LCD
Liquid crystal displays and backlighting
Incoherent lightsources
Christian Manß
27.04.2016
Structure
Historical background
Liquid crystals
Mesophase
Typical materials
Phase appeareances
Molecular properties
Functional principle of TN-LCDs
Polarization
TN-LCD
Triggering & activation
ITO, AMLCD, PMLCD
Backlighting
Edge/areal lighting
CCFL, LED(BY), LED(RGB)
Variations of LC structures in displays
Further applications of LCs
Conclusion
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B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016
Historical background
1888 F.Reinitzer
Entdeckung des Klärpunktes von Cholesterinbenzoat
1963 R.Williams
Ausrichtung von LCs in E-Feld > 3kV/m
1967 J.Fergason erstes LCD im Labor -> thermometer, mood ring
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2007
weltweit >400 Mio. Stück/Jahr produziert
2014
225.000 m² LCD Produktion
2016
OLED günstiger als LCD
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B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016
Liquid crystals – mesophase
liquid crystal phase: solid crystals in liquid matrix
mesophase
Tm
solid
Tc
liquid crystal state
liquid
melting temperature Tm for matrix
clearing temperature Tc = melting temperature for crystals
Tm < Tc
cholesteryl benzoate from Tm (left) to Tc (right), 1888 F.Reinitzer
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B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016
Liquid crystals – typical materials
general structure
G, G‘ -
aromates
X
bridge
-
R,R‘ -
some typical materials
residual groups
technically used are
mixtures of up to 30
different structures
property matching
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B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016
Liquid crystals – phase appeareances
smectic (gr. „soapy like“)
highest order
discrete parallel layers
crystals mostly parallel
nematic (gr. „string like“)
no discrete layers
crystals less parallel (than smectic)
cholester(in)ic
crystals parallel inside layers
layers rotating
first discovered
(super) twisted nematic
director
twisted (270°) 90°
nematic phase mixed with parts of a cholesteric phase
most used liquid crystal type (display technology)
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B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016
Liquid crystals – molecular properties
great electric/magnetic dipole moment by fitting residual groups
(e.g. F-alloys)
great displacement in electric/magnetic fields
display technology
birefraction due to anisotropy of refractive index
thermotropy
reversible temperature dependant phase switching
important for operating range -> room temperature
Video
example: 5CB 24 – 35°C, 8OCB 67 – 80°C
eutectical combination: 35% 80CB + 65% 5CB) 5 – 50°C
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B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016
Functional principle of TN-LCDs – Polarization
overview:
polarization of light
polarization filters
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B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016
Functional principle of TN-LCDs
~100nm
~0,5mm
self organisation
~5µm
0,5V
V@0%
V@50%
V@100%
AMLCD
„normally white“
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B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016
Triggering & activation
LCD generally a passive display
passive matrix
PMLCD
linewise image buildup
ITO-grid
-> motion blur in fast movement in video
small voltage transfer to neighbouring pixels
active matrix
AMLCD
pixelwise image buildup
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B.Sc. Christian Manß | Liquid crystal displays and backlighting
-> blur
separate ITO-transistor per px
-> very fast switching time
26.06.2016
Backlighting – Edge/areal lighting
electrolumin.
Foil (- 2008)
U too high
edge lighting
lightguide
needed
diffusor
needed
electroluminescent foil, U~200V
areal
T1-CCFL areal ~200mW/cm²
thinner display
areal lighting
diffusor
needed
LED areal ~200W/cm²
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LED edge-lit
B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016
Backlighting – CCFL, LED, diffusor/extractor
CCFL(Hg) array
HG-CCFL array with phosphorous
lumination (6500K)
LED RGB/BY
Diffusor/extractor unit
- diffuse scattered light from back
can almost homogeneously pass
through extraction spots
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B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016
Variations of LC structures in displays
(S)IPS
(Super) In-Plane-Switching
Angular independence
– Very high viewing angle (up to 170°)
Expensive (patterned electrodes..)
Slower than TN
VA
Vertical-Alignment
MVA
Multi-Domain-Vertical-Alignment
(S)PVA
(Super) Patterned-Vertical-Alignment
– Very high viewing angle (up to 170°)
– Slower than TN
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B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016
Further applications of LCs
thermometer strip – cholesteric
powerless temperature measurement by colour 1967
„moodring“ – cholesteric 1967
amplitude- and phase modulation with TN-LCD
e.g. realization of modifiable fresnel zone plates/ fresnel lenses
cancelling destructive interference by inserting patternded LCD into beam path
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B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016
Conclusion
LCD cheap and easy to produce
Also applications in medicine, biology, optics
LCD itself: Capacitive curcuit -> almost powerless
High intensity losses due to polarizers and filters
Areal backlighting with RGB LED best so far
Very homogenous
High efficiency
Possibility for „scanning backlight“ R-G-B in turn
Further reduction of motion blur
OLED technology will replace LCD in display technology
Higher contrast, better colour rendering, less losses -> less lighting
No backlighting, ito (->front emitter) needed
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B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016
Thank you for your attention.
Are there any questions?
B.Sc. Christian Manß | Liquid crystal displays and backlighting
26.06.2016