Page 785 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 2 3. 1 Automotive instrumentation and telematics

they gradually rotate through the liquid crystal structure light that enters the cell in the vicinity of energized
until the molecules at the back are horizontally polarized. segments is not returned to the front face. These seg-

Thus, the molecules of the liquid crystal rotate 90 from ments will appear dark to the viewer, the surrounding
the front plate to the back plate so that their polarization area will be light, and the segments will be visible in the
matches that of the front and back polarizers with no presence of ambient light.
voltage applied. The LCD is an excellent display device because of its
The operation of the LCD in the absence of applied low power requirement and relatively low cost. How-
voltage can be understood with reference to Fig. 23.1-17a. ever, a big disadvantage of the LCD for automotive ap-
Ambient light enters through the front polarizer so that plication is the need for an external light source for
the light entering the front plate is vertically polarized. viewing in the dark. Its characteristic is just the opposite
As it passes through the liquid crystal, the light polari- of the LED; that is, the LCD is readable in the daytime,
zation is changed by the orientation of the molecules. but not at night. For night driving, the display must be
When the light reaches the back of the crystal, its po- illuminated by small lamps inside the display. Another

larization has been rotated 90 so that it is horizontally disadvantage is that the display does not work well at the
polarized. The light is reflected from the reflector at the low temperatures that are encountered during winter
rear. It passes back through the liquid crystal structure, driving in some areas. These characteristics of the LCD
the polarization again being rotated, and passes out of have limited its use in automotive instrumentation.
the front polarizer. Thus, a viewer sees reflected ambi-
ent light. 23.1.10.1 Transmissive LCD
The effect of an applied voltage to the transmission of
light through this device can be understood from An LCD display can also function as an optical trans-
Fig. 23.1-17b. A voltage applied to any of the segments of mission device from a light source at the rear of the
the display causes the liquid crystal molecules under structure to the front face. A configuration such as this
those segments only to be aligned in a straight line rather permits an LCD to display messages in low ambient light
than twisted. In this case, the light that enters the liquid conditions (e.g., nighttime). In low ambient light condi-
crystal in the vicinity of the segments passes through the tions, a reflective LCD does not provide a visible display
crystal structure without the polarization being rotated. to the driver. The intensity of the back light is auto-
Since the light has been vertically polarized by the front matically adjusted to produce optimum illumination as
vertical polarizing plate, the light is blocked by the a function of the signal from an ambient light-even sensor
horizontal polarizer so it cannot reach the reflector. Thus, located inside the passenger compartment.

Fig. 23.1-17 Liquid crystal polarization.

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