Flat-screen technology has become ubiquitous. It has almost completely replaced CRTs in test instruments as well as in other appliances. Unfortunately, flat-screen tech can be a bit confusing because of vendors that use their own trade names to refer to their specific type of flat display technology. Here, we’ll try to clear up some of the nomenclature problems.
The principal types of flat-screen panels are:
• Liquid crystal display (LCD), including TFT and IPS
• Plasma panel
• Electroluminescent panel
• Organic light-emitting diode (OLED), including AMOLED
• Quantum-dot light-emitting diode
By far the most common type of flat-screen is the liquid crystal display (LCD). The image is high-quality, surpassing in resolution the CRT, and in all respects quite user-friendly. It is based upon unusual physical properties of liquid crystal that regulates the flow of light, comprising the image seen by the user. Classical science, before the 20th century, recognized three states of matter: solid, liquid and gas. In 1888, Friedrich Reinitzer discovered liquid crystal, which like plasma, does not fall into any of the conventional states of matter.
The LCD is composed of thin layers, each performing one of the essential functions needed to produce the image. At the back is a light source, known as the backlight. A CCFL LCD panel is backlighted by two cold-cathode florescent lamps sitting at opposite edges of the display. For large panels, there is an array of parallel fluorescent lamps. In an EL-WLED panel, backlighting is via a row of LED lamps. The next layer in both types is a diffuser that, like a sheet of frosted glass, blends the light emitted by the separate lamps so it forms a uniform source. The LED design is superior to the fluorescent backlight and it is more widely used because, without the inverter required for fluorescent lights, it results in a thinner profile. It is also less expensive to manufacture.
Continuing toward the viewer, there is an array comprised of millions of pixels, each made up of red, blue and green subpixels, which in various proportions create an excellent array of colors and shades. These pixels are unlike the pixels in a digital camera, which accumulate and output voltage in proportion to incident light intensity and duration, until the well is full. Pixels in an LCD array transmit varying amounts of light, and this is what the viewer sees. Each pixel has a polarizing glass filter behind and another in front of it. Prior to the first filter, of course, the backlight is unpolarized.
Polarizing filters have opaque parallel lines at sub-wavelength intervals. They were mechanically inscribed on early polarizing filters but are now made by chemical means. Beyond this filter, still with no video content, is now-polarized backlight. (We say no video content, but it should be noted that external circuitry varies the voltage on the lamps so as to darken or lighten the picture as a whole in response to programming changes. This feature is marketed as High Dynamic Range TV, and it also reduces power consumption.)
The two polarized filters are aligned 90° with respect to one another. The net effect of these two filters is to block all light, making for a black pixel. However, this effect is intermittently negated by the presence of the liquid crystal, so that the pixel can transmit white, black and intermediated shades. The action of the liquid crystal now requires an explanation.
A solid typically retains its shape and size except for slight thermal effects. A crystal is generally thought of as a solid having an interior lattice structure that does not change unless fractured by an external force or exposed to high temperature. How, then, is a liquid crystal possible? The fact is that liquid crystals flow like water but their molecules retain a crystal alignment. Many materials exhibit phase changes where they pass in and out of liquid crystal status. These bodies are thermatropic. A common liquid crystal phase is nematic. These materials are composed of rod-like structures. When they are poured or jostled in any way, the rods maintain their directional order. A subgroup is composed of chiral nematic liquid crystals, in which the rods twist or straighten out under certain conditions.
These materials undergo magnetic and electric effects due to the electric nature of the molecules. They are permanent electric dipoles because one end of the molecule has a consistent net positive charge while the other end has a net negative charge. The bottom line is that when small voltages are applied to the liquid crystals between the two polarizing filters that are situated 90° apart in an LCD pixel, the bundles of rods twist or untwist depending upon polarity. This changes the polarization of the light with respect to the second filter. To the viewer the pixels appear alternately white and black, or some shade or hue in between. Because each of the millions of pixels in the array is responding to various electrical signals, the overall picture in a TV or waveform in an oscilloscope is intelligible to the viewer.
To convey signal information to each of the six million pixels, a wire network is embedded in the screen. The fine conductors are arranged and connected so as to power each pixel with the correct signal at the correct time. For this purpose, horizontal wires are deployed on one side of the screen and vertical wires on the other side. Each pixel has a positive connection on one side and a negative connection on the other side. The pixels are powered by LCD drivers located at the panel edges. Smartphones and other mobile devices are constructed in a similar fashion.
One often sees several acronyms used within LCD technology. TFT stands for Thin Film Transistor. This refers to an active matrix configured such that each pixel is attached to a transistor and capacitor individually, allowing the individual control of pixels. TFT has a relatively low production cost and better contrast than traditional LCDs. But TFT LCDs consume more energy than some other LCDs, have shallower viewing angles and may have color reproduction shortcomings.
IPS stands for In-Plane Switching and is an improved version of TFT LCDs. It reproduces color better and provides wider viewing angles by using two transistors for each pixel combined with a more powerful backlight. Consequently, IPS LCDs need more power than other flat display types though generally less than garden variety TFTs.
One also may see the acronym IPS-NEO. This is a proprietary name for a Japan Display Inc. technology claimed to eliminate backlight leakage. It functions basically the same way as other IPS-LCD displays.
OLED stands for Organic Light Emitting Diode. An OLED is an LED characterized by an emissive electroluminescent layer comprised of a thin film of organic material. It emits light when power is applied. The organic layer sits between two electrode layers, one of which must be transparent as in a solar cell.
OLEDs can be used in devices that conventionally are based on liquid crystal cells. In an OLED, a backlight is not required because the cells emit light as in a plasma display, resulting in enhanced dynamics. As opposed to LCD panels, which are back-lit, OLED displays are always off unless the individual pixels are energized. Because the black pixels are off in an OLED display, the contrast ratios are also higher than that available on LCD screens. So OLED displays have much purer blacks and consume less energy when displaying black or darker colors.
AMOLED stands for Active Matrix Organic Light-Emitting Diode. The reproduction of lighter colors on AMOLED screens uses considerably more power than doing so on an LCD. AMOLED displays have a fast refresh rate but don’t have quite as much visibility in direct sunlight as backlit LCDs. Screen burn-in and diode degradation are other factors sometimes mentioned. But AMOLED screens can be thinner than LCDs because they need no separate backlighting and they can also be made flexible.
Another term one may encounter is Super AMOLED. This is what Samsung calls some of its displays used in smartphones. Super AMOLED integrates a touch-response layer into the display itself, rather than as an extra layer on top. Consequently, Super AMOLED displays handle sunlight better than AMOLED displays and also consume less power.
Retina is another term used to refer to displays on Apple phones. It is not a technology but is a marketing term used in reference to display pixel density.
In the plasma display, liquid crystal material is not required because the viewer sees light that originates within the pixels rather than light transmitted through them from LED or fluorescent backlight lamps. The pixels are sealed cells containing noble gases mixed with a small amount of mercury vapor or similar gas. When the signal requires light emission, voltage is applied to a pixel and electrons strike the mercury vapor particles increasing their energy so orbiting electrons migrate to an inner atomic shell. Immediately dropping back, they emit invisible high-energy (ultraviolet) photons. When they strike the phosphor layer, lower-frequency photons are emitted and seen by the viewer.
The conductors that carry TV broadcast information or signals from an oscilloscope processor are located between the glass plates in front of and behind the cells. Because backlighting is not used, the rear plate can be opaque, and the conductors may be thicker than in an LCD panel. Every pixel in a plasma display is made up of three different colored subpixels. The emitted light combines, constituting the many hues seen by the viewer. Brightness is regulated by pulse-width modulation (PWM). The plasma display is a viable technique, but it has been replaced by the less expensive and thinner LCD panel, which is incredibly user-friendly and finds wide application in TVs, computers, oscilloscopes and smartphones, among other instrumentation and consumer devices.
Finally, a new display technology called Micro LED could find use in flat panels. Micro LED displays work like OLED panels but can be thinner. They use gallium-nitrite semiconductor technology. They need no backlighting nor a polarization filter. The glass layer above the panel can be slimmer than in other flat displays now found in smartphones. They produce a higher brightness per watt than that of OLED panels, consuming only half as much energy as an equivalent OLED screen. The small diodes used in the display also yields a high resolution — industry analysts predict a 4K smartwatch would be practical with micro LED displays.