Photopic brightness controller for fluorescent backlights

• US 6,255,784 B1
• Filed: 12/02/1999
• Issued: 07/03/2001
• Est. Priority Date: 12/02/1999
• Status: Expired due to Fees

• Current Assignee: Visteon Global Technologies Incorporated (Visteon Corporation)
• Original Assignee: Visteon Global Technologies Incorporated (Visteon Corporation)
• Inventors: Weindorf, Paul Frederick Luther
• Primary Examiner(s): Philogene, Haissa
• Application Number:
  US09/452,978
• Time in Patent Office: 579 Days
• Field of Search: 315/291, 315/307, 315/224, 315/294, 315/149, 315/151, 315/155, 315/157-159, 315/DIG.4, 250/214.1, 250/214.AL, 250/205
• US Class Current: 315/291
• CPC Class Codes: G02F 2201/58 Arrangements comprising a monitoring photodetector
• CPC Class Codes: G09G 2320/0626 for control of overall brightness
• CPC Class Codes: G09G 2360/144 the light being ambient light
• CPC Class Codes: G09G 2360/145 the light originating from the display screen
• CPC Class Codes: G09G 3/3406 Control of illumination source illumination devices structurally associated with liquid crystal cells G02F1/1336
• CPC Class Codes: Y10S 315/04 Dimming circuit for fluorescent lamps

Abstract

A photopic light sensor for controlling the apparent brightness to a human observer of a fluorescent backlight uses a broad spectrum photodiode and a daylight-filtered photodiode, the latter sensing predominantly the infrared region of the broad spectrum. By properly scaling and subtracting signals from these two photodiodes, an effective response of a photopic sensor is created which may be used in a feedback loop to control the brightness of a fluorescent tube in automotive applications without the expense or package constraints incident to an infrared blocking filter of a true photopic sensor.

Claims

1. An illumination control for a fluorescent backlight comprising:

a first light sensor producing a first electronic signal dependent on sensed light in a first and second frequency range, the first frequency range corresponding substantially to light frequencies that the human eye sees and the second frequency range corresponding substantially to light frequencies that the human eye does not see;

a second light sensor producing a second electronic signal dependent on sensed light in a third frequency range substantially overlapping the second frequency range but not the first frequency range;

a fluorescent bulb illuminating the first and second light sensors according to a received drive signal;

a fluorescent driver circuit varying the drive signal to the fluorescent bulb according to a feedback control signal input; and

a subtractor subtracting the second electronic signal from the first electronic signal to produce the feedback control signal whereby the feedback control signal is dependent substantially on light only in the first frequency range.

2. The illumination control of claim 1 wherein the first and second light sensors are silicon photodiodes.

3. The illumination control of claim 1 wherein the second light sensor includes a filter blocking light in the first range.

4. The illumination control of claim 3 wherein the filter passes infrared light.

5. The illumination control of claim 1 wherein the fluorescent driver circuit also receives a desired brightness signal and also controls the drive signal of the fluorescent bulb according to the desired brightness signal.

6. The illumination control of claim 1 wherein the fluorescent driver circuit also receives an bulb temperature signal and also controls the drive signal level of the fluorescent bulb according to the bulb temperature signal.

7. The illumination control of claim 1 including further a printed wiring board;

and

wherein the fluorescent bulb is mounted against a first face of the printed wiring board and the first and second detectors are mounted on the first face of the printed wiring board between the fluorescent bulb and the printed wiring board to receive light directly from the fluorescent bulb.

8. The illumination control of claim 1 including further a printed wiring board having at least one aperture;

and

wherein the fluorescent bulb is mounted against a first face of the printed wiring board and the first and second detectors are mounted on a second obverse face of the printed wiring board over the aperture to receive light from the fluorescent bulb through the aperture.

9. The illumination control of claim 1 wherein the first frequency range is substantially 380 nm to 750 mn.

10. The illumination control of claim 1 wherein the second and third frequency ranges are substantially 750 mn to 1100 nm.

11. The illumination control of claim 1 including further a scaling circuit scaling at least one of the first and second electronic signal prior to receipt by the subtractor.

12. The illumination control of claim 1 wherein the fluorescent driver circuit integrates the feedback control signal.

13. A method of controlling a fluorescent backlight comprising the steps of:

(a) detecting light from a fluorescent bulb with a first light sensor producing a first electronic signal dependent on sensed light in a first and second frequency range of sensitivity, the first frequency range of sensitivity corresponding substantially to light frequencies that the human eye sees and the second range of sensitivity corresponding substantially to light frequencies that the human eye does not see;

(b) detecting light from the fluorescent bulb with a second light sensor producing a second electronic signal dependent on sensed light in a third frequency range of sensitivity substantially overlapping the second range of frequencies but not the first range of frequencies; and

(c) subtracting the second electronic signal from the first electronic signal to produce a drive signal for the fluorescent bulb controlling the brightness of the fluorescent bulb based on light only in the first frequency range of sensitivity.

14. The method of claim 13 wherein the first and second light sensors are silicon photodiodes.

15. The method of claim 13 including the step of filtering light to the second light sensor to block light in the first range.

16. The method of claim 13 including the step of modifying the drive signal according to a desired brightness signal independent of the light received from the first and second detectors.

17. The method of claim 13 including the step of modifying the drive signal according to an bulb temperature signal.

18. The method of claim 13 including the steps of:

mounting a fluorescent bulb against a first face of a printed wiring board; and

mounting the first and second detectors on the first face of the printed wiring board between the fluorescent bulb and the printed wiring board to receive light directly from the fluorescent bulb.

19. The method of claim 13 including the steps of:

cutting at least one aperture in a printed wiring board; and

mounting a fluorescent bulb against a first face of the printed wiring board; and

mounting the first and second detectors on a second obverse first face of the printed wiring board over the aperture to receive light through the aperture from the fluorescent bulb.

20. The method of claim 13 wherein the first frequency range is substantially 380 nm to 750 nm.

21. The method of claim 13 including further scaling at least one of the first and second electronic signals prior to subtracting them at step (c).

22. The method of claim 13 further including the step of integrating the difference between the first electronics signal and the second electronic signal to produce the drive signal.