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Reflective Mode
Color reflective STN LCDs use ambient light, which enters through the front (top in the sketch below). The polarizer transmits only light of a certain polarization which is twisted by the liquid crystal material. The mirror shifts the polarization as it reflects the light. The light is twisted again by the liquid crystal on its way back out. The twisting compensates for the reflective shift so that only light that has been twisted twice can pass again through the polarizer. The color filter for a reflective LCD is typically more transmissive and less saturated, since light passes through it twice.
Typical Reflective LCD
Reflective STN LCDs have some differences that do not show in this sketch. Since they are typically for hand held devices, the user will automatically tilt the device to the best viewing angle. So the optical films can trade away wide viewing angle to obtain more contrast. Brightness is not specified for reflective LCDs because their brightness depends on the brightness of the ambient light. Instead, the property of reflectance becomes very important. Kyocera typically achieves reflectance values of 35% Additional retardation film is used to bring in more ambient light from wider angles of incidence. Reflective LCDs can be very thin because there is no backlight. On some products, Kyocera offers frontlights, which again make the LCD thicker. LED frontlights are not as bright as CFL backlights, but they consume much less power.
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