Computer displays: points of light

June 1/8, 2005
All displays form images from pixels, or spots of light that are small enough to blend together. Displays also must have some means of controlling the color and brightness of individual pixels. Different types of displays use different means to produce and control pixels.

Each pixel of the boxy cathode ray tube (CRT) display is produced by a set of three colored phosphor dots painted on the inside of the screen. The phosphor dots glow when hit by a beam of electrons. An electron beam sweeps across the inside of a cathode ray tube to illuminate the correct red, blue and green combinations at the right moment to form an image.

The liquid crystal displays (LCDs) used in many laptop and thin desktop monitors generate pixels using tiny containers of liquid crystal that can be oriented electrically to allow light through. Each liquid crystal pixel is switched by an electric circuit, which is considerably more accurate than controlling phosphor glow using electron beams.

The plasma displays used in some large format computer monitors and televisions generate pixels using tiny containers filled with inert gases that form a plasma when electrified. The plasma emits ultraviolet light that excites phosphors like those used in cathode ray tubes.

Using mechanics

Another way to produce images is by manipulating light mechanically rather than electronically.

Micro-opto-electro-mechanical systems, or MOEMS, generate pixels by moving microscopic machine parts like mirrors, shutters or strips of metal in order to transmit, block or reflect light.

Micromirror displays use computer-controlled mirrors to reflect pixels, and are found in some of today’s large screen televisions and digital film projectors. Each mirror is as small as a few tens of microns across. This type of display uses a chip that contains thousands of micromirrors that can be individually tilted to reflect light into or away from a magnifying lens.

Micromechanical diffraction gratings generate pixels using a set of six microscopic aluminum-coated semiconductor ribbons suspended above a surface. Applying a voltage to a ribbon causes it to bend down toward the surface. When all six ribbons are unbent the device acts as a mirror, and when every other ribbon is bent it diffracts, or bends, light at precise angles. Reflected light is blocked and diffracted light is directed to a display screen.

A third type of micromechanical display uses a microscopic version of the age-old shutter to generate pixels. Arrays of shutters built into computer chips can be opened and closed thousands of times a second to control the amount of light that passes through the chip any given point.

The key to micromachine displays is timing. In all of these technologies, using a white light source and switching each pixel on and off at the right time produces black and white images. Video images are usually switched 24 or 30 times per second. The displays can produce grayscale images by switching more quickly. A pixel that is on only some of the time during a single video frame will appear gray. Devices that switch 240 times a second can produce eight-level grayscale images at 30 frames per second.

Color images can be produced by increasing the switching speed threefold and cycling the light source through red, green and blue. The method replicates the effect of the separate red, green and blue dots of cathode ray tube and plasma displays.

A fourth type of micromechanical display controls precise gaps between pairs mirrored surfaces to allow light of only a certain color to be reflected.

The gaps correspond to the wavelengths of the three colors the device allows through: red is about 700 nanometers, green 500 nanometers and blue 400 nanometers. A nanometer is one thousandth of a micron. When a gap is closed the device absorbs light and that area of the screen appears black. When the gap is open light of the appropriate color is reinforced and reflected. The surface of the device is coated with an iridescent material that makes the reflected color appear saturated and print-like.

Electronic ink

Making computer screens as thin as paper means working without a built-in light source. Instead of transmitting light through the screen, electronic paper reflects ambient light like ordinary paper. The lack of a light source makes it possible for electronic paper to be flexible and use little power.

The key to electronic paper is ink that reflects light and can be manipulated using electricity. Electronic ink consists of tiny amounts of substances sandwiched between thin layers of flexible material.

One prototype electronic ink consists of capsules that each contain black and white particles of pigment. A negative voltage causes the white particles in each capsule to move to the surface; a positive voltage causes the black particles to come to the surface.

The ink provides a contrast ratio of around 15, which rivals that of paper, and a reflectivity of around 40 percent, which is brighter than many screens. It takes about a quarter of a second to refresh a page. The capsules measure as small as tens of microns across, or a few times larger than a red blood cell.

A second, very different scheme uses a sandwich of insulator, colored oil and water to produce a pixel. Because the insulator is hydrophobic, the oil resides between the insulator and water. When electric current is applied to a portion of the insulator, however, that portion of the surface becomes hydrophilic, and as water is drawn to the surface the colored oil is shunted to one corner, exposing the white background.

The method also has a contrast ratio of 15, and a 13 millisecond refresh rate, which is fast enough to display high-quality video.

The system generates color images using pixels generated by a set three sandwiches that each contain two layers of oil and a color filter, and employ the cyan, magenta and yellow subtractive color scheme used in printing. Displays usually mix red, green and blue. Having each pixel area able to represent any of the three primary colors boosts the second ink scheme’s reflectivity to 67 percent, compared to 40 percent for standard liquid crystal displays.

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