Active Matrix Liquid Crystal Displays
A definitive guide to the theory and applications of AMLCDs, this book contains numerous design and applications examples to help illustrate key points and make them relevant to real-world engineering tasks.
TABLE OF CONTENTS Active Matrix Liquid Crystal Displays
6.1: Brightness Improvements
6.1 Brightness Improvements
The brightness of transmissive LCDs depends to a large extent on the backlight intensity. The most straightforward way to increase display luminance is therefore to turn up the backlight or to design brighter backlights. The display luminance is then proportional to the backlight intensity and can reach more than 1000 cd/m 2. However, there are drawbacks to raising backlight luminance. First, it increases power consumption, which is undesirable (particularly for portable applications). Second, it may require a different construction of the backlight (for example, multiple lamps rather than single lamp edge lighting) and can also have a negative impact on backlight lifetime. Third, high backlight intensity can cause heating of the display beyond 40 50 C, where display image quality may start deteriorating. Alternatives to raising the backlight intensity are addressed in the following subsections.
6.1.1 Increased Color Filter Transmission
Besides the backlight, the components of the assembly determine the final display luminance, as shown in the previous chapter in Fig. 5.5. The first polarizer transmits about 42% of the light. The black matrix opening on each pixel (the aperture ratio) is typically 40 70% and the color filters transmit about 25% of white light (averaged over the red, green, and blue filter). Other layers and the exit polarizer further reduce luminance so that total transmittance is only 5 10%. It is difficult to increase the transmittance of the polarizers or color filters without affecting contrast ratio or color saturation, respectively. There is a trade-off between color gamut and brightness.
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