Liquid crystals are the granddaddy of what we think of as "displays"—before their advent in wristwatches and calculators we had cathode ray tubes and other "monitor"-like tech. Now clever engineering has become ubiquitous, overcoming many of display tech's earlier problems (easily damaged, very poor viewing angles), particularly with the "in-plane switching" tech used in the LCD displays on Apple's iPad.

Its history stretches back to the 1888 discovery of the "liquid crystalline" nature of cholesterol molecules by Friedrich Reinitzer, through pioneering work by Dr. George Gray at the Royal Radar Establishment in Malvern (whose successor organization QinetiQ held the patent for many years) in the 1960s, to the first viable displays in the early 1970s.

Now pretty much every laptop you see has an LCD screen, as do the majority of HDTVs, cellphones and the next-big-thing tablet PCs

Image from wikimedia user plavius

LCDs work by a trick of optics—polarization. A white light source is passed through a polarizer that selects only light waves that oscillate in a particular direction. This light then shines through a transparent electrode and a layer supportive glass or plastic into a gooey layer of liquid crystals. In their natural "twisty" state, the crystals spin the polarization of the light as it passes through them so it can exit through the cross-polarized layer at the top, then the structural glass and to your eye.

When a voltage is applied through the electrodes above and below a particular pixel, the liquid crystal molecules untwist, and thus don't rotate the polarization of the light, which then gets blocked at the top polarizer, and the pixel is black. Color displays have a final layer of red, green and blue optical filters, and creating a full-color pixel is just a matter of shining the right combination of brightnesses through these colored windows.

Electronic paper is the enabling technology behind the current craze for digital book e-readers—without its "paper-like" optical qualities and extremely low power consumption, e-readers wouldn't be quite so attractive.

The first electronic paper was dreamed up in Xerox's Palo Alto Research Center, where Nick Sheridon created a rotating array of "janus particles" (one face white, one black) out of polyethylene spheres. This concept was developed into newer electrophoretic displays, as technology of making micro-encapsulating spheres was improved. A version of this electrophoretic tech is used by maker E-Ink, behind the biggest selling range of e-reader devices: The Amazon Kindle.

Those of you who're fans of Rube Goldberg will be definitely interested in E-Ink--it's such a pleasantly "physical" system.

Tiny microcapsules the size of a human hair and containing a transparent oil are trapped between an array of transparent electrodes, a reflecting and structural rear surface and a transparent front surface which may be plastic or glass

Inside the oil, tiny particles of white pigment and black pigment float around—the black ones carrying a negative electrical charge, and the white ones a positive one. In their randomized state, the resulting sphere appears grey.

To change the state of a pixel, a tiny voltage is applied across it, and the pigment particles inside the microcapsules in that pixel move through the oil and arrange themselves accordingly—white ones repelled by the positive electrode. If the white ones are at the top, they "hide" the black particles so the pixel seems white, and vice-versa.

The voltage only needs to be applied to change the state of the pixel, giving e-ink its low power consumption, and natural or artificial light shone onto the screen is all you need to see it.

OLED may or may not be the next big thing in display tech. Its super-brightness and extremely high contrast make it excellent for vibrant displays such as you may want on a tablet, TV, or smartphone screen. It has its particular setbacks, such as complexity of production, potentially high cost, and poor direct-sunlight visability, but that hasn't stopped companies like Samsung from trumpeting their "super oled" tech in their phones as out-performing any rival screen.

The origins of the tech go back to the earliest observations of electroluminescence in organic materials in the 1950s. Pioneering work into the science happened in Martin Pope's group in New York University in the 1960s, and thin-film electroluminescence was patented after a discovery in the UK in 1975. Polymer-based systems were given a huge boost in 1990 with a discovery at Cambridge University.

A "normal" light-emitting diode is based on inorganic semiconductors like silicon and gallium which are arranged in complex layers much like a silicon chip is. A physical trick of the structure turns electrical energy into light of a particular color (dependent on the materials chosen), with very high efficiency. Red LEDs came first, and blue ones only recently, due to the complexities of materials science

Organic LEDs work using very similar principles: A complex layered structure of materials, this time containing organic molecules based on carbon, is layered between transparent electrodes. When a voltage is applied, the same conversion of electrical energy into light occurs, with a color chosen by the characteristics of the materials.

OLED pixels are self-illuminating, so you don't need a backlight, and thus devices using the display can be very thin. The power consumption of a whole display also depends on how many pixels are "lit" at any one time, so it's variable. But the entire structure can be mounted on plastic and hence can be made into rollable, foldable screens—enabling all sorts of potential designs for portable devices to be realized.

Pixel QI just may be the display tech that takes the e-reader into the next-generation and could transform the current crop of tablet PCs into true jack-of-all-trades. This is because its a dual-mode tech: With backlight off, it's very daylight-readable, and has "e-paper-like" qualities. With backlight on, it's like a normal LCD and can cope with full color rendering and fast-moving graphics or videos.

The tech has origins in the early days of the One Laptop Per Child program, and a disagreement about how the design of the computer should proceed caused the formation of E-ink and rival Pixel Qi. Until recently, however, E-ink looks to have been the winner of this spat, since no big-scale roll-out of PQI screens has occurred. That may be about to change, and 7-inch Pixel Qi screens may be appearing on next year's crop of tablet PCs.

A Pixel Qi screen is a true hybrid of design, the product of some lateral thinking and advances in screen production technology that make its intricate structure possible.

Each pixel in a Qi screen has two main parts--both based on LCD systems. One part is exactly how a "normal" LCD works, and is how the screen displays when the backlight is on. But other segment is backed by a silvered mirror surface--and has a clear window on the face the user can see. Sunlight enters here, passes through the LCD layers, bounces off the mirror and back to the viewer--adjustments to the pixel made by varying the electrode voltages create a sunlight-viewable grey display when the backlight is off.

Hence, you have the best of both worlds married together in one very complex layered device.