Transcript Document
Introduction What is a Display ? (1) 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 Types of Displays Direct-View Backlight Emissive Reflective display that modulates backlight (light shutter) display that generates its own light display that rejects/reflects ambient light active matrix, CRTs, FEDs, cholesteric LC, STN, FLCD LEDs, plasma STNs, MEMs, FLCDs EL, VPD Projection Transmissive SLM Reflective SLM display (SLM) that modulates projection lamp (transmissive pixel) active matrix light valve display (SLM) that reflects projection lamp (reflective pixel) active matrix reflecting pixel, digital micromirrors 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) Display Applications Direct-View Displays Projection Displays (3-pass) white light dichroic mirrors LCD backlight transmissive room light LCD LCD LCD LCD projection optics mirrors Emissive - CRT Emissive - CRT Advantages: Mature Technology (>100 years old) Cheap to manufacture Good Viewing Angle Disadvantages: Heavy Bulky Power hungry Emissive – Plasma Light Output transparent electrode seal + Neon-Argon Gas cathode - • Neon glow discharge principle • Neon or Noble gas is ionized when sufficient voltage if applied • Ionization of gas results in visible glow (orange or red) • DC operation shown; AC shemes are also popular 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 Emissive - EL seal dielectric film phospher layer VAC electrode glass Orange-yellow light output for ZnS Application of an electric field causes visible light to be emitted from the phospher layer Emissive - EL • Metal electrode-insulator-phospher layer (EL), insulator, conducto • 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. 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 Emissive - VFD glass grid lead wire grid filament cathode anode coated with phospher seal filament lead glass tube tipped offevacuated space 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 bombardin 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 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 Emissive – Field Emitter Displays faceplate ITO driver ++ evacuated phosper electrons spacer + extractio grid - baseplate Insulator emitter tip emitter electrodes 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: j 120T exp( e / kT ) 2 work function e charge k Boltsman constant T absolute temperature 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 on • 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: 1/ 2 Ef is fermi energy ( E / ) 7 f 6 2 ( 6.810 3 / F ) j 6.2 10 F e F is electric field Ef 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 Emissive – Light Emitting Diodes Top Electrode Light emission layer Battery Hole transport layer ITO glass Light 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 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 (?) Transmissive –Twisted Nematic LCD 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. Transmissive –Super Twisted Nematic LCD 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 Reflective – Electrophoretic viewer sees white transparent electrode seal - - - - viewer sees black - metal electrode - - - - - + colloid suspension particles (surfactants, solvent) - Negatively charged white pigment particles Blue dye for emample 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 Reflective-Gyricon Reflective-Gyricon Advantages •Cheap •Cool Disadvantages • High voltage • needs active matrix • sticky balls Reflective - PDLC Polymer Dispersed Liquid Crystal (PDLC) ne • Easy to manufacture • Good viewing angle • Bright - no polarizers • No rubbing layers • Good projection displays • Slightly high driving voltages • Contrast only 10:1 • Poor reflectance • Off-axis haze • Direct-view (?) no np V ne no V np np ~ no Reflective – H-PDLC Holographic Polymer Dispersed Liquid Crystal (H-PDLC) ne np no V ne np V no Reflective – H-PDLC ADVANTAGES: • High reflection efficiency • Great color purity • No polarizers DISADVANTAGES: • High driving voltage • Still research • Fabricate with laser scanning Reflective – Cholesteric LCD Cholesteric Texture Displays Polymer Stabilized Cholesteric Texture (PSCT) • High contrast for reflective • Good viewing angle • Bistable memory • No polarizers - easy to manufacture V1 • Slow (not video compatible) • Bragg Color Shift V V2 Flexible Displays What technologies are adaptable to a flexible type substrate ? Threshold vs. Non-Threshold Addressing: How do we supply voltages to Render an image ? Threshold vs. Non-Threshold Examples of Threshold, Non-Threshold Materials Threshold No Threshold all LCD’s electrophoretics electroluminescent plasma light emitting diode Gyricon 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 NM electrical connections • For lower resolutions only <50 pixels inch Direct Drive Samples of Fixed Format Alpha Numeric Matrices 7-bar 10-bar 13-bar 14-bar Multiplexed Addressing • Can address NM 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 • For medium to high resolution ( 400 rows) Multiplexing 2D Array • Consider MN 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 • The instantaneous drop at the pixel electrode is ON state V=VS-(-VD) or V=VS-VD 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 2 ON V 2 OFF V 1 N -1 2 2 = VD VS +VD + N N 1 N -1 2 2 = VD VS -VD + N N 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 VON = VOFF NMAX +1 For NMAX>>1 VON =1+ VOFF NMAX -1 ‘ Selection Ratio’ 1 NMAX Selection Ratio Selection Ratio 2.5 2 1.5 1 0 200 400 600 NMAX 800 1000 Multiplexing: Practical Applications VOFF VTH VON VTH + Δ Δ P VTH Δ 1 VTH NMAX VTH: threshold voltage (turn on begins) D: steepness parameter Passive Multiplexing: Amplitude Modulation time pixel voltage (row-column) Frame 1 +S S+D T +S +D Row Signals +D // // +S +S S-D // +S S+D +S +S +D // +D +D // // +S D t S-D +D +D // 1 2 3 ... +D // N -D +D +D // -D -D -D Column Signals Passive Multiplexing: Pulse Width Modulation time 1-f Frame 1 Frame 2 Row Signals +S f // +S // +S +S // // +S Dt // 3 ... N 1-f +D // -D f Column Signals S+D S-D +D -D +S // 2 (row-column) +S T // 1 pixel voltage // Examples of Multiplexing 2 NMAX VTH = Δ 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 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 Active Matrix Circuit Scan Line Drain Source Liquid Crystal Active Matrix: A Complex Device Drain Principle of Operation-Active Matrix • One line at a time addressing • A positive voltage pulse (duration T/N, N # rows, T frame time) is applied to the line turning on all TFT’s • The TFT’s act as switches allowing electrical changes to the LC capacitors from the columns (data or source) • When addressing subsequent rows a negative voltage is applied to the gate lines thereby turning off the transistors for one frame time T, until ready to readdress it • For AC drives schemes (LCD’s) the polarity is alternated on the data voltage 4 Basic Steps of TFT 1. At time 1, a positive voltage VG of duration T/N is applied to gate to turn on TFT. The LC pixel (ITO) is changed to VON at time 2 within T/N, due to the positive source voltage VSD=VON. 2. At time 2, the gate voltage VG becomes negative, turning off the source voltage VSD from VON to –VON. During the time period 2 and 3, of duration (N-1)/NT, the pixel voltage VP remains about >0.9 VON as the LC capacitor is now isolated from data lines. 3. At time 3 (the next addressing time), the TFT is turned on again by applying a positive gate voltage of duration T/N. The LC capacitor now sees a negative source-to-drain voltage VSD=-VON. The pixel electrode is discharged from VP=VON at time 3 to VP=-VON within the time duration T/N. 4. At time 4, the TFT is turned off by the negative gate voltage, and simultaneously the source voltage VSD changes from –VON to +VON. TFT Addressing VG T/N T T 0 1 2 VSD 0 VON time Gate Voltage time Source Drain Voltage time Pixel Voltage 3 4 VP Notice that VP is not constant during the duration (n-1)T/N because of a slight leakage current of LC cell. LC materials must have a high voltage holding ratio (VHR) to minimize this. Summary Active Matrix Multiplexed LC Mode TN STN Contrast >100:1 10-15:1 Viewing (horizontal) -600,+600 -300,+300 Viewing (vertical) -300,+450 -250,+250 Response time 20-40ms 100-150ms Addressable lines >1000 ~400 Gray-scale >16 low Basic Display Measurement Display technology is a very interdisciplinary science, combining basic principles from all the sciences and engineering, and in addition, human physiology. Three basic concepts should be remembered when working with light measurement and displays-spectral, spatial and temporal. Spectral Characteristics: The spectral, or color consideration is closely related to the frequency band pass characteristics of devices and systems in electronics. Initially one must decide if the spectral characteristics are to be considered for the human eye (photometry) or power (radiometry). Spatial Characteristics: The spatial characteristics are geometric considerations affecting emission, reflection, absorption, transmission, and sensing light. Basic Display Measurement Temporal Characteristics: Temporal considerations are time related. Analogous to electronic devices, optical devices have rise times and fall times and frequency bandwidths associated with them. Electromagnetic Spectrum: The electromagnetic spectrum depicts the range of electromagnetic radiation. The region identified as photometry corresponding to the visible spectrum- this is the range where the human eye is sensitive. Human Eye Ultimate Reception for Displays • Secured by six muscles. • Sclera is a dense white fibrous material,except where it becomes transparent (cornea). • Transparent gel-like substance filb the eye (viteous humor). • An elastic lens is situated in the viteous humor and secured by a muscle. • The lens shape is controlled by muscle action to focus image. • Outside in formation passes through cornea, lens, and the viteous humor, where the light is focused on a slight indentation on back wall, the fovea. Human Eye Ultimate Reception for Displays • The inner wall of the eye is covered with a layer of light sensing cells (retina). • Nerve fibers protruding from each cell form complex web networks, eventually forming the optic nerve. • Between light sensing cells and their network of nerve fibers and the sclera, is another pigmented membrane, the choroid to absorb a residual light not absorbed by the light sensitive cells. • The retina contains 120 millions photosensitive receptions, called rods and cones. The cones are concentrated in the fovea and responsive for color vision. There are 7 million cones. Human Eye Ultimate Reception for Displays • The rods are not present in the fovea, but populate other areas of the retina. • The information created in the rods is funneled out through the optic nerve to the brain. The human eye is not without limitations, creating design challenges for display engineers. Radiometry Radiometry is the basis for all light measurements. It is defined by the Institute of Electrical and Electronics Engineers (IEEE) as the measurement of quantities associated with radiant energy Radiant Flux [W] - The watt (W) is the fundamental unit of radiometry. All other radiometric units combine watts with units of area, distance, solid angle and time. Radiant Intensity [W/Sr] - A true point source is an isotropic radiator. If we assume we have a 100W lamp, which is an isotropic radiator then it radiate light into an imaginary sphere. Radiometry If we form a cone of 57.2960 (1 steradian, the unit of solid angle which encloses a surface area on the sphere equal to the square of the radius) with its surface of the sphere, the total radiation flowing through the cone will be radiant intensity. A full sphere contains, dΩ = sin θ dθd , 4 sr or 12.566 sr : Thus one sr will contain: 4 8W/Sr 13.56 The diameter of the sphere does not matter. As the sphere diameter increases, the total radiation within the circle remains the same. Radiometry Irradiance [W/m2] – is simply the amount of optical radiation incident upon a specified surface area. The preferred unit is the watt per square meter [W/m2]. The irradiance will change inversely with the square of the distance. If the radiation source is moved to twice the distance, the same amount o flight will be spread over four times the area and the irradiance will be reduced by a factor of four. Radiant Exitance [W/m2] – measured in watts per square meter (as is irradiance) is used to indicate the total radiation per unit area emitted, reflected, or transmitted by a 1m2 surface regardless of direction. Radiometry irradiance (1 Watt/m2) 1 square meter Radiometric Units (SI) radiant intensity (1 Watt/sr) radiant flux (Watt, power) isotropic radiation radiance (Watt/srm2) 1 steradian 1 steradian is the unit of solid angle that encloses a surface area on sphere equal to the square of the radius. Photometry Photometry is a subset of radiometry. In radiometry, the detector has a flat spectral response. In photometry, on the other hand, the spectral response useful to the visual system is considered. To accomplish that, the detector should be closely matched to the spectral response curve of the eye. The spectral sensitivity of the human eye, also known as the photo-optic response curve. Photometry Luminous Flux [lm] – The lumen is essentially a unit of power useful to the human eye. It is closely related to the watt as the spectral luminous efficacy (km) for monochromatic light at the peak visual response wavelength of 555nm. It has been standardized at 683 lm/w. A standard 200W light bulb produces a broad band radiation as well as heat in the form of infrared radiation. The radiant flux produced by the lamp is 100W. If all of its radiation were concentrated at 555nm, it would have an output of 200W 683 lm/w = 68,300 However, only 10% of the total radiant power radiated by the lamp is within the visible and even less (2%) is useful to the human eye because of the eye’s insensitivity to blue and red wavelengths. A typical output for a 200W bulb is 1750lm. The luminous efficacy of the lamp is lumens per watt, 1750lm/100W = 17.5lm/W Photometry Luminous Intensity [lm/sr or candelas] – Assume the luminous flux is radiated in all directions, like a point source. If we form a cone of 57.2960, or 1 steradian (the unit of solid angle that encloses a surface area on the sphere equal to the square of the radius) with its origin at the lamp and extending to the surface of the sphere, the total visible light flowing through the cone is luminous intensity. Luminous intensity is expressed in lm/sr or candelas. A full sphere contains 4 or 12.56 steradians. So a light bulb of 1750lm/12/56=136cd. Again, the diameter of the sphere is irrelevant. The luminous intensity in cd is the basic unit of photometry, all other units are derived by combining the candelas with units of are, distance, solid angle and time. Photometry Illuminance [lm/m2] – Illuminance is the amount of visible radiation incident upon a specified surface area. The preferred unit is the lux (lumen per square meter). The deprecated Footcandle (lumen per square foot) is still used and can be converted to lux by simply multiplying it by 10.764. The inverse square law determines the illuminance. Luminance [cd/m2] – Luminance is candelas per square meter, is the unit to indicate how much light is reflected, transmitted or emitted by a diffusing surface. The deprecated unit, the footlambert () is still used. Luminance Exitance – is also measured in lumens per square meter, analogous with illuminance, is used to indicate the total light per unit area emitted, reflected, or transmitted by a surface regardless of direction. Photometry illuminance (1 lm/m2=1 lux) 1 square meter Photometric Units (SI) luminous intensity (1 lm/sr=1 cd) luminous flux (power) luminance (cd/m2, nit, lm/srm2) isotropic radiation 1 steradian Lambertian Reflector 1 steradian is the unit of solid angle that encloses a surface area on sphere equal to the square of the radius. Photometry illuminance (1 lm/ft2) 1 square foot Photometric Units (English) luminous intensity (1 lm/sr=1 cd) luminous flux (power) isotropic radiation luminance (fL) 1 steradian Lambertian Reflector 1 steradian is the unit of solid angle that encloses a surface area on sphere equal to the square of the radius. Examples of Illuminance/Luminance Examples of Natural Illuminance Levels Direct Sunlight 105 lx Daylight (excluding direct sunlight) 104 lx Overcast Sky 103 lx Heavy Overcast 102 lx Twilight 1-10 lx Full Moon 10-1 lx Overcast night sky (no moon) 10-4 lx Examples of Illuminance/Luminance Examples of Luminance Levels Sun’s disk 1.5108 cd/m2 100W soft white light bulb 3104 cd/m2 Fluorescent lamp surface 104 cd/m2 Overcast Sky 3103 cd/m2 Blue Sky 103 cd/m2 White paper (in office) 102 cd/m2 CRT 60-150 cd/m2 Radiometry/Photometry Radiometry Photometry Radiant Flux (Watt) Luminous Flux (lumen) Radiant Intensity (Watt/Steradian) Luminous Intensity (lumen/Steradian) Irradiance (Watt/m2) Illuminance (lumen/m2, lux) Radiance (Watt/Steradian m2) Illuminance (cd/m2, nit) Conversion Factors between Photometric units in SI system and English system. Footlambert candela/m2 Footlambert 1 0.2919 candela/m2 3.426 1 Footcandles lux Footcandles 1 0.2919 lux 3.426 1 Quantify Color • Most displays operate on color addition (red, green, blue), but a few do work on color subtraction (cyan, yellow, magenta). • Need to stimulate the stimulus, or spectral power arriving at the back of the eye. • Mathematical functions, called color matching functions that do just that. • Color matching functions model the receptors responsible for color vision. Deriving Color Matching Functions screen mask white screen RGB mix test lamp test Observer adjusts RGB until the mix matches the test lamp The color matching functions are derived from a basic color matching experiment, to define a linear mapping from a test light spectral power distribution test lamp. The test light is set to unit energy at lnm test wavelengths. The observer adjusts the primary intensities (RGB) until test and mixture fields match. The relative weights are termed tristimulus values, and the color matching functions are spectral plots of the tristimulus values. The Color Matching Functions z y x y-axis (Relative Response) : x-axis (Wavelength in nm) Tristimulus Values 780 X r,g,b = k Sr,g,b λ xdλ Sr,g,b λ ydλ Sr,g,b λ zdλ 380 780 Yr,g,b = k 380 780 Zr,g,b = k 380 k=683 lm/watt (normalizing factor), Sr,g,b is the spectral power distribution of source. Chromaticity Coordinates Xr Xr = X r +Yr + Zr Yr Yr = X r + Yr + Zr Zr Zr = X r + Yr + Zr CIE 1976 Chromaticity Coordinates 4X r u = -2X r +12Yr + 3 9Yr v = -2X r +12Yr + 3 CIE 1931 Reflective Objects Depend on Ambient Illumination 780 Xr = k SPD λ R λ xdλ 380 780 Yr = k SPD λ R λ ydλ 380 780 Zr = k SPD λ R λ zdλ 380 SPD (l) is the spectral power distribution of source k is the normalizing factor Photo-optic Reflection 780 SPD(l ) R(l ) y (l )dl %RP == 380 780 SPD(l ) y (l )dl 380 SPD (l) R (l) Fluorescent Lamp CLC Theory y (l) Color Matching y-axis (Radiance in Watts/sr m2) Standard Spectral Power Distribution 380 550 780 380 550 780 380 550 780 380 550 780 x-axis (Wavelength in nm) Examples of Spectral Power Distribution Flourescent Lamp Sun Sylvania Bulb P-LED Generating Color An example of what you might see if you magnify a CRT screen. The primary and secondary colors are achieved by color addition. Color Addition The simple color addition scheme for electric displays. Examples Include R+G+B=W, R+G=Y, and B+G=C, where Red (R), Blue (B), Green (G), Yellow (Y), Cyan (C), Magenta (M), and White (W). Ways to Perform Color Addition Full Color Displays Color Addition-RGB Spatial Color Synthesis Full Color Displays Color Addition-RGB Addition -RGB Color Additive Integrated Intraged Stack Stack Full Color Displays Color Addition-RGB Temporal Synthesis 0<time<T1 t1<time<T2 t2<time<T3 Color Temperature Many times in the center of a chromaticity diagram (white region) you will see temperatures listed. An object to any temperature above 650-800K will produce a spectrum emission with its color related to temperature. This is known as blackbody radiation. The color progresses from a very deep red, through orange, yellow white, and finally bluish white. This path is often plotted on the chromatic diagram, and is known in the literature as the Plankian locus. Most natural light sources, such as the sun, stars and fire fall close to this locus of points. Displays are often designed to meet these criteria. Light Source Color Temperature (K) Daylight, fluorescent lamp 6500 CRT Computer Displays ~6500 Contrast Lon Luminance of on - pixel CR = = Loff Luminance of off - pixel Lon: Luminance of the on-pixel Loff: Luminance of the off-pixel Contrast The derivation of PCR is intuitive and can be performed heuristically. The display row lines must be strobed sequentially when refreshing the display image. The pixel in a row will have a luminance of Lon and all pixels intended to be off, Loff will experience a partial signal. When the next row is addressed, the previous row will experience a partial signal and will be stimulated to Loff for the remaing M-1 rows. Over the entire frame is the sum of individual light pulses, therefore the pixel has a luminance of Lon+(M-1)Loff. Chromaticity Contrast The contrast ratio is a measure of the ratio of luminance between an on and off pixel. A more sophisticated approach is to incorporate both luminance and chromaticity contrast, where the total contrast is the root mean square of chromaticity and luminance contrast. To arrive at the chromaticity contrast, there have been many empirical studies to ascertain a normalized chrominance index. An empirical chrominance ratio u: Δu 2 + 2.224Δv 2 0.027 1/2 Chromaticity Contrast Where Du’ and Dv’ are the difference in chrominance between the two regions (a pixel) as plotted on the CIE chromaticity diagram. The 0.027 is an empirical factor based on just perceivable difference. The total contrast ratio, which includes both chrominance and luminance, can be combined as a root mean square. CR total = chrominance 2 + Luminance Contrast 2 Resolution Resolution: is the ability to delineate (resolve) picture detail. The smallest discernible and measurable detail on a visual presentation. This is not a quantifiable definition. Possibly the best way to quantify resolution is pixel density (PD), i.e. pixels per linear distance, how close pixels are together. The standard is # pixels per inch. ‘Ball Park’ definition: Ultra-high High Medium Low PD > 120 120 > PD > 70 70 > PD > 51 PD < 50 Summary • Display Technologies • Threshold vs. Non-Threshold • Direct Drive Addressing • Multiplexing Addressing • Active Matrix Addressing • Radiometry • Photometry • Chromaticity Coordinates • Contrast • Resolution