Overview

Many applications of displays involve installation in environments having a high humidity level. Of specific importance in this regard is the marine environment. In this application, an LCD, OLED or Plasma monitor, no matter how well sealed, will eventually “inhale” moisture within the enclosure. Once inside the enclosure, it is extremely difficult to remove the water vapor. As a result, the protective clear window installed in front of the display will develop a fogged appearance when it becomes chilled. Ways in which it can become chilled include direct impingement of air conditioned air onto the face of the display or spray impacting the outside. The fog which develops inside of the window is frequently difficult to remove without heating the glass or opening the display. General Digital™ Optical Bonding Laboratories™ has developed two distinctly different ways to prevent this from being an issue.

The first technique is to optically bond the window to the flat panel display. This eliminates the air space between the display and the window, thereby eliminating moisture entirely from the space, and hence, the problem of condensation.

The second technique it to provide a hermetically sealed window which includes a desiccating gasket as a part of the seal. This window is, in effect, a custom thermo-pane window tailored to the display. The following paragraphs will describe in more detail the trade-offs in this design.

Basic Design

Two pieces of glass are held in close proximity to one another and are sealed by a specially designed gasket. This gasket, in addition to providing a spacer for the protective window also excludes interchange with the ambient air. The adhesive used is designed to provide a moisture absorbing (desiccating) ability. Thus, as time goes on, the moisture content between the two pieces of glass will actually decrease. In addition, the dead air space between the two glass surfaces forms a thermal barrier, preventing the inner glass from becoming chilled when the outer surface is chilled. The overall effect is to eliminate the fogging which otherwise could occur.

Both our experience and the information from manufacturers of similar products show that the moisture content of a desiccant sealed optical window remains below 10% RH for years. Consequently, in the absence of mechanical damage to the protective window, the display will never be subject to condensation fogging.

Comparing the mechanical design of the sealed window to other designs, we find that this approach adds depth to the display. The thinnest design we would recommend is 7/16″. This is composed of two pieces of 3 mm glass, spaced apart by the thinnest desiccant gasket available (3/16″). In addition, there will need to be a small air gap between the inner layer of the window and the LCD, OLED or Plasma display. Compared with a 3 mm protective window, we need an additional 5/16″ inch in the display depth. Optical bonding does not add this extra space.