VIEWING MODE describes how a LCD manipulates polarized light to create an image. In order to select the proper viewing mode it is necessary to consider the LCD's typical ambient lighting conditions.
Typical
for indoor application. The backlight has to be always on
REFLECTIVE MODE
LCD's employing the reflective viewing mode produce dark characters on a light background. (Positive Image)
When a segment is turned off, ambient light travels through the following
layers:
1. The vertically aligned front polarizer located on the
face of the display.
2. The indium tin oxide segment pattern on the rear of the
front glass substrate.
3. Polyimide layer (alignment layer)
4. The liquid crystal material where it is twisted 90º
5. Polyimide layer (alignment layer)
6. The ITO defined commons on the front of the rear glass
substrate.
7. The horizontally aligned rear polarizer behind the rear
glass substrate.
8. And finally onto a reflector that returns it along the same path.
(See diagram E)
A positive image display has front and rear polarizers that are said to
be "OUT OF PHASE" with one another, or "crossed polarized" by 90º. When a segment is TURNED ON, the LC molecules won't twist polarized light. Therefore, the light is OUT
OF PHASE when it reaches the rear polarizer preventing it from passing through to the reflector. Because the light is not reflected, a dark or positive image is created for that particular segment.
Transflective displays are configured with a white or silver translucent material, which reflects a certain percentage of ambient light, but also permits the transmission of light emitted by a backlight. Since transflective mode displays both reflect and transmit light (as a function of the transflector/backlight assembly), they are useful when embedded in applications with varied lighting condition; aviation, automotive, petroleum pumps and notebook computers. A transflective display would have good visibility in direct sunlight with the added benefit of good visibility at night by virtue of its backlight.
Transflective displays do lose a slight amount of contrast when compared
to reflective mode displays because light will pass through.
Transmissive displays do not reflect light. Instead, they create images by modulating light from an artificial source as the light passes through the back of the display to the viewer. Backlit transmissive displays are configured with polarizers that are "IN PHASE" with one another. With a segment
turned off, the polarized light is twisted 90B by the liquid crystal molecules
and is "out of phase" when it reaches the front polarizer. The
front polarizer blocks the light resulting in a dark segment. With
the segment turned on, the light will not be twisted by the liquid crystal
material, so it will be "IN PHASE" with the front polarizer
and will pass through it, producing a light image on a dark background.
This is a negative image. Such displays must be backlit to insure visibility,
especially in low ambient light conditions. Transmissive negative mode
LCD's do not perform well in direct sunlight - even with a backlight since
artificial lighting cannot compete with direct sunlight.
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