Display technologies

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Raster scan
A raster scan CRT works much like a television set. Light occurs when an electron beam stimulates a phosphor.

Phosphors are inorganic crystalline materials that convert electron energy to light energy. They consist of a host material of high purity and one or more added materials called activators (see "Phosphors"). The electron energy beam is swept horizontally across the phosphor. After each sweep the beam is moved down an increment and swept across the phosphor again. After the bottom line is swept, the beam returns to the top and the sweep process begins again.

Symbols on the display are made by turning the beam on and off as it sweeps over the phosphor. At close range, this can be seen as a series of horizontal lines on a television set. As the electron beam sweeps over the lines, its current is raised or lowered to generate the correct luminance level for that specific part of the picture.

Some CRT raster scan VDTs are television monitors with added electronics. The system to which the VDT is connected provides the signals that would otherwise come from the antenna.

VDTs are not constrained by television standards. They need not operate at the same refresh rate or have the same number of lines or have the lines in the same places. Refresh rates on some raster scan VDTs may be much faster than on television monitors. Resolution also may be different. A CRT-type VDT need not use the same number of sweep lines, nor do the sweep lines have to fall in the same pattern as on a television set. The refresh technique may also be different.

Storage tube CRT
This CRT has two electron guns within it: a writing gun and a flooding gun. The writing gun charges the areas of the image that are luminous. The flooding gun then provides a flow of low-energy electrons over the whole storage surface. There is a secondary emission of electrons in those areas that were previously charged by the writing gun. That secondary emission causes the phosphor to glow in those areas.

Plasma panels
Plasma panels are two edge-sealed layers of glass separated by a thin layer of neon-based gas. There are horizontal conductors (wires) on one glass layer and vertical conductors on the other. When a voltage is applied across the intersection of a horizontal and vertical pair of conductors, the gas at the intersection becomes ionized and emits a spot of light. All the conductors may have a sustaining voltage applied to them that keeps the spot illuminated after the higher voltage causes the spot to glow. The sustaining voltage may have a high frequency, refreshing the dot several thousand times a second.

Liquid crystal displays
A liquid crystal is an organic compound in a transition state between solid and liquid forms that resembles a liquid in viscosity and a crystal optically and electronically. The most important optoelectronic type is the nematic system.

The nematic system consists of structured, molecular units that are commonly visualized as rods (having a long dimension several times the length of the smaller dimension). The direction along which rods align is called the director. The optical and electrical properties of the material differ in the plane along the director and in the plane perpendicular to it.

The molecules interact with the surface that contains the liquid. In a panel structure, the surfaces are prepared so the directors align in a single direction in the plane of each surface. In a typical design, the director is aligned right to left at the back plate and up down at the front plate.

Light is polarized with a film having fixed rod-like molecules and introduced into the back plate. The film polarizes the light in alignment with the back plate director. The director rotates through the cell, rotating the polarization 90 degrees. An analyzer polarizing film, aligned with the direction of the front plate director is placed between the viewer and the front plate. The light freely passes through this aligned film. Zero voltage gives a bright appearance.

The director in the liquid crystal is subject to a force proportional to the square of the applied voltage. When a voltage is applied, the directors rotate into a position perpendicular to the surfaces. In this situation, the light passes through the material with no effect on the polarization. Since the rear polarizer and the front analyzer are crossed (at ninety degrees), the light is blocked.

Two patterns of transparent conductors cross to form individual picture elements. The effect can occur at each intersection. This permits high content displays to be made.

A display unit based on this effect is called a Liquid Crystal Display. These displays can operate by reflecting room light, modulating a rear mounted light or in combination.

The ergonomic requirements and recommendations for this class of displays is covered extensively in the ISO Standard 13406 - 2: Ergonomic requirements for work with visual displays based on flat panels. Part 2 deals with the ergonomic requirements for flat panel displays.

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