The physical downsizing of microprocessor-based technologies has led to the development of portable personal computers, pocket secretaries, wireless phones, and pagers. All of these devices, and other devices such as clocks, watches, calculators, etc., have the common need for a data display screen having low power consumption in order to extend the useful working period between battery replacements or battery charging.
The most common type of display in such devices is the liquid crystal display (LCD). LCDs may be classified based upon the source of illumination. Reflective displays are illuminated by ambient light entering the display from the front. A reflective surface, such as a brushed metal reflector placed behind the LCD returns light that has been transmitted through the LCD while preserving the polarization orientation of the light incident on the reflective surface. Although reflective displays meet the need for low power consumption, the displays are only useful under good ambient lighting conditions. Under low levels of ambient light, the display often appears dark and is difficult to read. Therefore, the purely reflective display has limited utility.
Another type of LCD display is a backlit display, in which light is generated behind the display and is transmitted through the display to the viewer. Typically, the backlight assembly includes a light source, such as a light-emitting diode (LED), a fluorescent lamp, or other device that emits light, and some optical elements to direct the light from the light emitter to the LCD. Backlighting may also be used to supplement a reflective display so that it can be used over a wide range of ambient light conditions. However, the introduction of a backlit assembly increases the power drain on the battery, significantly reducing the useful battery life, or the time between battery chargings.
The combination of backlighting with an ambient reflective display introduces the need for a “transflective” film. The transflective film is placed between the LCD and the light source, and is used for reflecting ambient light that has been transmitted through the LCD and transmitting light from the light source to illuminate the LCD.