Gone are the days of ‘big idiot box’ type CRT TVs. While those old CRT TVs may still be seen in few homes, the era is of LCD and LED TVs. This article explains working principles of LCD and LED TVs.
What was a CRT TV?
CRT stands for ‘Cathode Ray Tube’ which is basically a vacuum tube consisting of one or more electron guns and a phosphorescent screen. A circuitry guides the electron gun to produce an electron beam that scans back and forth on the phosphorescent screen. When electrons collide with the phosphor coating on the screen, it emits visible light. That is how a CRT produces pictures. But, as CRTs are quite bulkier and less elegant and due to advancements in LCD and LED TVs, many manufacturers have stopped production of CRT TVs.
LCD (Liquid Crystal Display)
LCDs or Liquid Crystal Displays are a major part of technology. They can be found almost everywhere – watches, calculators, mobile phones, laptops, televisions, cars and many more. Obviously, LCD display on a calculator is different than an LCD TV, but the fact is that they all work on a same basic principle. An LCD screen basically consists of liquid crystal solution sandwiched between two transparent electrodes and two polarizing filter sheets. Following is the basic working principle of a Liquid Crystal Display.
The image at right shows the various layers of a typical LCD screen.
- Vertical polarization filter film
- Glass substrate with Idiom-Tin-Oxide (ITO) electrodes. ITO is a transparent and conducting oxide. Verticle ridges (or grooves) are etched on the side that does not have polarization film so that the liquid crystals align in line with the polarized light.
- Layer of twisted nematic liquid crystals.
- Glass substrate with common ITO electrode film with horizontal ridges to line up with polarizing filter.
- Horizontal polarization film.
- Reflective surface (mirror).
As light strikes the first layer of the vertical polarizer, it gets polarized. Because of the ridges on the glass substrate electrodes, the nematic liquid crystals are aligned with the vertical polarization plane. The liquid crystals layer is twisted and the molecules at the far side are aligned with the polarization plane of the horizontal polarization film. As the initially vertically polarized light passes through this twisted liquid crystals layer, angle of the plane of polarization also changes to match the angle of the molecules of the liquid crystals. When the light reaches the far side of the liquid crystals layer, the plane of polarization becomes horizontal and it passes through the polarized film. Thus, the light also passes back through all the layers after reflecting from the mirror.
When an electric charge is applied to the liquid crystals through the electrodes, they untwist. As the molecules straighten out, their alignment no more matches the plane of polarization or the farthest film. Consequently, the light gets blocked at the areas where electric charge is applied.
How LCD TV works?
An LCD TV basically works on the same principle. Here, the mirror at the back is replaced with fluorescent backlighting. If you look very closely, you can notice that the picture is made from millions of tiny blocks called pixels. Using one liquid crystal per pixel, each pixel can be made to allow or block the light. Thus, a black and white picture can be easily created using “active-matrix” technique. Basically, tiny switching transistors and capacitors are arranged in a matrix on a glass substrate. The rapidly switch the LCD’s pixels on or off.
But, how an LCD produces colorful pictures? To produce colorful images, each pixel actually consists of three sub-pixels. Red, green and blue color filter is applied to each sub-pixel. By controlling each pixel to glow red, green or blue, a colorful RGB picture is produced.
How LED TV is different than an LCD TV?
Technically speaking, an LED TV (not to be confused with LED matrix screens at malls, stadiums etc.) should be called LED-backlit LCD TV. An LED TV screen is also a Liquid Crystal Display. The only major difference, here, is the lights at the back is produced with LEDs instead of fluorescent backlights in LCDs. Generally, LED TVs have better black levels and contrasts. Individual zones of LEDs can be dimmed or brightened individually making image production more flexible. Color accuracy is also slightly better than LCDs. LED TVs are also more energy efficient than LCD TVs.