Display technologies - Cathode ray, LCD, LED, Plasma Monitors, OLED
Display device refers to an output device used for presenting information in visual text or image form. Display devices provide a way to show information, images, and videos to the outside world. When the information is supplied to electrical signals, the display is known as electronic display. Electronic visual displays are used in computer monitors, televisions, smartphones, tablets, gaming appliances etc. A number of display technologies have evolved for displaying the outputs of computers since the beginning of computer era. Important display technologies include Cathode Ray, LCD, LED, Plasma monitors, and OLED displays etc.
Cathode-ray tube (CRT)
- Cathode ray tube is a vacuum tube that has one or more electron guns and a phosphorescent screen for displaying pictures and videos. Electron gun modulates, accelerates, and reflects the electron beam on the phosphorescent screen for creating the images.
- Images can be displayed in the form of electrical waveforms, pictures radar targets etc. The image is created by controlling the intensity of the three electron beams with additive colours red, green and blue.
- Cathode ray were discovered in 1869 by Johann Wilhelm Hittorf. The cathode ray tube became commercial in 1922. The first cathode ray tube based television was manufactured in Germany in 1934.
- CRT was the single most display technology for television sets and computer monitors until the beginning of the 21st century when LCD began to replace them.
Advantages of the cathode-ray tube
- CRT can run at any resolution geometry and aspect ratio without having to rescale the images.
- These can run at the highest available pixel resolutions.
- CRTs have the ability to produce very dark black colour and highest available contrast levels.
- CRT produces the very best colours and grayscale which are used as the standard reference for other professional collaborations. Producers perfect smooth grayscale with infinite intensity levels.
- CRT is best for rapidly moving images due to its fast response time and no motion artifacts.
- CRT displays are comparatively lower in cost from other electronic display technologies such as LED LCD, OLED etc.
Disadvantages of cathode-ray tube display
- The images produced by CRT have softer edges which are not as sharp as that of the LCD display. It has imperfect focus and colour registration which reduces the sharpness of displayed images.
- Colour CRT displays suffer from the problem of Moiré pattern interference.
- CRT displays suffer from geometric distortion, screen regulation problems, and are impacted by the magnetic field of other equipment.
- CRT display produces electric, magnetic, and electromagnetic fields which have the controversy of posing health hazards.
- CRT displays have high electricity consumption rate and are large, heavy, and bulky.
Liquid Crystal display (LCD)
- LCD is a flat panel display which uses light modulating properties of liquid crystals for displaying images. Liquid crystals use a backlight for producing images in colour or monogram. The backlight provides light to individual pixels that are arranged to form a rectangular grid.
- Each pixel has red, green and blue sub pixel which can be turned on or off. If all the sub-pixels are turned off, the screen appears black and when all the sub-pixels are turned on, the display appears white. For getting images of different colours, the individual levels of red, green and blue are adjusted.
- LCD displays are used for displaying arbitrary or fixed images with low information content. The arbitrary images displayed on LCD screens are made up of a large number of small pixels.
- LCD display monitors are compact, lighter in weight, consume low power, occupy less space and are priced reasonably.
- LCD uses two types of technologies viz. Active Matrix LCD technology (TFT) and Passive matrix technology. The Active matrix technology produces better image quality and are more reliable.
- The resolution of LCD display monitors is equal to the total number of pixels contained in the matrix.
- LCD displays a wide range of applications which include computer monitors, LCD televisions, smartphones, digital cameras, gaming devices etc.
- LCD Technology has replaced the CRT displays in nearly all applications.
Advantages of LCD
- LCD is capable of producing very bright images due to high peak intensity.
- It consumes less power than CRT displays and has a lifespan of 50,000 to 100,000 hours.
- The electric, magnetic and electromagnetic fields produced by LCD is considerable lower than CRT displays.
- Since LCD displays do not use a glass screen, it eliminates the problem of glare.
Disadvantages of LCD
- LCD displays have fixed aspect ratio and screen resolution.
- Due to a poor back level, the contrast level in LCD displays is lower than that of CRT displays.
- LCDs suffer from slower response times and scan rate conversion which leads to the problem of ghosting.
- The HD monitors cannot be viewed at every angle. CRT displays have better viewing angle than the LCD displays.
- Light emitting diode (LED) display is a flat panel display which uses the light emitting diode as pixels for displaying images and videos. There are basically two kinds of LED displays available in the market viz. LED-backlit LCD displays, and microLED displays.
- In LED-backlit LCD displays, the light emitting diode is used as a backlight for powering the LCD panel. Basically, these are LCD displays with a LED backlight for powering the LCD panel.
- The LED display is placed behind the LCD panel for enhancing the luminosity of the LCD display. The LCD monitors used cathode ray as backlight whereas LED monitors replaced it with LEDs.
- Both LED-backlit LCD monitors and traditional LCD monitors use the same technology but the LED displays are better than LCDs. The colour accuracy, contrast, and brightness of LED backlit LCD displays are better than traditional LCD monitors.
- In 2012, Sony presented ‘Crystal LED TV’ with true LED display in which the light emitting diodes are used for producing actual images rather than being used as a backlight for the LCD panels as an LED backlit LCDs. It is a microLED display in which LED produces images rather than the liquid crystals of LCD panel.
- MicroLED is an emerging flat panel display technology in which arrays of microscopic LEDs from the individual pixel elements for displaying images. Micro LED displays are better in contrast, response time, and energy efficiency than the widespread LCD Technology.
- The plasma display panel (PDP) is a flat panel display that uses small plasma cells containing ionized gases such as Xenon and Neon that emits light when current flows through it.
- On the flow of electric current through the plasma, the negatively charged particles move towards the positively charged area and vice versa. This leads to collisions which excite the ionized gas atoms present in the plasma cells which releases energy in the form of photons of light.
- Every pixel the plasma screen is made up of three coloured phosphors viz. red, green and blue.
- Plasma display panels are brighter and have a wide colour gamut. These display panels can be produced in large sizes up to 150 inches diagonally.
- Plasma panels have very low luminance ‘dark-room’ black levels. Unlike LCD display, plasma display panels do not need backlight. Black colour looks blacker on Plasma display and greyer on LCD displays.
- The thickness of Plasma monitors is generally less than 10 cm.
- Electricity consumption depends upon the content displayed on the screen, bright scenes use more power than the dark ones. The power consumption is around 400 watts for a 50 inch plasma display screen.
- The temperature of Plasma can reach up to 1200 degree centigrade. The plasma displays have an estimated lifetime of around 100,000 hours after which the display brightness falls to the half of the original value.
- Plasma screens can cause glare from reflected objects because the screen is made out of class.
- Plasma displays are not economical on screen sizes below 32 inches.
Advantages of Plasma display
- Plasma display can produce deeper blacks that allow a superior contrast ratio.
- Plasma display colour reproduction is similar to that of cathode ray tube displays because they use similar phosphors for illumination.
- The viewing angle of the plasma display is wider than that of LCD, and images do not degrade at less than straight angles which happens in case of LCD displays.
- The motion blur is less visible due to very high refresh rates and faster response time that leads to superior performance while displaying rapid motion.
Disadvantages of Plasma display
- Plasma display is susceptible to screen burning and image retention. Improvements in technology have reduced the burn in issue but do not prevent it.
- Due to the bistable nature of colour and intensity generating method, Plasma displays can suffer from flickering effect.
- The plasma display uses more electricity than LED backlit LCD displays.
- Due to pressure differential between the gas inside the screen, plasma displays do not work at altitude above 2000 metres.
- The overall weight of Plasma displays are more than that of LCD displays and it requires more careful handling.
Organic Light Emitting Diode (OLED)
- OLED is a Light emitting diode (LED) in which the emissive electroluminescent layer is a film made of organic material containing carbon like wood, plastic, polymers etc which emits light on the flow of electric current.
- The organic layer is placed between two electrodes - a transparent anode and a metallic cathode.
- OLED is capable of producing light of different colours and they do not require a backlight and directly produce a correct colour which also saves power and space.
- In low light conditions, OLED displays provide a higher contrast ratio compared to the LCD displays.
- OLED displays are considered better than the existing display technologies due to their fast response time, better contrast levels, wide viewing angles, and perfect brightness.
- OLEDs are used for creating digital displays on television screens, smartphones, computer monitors, handheld game consoles, and personal digital assistants.
- An OLED display can be based on the passive matrix (PMOLED) or active matrix (AMOLED) control schemes. In PMOLED, each row in the display is controlled sequentially one by one.
- In the AMOLED scheme, a thin film transistor backplane is used for directly accessing and switching each individual pixel on or off. This provides higher resolution and larger display sizes.
Advantages of OLED
- Lower cost in future: OLED can be printed out through screen printing by using a suitable screen printer which makes them theoretically cheaper to produce. Roll-to-roll vapour deposition methods ensure that mass production can be done for minimal cost, but this technique have some challenges of accuracy.
- Lightweight and flexible substrates: these can be fabricated on flexible plastic substrates which makes the fabrication of flexible OLED for applications such as role of displays possible etc. The plastic substrates have the advantage of being shatter resistance and like the glass displays that are used in LCD monitors.
- Better picture quality these can provide greater contrast ratio and wider viewing angle because OLED pixels have the capability to emit lights directly. OLED colours appear correct and unshifted even when the viewing angle approaches 90 degree from the normal. Since these displays do not use any backlight, black levels appear more deeper.
- Better power efficiency and thickness: OLED display do not use backlight due to which the power consumption is less. Removal of backlight makes OLED display lighter as some substrates are not required. These devices are thinner than their LCD counterparts.
- Faster response time: OLED have much faster response time than LCD displays. The response time can be up to 1000 times faster than LCD displays which can be under 10 μs. OLED displays can be designed to removes the problem of sample-and-hold-behaviour seen on both LCD and some OLED devices.
- Environment friendly: the materials used are environment friendly and do not use Lead or such other material.
Disadvantages of OLED
- Lifespan: have lower lifetime due to limited lifetime of organic materials. Historically blue OLED had the lowest lifetime of around 14000 hours to half brightness level. This degradation occurs due to the accumulation of nonradiative recombination centers, and illuminations quenchers in the image safe zone. However, new technologies are expected to make OLED with expected life at par of LCD displays.
- Colour balance: historically blue OLEDs were the first to lose their efficiency which impacted the colour balance of pixels. Improvements in lifetime and deficiency of OLED is necessary for replacing the LCD Technology.
- Water damage: water can cause instant damage to the organic materials of the OLED displays. It can limit the longevity of more flexible displays. Thus, an improved ceiling is necessary.
- Outdoor performance: OLED uses metallic cathode which acts as a mirror with reflectance approaching 80%. This leads to poor readability in the bright light outdoor. Thus, circular polarizer and anti reflective coating are needed to reduce the reflectance to less than 0.1 %.
- Power consumption: power consumption in OLED displays are uneven. These consume around 40% of the power of LCD for Black images, 60 to 80% for the majority of images but more than three times as much power for displaying white background such as a website. This can reduce battery life in mobile devices when white backgrounds are used.
- Electronic paper and e-paper are display devices that mimic the appearance of ordinary ink on paper. Unlike conventional LCD displays that EMIT light, electronic paper displays reflect light like conventional paper. An ideal electronic paper display can be read in direct sunlight without fading of images.
- The electronic paper display has millions of minuscule capsule filled with a clear fluid having microscopic particles of different colours and electrical charges. The electrodes are placed above and below the capsules that move up and down on application of positive or negative electric field. This makes it possible for electronic paper display to reflect certain colours.
- Some electronic paper technologies can hold static text indefinitely without using any power. Thus, e-paper can provide paper like readability at extremely lower power consumption.
- The electronic paper can have multiple applications that include displaying real time bus arrival information, maps etc. Further, solar and battery powered electronic paper displays can be developed which do not require additional cables.