Hardware Reference
In-Depth Information
signals, depending on how you have set up the power management settings.
Table 12.22
shows the various signal states and relative power consumption according to the DPMS
state selected. Normally the monitor is placed in Suspend mode after a period of inactivity specified
in the OS power management settings.
Table 12.22. Display Power Management Signaling States
Virtually all CRT monitors with power management features meet the requirements of the United
States EPA's Energy Star labeling program, which requires that monitor power usage be reduced
from up to 100 watts or more (when operating normally) to 15 watts or less in standby mode. LCD
monitors comply with the far more stringent Energy 2000 (E2000) standard developed in
Switzerland. E2000 requires that monitors use less than 5 watts when in standby mode. Note that
LCD displays generally use one-third less power than CRTs in either operating or standby mode, and
LED displays use about one-third the power of LCD displays.
LCD Technology
Because of their light weight, smaller overall size, and much greater clarity, LCD panels have
replaced CRT displays in all new computer installations. Desktop LCD panels use the same
technology that first appeared in laptop computers. Compared to CRTs, LCDs have completely flat,
thin screens and low power requirements (30 watts versus up to 100 watts or more for CRT
monitors). The color quality of a good active-matrix LCD panel can exceed that of many CRT
displays, particularly when viewed from head on.
How LCD Panels Work
In an LCD, polarizing filters allow only light waves that are aligned with the filter to pass through.
After passing through one polarizing filter, all the light waves are aligned in the same direction.
A second polarizing filter aligned at a right angle to the first blocks all those waves. By changing the
angle of the second polarizing filter, you can change the amount of light allowed to pass accordingly.
It is the role of the liquid crystal cell to act as a polarizing filter that can change the angle of
polarization and control the amount of light that passes. The liquid crystals are tiny rod-shaped
molecules that flow like a liquid. They enable light to pass straight through, but an electrical charge
alters their orientation, which subsequently alters the orientation of light passing through them. In a
color LCD, there are three cells for each pixel—one each for displaying red, green, and blue—with a
corresponding transistor for each cell. The red, green, and blue cells that make up a pixel are
sometimes referred to as
subpixels
. See
Figure 12.23
.
