A television produces a series of tiny dots on a screen that, when seen as a whole, appear as an image. Older televisions rely on a cathode-ray tube to produce images, and operate with an analog signal. As technology has advanced and broadcast signals transitioned from analog to digital, plasma and LCD (liquid crystal display) televisions were created. These TVs are more compact and have crisper pictures than their cathode-ray counterparts because they use a thin grid of pixels to create images rather than a vacuum tube.
The Eyes and the Brain
Most kinds of television work from the same basic principle. The tiny dots of light produced on the TV screen, called pixels, flash according to a specific pattern provided by the video signal. A person’s eyes transmit this pattern to the brain, where it is interpreted as a recognizable image. The television set refreshes these patterns hundreds of times per second — faster than the human eye can see — which gives the illusion of movement.
The Cathode-Ray Tube
The cathode-ray tube (CRT), the oldest version of the television, consists of a vacuum tube with a narrow end and a wide end. The narrow end contains an ion gun, which shoots out a series of charged particles of electricity. A series of electromagnets guide the particles to specific points on the wide end of the tube, the screen that viewers look at. Phosphors, substances that light up when a charged electrical particle hits them, coat the screen’s inner surface. The ion gun essentially sprays the image at the screen, much like a paint gun sprays paint onto a surface.
Different kinds of phosphors produce different colors, but for color television, only red, blue, and green are needed. Using these colors in various combinations and intensities can create all the colors the human eye can see. As energy travels from the ion gun to the phosphors, it is filtered to strike the exact point on the screen needed to produce a specific hue. In combination, all of these colored pixels create a color image.
Cathode-ray tubes are quite heavy due to the large amount of glass they contain, and relatively inefficient, especially when used in large-screen televisions. For this reason, new technologies were developed to make lighter sets with crisper images. In addition, the development of high definition (HD) digital broadcast signals made bigger screens more popular since the images were of higher quality. Plasma and LCD televisions were
created in response.
The Plasma Screen
A plasma screen television consists of a number of tiny cells filled with neon and xenon gases. Each cell is linked to an electrode, which, when fired, excites the gases contained in the cell. The gases emit charge particles, much like the ion gun, that interact with phosphors coating the glass inside each cell. The phosphors light up, creating the image seen on the television screen. The large number of cells in a plasma screen makes for a great number of pixels, rendering a clearer and brighter image.
Compared to other technologies, plasma TVs produce some of deepest blacks, which means that the contrast ratio is very high. They also have very high refresh rates, so images with a lot of motion don’t blur as they can on other televisions. If the image remains static, however, it can burn into the screen, creating a permanent discoloration; this is more common in older plasma TVs, and can also occur with CRT screens. Plasma screens can be set to be very bright, which requires a lot of electricity. They also tend to be thicker than LCD televisions, although much thinner than CRTs.
The LCD Screen
LCD televisions also use cells to create images. Rather than exciting gases as plasma TVs do, however, the cells contain a set of red, blue, and green filters covered by a layer of liquid crystals sandwiched between two pieces of glass. Depending on the display type, each cell is linked to either electrodes or thin film transistors (TFT), which trigger the necessary cells to create the image. A backlight — most often cold-cathode fluorescent lamp — lights up the screen so the image can be seen.
While LCDs are very light and thin, they are subject to “dead” pixels, where one or more cells on the screen do not change. Viewing LCD screens from an angle can also lower the picture quality. They have slower response times than plasma or CRT televisions as well, so images can “ghost” or blur in movement.