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Introduction

Introduction

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Introduction

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  1. Introduction

  2. What is a Display ? • A complex optical device that renders an image, graphics and • text by electrically addressing small switching elements (pixels) • (2) Serves as an interface between human being and machine Let us survey some of the display technologies

  3. Types of Displays Emissive display that modulates backlight (light shutter) display (SLM) that reflects projection lamp (reflective pixel) display (SLM) that modulates projection lamp (transmissive pixel) display that generates its own light active matrix, STN, FLCD CRTs, FEDs, LEDs, plasma EL, VPD Direct-View Projection Backlight Reflective TransmissiveSLM Reflective SLM display that rejects/reflects ambient light active matrix reflecting pixel, digital micro- mirrors active matrix light valve cholesteric LC, STNs, MEMs, FLCDs Super-twisted nematic(STN); ferroelectric liquid crystal display(FLCD); cathode-ray-tube(CRT); light emitting diode(LED); vacuum fluorescent display(VPD); field emitter display(FED); electroluminescent (EL); micro-electro-mechanical(MEM); spatial light modulator(SLM); liquid crystal(LC)

  4. white light Display Applications Direct-View Displays Projection Displays (3-pass) dichroic mirrors LCD backlight transmissive room light LCD LCD LCD LCD mirrors projection optics

  5. Emissive - CRT

  6. Emissive - CRT Advantages: Mature Technology (>100 years old) Cheap to manufacture Good Viewing Angle Disadvantages: Heavy Bulky Power hungry

  7. Emissive – Plasma

  8. Emissive – Plasma • ADVANTAGES: • Established technology • Simplified driving schemes • Low cost, high volume because of simplicity • Color is feasible • Long lifetime • DISADVANTAGES: • High voltage drivers • Low contrast ratio • Residual background glow

  9. Emissive - EL

  10. Emissive - EL • Metal electrode-insulator-phospher layer (EL), insulator, conductor • All deposited by thin film techniques • Host material - zinc sulfide (ZnS) and activator manganese (Ms) • Manganese (yellow); terbium (green), cerium (blue) • High field is applied to phospher layer • Stack of insulators and phospher become charged, current flows • in phospher layer • Resulting in ‘excitation’ of activator atoms raising them to higher • energy level • Electric field is transferred to the electrons in activator atoms, • raising them to higher energy level for short period of time. • Electrons relax to ‘ground state’ energy is released in the form of • VISIBLE Light • The field in the phospher layer is then reduced and conduction • stops until field is reversed.

  11. Emissive - EL • ADVANTAGES: • Thin and compact • Fast writing speeds (video compatible) • Good readability & brightness • Gray scale ability • DISADVANTAGES: • High voltage drivers (170-200 volts) • Washout in bright ambient light (phosper layer scatters) • Color progressing but slow

  12. Emissive - VFD

  13. Emissive - VFD • Cathode filament is to 600oC to facilitate emission of thermal electrons • Anode voltage of 10-50 V is supplied to anode • At the same time voltage is applied to the grid of selected segments • Electrons from the filament are accelerated by the grid and sent • to phospher coated anode • Activators in the phospher are ‘excited’ from the electrons bombard- • in the phospher. • Energy from electrons transferred to phosphers raising the electrons • to a higher energy level for a short period of time • When the electrons relax to their ground state, energy is released in • the form of visible light • ZnS is often used as the phospher layer

  14. Emissive - VFD • ADVANTAGES: • High brightness • Low cost for low information content displays • Full color available • Manufacturing is well established • DISADVANTAGES: • Large screen & high resolution hard to do • Not for portable applications - high power • High voltage drivers needed

  15. Emissive – Field Emitter Displays

  16. Emissive – Field Emitter Displays • Original theory of Richardson (1934) • Electrons treated as substance that escapes from the solid state • into a vacuum • Some electrons are reabsorbed into the surface • Equilibrium is established • Equilibrium changes with temperature • Increased temperature, electrons escape faster than they find • themselves being reabsorbed by the surface • Electrons at the highest energy levels are allowed to escape • (no very many) Richardson-Dushman equation for current emission j: f work function e charge k Boltsman constant T absolute temperature

  17. Emissive – Field Emitter Displays • Quantum mechanics - electron position viewed in terms of probability • Finite probability that electron will find itself outside energy barrier • in spite of the fact if it has enough energy to ‘leap over’ the barrier • Tunneling • Small % of electrons will tunnel between emitter and vacuum • Increase % by narrowing the width of energy barrier • Higher probability that electrons tunnel through thin wall than thick one • Vary width with high electric field at surface of emitter • An electron that finds itself an infinitesimal distance outside emitter • escapes • High electric fields are needed 3-6 x 107 eV/cm Fowler-Nordheim Equation for current emission j: Ef is fermi energy F is electric field

  18. Emissive – Field Emitter Displays ADVANTAGES: Potentially high luminous A lot of CRT phosphors High speed addressing and response No temperature sensitivity Analog gray scale and full color possible Limited photolithography requirements DISADVANTAGES: No low voltage phosphors developed yet No manufacturing infrastructure High driving voltages needed High temperature fab equipment needed Phosphors scatter sunlight (portable ?) Cross talk of electrons in adjacent pixels Still reseach projects for most

  19. Emissive – Light Emitting Diodes

  20. Emissive – Light Emitting Diodes Mechanism of p-n Junction Operation • When no voltage or reversed voltage is applied across a • p-n junction, an energy barrier is formed preventing the • flow of electrons and holes • When a forward bias is applied across the p-n junction, the • energy barrier is reduced allowing electrons to be injected into • p regions and holes to be injected into n regions • The injected carriers recombine with carriers of opposite sign • resulting in the emission of light

  21. Emissive – Light Emitting Diodes • ADVANTAGES: • Low voltage operation • Low cost for low information content • Multiple colors • Manufacturing well established • Large screen message screens available • Organic LED materials potentially easier to process • Organics now possible with flexible substrates • DISADVANTAGES: • High power consumption for portable products • High cost for high information content • Blue LEDs have low brightness • Full-color displays (?)

  22. Transmissive –Twisted Nematic LCD

  23. Transmissive –Twisted Nematic LCD • Advantages • Well established technology (early 1970’s) • Created the portable computer market • High resolution with active matrix • Excellent color purity • Disadvantages • Needs active matrix • backlight is the power sink • A lot of layers, both optical and electronic • Viewing Angle is said to be a problem but • many solutions are practiced to fix it.

  24. Transmissive –Super Twisted Nematic LCD

  25. Transmissive –Super Twisted Nematic LCD • Advantages • Well established technology • Great for inexpensive low-medium resolution displays • No need for active matrix, cheap passive solutions • Disadvantages • Poor color performance • Poor viewing angle • Medium resolution with passive addressing

  26. Reflective – Electrophoretic

  27. Reflective – Electrophoretic • ADVANTAGES: • Low power consumption - reflective • Adequate contrast • Wide viewing angle • High resolution possible • Inherent memory • New encapsulation techniques for stabilization (E-Ink) • DISADVANTGES: • Stability of suspension unclear • Higher drive voltage than available drivers • Slow switching speed • Complex chemistry

  28. Reflective-Gyricon

  29. Reflective-Gyricon • Advantages • Cheap • Cool • Disadvantages • High voltage • needs active matrix • sticky balls

  30. Reflective - PDLC

  31. Reflective – H-PDLC

  32. Reflective – H-PDLC • ADVANTAGES: • High reflection efficiency • Great color purity • No polarizers • DISADVANTAGES: • High driving voltage • Still research • Fabricate with laser scanning

  33. Reflective – Cholesteric LCD

  34. Flexible Displays What technologies are adaptable to a flexible type substrate ?

  35. Threshold vs. Non-Threshold Addressing: How do we supply voltages to Render an image ?

  36. Threshold vs. Non-Threshold

  37. Examples of Threshold, Non-Threshold Materials all LCD’s electroluminescent plasma light emitting diode Threshold No Threshold electrophoretics Gyricon

  38. Direct Drive Addressing • Thresholdless nature of material is irrelevant • Every pixel is independently addressed • Every pixel has a connection for a N+M display, there • are NM electrical connections • For lower resolutions only <50 pixels inch

  39. Direct Drive

  40. Samples of Fixed Format Alpha Numeric Matrices 7-bar 10-bar 13-bar 14-bar

  41. For medium to high resolution ( 400 rows) Multiplexed Addressing • Can address NM pixels using only N+M electrical • connections • Strict limitation on threshold voltage and T-V • steepness curve • Voltages applied to one pixel cannot be arbitrarily • changed without affecting the applied voltage of the • other cells

  42. The instantaneous drop at the pixel electrode is ON state V=VS-(-VD) or V=VS-VD Multiplexing 2D Array • Consider MN Array, addressed with N rows and M • columns • The M elements in the first row can be turned ON or • OFF depending on the voltages applied to each • element. Let VS denote the row voltage and VD denote • the column voltage • The row voltage is always VS, and the column voltage • can be VD

  43. Conditions for RMS Responding Material • Response time, governed by viscoelastic properties, must be • >> than period of driving waveform • Interaction between LC molecule and applied electric field • must be a=E2 (induced polarization) • In each multiplexing cycle, each row is selected on during 1/N • of the cycle time T

  44. RMS Responding Material Alt and Pleshko IEEE Trans. Electronic Devices ED-21, 146-155 (1974) Using the previous equations, one can derive the maximum number of rows ‘ Selection Ratio’ For NMAX>>1

  45. Selection Ratio 2.5 2 Selection Ratio 1.5 1 1000 400 200 600 800 0 NMAX

  46. VTH: threshold voltage (turn on begins) D: steepness parameter Multiplexing: Practical Applications

  47. S+D +D +D time pixel voltage // S-D (row-column) +D Frame 1 // S+D +S +D +D // +S T +S S-D +D +D // // -D +S +S Row Signals // +S +S // +S D t // 1 2 3 . . . N +D +D +D // -D -D -D Column Signals Passive Multiplexing: Amplitude Modulation

  48. pixel voltage time 1-f (row-column) Frame 1Frame 2 f +S +S T // +S +S Row Signals // +S // +S S+D D t S-D +D // // -D 1 2 3 . . . N // +S 1-f // // +D f // Column Signals -D Passive Multiplexing: Pulse Width Modulation

  49. Examples of Multiplexing Display Configuration VTH D NMAX (NMAX)2 TN 2 Volts 0.4 Volts 25 625 STN 4 Volts 0.2 Volts 400 1.6  104 PDLC 8 Volts 3 Volts 7 50 electrophoretic none undefined 0 0

  50. Active Matrix Displays • Multiplexing is limited and not adequate for high • resolutions (slow response, poor viewing angle, no • gray scale) • A non-linear element is build into the substate at each • pixel, usually a thin-film-transistor • Being isolated from other pixels by TFT’s, the voltage • remains constant while the other pixels are being • addressed • Not subject to Alt-Pleshko Formalism