The source is usually a cold-cathode fluorescent lamp (CCFL) or a series of light-emitting diodes (LED). A reflector is placed behind the source to increase the efficiency of the system. A wedged light guide exploits total internal reflection (TIR) to distribute light more evenly across the display area. Mirrored surfaces surround the light guide, also for the purpose of increasing efficiency. A patterned array, referred to as brightness enhancement film (BEF), is used to control the luminous intensity and polarization of the emitted light.

In this design case, we will assume certain constraints. The area of the display will be based upon a standard cellular phone. The light guide thickness will be chosen to limit the overall package height. The BEF will be scaled from the Vikuiti™ design modeled in another knowledge base article.

Display area: 75 mm x 75 mm
Wedge thickness: 4 mm input face, 1 mm end face
BEF: Vikuiti™ T-BEF 90/24

Download the necessary files located on the last page of this article. Place the glass catalog in the ZEMAX\Glasscat directory. This catalog contains modified acrylic and PMMA to model the approximate internal transmission values (93% over 25 mm) of these plastics. The basic design and parameters are defined in the “Starting Point.zmx” file. Note the source/object types in the non-sequential components editor (NSCE) used to model the different backlight components.

A CCFL emits light when excited electrons strike the coating material on the tube surface. The “source tube” is ideal for this type of source emission. We could alternately use the “source diode” to model a 1D array of diodes as the source.

The wedged light guide is modeled with a rectangular volume object made of acrylic material. This object allows different end face dimensions and tilts. Note that we want to keep the upper face of the light guide parallel to the X-Z plane and must tilt the object to do so. A slight positional change in Y is also needed because we are rotating about the center of the input face, not the top edge. Pickup solves on the tilt of the front and rear faces ensure that they remain parallel to the Y-Z plane.

The BEF is the most complex component in the system. Replicating the parent prism by hand would be time consuming and require extensive memory during ray tracing. The array object is a much better choice because it requires only as much memory as the parent object, and the entire array can be altered by adjusting the parameters of the parent. Note the ray tracing speed of the array even though it is still a geometry object.