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Colour stability scheme

IP.com Disclosure Number: IPCOM000010039D
Publication Date: 2002-Oct-14
Document File: 1 page(s) / 6K

Publishing Venue

The IP.com Prior Art Database

Abstract

ID610149

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ID610149

Colour stability scheme

    LEDs are suitable for use as a light source in a backlight system for an LCD screen. Such a light source can consist of red, blue and green LEDs, that are independently driven by three LED drivers. Each of the LED drivers controls the amount of light by periodically switching the current through the LEDs on and off with a certain duty cycle. The time duration of one period is further referred to as a driver cycle. The primary colours generated by these LEDs are mixed so that the light source generates white light. Problems associated with the use of LEDs, are colour shift, temperature sensitivity and degradation. To compensate for these effects the light outputs of the red, green and blue LEDs is often monitored by means of three sensors, each comprising a photo diode with a colour filter in front. The signals generated by these sensors are used in a control loop for maintaining the colour point of the white light by adjusting the duty cycles of the LED drivers. However, the filters have a high transmission in the area of dark red to infrared (λ > 650 nm). Due to this property of the sensors, the wavelength shift of the light generated by the red LEDs due to a temperature change is not compensated by the control loop. Since none of the duty cycles of the LED drivers is 100% under normal operational conditions, this problem can be overcome in the following way: the red LEDs are driven exclusively during a small time lapse in each...