Presentation kit

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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
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(NY Times)
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Outline

• Background
• Psychophysics
• Chromaticity scaling
• Conclusions

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Brightness/lightness Demo
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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

?
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Reduce Backlight Power Consumption
Power Backlight x Transmittance
Luminance
LCD Panel
What is the optimal enhancement?
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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
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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?

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Hue Adjustment
Blue
Red

• Weak color memory

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• Whats the time?
• Few people noticed the clock
• The brightest object
• The least important object

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Psychophysical Study

• Brightness ? Contrast

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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

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ContrastL lt ContrastR?
Yes
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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
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Chromaticity Scaling
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Chromaticity Scaling

• MINMAX(Luminance)
• MINMAX(Red)MAX(Green)MAX(Blue)

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Chromaticity Scaling

• CIELAB Color Difference ?E
• Industry standard

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RGGB LED backlights
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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)

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Thank You
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Luminance Scaling

• Transfer function f

Contrast fidelity f
X
Image quality
Histogram h
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Viewing Angle
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Sample Pictures
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RGB LED Backlighting

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

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Backlight Dimming vs. Image Enhancement
(b) Dim backlight to 50 without compensation
Identity transfer function

• (a) Original image

(d) Optimal enhancement
(c) Enhance contrast to recover brightness but
pixels over-saturated
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Light Efficiency of TFT-LCD

• Transmittance

Front polarizer 90
Color filter 30
TFT array 80
Liquid crystals 95
Rear polarizer 50
Total lt5
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Apple iPod Video 60G
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Power
Energy
HD 10
BL 16
BL 49
HD 54
µP 14