A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals. Liquid crystals do not emit light directly, instead using a backlight or reflector to produce images in color or monochrome.
LCDs are available to display arbitrary images (as in a general-purpose computer display) or fixed images with low information content, which can be displayed or hidden, such as preset words, digits, and seven-segment displays, as in a digital clock. They use the same basic technology, except that arbitrary images are made up of a large number of small pixels, while other displays have larger elements.
How LCD Works
LCDs can either be normally on (positive) or off (negative), depending on the polarizer arrangement. For example, a character positive LCD with a backlight will have black lettering on a background that is the color of the backlight, and a character negative LCD will have a black background with the letters being of the same color as the backlight. Optical filters are added to white on blue LCDs to give them their characteristic appearance.
Why LCD Obsolesced CRT
The LCD screen is more energy-efficient and can be disposed of more safely than a CRT can. Its low electrical power consumption enables it to be used in battery-powered electronic equipment more efficiently than CRTs can be. By 2008, annual sales of televisions with LCD screens exceeded sales of CRT units worldwide, and the CRT became obsolete for most purposes.
LCD Illumination
The brightness of an LCD display screen is measured in cd/m2 which is Candela Per Square Meter.
Since LCD panels produce no light of their own, they require external light to produce a visible image. In a transmissive type of LCD, this light is provided at the back of the glass stack and is called the backlight. The common implementations of LCD backlight technology are:
- CCFL
- EL-WLED
- WLED Array
- RGB-LED Array
Today, most LCD screens are being designed with an LED backlight instead of the traditional CCFL backlight, while that backlight is dynamically controlled with the video information (dynamic backlight control).
LCD Advantages
- Very compact, thin and light, especially in comparison with bulky, heavy CRT displays.
- Low power consumption.
- Little heat emitted during operation, due to low power consumption.
- No geometric distortion.
- The possible ability to have little or no flicker depending on backlight technology.
- Usually no refresh-rate flicker, because the LCD pixels hold their state between refreshes.
- Sharp image with no bleeding or smearing when operated at native resolution.
- Emits almost no undesirable electromagnetic radiation (in the extremely low frequency range), unlike a CRT monitor.
- Can be made in almost any size or shape.
- No theoretical resolution limit.
- Unaffected by magnetic fields.
- As an inherently digital device, the LCD can natively display digital data from a DVI or HDMI connection without requiring conversion to analog.
LCD Disadvantages
- Limited viewing angle in some older or cheaper monitors, causing color, saturation, contrast and brightness to vary with user position, even within the intended viewing angle.
- Uneven backlighting in some monitors (more common in IPS-types and older TNs), causing brightness distortion, especially toward the edges (“backlight bleed”).
- Black levels may not be as dark as required because individual liquid crystals cannot completely block all of the backlight from passing through.
- Low refresh rate.
- Input lag, because the LCD’s A/D converter waits for each frame to be completely been output before drawing it to the LCD panel.
- Dead or stuck pixels may occur during manufacturing or after a period of use. A stuck pixel will glow with color even on an all-black screen, while a dead one will always remain black.
- Loss of brightness and much slower response times in low temperature environments.
- Loss of contrast in high temperature environments.
- Defective transistors can cause permanently lit or unlit pixels which are commonly referred to as stuck pixels or dead pixels respectively.
LCD Specifications
Resolution
The resolution of an LCD is expressed by the number of columns and rows of pixels (e.g., 1024×768).
Spatial Performance
For a computer monitor or some other display that is being viewed from a very close distance, resolution is often expressed in terms of dot pitch or pixels per inch, which is consistent with the printing industry.
Temporal Performance
The temporal resolution of an LCD is how well it can display changing images, or the accuracy and the number of times per second the display draws the data it is being given.
Color Performance
There are multiple terms to describe different aspects of color performance of a display. Color gamut is the range of colors that can be displayed, and color depth, which is the fineness with which the color range is divided.
Brightness and Contrast Ratio
Contrast ratio is the ratio of the brightness of a full-on pixel to a full-off pixel.
Active Matrix Technologies
The following list of technologies belong to the list of LCD matrices.
TN (Twisted Nematic)
Twisted nematic displays contain liquid crystals that twist and untwist at varying degrees to allow light to pass through. When no voltage is applied to a TN liquid crystal cell, polarized light passes through the 90-degrees twisted LC layer. In proportion to the voltage applied, the liquid crystals untwist changing the polarization and blocking the light’s path. By properly adjusting the level of the voltage almost any gray level or transmission can be achieved.
TN is known for low color quality, low viewing angles, fast response times.
IPS (In-Plane Switching)
In-plane switching is an LCD technology that aligns the liquid crystals in a plane parallel to the glass substrates. In this method, the electrical field is applied through opposite electrodes on the same glass substrate, so that the liquid crystals can be reoriented (switched) essentially in the same plane, although fringe fields inhibit a homogeneous reorientation. This requires two transistors for each pixel instead of the single transistor needed for a standard thin-film transistor (TFT) display. Before LG Enhanced IPS was introduced in 2009, the additional transistors resulted in blocking more transmission area, thus requiring a brighter backlight and consuming more power, making this type of display less desirable for notebook computers. Currently Panasonic is using an enhanced version eIPS for their large size LCD-TV products as well as Hewlett-Packard in its WebOS based TouchPad tablet and their Chromebook 11.
IPS is known for high color quality, wide viewing angles, slow response times.
Ilumination
Since LCD panels produce no light of their own, they require external light to produce a visible image. In a transmissive type of LCD, this light is provided at the back of the glass stack and is called the backlight. While passive-matrix displays are usually not backlit (e.g. calculators, wristwatches), active-matrix displays almost always are. Over the last years (1990 — 2017), the LCD backlight technologies have strongly been emerged by lighting companies such as Philips, Lumileds (a Philips subsidiary) and more.
CCFL (Cold Cathode Flourescent Lamps)
CCFL: The LCD panel is lit either by two cold cathode fluorescent lamps placed at opposite edges of the display or an array of parallel CCFLs behind larger displays. A diffuser then spreads the light out evenly across the whole display. For many years, this technology had been used almost exclusively. Unlike white LEDs, most CCFLs have an even-white spectral output resulting in better color gamut for the display. However, CCFLs are less energy efficient than LEDs and require a somewhat costly inverter to convert whatever DC voltage the device uses (usually 5 or 12 V) to ≈1000 V needed to light a CCFL. The thickness of the inverter transformers also limits how thin the display can be made.
OLED (Organic Light Emitting Diode)
Static Contrast ratio is unable to be calculated as black levels are zero. Peak luminosity is 100-700 cd/m2. Color depth is 8 to 10-bit per subpixel, with some HDR models capable of 12-bit per subpixel.
OLED response times are up to 1,000 times faster than LCD, putting estimates varying from under 10 μs (0.01 ms) to as low as 0.001 ms. Frame rate is 60 fps typically. OLED can have poor brightness.
Risk of image retention or ‘burn-in’. Coloured sub-pixels may age at different rates, leading to a colour shift. Sensitive to UV light from direct sunlight.
Energy consumption and heat generation
Varies based on image brightness and color.For the majority of images it will consume 60–80% of the power of an LCD.
OLED displays use 40% of the power of an LCD displaying an image that is primarily black as they lack the need for a backlight, while OLED can use more than three times as much power to display a mostly white image compared to an LCD.