The Basics of Television Display Technologies
the first half century that television sets have been a permanent
fixture in our homes there was nothing but cathode ray tubes to display
the moving images that viewers stared at. It all started to
change in the late 1990's when Plasma panels and digital light
projection sets made their way into living rooms. They were
followed by LCD panels a few years later. Technologies such
as Organic Light Emitting Diode will add to the choices for television
displays in the next few years to come. For today the options
for between display technologies can may buying a television difficult
to say the least. Knowing how each type of display works and
the advantages and disadvantages is the best way of know which kind of
television is right for you.
Liquid Crystal Display (LCD)
Far and away the most popular technology for flat panel TV displays LCD now offers great picture quality with wider viewing angle thought to be impossible just a few years ago. The working of a LCD panel is pretty basic. A back light using ether florescent bulbs or light emitting diodes shines through a diffuser panel to avoid a bright spot appearing directly in front of where the back light is located. The light shines through a plastic layer that has the red, blue and green pixels imprinted on it. The light shines through a layer of liquid crystal elements. The elements each the size of a pixel are activated to turn black similar to the liquid crystal display in a watch or calculator turns black to block light coming from pixels that are not needed. While LCD displays are capable of high resolution and high refresh rates, The range of colour and contrast can be limited. The backlight system has been blamed on poor colour reproduction for dark shades of blue, green, brown and black. Back light systems using Light Emitting diodes (LED's) remedy this by dynamically turning off individual LED's for any area of the image displaying dark shades of colour. LCD TV's are good choice for most viewing locations where there is ambient light from windows or lighting fixtures.
One of the most mature technologies for large screen flat panel TV's is plasma. While LCD is the dominant technology in the market plasma screens maintain consistent popularity due to a loyal fan base. Plasma screens reproduce a wide range of colour and contrast that LCD has yet to match. Plasma screens can refresh it's pixels much faster than LCD screens are able to which makes plasma screens popular with sports fanatics, and ardent movie buffs. Detracting from Plasma's advantages over LCD is the highly reflective glass front surface that reflects light from ambient sources. Plasma screens work best in a dedicated space where the lighting can be lowered.
Each of the pixels of a plasma screen work like a florescent bulb. Charged electrodes pass electrons through a chamber of highly compressed xenon and neon gasses. The charge causes the gasses to ionize. The sides and bottoms of the chambers of each pixel are coated with phosphorous that is dyed red, blue and green, The ionized gasses excites the phosphorous and causing it to give off visible light.
Organic Light Emitting Diode (OLED)
The technology that is quickly emerging for flat screen television display is Organic Light Emitting Diode or OLED. Digital Cameras and Smartphones are appearing on the market with OLED screens as they are a much more energy efficient alternative to LCD screens typically found on these devices. Sony and LG are the first manufacturers to produce television sets using OLED screens. OLED's principal advantage over LCD is that OLED can produce contrast and colour in a much greater range. Contrast ratio of 1,000,000:1 have already been achieved on the first models of OLED TV's. Like plasma screens OLED screens are not effected by the angle the screen is viewed at. The main disadvantage is going to be the cost of OLED TV's. As manufacturing capacity improves the cost will come down. In order for OLED to become commercially viable the manufacturing capacity and the price will have to improve very quickly.
OLED uses a thin sheet of organic compounds which itself has two layers, a conductive layer and an emissive layer. Each pixel has an anode and a cathode, when an electrical charge is applied to the pixel, electrons supplied from the cathode flow through the emissive layer handed off to the conductive layer and return to the anode. It's the hand off of electrons from the emissive layer to the conductive layer causes light to be emitted from the pixel.
Surface-Conduction Electron-Emitter Display (SED)
One technology that is has yet to move past the prototype stage but is worth mentioning is called Surface Conduction Electron Emitter Display or SED. The best way to describe an SED is a flat panel version of the tube TV's that we grew up with. Like a cathode ray tube, electrons are fired at phosphorous coloured with red, green and blue dyes to create the pixels. SED creates a picture with the wide contrast range as CRT but in a flat panel form factor.
SED TV's were a hit with both technology journalists and fans when it was first shown at the Consumer Electronics Show in 2006, since then development has ground to a stand still due to litigation. A company called Applied Nanotech sued Canon, which was the biggest developer of SED display technology. At the time that the lawsuit was launched Applied Nanotech claimed that agreement with Toshiba that Canon entered into was an unlawful transfer of intellectual property owned by Applied Nanotech. SED technology probably won't make it into the living rooms of consumers for years until the legal issues get sorted out but it worth mentioning it simply because it goes to show that today's flat panel display technologies won't necessarily will be tomorrow's..