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Reflective and transflective mode display

sentiment:2563 Published time :2017-11-15 22:53

For outdoor applications, a reflective or transflective LCD is preferred because of their energy efficiency and quite good sunlight readability.

Reflective mode LCD

Some displays only use under ambient light rather than a backlight, this mode of operation is called reflective. In reflective mode, a mirror is installed behind the liquid crystal layer, either inside the LCD cell or on the rear polarizer. Ambient light passes through the LCD cell from the front side and is reflected by the mirror back to the viewer. The advantage is lower power consumption and excellent visibility in direct sunlight, making such displays excellent solutions for outdoor daytime applications. To be visible at night or in dark settings, reflective LCDs require additional lighting.

1). The relationship between reflector and display performance

To make the ambient light reflect on the LCD panel, a reflective plate is needed in the LCD structure. The simplest structure is that the reflector is attached to the outside of the LCD liquid crystal cell, but when the pixel pitch is small, the emitted light interferes with adjacent pixels, and a double image will be observed. Inserting a mirror reflector into the liquid crystal cell can solve the double image problem, but the image of the light source or the reflected image of the observer can be observed from the display screen. The regular uneven surface transforms the incident light into a diffused light structure. In this structure, the regular irregularities on the surface cause the incident white light to produce a dispersion effect. When the reflector is formed with an irregular uneven surface structure, the wavelength dependence of the reflected light disappears. When incident light hits the irregular reflector, the reflected light includes normal reflected light and diffused light. The ratio of the diffused light of the normal reflected light has a greater impact on the reflected display characteristics of the product. The above is for the case where the incident light is parallel light. If the incident light is non-parallel light, in order to make the reflective LCD display a good image, design and control the surface of the reflector according to the use purpose and the size of the screen. For example, the incident light indoors is composed of parallel light directly illuminated by a fluorescent lamp and diffused light formed after reflection on the wall, and the ratio of parallel light to diffused light is 1:1.

2). Color display

The reflective LCD of the color mode has the following two structures, the color film structure and the laminated structure. The structure of the color film is the same as that of the transmissive LCD, but is different in color purity and brightness control. In the transmissive LCD, the color purity is determined by the color film and the backlight. The brightness can be improved by increasing the intensity of the backlight. However, in the reflective LCD, the light depends on the ambient light and passes through the color film twice, so it is necessary to design Dedicated color film.

Transflective mode LCD

Transflective LCD displays have both transmissive and reflective characteristics. They contain an integrated backlight unit and a semi-transparent reflector or a reflector with a hole for each pixel. Again, the reflector can be behind the rear polarizer or inside the LCD cell behind the liquid crystal layer. Light from the backlight can pass the semi-transparent reflector and operate the display in the transmissive mode. At the same time, ambient light can be reflected so that the display is visible in direct sunlight as well. Care must be taken to account for the fact that in the transmissive mode of operation the light passes the liquid crystal layer once, while in the reflective mode it passes the liquid crystal layer twice. The appearance of transflective displays is a compromise. It is the most flexible solution as it allows for lower power consumption in bright environments and readability in any lighting condition. This comes at the expense of top performance in the pure illumination modes and sometimes significant additional manufacturing cost.

Reflective LCD relies on ambient light, so the most fatal weakness is that it cannot be used in a dark environment. Transflective LCD is referred to as TR-LCD, which is a combination of transmission and reflection. In the TR-LCD structure, since the reflected light passes through the liquid crystal layer twice, the optical path difference between the reflected light and the transmitted light area is twice.

The disadvantage of TR-LCD is that when the surrounding ambient light is not too dark, when you turn on the backlight, you can see the images in the transmissive area and the reflective area at the same time, which reduces the sharpness of the image quality. In addition, in the reflection area, the light passes through the color film layer twice, and the images in the reflection area and the transmission area have color deviations. Aiming at the problem of color shift, Sharp has developed a TR-LCD structure in which the color film thickness of the reflective area is reduced by 1/2.

The manufacturing process of producing different thicknesses of reflective and transmissive regions in the liquid crystal cell is relatively complicated, and the driving characteristics of the transmissive and reflective regions are different, which brings many difficulties in actual manufacturing and application.