Main Difference – LCD vs. Plasma
LCD and plasma are two different types of screens in widespread use. They were created in order to make larger screens without having them be too bulky, as with the older CRT displays. The main difference between LCD and plasma is that LCD uses the twist of liquid crystals to display an image, whereas plasma displays utilize excitations of atoms in a plasma.
What is LCD
LCD stands for “Liquid Crystal Display”. Liquid crystals are a special type of material, where the molecules are oriented as if the material were a solid crystal, but the molecules can also move about each other, giving the substance the ability to “flow” as though it were liquid. To construct LCDs, a specific type of liquid crystals called twisted nematic liquid crystals are used. As the name suggests, they have the ability to “twist”, and this is important to how an LCD screen works.
A liquid crystal display is constructed by placing a layer of liquid crystals between two polarised filters. The two filters are arranged so that their axes of polarisation are perpendicular to each other. If there were nothing between the two filters, the light passing through one filter would be completely blocked by the other filter. When twisted nematic LCDs are placed between the two filters, they can twist, making the electric fields in light also twist in such a way that enables light coming from one polaroid filter to pass through the other filter as well.
If an electric field is applied across the layer of liquid crystals, they can be made to untwist. In LCD screens, there is a layer of electrodes between the crystals and each polaroid filter. These electrodes are arranged to form a grid so that a specific point on the grid can be activated by applying a potential difference across the two electrodes that cross to make that point. Activating a specific place causes the crystals there to untwist and for the cell to become darker. To display colour, each pixel is made of three red, green and blue subpixels. A thin film transistor (TFT) is attached to each subpixel. At any given time, a single row of pixels is activated. Soon afterwards the row becomes deactivated and the next row becomes activated,… and so on. When a particular row is activated, each column of electrodes sends a voltage corresponding to the level of illumination required by each coloured subpixel of that row. The voltage of the electrodes determines to what extent the subpixel passes light through it. Each subpixel also consists of a capacitor, which retains charge and keeps the subpixel illuminated until the next time the row is refreshed.
Note that the crystals themselves do not produce light, they merely control the levels of light passing through them. some LCD screens, such as those in calculators, are reflective: they simply reflect light falling on to them. LCD screens in laptops and televisions are often backlit by a light source.
What is Plasma
A plasma is a state of matter which consists of a large number of unbound charge carriers, which freely move about. A plasma can be formed by applying an electric field across gases. In a plasma TV, the display is made of a number of tiny pixels, which are in turn made of red, green and blue subpixels. Each subpixel consists of a cell: a small container holding a mixture of a noble gas and another gas. There are electrodes that run along each row of cells. Similarly, on the other side of the cells, there are more electrodes running vertically along each column of electrodes. Each subpixel can be activated by giving a voltage to the two electrodes that “cross” at each cell.
When a cell is activated, a current flows through the cell. This causes the gases in the cell to become ionized. The ionized gas molecules and electrons are accelerated by the electrodes. When they are accelerated, they collide with the noble gas atoms and this causes ultraviolet photons to be released.
Each cell is coloured and coated with a phosphor material. When ultraviolet photons in an activated cell strike the phosphor, they cause the atoms making up the phosphor to become excited. These atoms, when they are getting de-excited, release visible light photons and cause the cell to “glow”.
Difference Between LCD and Plasma
LCD uses two polarizing filters, and the twisting of liquid crystals between these two layers determine how light passes through.
Plasma works by exciting atoms in a plasma so that they emit ultraviolet photons. These photons strike a phosphor and creates a glow.
LCD screens cannot produce their own light. Instead, they rely on external sources for illumination.
In plasma, the glowing cells produce their own light.
LCD screens consume comparatively less power.
Plasma screens consume comparatively more power.
LCD screens have a limited viewing angle. When viewed from other angles, the images produced by LCDs appear less bright and with distorted images.
Plasma displays have a wider viewing angle.
LCD screens may be susceptible to ghosting. When the screen is displaying very fast motion, sometimes the pixels cannot refresh fast enough so that the image looks blurred.
Plasma screens refresh much faster, so the blurring effects are not present.
LCD screens have low levels of burn-in. That is, once a pixel is refreshed, there is not much of an effect left behind from the previous image.
Plasma screens (especially older ones) could have more noticeable burn-in. If a static image is displayed on a screen for a long time, the old image still seems to “remain behind” on the screen even when the pixels have been refreshed to produce a newer image.
“Reflective twisted nematic w:liquid crystal display.” by ed g2s (Own work) [CC BY-SA 3.0], via Wikimedia Commons
“Simple composition of the alternating current plasma display panel with matrix electrode design.” by Jari Laamanen (Own work) [Free Art License 1.3], via Wikimedia Commons