the display, since the amount of power required to turn on each digit segment is
negligible. Backlit LCDs are typically found in automobile dashboards, where their
power usage is not a particular concern and where readability at night is important.
A reflective LCD, such as the one in the watch above, consumes less power. We show
how a reflective LCD works in Concept 5; it is a bit more complicated than a backlit
LCD. A mirror that can reflect ambient light coming from in front of the LCD replaces the
backlight behind the polarizer. As with the backlit display, the transmission axes of the
polarizer and analyzer are at right angles to each other.Concept 5 shows what happens when a liquid crystal digit segment in this type of LCD
is turned off. In this state, light passes through each of the analyzer, the liquid crystal
segment, and the polarizer twice. Unpolarized light enters the display from the right and
becomes horizontally polarized as it passes through the right-hand filter (the analyzer).
It rotates 90° to a vertical orientation as it passes through the optically active liquid
crystal component. In this orientation it can pass through the polarizer, reflect off the
mirror, and come back out through the polarizer without hindrance. (A keen-eyed
observer will note that the angular symmetry of reflection is apparently being violated
where the light strikes the mirror: We drew the incident ray at a different angle in order
to fit all the details into the diagram.)
After passing through the polarizer, the light retraces its path through the liquid crystal,
again rotating and again becoming horizontally polarized. It passes back out through
the analyzer without absorption, creating a region that blends into the background color
of the display.
When the segment is turned on, it becomes optically inactive; optically, it can be treated
as if it were no longer there. Light entering from the right gets horizontally polarized by
the analyzer, and propagates to the left until it strikes the polarizer, where it is
absorbed. This light never even reaches the mirror, and the segment appears dark. You
see this happening in the watch above, where the dark segments spell out the time of
day.
More elaborate LCDs of both types can be manufactured with “segments” having any
shape, not just the parts of digits. For example, the battery and signal strength icons on
a cellular telephone display, or the letters “H” and “M” on the watch above, use specially
shaped segments. Some flat panel display screens use an array of thousands or even
millions of tiny LCD dot-segments to produce virtually any image. With the appropriate
refinements, color images can be produced.
As with polarized light from the sky, you can use a pair of Polaroid sunglasses to
experiment with the polarized light emitted by a liquid crystal display. For example, you
will find that a watch, viewed through the glasses, looks quite different depending on its
orientation. At some angles the display looks more or less normal, while at others it
becomes completely unreadable.Power ON
Liquid crystal is optically INACTIVE
Does not rotate polarized lightBacklit LCD íON
Analyzer absorbs unrotated light
·Segment appears blackBacklit LCD - OFF
Analyzer admits rotated light
·Segment blends into backgroundReflective LCD
When power OFF, segment is not seen
When power ON, segment is black(^568) Copyright 2000-2007 Kinetic Books Co. Chapter 30