Sharp’s LCD panel viewable in sunlight
When used outdoors, the LCD panel of a smartphone reflects sunlight, making it very difficult to view the screen. Sharp Corp has developed a display technology to solve this problem.
For preventing the reflection of natural light such as sunlight, it is effective to attach a low-reflection film called AR film to the surface of an LCD panel. An AR film has a structure consisting of multiple layers and reduces light reflection by interference. By attaching an AR film to an LCD panel, the surface reflectance of the panel decreases from 4.0% to 0.2%, and the reflectance of the entire panel decreases from 5.7% to 1.9%, according to Sharp.
As a result, the LCD panel’s contrast ratio in bright light also improves. However, with a brightness of 20,000lx (outdoors), the contrast of an LCD panel whose white light has a brightness of 400cd/m2 increases only from 3:1 to 8:1, Sharp said.
Therefore, considering that it is necessary to reduce not only the surface reflection but also internal reflection of an LCD panel, Sharp developed a low-reflection LCD panel having a new structure. According to the company’s calculation, under the abovementioned conditions, the internal reflectance is lowered from 1.7% to 0.1%, and the reflectance of the entire LCD panel decreases to 0.3%, realizing a contrast ratio of 42:1 in bright light.
To reduce the internal reflection of the LCD panel, Sharp decided to attach a circular polarizing plate, in place of a linear polarizing plate, to a color filter substrate. The circular polarizing plate was made by attaching a 1/4-wavelength plate to a linear polarizing plate.
For designing an LCD panel, the retardation (phase difference) of the entire panel needs to be zero so that it does not affect the display. In the case of the VA mode, which is used for large-size TVs, etc, when a circular polarizing plate is attached to a color filter substrate, the retardation of the entire display is reduced to zero by attaching the same circular polarizing plate to the opposing TFT substrate.
On the other hand, the new low-reflection LCD panel developed by Sharp features a wide-viewing-angle, low-power-consumption IPS mode and, therefore, is suited for use in mobile devices. To reduce the retardation of the entire panel to zero with the IPS mode, Sharp employed a structure in which a 1/2-wavelength plate is sandwiched between the TFT substrate and linear polarizing plate.
In a demonstration, Sharp placed a conventional panel and the newly-developed panel under bright light of an LED lamp. And their difference was very clear.
The contrast ratio in bright light (brightness: 20,000lx) improved from 5:1 to 40:1. The surface reflectance of natural light, internal reflectance of natural light and the reflectance (of natural light) of the entire panel were reduced from 4.6% to 0.3%, 1.1% to 0.1% and 5.7% to 0.4%, respectively. The screen size, pixel count, resolution and brightness of the LCD panels used in the demonstration are 3 inches, 1,080 x 682, 405ppi and 600cd/m2.
Sharp also mentioned the improvement of the contrast ratio of an automotive LCD display. It improved contrast ratio in bright light by using the latest low-reflection technology and contrast ratio in dark places by using local dimming.
When those technologies were applied to a 12.3-inch automotive IPS LCD panel, the contrast ratio in a dark place (brightness: 0.1lx) increased from 1,500:1 to higher than 100,000:1, and the contrast ratio in bright light (brightness: 20,000lx) improved from 3:1 to 42:1, according to the company.
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