Power Minimization for LED-backlit TFT-LCDs

Power Minimization for LED-backlit TFT-LCDs

Wei-Chung ChengJuly 26, 2006

Wei-Chung ChengJuly 26, 2006

PODLAB – Perception Oriented Design LabDepartment 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 = 7030W

Power Backlight x Transmittance = Luminance

LCD Panel

70% x 70 = 4925W 70% x 100 = 7025W

What is the optimal enhancement?

Previous Works

N. Chang, IEEE TCAD 2004Dynamic backlight luminance scaling

N. Chang, IEEE DTC 2004Battery-powered multimedia systems

W.-C. Cheng, DATE 2004Concurrent brightness and contrast scaling

N. Dutt, IEEE DTC 2004For video streaming

A. Iranli, DATE 2005Histogram equalization

A. Iranli, DAC 2005Dynamic tone mapping

L. Zhong, N. Jha, MobiSis 2005Minimum 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 StudyPsychophysical Study

Brightness =? ContrastBrightness =? Contrast

Psychophysical Study

37 observers; 2 excluded after statistical analysis

The Method of Adjustment, a standard psychophysical methodby Gustav Fechner

Original100% backlightOriginal contrast

Contrast enhanced70% backlightAdjusted contrast

“ContrastL < ContrastR?”

“Yes”

Experimental Data

0

4

8

12

16

20

0.21 0.42 0.58 0.79 1.00 1.25 1.67 2.50 5.00

slope

co

un

t

Ascending

Descending

"Interaction between brightness and contrast of complex stimuli," Journal of Vision, 6(8), 2006.

Yes

Yes

No

No

1.5

1.3

0.9

0.7

Chromaticity ScalingChromaticity Scaling

Chromaticity Scaling

MIN{MAX(Luminance)}

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

0.00

10.00

20.00

30.00

40.00

50.00

0 50 100 150 200 250 300 350

Luminance (nit)

Pow

er (w

att)

R+G+B

Red

Blue

Green

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 directionsConsider viewing directionsHuman-display interaction

More details can be found in our related articlesIEEE J. Display Technology, 2006 (JDT)Color Imaging Conference, 2006 (CIC14)Society for Information Display, 2006 (SID06)

Thank YouThank You

Luminance Scaling

Transfer function f

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.2083 0.2917 0.4167 0.5 0.5833 0.7083 0.7917 0.9167 1 1.125 1.25 1.4167 1.6667 2 2.5 3.2917 5

Contrast fidelity f’

Histogram hX Image quality

Viewing Angle

Sample Pictures

RGB LED Backlighting

Superior to CCFLLarge gamut (high color saturation)Mercury-freeFast responseCompact sizeDC driving

ChallengesCostVariationHeat 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

L(x)

x

L(x)

x

L(x)

Identity transfer function

Light Efficiency of TFT-LCD

Transmittance

Front polarizer 90%

Color filter 30%

TFT array 80%

Liquid crystals 95%

Rear polarizer 50%

Total <5%

Apple iPod Video 60G

0.0

0.5

1.0

1.5

2.0

2.5

660 710 760 810 860 910 960 1010

Time (sec)

Pow

er (

W)

HD: 10%

BL: 49%

HD: 54%

BL: 16%

Power

µP: 14%

Energy