Optimizing LCDs for Outdoor or Direct Sunlight Viewability
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A display becomes less viewable in a bright ambient light. This is true for both light emitting panels such as an OLED or for a transmissive LCD display that uses a backlight. In a bright ambient environment, the light from the sun competes with the light from the display and the perceived contrast goes down. There are several ways to deal with this intrinsic problem. |
Backlighting Methods
One solution is to build a bigger backlight behind the display. Several companies have produced displays using this brute-force approach. This has been done, for example, in kiosk applications where the colors are quite viewable in bright sun. It is difficult, however to dim such a display, so if you are in a variable environment it is a challenge to turn the brightness down for lower ambient conditions.
Contrast Enhancement Films
Another approach is to take a moderately bright backlight and add contrast-enhancement films to the front. Interference films exist that can be applied so that reflections from the face of the display are reduced. Using index coupling fluids as an alternative to these films further reduces the reflections and allows relatively more of the display light to pass.
Reflective Mode
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The next approach to working in bright ambient light levels is to make a purely reflective display. Sharp has developed a technology called HR-TFT or Highly Reflective TFT most commonly seen in hand held video games. This works in the range from bright sunlight down to a relatively well-lit room. Of course, in a darkened room, the display performance is not sufficient. |
Transflective Mode
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The next approach is the transflective display. Beginning with a reflective display, one can make a display that works well in a bright ambient or moderately bright environment. Then by putting an aperture in the reflector (this reflector is the drain pad of the field effect transistor) it is possible to get enough light from a backlight (either CCFT or LED) to see the display in darker environments. As you go to brighter ambient conditions, the reflected light begins to dominate. In a well designed system, you have a seamless transition between the backlit and reflective modes. Sharp's Advanced TFTs have one further innovation – selective cell-gap control. In the transmissive region (where light only passes in one direction) the thickness of the cell gap is equal to 2N. In the reflective region, the cell gap is equal to N. So light coming through the display is reflected and goes back a second time and therefore the optical path length is uniform between the transmissive and reflective areas. SHARP also deposits different color filters in the reflective and transmissive regions. The combination of these two techniques makes Sharp’s transflective technology (Advanced TFTs) superior to all other transflective products on the market — giving it the best uniformity of color. |
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