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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
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2
sin
cos1*sincos
MRMRR
For reflective display with one polarizer
Reflective displays
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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
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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
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Both NW and NB modes are possible (//-// polarizers)
nd
Twist Angle
A
B
C
NB
NW
Better nomenclature: In-well and out-well modes
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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
)()(
)()(
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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
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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
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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
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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
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Viewing angle of MTB mode - importance of operating voltage
125 250
3V 4V
±10o for CR = 250± 4o for CR = 125
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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
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Compact Projection System
MH lamp
PBS
B-LCD
G-LCD
R-LCDTrichroic prismA
A
B
c
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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
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= 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
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has to be at least 24o for conventional TPA
NAc1sin
2
1
Blue
Red + Green
coating
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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
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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