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Wei-Chung Cheng, Member, IEEE, Massoud Pedram, Fellow, IEEE - page 2 / 8





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In this paper, we use backlight factor to express the







transmissivity to express the translucence of the TFT- LCD. The backlight factor and TFT-LCD transmissivity determine the perceived luminance from the TFT-LCD display.

B. Backlit TFT-LCD display

The major components of a backlit (or transmissive) TFT-LCD display subsystem include the video controller, frame buffer, video interface, TFT-LCD panel, and backlight. The frame buffer is a portion of memory used by software applications to deliver video data to the video controller. The video data from the application is stored in the frame buffer by the CPU. The video controller fetches the video data and generates appropriate analog (e.g. VGA) or digital (e.g. DVI) video signals to the video interface. The video interface carries the video signals between the video controller and the TFT-LCD display. The TFT-LCD display receives the video data and generates proper shade – transmissivity – for each pixel according to its pixel value. All of the pixels on the transmissive LCD panel are illuminated by the backlight from behind. To the observer, a displayed pixel looks bright if its transmissivity is high (i.e., in the 'on' state), meaning it passes the backlight. On the other hand, a displayed pixel looks dark if its transmissivity is low (i.e., in the 'off' state), meaning it blocks the backlight. If the transmissivity can be adjusted to more than two different levels between the 'on' and 'off' states, then the pixels can be displayed in grayscale. If the shade can be colored as red, green, or blue by using different color filters, then pixels can be displayed in color by mixing three sub-pixels in different colors at different grayscales. In other words, the perceived brightness of a pixel is determined by its transmissivity and the backlight illumination.

Most of current TFT-LCD displays use CCFL backlighting thanks to its unrivaled luminance density – emitting the most light within the minimum form factor. The CCFL can be designed to generate arbitrary color, which is critical to reproducing pure white in the backlighting applications. The technology of manufacturing CCFL is mature so that its cost has been minimized. The power consumption of the CCFL backlight, however, is considerably high compared with that of the TFT-LCD panel.

The observed luminance of a transmissive object L is the product of the luminance of the light source b and the transmissivity of the object t [5]. For a pixel on a backlit TFT-LCD display, its transmissivity is a function of its pixel value x. Thus, its observed luminance L is

L = t(x)b









1 b



  • 0



CCFL Backlight Factor

TFT-LCD Transimissivity Function

Luminance Function

Fig. 2: The luminance function of normalized pixel value (right) is the product of the backlight factor b and the TFT-LCD transmissivity function (center).

The ambient light is not considered here because it has little effect for a transmissive TFT-LCD when compared with a reflective or transflective one. Fig. 2 depicts the relation in (1) assuming that the transmissivity is a linear function of the pixel value.

In a non-backlight-scaled TFT-LCD display, the backlight b is always fixed at full power. The backlight scaling techniques in [2][3] reduce b while increasing the pixel value from x to x' by

x =x+b


x =x/b,


to maintain the same L. These approaches have the following drawbacks:

Equation (2) cannot preserve brightness invariance according to (1). The contrast distortion among the unsaturated

pixels is not considered. The software-based approach energy/performance overhead.



The CCFL illumination is incorrectly modeled as a linear function of power.

In this paper, we propose solutions to surmount the above-mentioned drawbacks. In Section II we characterize the CCFL illumination and power consumption. In Section III we propose adjusting the transmissivity function t rather than the pixel value x. The optimal CBCS problem is introduced in IV. Section V presents the experimental results followed by conclusions in Section VI.


A CCFL backlight unit consists of the fluorescent lamp, the driving DC-AC inverter, and the light reflector. A CCFL is a sealed glass tube with electrodes on both ends. The tube is filled with an inert gas (argon) and mercury. The inner glass surface of the tube is coated with phosphor, which emits visible light when excited by photons. The wavelength or color of the visible light depends on the type of the gas and phosphor. In the LCD backlighting application, a proper mix of red, green, and

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