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

display that modulates backlight (light shutter) Types of Displays Direct-ViewProjection Backlight Emissive ReflectiveTransmissive SLMReflective SLM display that generates its own light display that rejects/reflects ambient light active matrix, STN, FLCD CRTs, FEDs, LEDs, plasma EL, VPD cholesteric LC, STNs, MEMs, FLCDs display (SLM) that modulates projection lamp (transmissive pixel) display (SLM) that reflects projection lamp (reflective pixel) active matrix light valve active matrix reflecting pixel, digital micro- mirrors 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 backlight LCD room light transmissive Projection Displays (3-pass) white light LCD mirrors projection optics dichroic mirrors

Emissive - CRT

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

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

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.

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

Emissive - VFD

Cathode filament is to 600 o C to facilitate emission of thermal electrons Anode voltage of 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

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

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:  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 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 10 7 eV/cm Fowler-Nordheim Equation for current emission j: E f is fermi energy F is electric field

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

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

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

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

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

Advantages Cheap Cool Disadvantages High voltage needs active matrix sticky balls

Reflective - PDLC

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

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

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

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

Direct Drive

Samples of Fixed Format Alpha Numeric Matrices Samples of Fixed Format Alpha Numeric Matrices 7-bar10-bar13-bar14-bar

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 For medium to high resolution ( 400 rows)

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 V S denote the row voltage and V D denote the column voltage The row voltage is always V S, and the column voltage can be  V D The instantaneous drop at the pixel electrode is ON state V=V S -(-V D ) or V=V S -V D

Response time, governed by viscoelastic properties, must be >> than period of driving waveform Interaction between LC molecule and applied electric field must be  =E 2 (induced polarization) In each multiplexing cycle, each row is selected on during 1/N of the cycle time T Conditions for RMS Responding Material Conditions for RMS Responding Material

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

Selection Ratio N MAX

Multiplexing: Practical Applications V TH : threshold voltage (turn on begins)  : steepness parameter

Passive Multiplexing: Amplitude Modulation

Passive Multiplexing: Pulse Width Modulation

Examples of Multiplexing Display Configuration V TH  N MAX (N MAX ) 2 TN STN PDLC electrophoretic 2 Volts 4 Volts 8 Volts none 0.4 Volts 0.2 Volts 3 Volts undefined 

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 Source Drain 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 V G of duration T/N is applied to gate to turn on TFT. The LC pixel (ITO) is changed to V ON at time 2 within T/N, due to the positive source voltage V SD =V ON. 2.At time 2, the gate voltage V G becomes negative, turning off the source voltage V SD from V ON to –V ON. During the time period 2 and 3, of duration (N-1)/NT, the pixel voltage VP remains about >0.9 V ON 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 V SD =-V ON. The pixel electrode is discharged from V P =V ON at time 3 to V P =-V ON 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 V SD changes from –V ON to +V ON.

0 0 time T/N TT VGVG V SD VPVP V ON Gate Voltage Source Drain Voltage Pixel Voltage Notice that V P 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. TFT Addressing

Active Matrix Multiplexed LC Mode Contrast Viewing (horizontal) Viewing (vertical) Response time Addressable lines Gray-scale TN >100: , , ms >1000 >16 STN 10-15: , , ms ~400 low Summary

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. Basic Display Measurement

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 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 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. Human Eye Ultimate Reception for Displays Human Eye Ultimate Reception for Displays

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 (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, or : Thus one sr will contain: 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/m 2 ] – is simply the amount of optical radiation incident upon a specified surface area. The preferred unit is the watt per square meter [W/m 2 ]. 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/m 2 ] – 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 1m 2 surface regardless of direction.

Radiometry Radiometric Units (SI) irradiance (1 Watt/m 2 ) 1 square meter isotropic radiation radiant flux (Watt, power) radiant intensity (1 Watt/sr) radiance (Watt/sr  m 2 ) 1 meter 1 steradian is the unit of solid angle that encloses a surface area on sphere equal to the square of the radius. 1 steradian

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 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 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.

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 , 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 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/m 2 ] – 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 The inverse square law determines the illuminance. Luminance [cd/m 2 ] – 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 Photometric Units (SI) illuminance (1 lm/m 2 =1 lux) 1 square meter isotropic radiation luminous flux (power) luminous intensity (1 lm/sr=1 cd) luminance (cd/m 2, nit, lm/sr  m 2 ) 1 meter 1 steradian is the unit of solid angle that encloses a surface area on sphere equal to the square of the radius. 1 steradian Lambertian Reflector

Photometry Photometric Units (English) illuminance (1 lm/ft 2 ) 1 square foot isotropic radiation luminous flux (power) luminous intensity (1 lm/sr=1 cd) luminance (fL) 1 foot 1 steradian is the unit of solid angle that encloses a surface area on sphere equal to the square of the radius. 1 steradian Lambertian Reflector

Examples of Illuminance/Luminance Direct Sunlight Daylight (excluding direct sunlight) Overcast Sky Heavy Overcast Twilight Full Moon Overcast night sky (no moon) 10 5 lx 10 4 lx 10 3 lx 10 2 lx 1-10 lx lx lx Examples of Natural Illuminance Levels

Examples of Illuminance/Luminance Sun’s disk 100W soft white light bulb Fluorescent lamp surface Overcast Sky Blue Sky White paper (in office) CRT 1.5  10 8 cd/m 2 Examples of Luminance Levels 3  10 4 cd/m cd/m cd/m 2 3  10 3 cd/m cd/m cd/m 2

Radiometry/Photometry Radiometry Photometry Radiant Flux (Watt) Radiant Intensity (Watt/Steradian) Irradiance (Watt/m 2 ) Radiance (Watt/Steradian m 2 ) Luminous Flux (lumen) Luminous Intensity (lumen/Steradian) Illuminance (lumen/m 2, lux) Illuminance (cd/m 2, nit) Conversion Factors between Photometric units in SI system and English system. Footlambert candela/m Footcandles lux

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 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 nm 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. white screen test lamp screen mask Observer adjusts RGB until the mix matches the test lamp test RGB mix

The Color Matching Functions y-axis (Relative Response) : x-axis (Wavelength in nm)

Tristimulus Values k=683 lm/watt (normalizing factor), S r,g,b is the spectral power distribution of source.

Chromaticity Coordinates

CIE 1976 Chromaticity Coordinates

CIE 1931

SPD ( ) is the spectral power distribution of source k is the normalizing factor Reflective Objects Depend on Ambient Illumination Reflective Objects Depend on Ambient Illumination

Photo-optic Reflection SPD ( ) R ( ) y ( ) Fluorescent Lamp CLC Theory Color Matching %R P =

Standard Spectral Power Distribution y-axis (Radiance in Watts/sr m 2 ) x-axis (Wavelength in nm)

Examples of Spectral Power Distribution Examples of Spectral Power Distribution Flourescent Lamp Sun P-LED Sylvania Bulb

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 Color AdditiveIntragedStack Full Color Displays Color Addition- RGB Color AdditiveIntegratedStack

Color Temperature Many times in the center of a chromaticity diagram (white region) you will see temperatures listed. An object to any temperature above K 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. Color Temperature (K) 6500 ~6500 Light Source Daylight, fluorescent lamp CRT Computer Displays

Contrast L on : Luminance of the on-pixel L off : Luminance of the off-pixel

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 L on and all pixels intended to be off, L off will experience a partial signal. When the next row is addressed, the previous row will experience a partial signal and will be stimulated to L off for the remaing M-1 rows. Over the entire frame is the sum of individual light pulses, therefore the pixel has a luminance of L on +(M-1)L off. 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: Chromaticity Contrast

Where  u’ and  v’ are the difference in chrominance between the two regions (a pixel) as plotted on the CIE chromaticity diagram. The 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. Chromaticity Contrast

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-highPD > 120 High120 > PD > 70 Medium70 > PD > 51 LowPD < 50 Resolution

Summary Display Technologies Threshold vs. Non-Threshold Direct Drive Addressing Multiplexing Addressing Active Matrix Addressing Radiometry Photometry Chromaticity Coordinates Contrast Resolution Display Technologies Threshold vs. Non-Threshold Direct Drive Addressing Multiplexing Addressing Active Matrix Addressing Radiometry Photometry Chromaticity Coordinates Contrast Resolution