HKUST

Generalized Mixed-Mode Reflective LCDs for Three Panel Color Projection Applications

(+Improved Projector Optics)

J Chen, P W Cheng, H C Huang and H S Kwok

Center for Display ResearchHong Kong University of Science & Technology

S K Kwok, C S Li and Steve Yeung

Varitronix Ltd, Hong Kong

HKUST

Silicon Microdisplays

• High aperture ratio at high resolution• Twisted nematic modes or ECB mode• More difficult reflective optics: e.g. PBS -

small acceptance angle

HKUST

Observation

At any voltage T = T( dn)

For a single polarizer reflective display

R = R( dn)

Therefore, T or R can be plotted as a function of 2 variables by fixing the third or fourth variable - parameter space

HKUST

2

sin

cos1*sincos

MRMRR

For reflective display with one polarizer

Reflective displays

HKUST

Parameter Space Diagrams for Reflective Displays

HFE

RTN

RSTN

TN-ECB-1

TN-ECB-2

-360 -240 -120 0 120 240 3600

0.3

0.6

0.9

1.2

1.5

Twist angle

dn

º

TN-ECB-3

HKUST

Parameter Space Diagrams for Reflective Displays - effect of change in polarizer angle

HFE

RTN

RSTN

TN-ECB-1

TN-ECB-2

-360 -240 -120 0 120 240 3600

0.3

0.6

0.9

1.2

1.5

Twist angle

dn

º

TN-ECB-3

RSTN

-360 -240 -120 0 120 240 3600

0.3

0.6

0.9

1.2

1.5

Twist angle

dn

º

SCTN

-360 -240 -120 0 120 240 3600

0.3

0.6

0.9

1.2

1.5

Twist angle

dn

º

MTN

-360 -240 -120 0 120 240 3600

0.3

0.6

0.9

1.2

1.5

Twist angle

dn

º

ECB

HKUST

Both NW and NB modes are possible (//-// polarizers)

nd

Twist Angle

A

B

C

NB

NW

Better nomenclature: In-well and out-well modes

HKUST

In-well and out-well modes

Note: No retardation film

Normally white(NW)

Normally black(NB)

Direct view Out-well, dark state atintermediate voltage

In-well, bright state at highvoltage

Projection In-well, dark state at highvoltage

Out-well, bright state atintermediate voltage

HKUST

Electro-optic properties of in-well and out-well modes

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.400.45

0.50

0 1 2 3 4 5 6Voltage

Refl

ecta

nce

70_R70_G70_B

dn=0.31

00.10.20.30.40.50.60.70.80.9

1

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0Voltage

Ref

lect

ance

HFERTNTN-ECB

0.00.10.20.30.40.50.60.70.80.91.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Voltage

Ref

lect

ance

HFERTNTN_ECB

HKUST

0 20 40 60 80 1000

0.1

0.2

0.3

0.4

0.5

Twist Angle

dn

0.9 reflectance contour for various polarizer angles

TN-ECB

MTN

SCTN

Relationship between various MTB modes (Mixed TN-Birefringence modes)

HKUST

Can optimize these MTB modes using various objectives: e.g. least dispersion (for B/W displays) or highest contrast (low voltage operation).

Least dispersion criteria:

Define Y = R/R within 450-700nm for the bright state. Search in parameter space to find (dn) with smallest Y.

Highest contrast criteria:

Define

dLR

dLR

off

onCR

)()(

)()(

HKUST

Least Dispersive Modes

• Needed for B/W displays• Also for color displays with one type of panel

0.00

0.05

0.10

0.15

0.20

0.250.30

0.35

0.40

0.45

0.50

400 450 500 550 600 650 700 750Wavelength

Ref

lect

ance

90_off90_on80_off80_on70_off70_on60_off60_on

off

on

HKUST

High Contrast Modes - Contrast Parameter Space

• PS is obtained for V=0

• Take Vns = 0 and use appropriate Vs from RVC and calculate new PS

• Calculate Rns and Rn and take the ratio to get the Contrast PS (CPS)

• Contrast is given by Rs/Rns for NB mode or Rns/Rs for NW mode

HKUST

Example of CPS for reflective LCD

0 0.5 10

0.2

0.4

0.6

0.8

1

0 15 30 45 60 75 900

0.1

0.2

0.3

0.4

0.5

Polarizer angle

d

n

70

0 0.5 10

0.2

0.4

0.6

0.8

1

0 15 30 45 60 75 900

0.1

0.2

0.3

0.4

0.5

Polarizer angle

d

n50

0 0.5 10

0.2

0.4

0.6

0.8

1

0 15 30 45 60 75 900

0.1

0.2

0.3

0.4

0.5

Polarizer angle

d

n

90

SCTN mode has highest contrast for the same voltage, but dispersion is large

HKUST

Viewing Angle of Light Valves

• Not an issue with present PBS designs (10o acceptance angle)

• Will be an important issue with new improved optical elements

• Strongly dependent on operating voltages and LCD modes

HKUST

Viewing angle of MTB mode - importance of operating voltage

125 250

3V 4V

±10o for CR = 250± 4o for CR = 125

HKUST

Normally Black Modes with PBS (Out-well modes)

Dark state(V=0) of NB RTN modes with diff. twist angles

0%

2%

4%

6%

8%

10%

12%

14%

350 450 550 650 750

Wavelength

Ref

lect

ance

Bright state(V=2v) of NB RTN modes with diff. twist angles

0%5%

10%15%20%25%30%35%40%45%50%

350 400 450 500 550 600 650 700 750 800

Wavelength

Re

fle

cta

nce

HKUST

Compact Projection System

MH lamp

PBS

B-LCD

G-LCD

R-LCDTrichroic prismA

A

B

c

HKUST

S-P polarization split of color filters

• The reflectance spectrum of a color filter coating depends on the polarization of the incident light

• The polarization of light is modified by the reflective LCOS light valve

• -> The color separator and recombiner operate at reverse polarizations

• -> Have to reduce s-p polarization split

HKUST

= 45o

400 450 500 550 600 650 7000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength (nm)

Refle

cta

nce

This is actually the principle of PBS!

Needs smaller incidence angles

p sReflectance

Polarization split is strongly dependent on incident angle

400 450 500 550 600 650 7000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength (nm)

Reflecta

nce

HKUST

has to be at least 24o for conventional TPA

NAc1sin

2

1

Blue

Red + Green

coating

HKUST

The PBS does not have to be cubic!

blue

green

Both coatings have = 16o

*patent pending

3

2

12

22

B

D

A

C

red

HKUST

400 450 500 550 600 650 7000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength(nm)

Rela

tive In

ten

sity

- S Polarization

-- P Polarization

400 450 500 550 600 650 7000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Wavelength(nm)

Rela

tive In

ten

sity

- S Polarization

-- P Polarization

Comparison of 24o and 16o TPA

Measured with prisms made at Zhejiang University

HKUST

Summary

• Have shown that PS is a good tool for the optimization of MTB modes for reflective displays

• For 3 panel color projector, contrast is more important than dispersion

• Viewing angle will be an important issue for wide acceptance angle PBS

• Have demonstrated a new trichroic prism assembly which is compact and can perform color separation and recombination satisfactorily