light. The light passing through the first filter results in
polarized light. Polarized light consists of light waves all
the same shape and of a single frequency rather than the
entire spectrum of light frequencies generated by the
backlight. The light passing through the first filter con-
sists only of vertical waves.
A liquid crystal sits between the two filters. When a
voltage is applied to the liquid crystal, the molecules in
the crystal rotate from the vertical position. When the ver-
tical light passes through the energized liquid crystal, it
too rotates, changing into a horizontal light wave that is
blocked by the second filter. The second filter allows only
vertical waves to pass through. The amount of voltage
applied to the crystal determines the amount of rotation
from the vertical position to a horizontal position. The
more voltage applied, the less light that will pass through
the second filter.
Passive-matrix display
There are two types of electrical circuitry used to
energize the crystal area, active and passive, Figure 23-45.
In a passive-matrix display, a grid of semitransparent con-
ductors run to each crystal used as part of the individual
pixel area. The grid is divided into two major circuits,
columns and rows. Transistors running along the top and
the side of the display unit head the columns and rows.
A ground applied to a row and a charge applied to a
column activates a pixel area. The voltage is applied
briefly and must rely on screen persistence and a fast
refresh rate. Because current must travel along the row and
column until it arrives at the designated pixel, response
time is slow.
Active-matrix display
In an active-matrix display, each individual pixel in
the grid has its own individual transistor. The active-
matrix provides a better image than the passive-matrix.
The active-matrix image is brighter because each cell can
have a constant supply of voltage.
The most common active-matrix display is the thin
film transistor liquid crystal display (TFT-LCD).
Often, this type of display is referred to simply as a TFT
display. The TFT display consists of a matrix of thin film
transistors spread across the entire screen. Each transistor
controls a single pixel on the display. There are over
one million transistors in a display, three transistors at
each pixel area, and one transistor for each color pixel,
Figure 23-46. The liquid crystals in the TFT display are
energized in a pattern representing the data to be displayed.
The conventional television has used the CRT to
display images because the original LCD design had
limitations that could not compete with larger display
units. As the size of the display unit grew to over
18 inches, problems developed with the brightness of the
display and in converting the analog television signal to a
digital signal and to a wide-angle viewing area without
image distortions. These problems were solved with the
introduction of thin film transistor LCD technology.
414 Electronic Communication and Data Systems
Transistor
Screen area
grids
Passive-Matrix Active-Matrix
Pixels are activated at intersections
Figure 23-45. In an active-matrix display, each individual cell in the grid has its own individual transistor. The active-
matrix provides a better image than does the passive.