LCD (Liquid Crystal Display) is a type of flat panel display which uses liquid crystals in its primary form of operation. LEDs have a large and varying set of use cases for consumers and businesses, as they can be commonly found in smartphones, televisions, computer monitors and instrument panels.
LCDs were a big leap in terms of the technology they replaced, which include light-emitting diode (LED) and gas-plasma displays. LCDs allowed displays to be much thinner than cathode ray tube (CRT) technology. LCDs consume much less power than LED and gas-display displays because they work on the principle of blocking light rather than emitting it. Where an LED emits light, the liquid crystals in an LCD produces an image using a backlight.
As LCDs have replaced older display technologies, LCDs have begun being replaced by new display technologies such as OLEDs.
How LCDs work
A display is made up of millions of pixels. The quality of a display commonly refers to the number of pixels; for example, a 4K display is made up of 3840 x2160 or 4096x2160 pixels. A pixel is made up of three subpixels; a red, blue and green—commonly called RGB. When the subpixels in a pixel change color combinations, a different color can be produced. With all the pixels on a display working together, the display can make millions of different colors. When the pixels are rapidly switched on and off, a picture is created.
The way a pixel is controlled is different in each type of display; CRT, LED, LCD and newer types of displays all control pixels differently. In short, LCDs are lit by a backlight, and pixels are switched on and off electronically while using liquid crystals to rotate polarized light. A polarizing glass filter is placed in front and behind all the pixels, the front filter is placed at 90 degrees. In between both filters are the liquid crystals, which can be electronically switched on and off.
LCDs are made with either a passive matrix or an active matrix display grid. The active matrix LCD is also known as a thin film transistor (TFT) display. The passive matrix LCD has a grid of conductors with pixels located at each intersection in the grid. A current is sent across two conductors on the grid to control the light for any pixel. An active matrix has a transistor located at each pixel intersection, requiring less current to control the luminance of a pixel. For this reason, the current in an active matrix display can be switched on and off more frequently, improving the screen refresh time.
Some passive matrix LCD's have dual scanning, meaning that they scan the grid twice with current in the same time that it took for one scan in the original technology. However, active matrix is still a superior technology out of the two.
Types of LCDs
Types of LCDs include:
- Twisted Nematic (TN)- which are inexpensive while having high response times. However, TN displays have low contrast ratios, viewing angles and color contrasts.
- In Panel Switching displays (IPS Panels)- which boast much better contrast ratios, viewing angles and color contrast when compared to TN LCDs.
- Vertical Alignment Panels (VA Panels)- which are seen as a medium quality between TN and IPS displays.
- Advanced Fringe Field Switching (AFFS)- which is a top performer compared IPS displays in color reproduction range.
LCD vs OLED vs QLED
LCDs are now being outpaced by other display technologies, but are not completely left in the past. Steadily, LCDs have been being replaced by OLEDs, or organic light-emitting diodes.
OLEDs use a single glass or plastic panels, compared to LCDs which use two. Because an OLED does not need a backlight like an LCD, OLED devices such as televisions are typically much thinner, and have much deeper blacks, as each pixel in an OLED display is individually lit. If the display is mostly black in an LCD screen, but only a small portion needs to be lit, the whole back panel is still lit, leading to light leakage on the front of the display. An OLED screen avoids this, along with having better contrast and viewing angles and less power consumption. With a plastic panel, an OLED display can be bent and folded over itself and still operate. This can be seen in smartphones, such as the controversial Galaxy Fold; or in the iPhone X, which will bend the bottom of the display over itself so the display’s ribbon cable can reach in towards the phone, eliminating the need for a bottom bezel.
However, OLED displays tend to be more expensive and can suffer from burn-in, as plasma-based displays do.
QLED stands for quantum light-emitting diode and quantum dot LED. QLED displays were developed by Samsung and can be found in newer televisions. QLEDs work most similarly to LCDs, and can still be considered as a type of LCD. QLEDs add a layer of quantum dot film to an LCD, which increases the color and brightness dramatically compared to other LCDs. The quantum dot film is made up of small crystal semi-conductor particles. The crystal semi-conductor particles can be controlled for their color output.
When deciding between a QLED and an OLED display, QLEDs have much more brightness and aren’t affected by burn-in. However, OLED displays still have a better contrast ratio and deeper blacks than QLEDs.
This video shows pixels operating on CRT, LCD, and OLED televisions in slow motion, and how they all work together to create a working image.