system of claim 1 further including a controller providing the modulator with a first and second modulation signal for controlling jointly the brightness of the lamps of the groups, wherein the first and second modulation signals are derived from a desired brightness signal independently of the measurement signal from the photodetector.
10. The backlight system of claim 9 further including at least one feedback circuit providing a third modulation signal for controlling independently the brightness of the lamps of the groups, wherein the third modulation signal is derived from the second modulation signal and the measurement signal from the photodetector.
11. The backlight system of claim 9 wherein the modulator provides duty cycle modulation of the lamps according to the first modulation signal and continuous current control of the lamps according to the second modulation signal.
12. The backlight system of claim 9 wherein the first modulation signal changes to control a brightness of the lamps during a high range of the desired brightness signal, and the second modulation signal changes to control the brightness of the lamps during a low range of the desired brightness signal.
13. The backlight system of claim 1 further including a light spreader providing light from multiple lamps of each group to the photodetector.
14. The backlight system of claim 1 wherein the predetermined color and brightness is white.
15. The backlight system of claim 1 further including a light diffuser positioned adjacent to the set of solid state lamps and an LCD screen placed on an opposite side of the light diffuser from the solid state lamps.
16. A backlight system comprising:
a set of groups of solid-state lamps, the lamps of each group providing a different color of light;
at least one photodetector positioned to receive light from the groups of solid state lamps to produce at least one measurement signal; and
a modulator communicating with each group to modulate a brightness of light from each group during:
(i) an illumination period wherein the groups are controlled to provide a multispectral backlight source of predetermined color and brightness, wherein the brightness is controlled by pulse width modulation of the groups; and
(ii) a measurement period separate from the illumination period wherein the groups are controlled to provide measurement signals revealing relative intensities of each color;
wherein the modulator controls the brightness of the groups by control of instantaneous maximum current to each lamp during the illumination period according to measurement signals made during the measurement period.
17. A backlight system comprising:
a set of groups of solid-state lamps, the lamps of each group providing a different color of light;
at least one photodetector positioned to receive light from the solid state lamps to produce at least one measurement signal; and
a modulator communicating with each group to modulate a brightness of light from the groups to vary a backlight brightness throughout a range of brightness by:
(i) controlling a duty cycle of the solid state lamps according to a desired brightness signal without regard to measurement signals; and
(ii) controlling current of the solid-state lamps by changing a non-zero level of current according to the measurement signals.
18. A high dynamic range solid state backlight comprising:
a set of groups of solid-state lamps, the lamps of each group providing a different color of light;
at least one photodetector positioned to receive light from all of the groups to produce a measurement signal; and
a modulator communicating with each group to modulate a brightness of light from the groups to vary a backlight brightness throughout a range of brightness using:
(i) duty cycle modulate the solid state lamps during a first range of brightness without reference to the measurement signal;
(ii) current modulate of the solid state lamps by continuously varying the current to the solid state lamps to control their brightness during a second range of brightness that is less than the first range; and
(iii) further control the current modulation with feedback control using the measurement signal.
19. The high dynamic range solid-state backlight of claim 18 comprising: wherein the first range of brightness is in excess of 1000:1 and the second range of brightness is less than 1000:1.
20. The high dynamic range solid-state backlight of claim 18 comprising: wherein the first range of brightness and second range of brightness are divided at a brightness level of less than one foot-lambert.