Light Pipes Doing Triple Duty: In hand-held display modules, weight is a key consideration. Thus,
any dual or triple functionality that can be realized from existing
mechanical components is mandated. With this in mind, the light
pipe serves to take on a few more functions than just light distribution.
First, the light pipe serves to anchor the glass (Fig. 6, again). Features can be molded into the light pipe, which will capture
the glass and hold it in place. This is especially useful in the
vibration environment. The mass of the display glass is usually
insufficient to deform or break the features clamping the glass
to the light pipe. The light pipe provides the necessary support
to keep the glass in place during vibration and shock.
The light pipe also is used to anchor the pc board. Using a heat-seal
connector type of electronic module, the pc board can be wrapped
around to the other side of light pipe. Features are molded into
the light pipe, which hold the pc board in place. As with the
glass, the pc board is held in place during shock and vibration.
Finally, the light pipe is used to hold the entire display module
in place within the housing. Tabs, rings, holes, or other features
can be molded into the light pipe to allow alignment of the module
into the housing. These features can also serve as positive connection
points between the housing and the display module. Special consideration
must be given, however, to these points as mounting features.
While the mounting features on the light pipe may be strong enough
to hold the display module in place during vibration or shock,
they may not be strong enough to prohibit deflection of the display
module when impacted by other mechanical components within, or
external to, the housing.
Experience shows that the housing must be designed to prevent
deformation of, and deflection into, the display module. The most
common occurrence is glass breakage due to direct impact of a
housing feature on the display module. The display module/housing
interface must be adequately designed and modeled to prevent high-mass
components from coming into contact with the display module. As
an additional precaution, the mounting features of the display
module, most notably ring or pin structures, must be modeled to
prevent stress fractures from occurring over time. This stress
leads to premature fatigue of the display module.