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Power Minimization for LED-backlit TFT-LCDs: A Perception-Oriented Approach

This study explores brightness and lightness perception in TFT-LCDs to reduce power consumption for LED-backlit displays. The research delves into chromaticity scaling, psychophysics, and conclusions drawn from luminance and brightness measurements. Previous works, such as dynamic backlighting and concurrent brightness and contrast scaling, are reviewed. The study also examines hue adjustment, psychophysical studies on contrast, and chromaticity scaling strategies. The findings emphasize the perceptual measures in optimizing display power and suggest future research directions.

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Power Minimization for LED-backlit TFT-LCDs: A Perception-Oriented Approach

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

  2. (NY Times)

  3. Outline • Background • Psychophysics • Chromaticity scaling • Conclusions

  4. Brightness/lightness Demo

  5. 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 ?

  6. 30W 100% x70= 70 70% x70= 49 25W 70% x100= 70 25W Reduce Backlight Power Consumption PowerBacklight x Transmittance = Luminance LCD Panel What is the optimal enhancement?

  7. 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

  8. 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?”

  9. Hue Adjustment Blue Red Weak color memory

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

  11. Psychophysical Study Brightness =? Contrast

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

  13. “ContrastL < ContrastR?” “Yes”

  14. Experimental Data 0.7 No 0.9 No 1.3 Yes • "Interaction between brightness and contrast of complex stimuli," Journal of Vision, 6(8), 2006. 1.5 Yes

  15. Chromaticity Scaling

  16. Chromaticity Scaling • MIN{MAX(Luminance)} • MIN{MAX(Red)+MAX(Green)+MAX(Blue)}

  17. Chromaticity Scaling • CIELAB Color Difference ∆E • Industry standard

  18. RGGB LED backlights

  19. 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)

  20. Thank You

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

  22. Viewing Angle

  23. Sample Pictures

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

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

  26. Light Efficiency of TFT-LCD • Transmittance

  27. Apple iPod Video 60G

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

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