Optional Design Features
Another drawback to the simple approach of using two pieces of glass is the introduction of extra reflecting surfaces. As noted in our Display Enhancement Tutorial, each air-to-glass transition will cause about 4% of the light incident on that surface to be reflected. Adding one piece of glass to the optical path will result in a loss of about 7% in light output (the light from the display is reflected back into the space) and an increase of about 7% in reflected light level. This is not a desirable situation for displays to be used in high brightness locations, such as on the bridge of a boat.
This situation can be substantially improved by the use of antireflection (AR) coatings on the glass. All of the surfaces used in the insulated protective window may be so coated. This will add to the cost of the display, but the reflection losses can be reduced from approximately 4% per surface (7-8% per glass plate) to less than 0.5% per surface (1% per glass plate).
Furthermore, the display face reflection can be reduced through the use of AR-coated plastic films laminated directly to the display. Using these, it is possible to increase the effective light transmission capability of the display monitor from 85% (two glass plates without coatings) to 98+% (all four surfaces AR coated). This improvement can be utilized by the designer to either increase the display luminance at fixed input power, or to reduce the input power at a fixed luminance.
Another option in this design is to place a heat barrier onto one of the glass surfaces. Placing a heat mirror onto the inner surface of the first glass piece will reduce the effect of solar loading on the display dramatically. This will allow the use of the display in full sunlight for extended time periods without the display turning black and useless.
General Digital Optical Bonding Laboratories has successfully demonstrated the advantages of this design approach and has supplied displays using this window to the new class of attack submarines being manufactured by Electric Boat Company. We are well prepared to provide monitors or desiccated windows for use by OEM display manufacturers. Please contact us for a quote on your application.