Monday, February 4, 2013

Unbundling the Optical Stack for Better Environmental Performance

LCDs used outdoors as digital signage must be protected from the environment. Typically, they will have a cover glass to protect them from being poked and prodded. They may also have air-conditioning to protect the LCD from reaching its thermal clearing temperature or otherwise suffering thermal damage. Objects in direct sun can reach temperatures as much as 185⁰ F, and more than 70⁰ F above the ambient air temperature. The cover glass may be plate glass or chemically strengthened glass. Chemically strengthened glass has the advantage of being thinner and lighter. Plate glass has an advantage in that if it does fracture, it typically cracks rather than shattering, a feature known as "frangibility". This is especially true if a laminated solution is used. For public spaces, especially confined spaces such as in transportation applications, management of the broken glass hazard may be critical.

However, the cover glass may also be part of the thermal solution. The polarizers of an LCD, by definition, absorb about 50% of the light that hits them. They are agnostic as to which direction the light comes from absorbing 50% of inbound sunlight as well. Removing the outer polarizer from the LCD and laminating it between the layers of the cover glass puts a few millimeters of glass and an air gap between the absorbed thermal energy and the liquid crystal. This would cut the solar radiation load on the LCD by 50% and also provide the opportunity for passive, chimney, cooling rather than a powered air conditioner. Here, I discus removing the color filter as well. In addition to the brightness benefit, removing the color filter would have some solar radiation benefits as well. Certainly it will obviate any issues with solarization or fading of the color filter.

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