Advanced Display Technologies Presented by: JonathanAlan Brawn, C. Brawn & CTSJonathan Brawn CTS, ISF, ISF-C, DSCE, DSDE, DSNE Principal, PrincipalsBrawn of Brawn Consulting Consulting [email protected] [email protected] Advanced Display Technologies • The central focus of the commercial industry is (and perhaps always will be) displays. • The topic of displays is broad, encompassing many elements built in to the final products that we buy. • This course is intended to take a detailed look at the technologies that go into each type of displays that are commonly available today. • To place it all in context, we will examine the operation and construction of the technologies themselves, and advances and trends that will characterize where we will be going in the future. • We hope you enjoy the journey! Liquid Crystal Display (LCD) Technology LCD Made Possible • There's far more to building an LCD than simply creating a sheet of liquid crystals. • The combination of four principles makes LCDs possible: • Light can be polarized. • Liquid crystal can transmit polarized light or change the plane of polarization. • The structure of liquid crystals can be changed by electric field. • There are transparent substances that can conduct electricity. Polarization of Light • Light is made up of electromagnetic waves, that can be reflected, transmitted, or absorbed by materials. • These waves will naturally have an axis, or a plane that they follow as they move through space. • When light is said to be polarized, all of the waves of light are following the same axis and orientation of the plane. • A polarizing filter is designed to block out light of certain planes, while allowing specific orientations through the filter. • As the axis of the filter rotates in relation to the plane of the incoming light, varying amounts of light are allowed through, reducing or eliminating the light. • A polarizer at 90° to the incoming light will block most of it from passing through. LCD Construction Principle • To create an LCD, you take two pieces of glass with polarizing films applied, which are assembled together with a carefully controlled gap between. • When liquid crystal material is introduced to this cell, the layers adjacent to the glass will align, resulting in a helical structure of molecules between the two glass plates. • As light strikes the first plate, it is polarized. • The molecules in each layer then guide the light through the display, changing the angle of polarization to match. LCD Construction Principle • When light reaches the final layer of the crystal material its plane of polarization will have been rotated through 90 degrees and since the plane of polarization matches the plane of the polarizing film, light will be transmitted. • If we apply an electric field to liquid crystal molecules, they untwist. • When they straighten out, they change the angle of the light passing through them so that it no longer matches the angle of the top polarizing filter. • Consequently, no light can pass through that area of the LCD, which makes that area darker. LCD Pixel Circuit Construction • Each pixel in an LCD display must contain a transistor and a capacitor, to discharge electricity into the liquid crystal material, in order to manipulate it’s degree of rotation. LCD Pixel Circuit Construction • In order to address each sub pixel independently, the transistors must be wired to a control circuit, that defines how bright each subpixel should be. • These control wires are built as a grid, with positive data lines, and ground addressing lines running to each transistor. Data Lines Transistor Switch Pixel LCD Flat Panel Color Production • For an LCD monitor to produce color, each pixel on the screen has to have three sub-pixels, each being a primary color (red, blue and green). By taking each of the three colors, and varying the intensity of each, then blending it all together, the color LCD has a large possible palette of colors. LCD Backlighting • The illumination source in most flat panel LCDs is a backlight, a light source placed behind the first polarizing layer, that provides the light needed for the display. • Today, this is commonly done with LEDs, hence why some call them “LED TVs”. • LEDs allow for a number of benefits: – Uniform light with no color decay over time – Low power consumption – Low heat output – Longer operational lifetime (50K hours or more) – Lower weight – Compact size for smaller display form factors LCD Backlighting • There are two kinds of LED backlight configurations: • Edge lit LED • Edge lit backlights use a ring of LEDs around the outside edge of the LCD panel. This allows for the display to be manufactured with a very shallow depth (hence the less than ½” displays on the market!) • Direct lit LED • Direct light LED backlights use an array of LEDs placed behind the LCD panel, like a standard CCFL backlight. This type of backlight is thicker than an edge lit, but offers the potential for better contrast. LCD Backlighting • Since the LED backlight is made up of many individual LED lamps, and LEDs can be varied in light output by manipulating the input voltage, an LED backlight can achieve what is known as localized dimming. • Localized dimming is the reduction in backlight output in coordination with the video signal. • Areas of the image that are darker will have the backlight dimmed to increase the level of black in the picture. • Direct lit LED backlights have an advantage here, because there are more discrete LEDs in the array behind the LCD panel. LCD Display Brightness • LCDs are typically measured in candela per square meter (cd/m2). • Nit is a common language name also used for this unit (1 nit = 1 cd/m2). • As a measure of light emitted over surface area, this unit is used to specify the brightness of a display device. • The higher the number of nits, the brighter the display. • Typical consumer liquid crystal displays have a luminance of 200 to 350 cd/m2 • Typical commercial LCD monitors range from 350 to about 1500 cd/m2 • In general, 1,500 nits, provides readable text in non-direct sunlight, direct sunlight can require up to 5,000 nits for acceptable color depth. Image Retention Considerations • Every LCD, when run for an extended period of time with a static image on screen, can suffer from a phenomenon known as image retention. • LCD operates by twisting the liquid crystal structure to block or transmit light through the panel. • When a static image is left on the screen for prolonged periods of time, the twist in the liquid crystal is kept fixed in a single position. • When the liquid crystal molecules are kept partially twisted for long periods of time, they can retain that new position, even after voltage is removed. • This leaves a residual ghost image on the screen. • Over time this effect may fade as the display is used, but it can happen repeatedly, and become worse each time. • Heat exacerbates this effect. LCD Duty Cycle • Due to heat and image retention issues, all LCD displays, regardless of manufacturer, will come labeled with a designated duty cycle rating. • This rating, typically between 8 and 16 hours per day, from 5 to 7 days a week, will show how long each day the display may be used without any ill effects. • Some displays are rated at 24/7/365. These displays are a requirement for applications that fit within that duty cycle! Do not use a lower duty cycle display. • Manufacturer warranties will often be tied to the duty cycle for a display model. Feel lucky? High Brightness and Outdoor LCD Flat Panels LCD Brightness • When we consider most commercial installations, the immediate display selection that comes to mind is either an LCD flat panel, or a projector. • The main issue with these technologies is one of brightness; most displays we view in public spaces simply are not bright enough for the application. • High brightness LCD displays and videowalls offer a major benefit compared to traditional flat panels, videowalls, and projection systems. • In a typical conference room where you have 30 foot candles of ambient light, a 350-400 nit LCD is fine, but outside of that kind of controlled environment, you may have many times that amount of ambient light falling on a display, necessitating a high brightness LCD. Display Brightness VS. Ambient Light • Direct sunlight comparison of LCD displays, from 1,500 nits to 5,000 nits: 1,500 Nits 2,500 Nits 3,000 Nits 5,000 Nits Liquid Crystal Properties and Outdoor LCDs • The liquid crystals in an LCD are interesting because they don't exactly fall under the three main states of matter: solid, liquid or gas. • Liquid crystals are actually in a unique intermediate phase between solid and liquid. Nematic, Smectic and Isotropic Properties • Crystals behave differently than most materials. They can exist as a liquid, in a phase called isotropic, or a solid phase called smectic. Liquid crystal exists in between, in a phase known as nematic. • The nematic phase of liquid crystals is what makes LCD screens possible. • During this phase, the molecules are in no positional order, but do have a notable pattern. • This allows the molecules to flow as if they were a liquid, but retain a general alignment to each other. Isotropic Nematic Smectic (Liquid) (Hybrid) (Solid) Nematic, Smectic and Isotropic Properties • When heated, most crystalline solids experience a transition to a different phase - to an isotropic liquid state. • When the nematic liquid crystal is heated, it turns to a true liquid – Isotropic isotropic. This transition temperature is called the clearing point. • When the clearing point is achieved, the liquid crystal loses it’s ability to control light because of the lack of common direction for each molecule.