LCD TVs have made up the vast majority of the range on the TV market for years. But how does an LCD TV work? We will explain it all to you.
LCD TVs: Liquid Crystal Display in short
LCD stands for Liquid Crystal Display. This screen technology owes its name to the Liquid Crystal panel that is responsible for switching each pixel on and off. How does that work in practice? A light source sends light through the LCD panel. That panel determines per pixel to what extent the light is transmitted. That is why we speak of a “transmissive” technology, as opposed to, for example OLED where each pixel itself emits light and what we call an “emissive” technology.
Light and polarization from LCD TVs
But how does an LCD panel actually block light? To explain that, you need to know what the polarization of light means. The oscillation of a light wave has a certain orientation. The wave motion can be in a vertical, horizontal plane, or in any other direction. Or it can even change constantly as the gold travels, we speak of circular polarization. In addition, light can be filtered on the basis of polarization, for example to allow only the vertically polarized light to pass through. Such a filter is also used in sunglasses, for example, to block strong reflections. When vertically polarized light subsequently reaches a horizontal polarizing filter, it is completely stopped. You can test this at home with two polarized sunglasses by placing them at right angles to each other.
How does an LCD TV work?
LCD TVs always need a light source. This is contained in a module that we call the backlight (Backlight Unit or BLU). How the background lighting is structured can vary, more on that below.
The white light from the backlight first passes through a vertical polarizing filter. Then it goes through the LCD layer. These liquid crystals can change direction. It is controlled via the TFT layer (Thin Film Transistor). Depending on the direction of the crystals, the light passes unchanged or the polarization direction is readjusted. Then the light passes through a color filter to convert the white light into the three basic colors. The last layer is a horizontal polarizing filter. If the light in the crystal layer has changed polarization direction, it will pass (partially or completely) through the last filter and the pixel will be on. If the light has passed through the crystal layer unchanged, it is stopped by the second polarization filter and that pixel is therefore off.
The above image is sufficient as a schematic explanation of the operation of LCD TVs. For the sake of completeness, we give you the complete structure of an LCD TV below.
I have an LED TV, is it the same as an LCD TV?
Yes, it is the same. An LED TV is simply an LCD TV with a modern backlight based on LEDs instead of CCFL (tube lamps). LED TV is a marketing term that was introduced when the first LED-based backlights were used. But the image technology is definitely LCD. The use of LEDs in the backlight did make a number of innovations possible. Tubular lamps as background lighting have now completely disappeared from the market.
Various LCD panels
The actual LCD panel in your television has evolved a lot over the years. There are many different types of LCD panels, each with different properties. In televisions we mainly find two types. VA (Vertical Alignment) and IPS (In Plane Switching).
VA panels provide the best contrast, typically around 3,000: 1, but have a relatively weak viewing angle. Especially the contrast decreases sharply if you are not perpendicular to the screen. The pixel of a VA panel looks like a rectangle, divided into four or even eight pieces.
IPS panels have to do with a typical contrast of about 1,000: 1. They are known for their relatively wide viewing angle, both contrast and color are much better preserved if you are not centrally located in front of the screen. From a corner, IPS panels can suffer from ‘IPS glow’ where you see a white glow over some parts of the screen. The pixel of an IPS panel looks like a chevron.
Every now and then we also find a PLS panel (Plane to Line Switching). This type of panel has the same characteristics as an IPS panel, but is manufactured by Samsung. (most IPS panels are from LG). The pixels of a PLS panel look like rectangles with notches here and there, and they are divided in half.
Keep in mind that it is best to consider VA and IPS as two families. Within each family there are different variants. This is one of the reasons why performance may differ even if two TVs are equipped with a VA panel (or IPS panel).
Types of backlighting
The backlight plays an important role in LCD TVs. Why? The main reason is that an LCD panel can never perfectly shut off all light. And since the backlight is always on, light can seep through even when an LCD pixel is closed. That’s why LCD TVs have less contrast.
With the introduction of LEDs as a light source, however, all kinds of new options became possible. Because LEDs have a very fast response time, the intensity of the backlight can be based entirely on what is being seen. We call this global dimming. One step further is to subdivide the backlight into different segments. These are then each controlled separately, which we call local dimming. Because LEDs are very small, they also made much thinner TVs possible, for example by putting the LEDs in the side of the screen.
Would you like to know more about local dimming, global dimming and Edge LED? Then read our background article about it the different types of backlight. A recent development is the use of the miniLEDs in the backlight. These enable even better controlled local dimming.
Colors and quantum dots
In addition to the structure, the nature of the white light is a very important property of the backlight. Since an LCD TV works with a color filter and therefore filters out light (and energy), you want the white light to be made up of red, green and blue as pure as possible. This not only gives you more intense colors, but also less filtering, so that the operation is more efficient. The first white LEDs used a blue LED with a yellow phosphor layer. With the advent of HDR and the requirement larger color gamut manufacturers looked for other solutions. One is to use blue LEDs with a red and green phosphor, another solution is to use quantum dots. Keep in mind that these are all still LCD TVs regardless of marketing terms such as QLED.
Do you want to know more about TV techniques, setting up a television or connecting a television? Then take a look at our tips and advice section.