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                                         AUTOMOTIVE INSTRUMENTATION                                        9




                    When current is not       The operation of the LCD in the absence of applied voltage can be
                    being applied to an LCD   understood with reference to Figure 9.17a. Ambient light enters through the
                    display, light entering   front polarizer so that the light entering the front plate is vertically polarized. As
                    the crystal is polarized   it passes through the liquid crystal, the light polarization is changed by the
                    by the front polarizer,   orientation of the molecules. When the light reaches the back of the crystal, its
                    rotated, passed through   polarization has been rotated 90˚ so that it is horizontally polarized. The light is
                    the rear polarizer, and   reflected from the reflector at the rear. It passes back through the liquid crystal
                    then reflected off the   structure, the polarization again being rotated, and passes out of the front
                    reflector.            polarizer. Thus, a viewer sees reflected ambient light.
                    When a voltage is applied   The effect of an applied voltage to the transmission of light through this
                    to a display segment, the   device can be understood from Figure 9.17b. A voltage applied to any of the
                    crystal’s molecules   segments of the display causes the liquid crystal molecules under those
                    change and do not rotate   segments only to be aligned in a straight line rather than twisted. In this case,
                    the polarized light. Since   the light that enters the liquid crystal in the vicinity of the segments passes
                    the light cannot align   through the crystal structure without the polarization being rotated. Since the
                    with the rear polarizer, it   light has been vertically polarized by the front vertical polarizing plate, the
                    is not reflected and the   light is blocked by the horizontal polarizer so it cannot reach the reflector.
                    segment appears dark.  Thus, light that enters the cell in the vicinity of energized segments is not
                                         returned to the front face. These segments will appear dark to the viewer, the



                    Figure 9.17
                    Liquid Crystal
                    Polarization









                                                         FPO





















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