CRT versus LCD Technology

A monochrome CRT is the simplest kind. A
heater is used to liberate electrons from a cathode tube. These negatively charged
electrons are attracted towards the positively charged anode which is near the front of
the CRT. The stream of electrons is focused into a fine beam by extra elements near the
cathode in an assembly known as the ‘gun’. The number of electrons, and hence
the brightness of the image is controlled by a voltage applied to the grid.

When the electrons get to the front of the
CRT, they trigger an impact with a special coating of phosphor and this collision results
in the release of light. The color of this light depends on the formulation of phosphor
used.
The beam scans from left to right and from top to bottom creating a grid or
‘raster’. An image is formed by increasing or decreasing the brightness of the
points on the raster. Where the brightness is switched on or off a dot is formed. This is
called a pixel or a picture element.

CRT
There are two major color CRT technologies, Shadow Mask and Aperture Grille. Color CRT
technology is based on using small areas of phosphors that emit these basic colors. Three
separate beams of electrons are used to make the respective phosphors glow.
Shadow Mask CRT: The most widely available technology is called ‘shadow mask’. A
perforated sheet of metal is positioned just behind the glass at the front of the monitor.
This masks the three separate beams which are each controlled by their own gun. The
masking effect refers to the phenomenon where each beam converges only on the appropriate
color of phosphor.

Originally shadow mask CRTs had a curved
appearance. This allowed better focus and reduced the negative effects caused by heating.
Now, the vast majority of specialist and ‘add-on’ monitors are of the Flatter
Square Tube (FST) type.

Croma clear is a type of shadow mask which
uses slots rather than round holes in the mask. The advantages claimed for shadow-mask
monitors include:

  • Clearer character formation.
  • More ‘realistic’ and accurate
    color rendition.
  • Better cost or performance.
  • No horizontal restraining wires.

Aperture Grille: In 1968, Sony introduced a
new technology for CRTs, the Trinitron. The most obvious difference from shadow mask
technology is at the front of the CRT, where instead of using a sheet of metal for the
shadow mask, vertical wires—the aperture grille—held under tension are used. The
phosphor is coated in strips rather than in dots. Aperture grille CRTs claim a number of
advantages.

  • More of the energy in the beam reaches the
    phosphor and less is turned into heat.
  • More light output for a given beam size.
  • A darker tint of glass can be used for more
    screen contrast.
  • Cylindrical face of CRT reduces reflections.

CRT monitor design is a mature technology
that brings unique properties of economy, flexible format and excellent visual
performance.

The key technology that is likely to be
used in the near future is Liquid Crystal Display (LCD) technology, which is already known
owing to its use in notebook displays. Color Plasma Display Panels (PDPs) may also be come
popular in the 21" to 25" size range.

LCD
LCDs rely on the special properties of a group of chemicals called liquid crystals. The
twist of the molecules changes the polarization of transmitted light. The angle of the
change can be controlled by subjecting the crystal to an electric field. These properties
have been used to develop displays that employ crystals to control the amount of light
that is passed through the display.

When no field is applied to the LCD, the
polarization of the light is changed as it passes through the LC material. The light then
meets a second polarizing filter that is at right angles to the first and light is
transmitted. If a field is applied to the crystal, the angle of twist changes and only a
portion of light is transmitted. In this way the brightness level is controlled, the gray
scale necessary for high quality images is made available.

Passive Matrix LCD: The simplest
form of LCD addressing is passive matrix addressing. Transparent conductive lines for the
rows and columns are applied to the glass above and below the liquid crystal material.

When a voltage is applied between the two
points, the crystal re-aligns, changing the light transmission. In order to set different
brightness levels for individual pixels, rows are set sequentially. When a row is
selected, the appropriate voltages are fed to individual column driver circuits. Current
flows through the column lines to the selected row and the LC materials align accordingly.
The drive circuits then move to the next row and repeat the operation. When the scanned
row reaches the bottom of the display, it starts again at the top of the display.

Active Matrix LCD: Active Matrix
LCDs use an electronic switch at every pixel position so that once a pixel is switched on,
the field can be maintained by the switch. This allows fast LC material to be used, so
that smearing is no longer a problem. The switch, which is usually a Thin Film Transistor
(TFT) also isolates the pixel from the influence of adjacent pixels and eliminates cross
talk.

Courtesy: The Computing Suppliers Federation

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