Power Minimization for LED-backlit TFT-LCDs Wei-Chung Cheng July 26, 2006 PODLAB – Perception Oriented Design Lab Department of Photonics and Display Institute National Chiao Tung University, Taiwan

(NY Times)

Outline Background Psychophysics Chromaticity scaling Conclusions

Brightness/lightness Demo

Luminance vs. Brightness Luminance is a physical measure Brightness and lightness are perceptual measures Luminance can be measured by a light meter How to measure brightness? Psychophysics ?

Reduce Backlight Power Consumption 100% x 70 = 70 30W Power Backlight x Transmittance = Luminance LCD Panel 70% x 70 = 49 25W 70% x 100 = 70 25W What is the optimal enhancement?

Previous Works N. Chang, IEEE TCAD 2004 Dynamic backlight luminance scaling N. Chang, IEEE DTC 2004 Battery-powered multimedia systems W.-C. Cheng, DATE 2004 Concurrent brightness and contrast scaling N. Dutt, IEEE DTC 2004 For video streaming A. Iranli, DATE 2005 Histogram equalization A. Iranli, DAC 2005 Dynamic tone mapping L. Zhong, N. Jha, MobiSis 2005 Minimum energy for displays luminance scaling

Concurrent Brightness and Contrast Scaling (CBCS) Cheng and Pedram, DATE 04 Scales brightness and contrast Brightness and contrast are different qualities Question unanswered: “Are brightness and contrast interchangeable?”

Hue Adjustment Red Blue Weak color memory

What’s the time? Few people noticed the clock The brightest object The least important object

Psychophysical Study Brightness =? Contrast

Psychophysical Study 37 observers; 2 excluded after statistical analysis The Method of Adjustment, a standard psychophysical method by Gustav Fechner Original 100% backlight Original contrast Contrast enhanced 70% backlight Adjusted contrast

“Contrast L < Contrast R ?” “ Yes ”

Experimental Data "Interaction between brightness and contrast of complex stimuli," Journal of Vision, 6(8), Yes No

Chromaticity Scaling

MIN{MAX(Luminance)} MIN{MAX(Red)+MAX(Green)+MAX(Blue)}

Chromaticity Scaling CIELAB Color Difference ∆E Industry standard

RGGB LED backlights

Conclusions Brightness and lightness are perceptual measures Human vision can be negotiated to save power Used psychophysical methods to guide low-power design Scaling in the chromaticity (RGB) space RGGB LED backlights; 30% to 75% power savings for USC-SIPI image database Future directions Consider viewing directions Human-display interaction More details can be found in our related articles IEEE J. Display Technology, 2006 (JDT) Color Imaging Conference, 2006 (CIC14) Society for Information Display, 2006 (SID06)

Thank You

Luminance Scaling Transfer function f Contrast fidelity f’ Histogram h X Image quality

Viewing Angle

Sample Pictures

RGB LED Backlighting Superior to CCFL Large gamut (high color saturation) Mercury-free Fast response Compact size DC driving Challenges Cost Variation Heat dissipation

Backlight Dimming vs. Image Enhancement (a) Original image (b) Dim backlight to 50% without compensation (d) Optimal enhancement (c) Enhance contrast to recover brightness but pixels over-saturated x L(x) x x x Identity transfer function

Light Efficiency of TFT-LCD Transmittance Front polarizer90% Color filter30% TFT array80% Liquid crystals95% Rear polarizer50% Total<5%

Apple iPod Video 60G

HD: 10% BL: 49% HD: 54% BL: 16% Power µP: 14% Energy