LCD. Liquid crystal displays and backlighting. Incoherent lightsources Christian Manß

Transcription

1 LCD Liquid crystal displays and backlighting Incoherent lightsources Christian Manß

2 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 1

3 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 2007 weltweit >400 Mio. Stück/Jahr produziert m² LCD Produktion 2016 OLED günstiger als LCD 2

4 Liquid crystals mesophase liquid crystal phase: solid crystals in liquid matrix mesophase melting temperature Tm for matrix clearing temperature Tc = melting temperature for crystals Tm < Tc Tm solid liquid crystal state liquid Tc cholesteryl benzoate from Tm (left) to Tc (right), 1888 F.Reinitzer 3

5 Liquid crystals typical materials general structure G, G - aromates X - bridge R,R - residual groups some typical materials technically used are mixtures of up to 30 different structures property matching 4

6 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 twisted (270 ) 90 nematic phase mixed with parts of a cholesteric phase most used liquid crystal type (display technology) director 5

7 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 C, 8OCB C eutectical combination: 35% 80CB + 65% 5CB) 5 50 C 6

8 Functional principle of TN-LCDs Polarization overview: polarization of light polarization filters 7

9 Functional principle of TN-LCDs ~100nm ~0,5mm self organisation ~5µm 0,5V AMLCD normally white 8 von 14 B.Sc. Christian Manß Liquid crystal displays and backlighting

10 Triggering & activation LCD generally a passive display passive matrix PMLCD ITO-grid linewise image buildup -> motion blur in fast movement in video small voltage transfer to neighbouring pixels -> blur active matrix AMLCD separate ITO-transistor per px pixelwise image buildup -> very fast switching time 9

11 Backlighting Edge/areal lighting electroluminescent foil, U~200V areal T1-CCFL areal ~200mW/cm² electrolumin. Foil (- 2008) U too high edge lighting lightguide needed diffusor needed thinner display areal lighting diffusor needed LED areal ~200W/cm² LED edge-lit 10

12 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 11

13 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 12

14 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 13

15 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 14

16 Thank you for your attention. Are there any questions? B.Sc. Christian Manß Liquid crystal displays and backlighting