Useful Notes / Display Technology

Displays are electronic devices intended to give humans a natural method of showing the output of an electronic device, the most common example being the television. Early display technologies, like mechanical television had such poor quality that images could barely be made out.

History of Displays (Or the Days of Analog Displays)

For the longest time, there was only one kind of display technology: the cathode ray tube (CRT). When it came with broadcasting television, there was a split on how to do it. North America and parts of Asia use 60 Hz AC electricity, so it made sense to run television signals at this frequency. The rest of the world, however, ran on 50Hz AC electricity and came up with their own standard. Since both used similar transmission techniques, the solution involved 60 Hz broadcasts repeat every fifth frame from 50 Hz broadcasts while 50Hz broadcasts drop every sixth frame from 60 Hz broadcasts.

Then in the The Fifties, color TV standards were being developed. To remain backwards compatible with the old monochrome TV, the same signal was sent with an embedded color signal, which monochrome televisions lacked the circuitry to decode. However, this method of decoding the color signal was problematic standards wise. The U.S. National Television System Committee was the first to propose a color TV standard, which was later called NTSC. In Europe the situation was even more complex, with two competing standards being proposed Germany promoted the Telefunken-developed Phase Alternating Line system (PAL), while France promoted the "Séquentiel couleur à mémoire" (SECAM) standard created by Thomson, which was later also adopted by the Soviet Union. Quality wise, NTSC was inferior (it had a lower resolutionnote  and was notoriously poor at reproducing colorsnote ) and prone to quality degradation but ran at 60Hz, and thus had less flickering. PAL has slightly better picture quality and higher resolution but was subject to flicker due to its lower refresh rate of 50Hz.note . SECAM, despite being chronologically first, was even better, but more complex and expensive to produce and still suffered from the flickering issue due to still being tied down to a 50Hz refresh rate.

When computers first started to appear, they borrowed from televisions and adopted the same technology. Well, mostly. When they were powerful enough to display colors, instead of just showing different brightnesses and a color, the displays were simply fed red, green, and blue signals that varied in intensity. Also by then, monitors were starting to use standard connectors and signals note , so there was no need for backwards compatibility.

The Transition From Analog to Digital

Computers were first to adopt a digital signal standard, mostly because computers were digital and needed an interface that didn't convert digital to analog and back to digital. While analog signals used discrete levels, these were still, well, analog interfaces in nature. The first true digital interface (DVI and LVDS) came about the same time as LCD monitors were starting to become mass produced.

Later after 2000, television broadcasts were going into digital broadcasting, as it was a more efficient use of airwaves with digital compression. Displays were also transitioning from the bulky CRT to the more space efficient plasma and LCD, which were more or less purely digital devices. When coming up with a cable to transmit these new HD digital signals, the computer's DVI standard was used to form HDMI. Today, there is a push for the DisplayPort standard for computer monitors, which uses packets of information rather than a stream of data, as DVI more or less mimics the signaling of CRTs.

Digital signals have since negated any quality difference between regions. The only difference is the 50 Hz/60 Hz refresh rates. Today, the high end features of display technology are very high refresh rates, sterescopic 3D, and higher resolutions.

Digital Color

A special side topic that affected both analog and digital displays is digital color. In early computerized systems, memory was a precious resource. Thus, only a certain number of bits could be reserved for colors when saving a display buffer. These bits mostly held the intensity for red, green, and blue signals.

For many systems that outputted to analog displays, there were tricks to support a lot of colors without eating so much memory. The frame could be rendered with some colors (called a pallete) out of a larger range of colors. For instance, the Nintendo Entertainment System had a color range of 8 bits, but it could only display 4-bits of color for each frame. It was up to the system's GPU and RAMDAC to mix these colors in the end.

Today practically all modern systems that support color stop at 24 bits (about 16 million colors) since this is on the upper edge of human perception. Some displays do offer more for artists in need of extra color accuracy, and image formats may also support higher bits of color to prevent errors from accumulating while applying effects.

Video transmission types

If not being broadcast from the air, there are a plethora of video connectors to choose from to hook up a device, such as a game console, VCR, or DVD player, to a display like a TV. They have their differences in quality, the worst to best are: RF (video + audio in one signal), composite (video only in one signal), S-Video (light and color separated, two signals), component (which is either light and two color signals, or three color signals, sometimes with a separate signal syncing signal), and the digital ones (DVI, LVDS, HDMI, DisplayPort, etc.)

Of the types, only component and digital are capable HD resolution. HDMI and DisplayPort are capable of 4K resolution, though DVI can do it at maximum of 30Hz.

For a list of display connectors, see the page on That Other Wiki.

Issues common to all displays

Screen burn-in/Image Persistence

When an image being displayed doesn't change significantly, the pixels may wear out faster than usual. A faint ghost of that image may appear if the display tries to show something else. This is called burn-in. You can commonly see this in digital signs (like the timetables at airports) or on video arcade cabinets. Only displays that emit light are susceptible to this permanent defect.

Displays that block or reflect light may have an issue called image persistence, where if the pixels are constantly showing the same image, they have a tendency to stick that way. Unlike burn-in, image persistence usually goes away on its own.

Input Lag

Input lag is the time it takes for the screen to show a response from an input, such as moving a cursor. Input lag is most relevant to games and applications where tight timing is important. However, significant input lag is unpleasant regardless.

Dead/stuck pixels

Dead pixels are pixels which will not respond to any input and usually remain either black, white, or some color. These are defective and cannot be repaired. Stuck pixels, on the other hand, are defective but may resolve itself over time. A popular method to resolve a stuck pixel is to flash it several colors rapidly for a few minutes.

Contrast Ratio

This is more of a problem with marketing than the display itself. Contrast ratio is the measurement of brightness from the display's pure white against pure black. The rating is under ideal conditions (i.e. a pitch black room). Displays that need another source of light to work (LC Ds for instance) cannot completely block light, so the manufacturer may fudge with the environment to get the ideal number.

The worst case is the term "dynamic contrast" on LCD-type displays, which is actually a marketing fluff term. Basically, the display may adjust its backlight and amplify the color input, giving the illusion of a higher contrast.

Display Technologies

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Obsoleted display technologies note 

    Cathode Ray Tube (CRT) 
Technology
  • CRTs shoot an electron beam at phosphors coated on a screen at a rate of 50 full images (or more) per second, topping out at 160 or 180 Hz for high-end VGA monitors. When an electron beam hits a phosphor, it glows for a moment then fades. This is the only display type primarily used for analog signals.
    • Arcade Light Gun games actually took advantage of the technology, by calculating the difference between the start of a scan and when the gun picked up light.
  • A common effect when filming a display using CRT is black bands that slowly scroll down. This is because the filming rate (usually 24FPS) only captures some of what the CRT is showing at the time. Human eyes have what is called persistence of vision, which allows humans to retain the image of something for a brief period of time.
  • Devices that used CRTs either filled the entire screen with an image or drew a line as a vector, often seen in analog oscilloscopes.
Pros
  • Can display any resolution without quality degradation.
  • Great color quality with true black
  • Practically no input lag, thus perfect for Video Games.
Cons
  • Contrast isn't as good, as maximum brightness is limited.
  • Subject to flickering if the refresh rate is too low.
    • This is why most people who claim to get headaches or eye problems from CRTs have them, especially if the room was lit with fluorescents, which have a 120hz flicker (in the United States) from the AC power. 60hz would interact with the fluorescent lights and make the flickering even worse. Setting the refresh to 72hz or more made the display clear and stable. It's even worse in Europe, Asia and the Pacific, where the AC cycle is 50hz, which makes the flickering more pronounced.
  • Image geometry can be distorted as the phosphor screen is curved (even on so called "flat" displays), often in a way that cannot be corrected by the On-Screen Display (OSD) menu.
  • Color convergence can be thrown off, resulting in color fringing.
  • CRT displays are bulky, heavy, power consuming, and they run hot.
  • Since CRTs contain what is essentially a linear particle accelerator, the screen can generate a non-trivial amount of X-rays, especially in color sets. CRT glass is leaded to protect against this, but the lead makes disposal a problem and adds to the already-unwieldy bulk of large tubes. Also, the lead protection can not eliminate it completely, but only reduce it to miniscule amounts.
  • Holding a strong magnet against the screen can permanently damage it by twisting or distorting the shadow mask (a piece of metal that keeps the colors separated) behind the screen. Light magnetic distortions are normal due to the Earth's own magnetic field, and are fixable using via degaussing (most CRTs have a built-in degaussing coil, and will automatically use it when powered up).
    • Old monochrome CRTs don't use a shadow mask, making it safer (though still not a good idea) to use strong magnets on their faces.
  • Most famously, CRT screens are subject to burn-in.

    Vacuum Fluorescent Display (VFD) 
Technology
  • Similar to plasma, but built on vacuum tube technology rather than the neon light. Phosphors painted in patterns on the back of the tube are excited by electrons emitted from a filament just behind the screen. A set of grids between the back and the filaments allow control over various areas of the screen; this allows for LCD/LED-style multiplexing.
  • Usually monochrome, specifically a pleasing turquoise color (though Sony has sometimes used a more whitish phosphor on their VFDs).
    • However, since it's based on phosphor technology, it is possible to have a VFD with multiple albeit segmented colors, as seen in various electronics from the 80s (i.e. the spectrum analyzer on some stereos, or the record icon on some VCRs). Read: a VFD can show multiple colors, but the color of a segment on the VFD cannot be changed once it's manufactured.
Pros
  • Very bright and readable, even in bright light.
  • Will run well in cold temperatures, making them popular for automotive use (digital dashboards, radios, and such).
Cons
  • Uses more power than LCD or LED-matrix displays, but not quite as much as plasma.
  • Limited range of colors available; almost all VFDs use the same few shades of turquoise, white, red and orange. Blue VFDs are usually achieved by passing White through a color filter, true blue VFDs are rare and expensive.
  • Not as thin as LCD or LED-matrix displays.
  • Like all vacuum tubes, they're fragile and the filaments have a limited life. However, since the filaments don't need to be run hot, their lifetimes can be very long (decades).
  • Due to limitations of the manufacturing process of VFDs, it's not possible to have a full color dot matrix VFD, let alone one capable of going beyond low-resolution graphics.

    Proto-plasma — Nixie and Panaplex tubes 
Technology
  • They work just like plasma screens, but instead of individual pixels, fully-formed numbers (Nixie) or seven-segmented digits (Panaplex) are used.
  • Very common in older digital equipment from the 1960s and early 1970s.
Pros
  • Largely the same as modern plasma screens; these were the brightest small displays around until VFDs and LED-matrix screens became popular in the late 1970s.
Cons
  • Require high voltages to work, making them impractical for battery-powered equipment (though some people have done it anyway).

Current display technologies

    Plasma Display 
Technology
  • Made of individual plasma cells. Like CRT, the screen is coated with phosphors that glow, but instead of being excited by electrons, they glow from plasma generated in the plasma cell.
  • Based on the same principle as the neon light; early plasma screens were monochrome and glowed with the bright red-orange color of neon.
Pros
  • Usually very bright with the highest contrast available.
  • Excellent color quality.
  • Boasts refresh rates up to 600Hz.
  • Thin profile.
  • Very good black levels (Effectively 0); the Pioneer Kuro line even has its main selling point right in the name!
Cons
  • Discrete pixels, thus only the native resolution can be displayed clearly. The issue with this is that they tend to produce blurry images if a non-native resolution is fed to the display. On the flip side, it eliminates geometry issues.
  • The front panel is glass (subject to breakage) which makes plasma displays deceptively heavy.
  • Plasma displays run hot and consume lots of energy. Modern displays though are approaching similar power consumption as early LCD sets.
  • Subject to image burn-in, though modern displays aren't as bad as early ones.
  • As of 2014, pixel density appears limited. One needs to buy a 50" TV before reaching 1080p.

    Liquid Crystal Display (LCD) 
Technology
  • The display has a material called liquid crystal, which depending on the voltage applied to it can either block light or let light through.
    • Reflective LCDs: ambient light is reflected back to the user while the liquid crystal blocks or allows some of that light back. Used in very low power consuming devices but has the problem of requiring a good light source to see well. Front lights can either be embedded or bought separately. The Game Boy and Game Boy Advance used reflective LCDs.
    • Backlit LCDs: an always-on backlight shines behind the liquid crystal. While lower power consuming than other display types, it also works poorly in bright light. This is the most common type of LCD you'll find.
  • The amount of energy consumed for what the LCD is displaying is inverted than what one would expect. Showing white consumes the least amount of energy, while showing black consumes the most. It can be thought of like a shutter, where the off state is open and the on state is closed.
Pros
  • Currently the cheapest, most versatile display technology available.
  • Thin and light profile. Suitable for portable displays.
  • Depending on the content, it consumes on average the least amount of energy.
Cons
  • Because LCDs have discrete pixels, it cannot display a resolution clearly except for the "native resolution." On the other hand, geometry is always perfect.
    • In order to display a non-native resolution, LCD screens have a scaler. However this can introduce a non-trivial amount of input lag.
  • Pixel response time is higher than most other technologies. Average response time is 8ms-16ms using the ISO standard test of full black to full white, with 2ms-6ms from gray-to-gray. Longer response times cause a smearing effect known as ghosting.
  • Some LCD panel types have narrow viewing angles which cause color shifting if not viewed head on. Though if the screen is too large, color shifting can still be perceived regardless of viewing angle.
  • Backlit LCDs have poor sunlit outdoor readability, requiring a very bright backlight to be readable.
  • Backlights in older or cheaper displays employ pulse-width modulation for brightness control. This can be perceived as uncomfortable flickering in some people. However, higher-end displays may use as a user set option this to minimize motion blur at the expense of maximum brightness.
  • Because the backlight is always on, it's impossible to show "true black". Though with certain backlight arrangement it's possible to show deep blacks.
  • Old and cheaply made LCD displays tend to have a problem with image persistence.
YMMV
  • LCDs may come in glossy or matte finish.
    • Glossy finishes offer a clearer picture, but there is bad glare in bright light.
    • Matte finishes have excellent glare reduction in all lighting conditions, but may appear fuzzy.
  • LCDs may have a glass covering over it to protect it. Consequently this makes it subject to cracking if the display takes a hard hit. Though glass is normally used in touchscreen or high-end applications. Otherwise plastic is used.

Panel Types
  • Twisted Nematic (TN) - Offers the best response time and is the cheapest to make, but cannot product 24-bit color (most panels are 18-bit color that employs some trick to make it look like 24-bit color) and the viewing angles are narrow. If you view a TN panel off center, colors and contrast start shifting. TN panels are also unsuitable for touch screens without a hard surface between the touch interface and the display, as images distort under small amounts of pressure. Early Twisted Nematic screens were also prone to "ghosting", an effect where images on a screen blurs due to the poor response time of earlier panels. Several companies attempted to rectify this through the use of flickering backlights, but the problem was only truly solved with introduction of the faster "Thin Film Transisor" panels, which rendered the flickering backlight method obsolete.
  • Vertical Alignment (VA) - VA offers superior color reproduction (true 24-bit colors are possible) and contrast ratios versus a TN panel, but suffers from poorer response time and still has color shifting if viewed off center.
  • In-Plane Switching (IPS) - Considered the high-end of the spectrum. Offers the widest viewing angles, the best color reproduction (true 30-bit color panels are possible), and have very good response times to be used in up to 165Hz applications as well as motion blur reduction technology (or a fancy way of saying the backlight strobes really fast, or alternatively the panel is really fast enough that there is no flickering backlight). Its main problem is the "dreaded" IPS glow, where the backlight can have a non-trivial amount of bleed when viewing dark content. IPS goes by other names such as PLS in Samsung models and AHVA in AUOptronics panels.

    LED Display 
Technology
  • What most people think is an LED display are actually an LCD using light emitting diodes as the lighting element, as opposed to cold cathode fluorescent lights (CCFL).
  • There are two types for LEDs to distribute light: edge lit and array.
    • Edge lit displays have the LEDs on the edge and the light is as uniformly distributed as possible across the display.
    • Array based has the LEDs behind the display itself in a uniform array. This may be in sections or down to the pixel level.
  • Displays made out of colored LEDs for each pixel (bypassing the LCD entirely) are real, but are used in very large displays like those seen for outside displays. Work is in progress to shrink the technology so that it's possible to have such displays for the living room, but apart from a few prototypes presented at electronic shows and aside from a couple of very expensive professional-grade displays (and even then they're still combined with LCD technology), there are no affordable full LED-based displays for the average consumer yet.
    • Older handheld devices (such as Mattel's Pocket Sports games and many early Texas Instruments calculators), and things that require a low-cost screen that's also bright and easy to read screen (alarm clocks, VCRs, etc.) actually use VFDs, although there are a handful out there based on true LED technology.
Tidbits
  • Thin profile. LED-backlit displays have been as thin as less than half an inch.
  • Array based LED displays can use local dimming to increase contrast ratios.
  • Some LCD displays do use RGB LED backlighting for an increased color gamut, rather than the usual white LED or CCFL. These are all invariably upmarket professional-grade displays where color accuracy is critical.
  • They are more reliable, robust, and energy efficient than CCFL based LCDs.

    Digital Light Processing (DLP) 
Technology
  • A light source (usually a white light lamp/bulb, but more recently, RGB lasers or an array of white LED bulbs) is shined upon a chip with microscopic mirrors that represent a pixel. Each mirror can tilt to adjust the amount of light that hits the screen. Color filters are used for color in single-chip designs. Three-chip designs can bypass the color wheel entirely, as do laser-lit designs.
  • Can be used either as an actual projector, or a self-contained rear projector TV.
Pros
  • Very good image quality reproduction, especially laser DLP models that allow for incredible color gamut.
  • Tends to be more user-serviceable. Lamps used in self-contained TVs can be replaced. Laser-based sets should not need regular replacement.
Cons
  • If used as a TV, it's bulky due to the throwing distance needed for the projector. Inherently not flat-panel. On the flip side, they're deceptively light.
  • Can suffer from "Rainbow effect", an issue from cheaper DLP sets that use one chip with a color wheel. Scanning your vision quickly across will allow you to briefly see the red, green, and blue filtered images. Likewise, the filtering has been known to cause headaches in some people, akin to CRT flickering. As mentioned this is not an issue with the laser lit or three chip designs. However laser lit and three chip DLP projectors are inherently more expensive.

    Projectors 
Technology
  • Basically, shoot bright light onto a screen. This makes projectors flexible as to where you can put them and as long as the surface is flat and of a neutral color, ideally white.
  • Comes in LCD (uses LCDs as the color filter), DLP (most projectors of this type use a rotating color filter wheel with a DLP chip, but later more expensive models do away with the color filter wheel for three separate DLP chips), or laser (a red, green, and blue laser rapidly shine the image on screen) varieties. Very old models may be CRT-based. If the projector uses DLP or LCD technology, the light source may be either a xenon lamp or LED array.
Pros
  • "Screen size" is as good as how bright the image is.
  • Portable, due to their inherently small size, in fact some cellphones have projectors in them.
Cons
  • Many projectors tend to run very hot and are loud as a result (there's usually a fan to keep the laser, lamp or CRTs cool). Projectors with a LED light source run much cooler however, but are very expensive at this point due to the novelty of using a LED array.
  • Requires cooling down after use. Suddenly disconnecting the power can damage the lamp. However this does not apply to LED lit projectors.
  • The room used for a front projector must be as dark as possible, or the contrast of the projected image will severely suffer. Additionally, the area that is to be used for the projection screen must be matte bright white and as smooth as possible, but not reflective, to achieve the best possible image quality.
  • Setting up a projector is more hassle than other technologies. The projector's distance, angle and focal point must be carefully adjusted or the picture will be distorted. While the higher-end projectors offer automatic keystone and focus adjustments using various technologies, the measurements tend to be inaccurate still need manual fine-tuning. This compared to the CRT and LCD display's "plug and play" setup.
  • DLP-based projectors also borrow a lot of pros and cons from DLP display panels mentioned above.

    Electronic Ink 
Technology
  • Similar to reflective LCD. The ink is electromagnetically sensitive and suspended in a fluid. When given a voltage, the ink moves from one side of the cell to another, which changes how much light is reflected back. The principle is old though, Magna Doodles used a similar technique.
Pros
  • Lowest power consumption. E-Ink retains its state even without power so it only needs power to change something.
  • Very thin profile. Typically used in portable E-Book readers
Cons
  • Image quality is acceptable, but currently available in Black and White only. Color based displays are in development.
  • Only useful for showing static images as response time is very poor.
    • The display has to black itself out first before displaying new content. This also has the issue of sometimes having image persistence.

    Organic Light Emitting Diode (OLED) Display 
Technology
  • This display consists of organic compounds that can produce bright light when given a voltage. Because the pixels themselves emit light, no always-on backlight is needed, increasing image quality.
    • A cousin of this called OLET, for transistor instead of diode, is practically the same with one difference. OLED still needs a thin-film-transistor to control each diode, whereas OLET is both the controlling transistor and lighting element.
Pros
  • Very thin profile, down to millimeters. It's possible to build flexible OLED displays too.
  • Very good brightness and contrast ratios. Comparable to plasma.
  • It's possible to actually print an OLED display, which when perfected, can lead to drastically lower costs.
Cons
  • Not as energy efficient as LCDs note 
  • For larger displays, like monitors and TVs, it's still a rather expensive technology, costing up to three times as much as the equivalent LCD.
  • Blue OLEDs have an issue with both lifespan (16,000 to half-brightness vs. 60,000 of other technologies) and efficiency at high brightness.
  • Will be damaged upon contact with water, thus the display has to be sealed.
  • Subject to burn-in.
YMMMV
  • A lot of consumer OLEDs use a PenTile Matrix to arrange the subpixels so that there are more (smaller) green and red subpixels and fewer (but larger) blue subpixels. This may give the display a subtly odd hue compared to an LCD.

Emerging display technologies

    Quantum Dot Display 
Technology
  • A particle called a quantum dot is given a voltage to give it a higher energy state. Then it gives off a photon of a specific color to return to a lower energy state. The color depends on the size of the quantum dot, rather than the voltage passing through it.
Pros
  • Since they are more inorganic, they can last longer than OLED.
Cons
  • Blues are less saturated, but it's being worked upon.

Despite its potential to be used as a display in and of itself, it's currently deployed as another backlight source for LCD displays. However, due to Techno Babble buzzwords about, a "quantum display" does not imply a quantum dot display. LG brands some of their displays this way.

    Interferometric Modulation (IMOD) Display 
Technology
  • Similar to DLP, it uses a microelectromechanical-system (MEMS) chip to reflect light. Unlike DLP, it works just fine in ambient light and doesn't require a color filter, as each MEMS element is designed to reflect a specific wavelength of light (which is where that Techno Babble name comes from).
Pros
  • Very good visibility in bright light.
  • Can retain state without power, or at least using very little power.
  • Small enough that the current major manufacturer of this technology is using it for a smart watch.
Cons
  • Refresh rate is a little poor, recent models have just gotten up to 30Hz.
  • Doesn't work in darkness, but a backlight can be used.
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