Agilent introduces optical feedback system for LCD backlights

Jan. 6, 2005
Agilent Technologies has announced a color management system for backlighting LCD flat-panel TVs using LEDs, comprising a color management controller IC and a color sensor module.
The system is designed to work with high-power Luxeon® red, green and blue LEDs from Lumileds Lighting. Lumileds is the joint venture between Agilent and Philips Electronics.

The Agilent system provides closed-loop optical feedback and makes it possible to maintain constant color and brightness over the lifetime of the display.

Initially, the primary application for this technology is to control RGB LED backlighting for liquid-crystal display (LCD) flat-panel televisions. However, Agilent says that the technology may also be used to dynamically adjust the color and brightness of mood lighting in homes and businesses, interior lighting, automobiles and many other applications.

A typical backlighting system consists of an array of red, green and blue LEDs; LED drivers; Agilent's HDJD-S831-QT333 tri-color photo sensor that samples the light output, and the HDJD-J822-SCR00 color management controller IC. The controller interfaces directly to the photo sensor, processes the color and brightness information and adjusts the light output from the LEDs until the desired color is achieved.

Agilent's HDJD-J822-SCR00 integrates a high-accuracy, analog-to-digital converter front-end, a color data processing logic core and a high-resolution 12-bit pulse-width modulation (PWM) output generator. The output PWM signals are connected directly to the LED drivers, controlling the on-time duration of the red, green and blue LEDs. That duration is continually adjusted in real-time to match the light output from the LED array to the specified color.

Using the feedback system, the light output produced by an RGB LED array maintains its color over time and with variations in ambient temperature. The desired color can be specified using a standard CIE (Commission Internationale de l'Eclairage or International Commission on Illumination) color space.

An important advantage of using an RGB LED array and color management is that the white point in a television can be set to, and will maintain, any of a wide range of color temperatures. While professional studio monitors in the US and televisions in Europe are set to a color temperature of 6,500 K, typically US televisions are set for a color temperature of approximately 7,100 K (a bluer tint), and Japanese televisions and most worldwide computer monitors are set to 9,300 K (even bluer).

Currently, most flat-panel LCD television displays are backlit using cold-cathode fluorescent lamps (CCFLs). While efficient, CCFLs gradually degrade in brightness and change color with time, and offer a lifetime in the range of 15,000 to 25,000 hours. In addition, the CCFL light source only covers 75 percent of the color space defined by the National Television System Committee (NTSC), which is responsible for setting television and video standards in the United States.

RGB LEDs with Agilent's illumination and color management solution exceeds 100 percent of the NTSC color space, providing sharper, more brilliant colors. The use of LEDs also aids European manufacturers who are seeking to eliminate the small amount of mercury used in fluorescent backlighting.