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Power Minimization for LED-backlit TFT-LCDs

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.

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Power Minimization for LED-backlit TFT-LCDs

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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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