Sun-readable display technology: How it works

21 October 2008

Sunlight visibility of a display depends on differences between the lumination (brightness) and the illumination (ambient light shining on a display). Lumination is the amount of light energy emanating from the display (expressed in Cd/m2).

Beijer Electronics released Exter Sun-Readable HMI
Beijer Electronics released Exter Sun-Readable HMI

The readability depends on the amount of light being reflected off the display. The simplest way to enhance visibility is to increase brightness. A standard screen emits 350 Cd/m2. Some offers on the market feature 8,000 Cd/m2. The result is good, but because of the added lamps, high bright LCDs have disadvantages including high power consumption, excessive heat generation, increased dimensions, electrical circuit reliability problems, and shortened LCD lifetime.

A second option is transflective technology. Some TFT LCDs are provided with a reflective mode of illumination without compromising transmissive illumination. With imposed reflective function, modified LCDs reflect ambient light passing the LCD cell and use reflected light beams as illumination. The stronger the ambient light, the brighter the LCD appears. The modified LCD is viewable under all lighting conditions including direct sunlight regardless of the LCD's original brightness. However, a regular transmissive LCD is difficult to read under strong ambient light without direct sunlight. Problems noticed in transflective LCDs include a narrow viewing angle, discoloration, low brightness, and loss of contrast.

The high bright LCD and the transflective LCD are solutions generally used for outdoor applications. However, both solutions are not really suited for industrial use. It is necessary to take another route in trying to control reflected lights. As light transfers from one material to another, such as air to glass, differences in the index of refraction will cause reflection. On an LCD screen, cumulative reflection of the different surfaces is nearly 15 per cent of ambient light. If the total reflection is close to the display´s brightness, the contrast of the display is reduced to the point where the display's readability is diminished to unacceptable levels.

Bond technology is a third option. It enables coupling of the front glass and the internal glass, but the cost is high compared to what it brings in terms of quality. Features include a standard display, an enhancement film applied on the internal screen improving brightness and contrast, and a polarising film applied on both sides of the touchscreen to reduce light reflection. The design includes a gap of 1.5 mm, between the two screens. This distance has the effect of reducing Newton rings or the circular rainbow effect which may be caused by non-uniform cell thickness, as well as other defects resulting in a flat surface in contact with a slightly curved one.

Brightness is better than a standard screen (550 Cd/m2 instead of 350 Cd/m2 for a standard Beijer Electronics screen). The touchscreen features anti-glare and reflection surface treatments. Anti-reflective coated protection glass minimises mirror image and maximises transmittance. Anti-glare coated protection glass scatters light directed on the surface and softens the image of direct light sources visible in the reflection of the viewing area.

– Control Engineering News Desk

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