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Power Minimization in a Backlit TFT-LCD Display by Concurrent Brightness and Contrast Scaling

Wei-Chung Cheng, Member, IEEE, Massoud Pedram, Fellow, IEEE

Abstract This paper presents a Concurrent Brightness and Contrast Scaling (CBCS) technique for a cold cathode fluorescent lamp (CCFL) backlit TFT-LCD display. The proposed technique aims at conserving power by reducing the backlight illumination while retaining the image fidelity through preservation of the image contrast. First, we explain how CCFL works and show how to model the non-linearity between its backlight illumination and power consumption. Next, we propose the contrast distortion metric to quantify the image quality loss after backlight scaling. Finally, we formulate and optimally solve the CBCS optimization problem subject to contrast distortion. Experimental results show that an average of 3.7X power saving can be achieved with only 10% of contrast distortion.

Index Terms — backlight scaling, concurrent brightness and contrast scaling, TFT-LCD, low power.

I. INTRODUCTION

Previous studies on battery-powered electronics point out that the cold cathode fluorescent lamp (CCFL) backlight of an LCD display dominates the energy consumption of the whole system [1]. In the SmartBadge system, for instance, the display consumes 28.6%, 28.6%, and 50% of the total power in the active, idle, and standby modes, respectively [1]. To reduce the power consumed by the backlight, researchers [2][3] have proposed the concept of Backlight Scaling. The backlight scaling technique dynamically dims the backlight to conserve its power consumption while increasing the transmissivity of the LCD panel to compensate for the image fidelity loss due to reduced backlight. Image fidelity is defined as the resemblance between the original and backlight-scaled image. If the backlight-scaled image is identical to the original image in terms of the brightness of each pixel (these approaches are called brightness-invariant backlight scaling), then there is no fidelity loss after backlight scaling. However, when a more aggressive backlight

scaling policy is used to gain greater power savings, the brightness invariance is no longer attainable and the induced distortion degrades the image fidelity. Image fidelity can be measured by the brightness variance after backlight scaling. However, the brightness metric is too strict for efficacious backlight scaling policies. Since dimming the backlight directly limits the dynamic range of the image brightness, a brightness-invariant backlight scaling policy is usually too conservative to deliver great energy savings. In this paper, we propose using the image contrast as a metric to measure the image fidelity after backlight scaling.

A. Terminology

The following photometric quantities are illustrated around a backlit TFT-LCD display in Fig. 1. Luminous flux (lumen) is the emission rate of light energy corrected for the standardized spectral response of human vision. Luminous intensity (candela) is defined as cd=lm/sr, one lumen of luminous flux per steradiam (unit of solid angle). Luminous intensity can be used to characterize the optical power emitted from a spot light source, such as a light bulb. Illuminance (lux) is defined as one lumen of luminous flux per area (lm/m2). Illuminance can be used to characterize the luminous power emitted from a surface. Most light meters (e.g., for photographic purpose) measure the illuminance quantity. The luminous flux may not travel in parallel after passing the surface, so that the light intensity decreases as the travel distance increases. Luminance (nit) is defined as lumen per area per steradiam (lm/m2/sr). Luminance is used to rate the maximum brightness of CRT or LCD monitors [4].

Illuminance

Luminance

lux=lumen(lm)/m2

nit=lumen(lm)/m2/sr

Luminous Intensity candala(cd)= lumen(lm)/sr

Luminous Flux lumen(lm)

This research was supported in part by DARPA PAC/C program under contract DAAB07-02-C-P302 and by NSF under grant no. 9988441.

Reflector

CCFL

TFT-LCD Panel

Light meter

Fig. 1: Illustration of CCFL backlight and photometric terms.

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