How to Model a Dual Brightness Enhancement Film

In this article, we will model a Dual Brightness Enhancement Film (DBEF) with the Dual BEF Surface object in non-sequential mode. To test its effectiveness, we will make a mock LCD system consisting of a light source, a reflective enclosure, a diffusive surface, and a polarizer to analyze the output power of the system with and without the DBEF.

03/30/2017
OpticStudio

Introduction

Liquid crystal displays (LCDs) are a fundamental part of today’s technology, allowing people to transmit visual information. To do so, LCD screens use the optical properties of liquid crystals in combination with polarizers to control the light that exits a screen. These screens consist of layers of components including a backlight, display enhancement films, and an LCD cell with bottom and top polarizers. The figure below shows an example of the components in a typical notebook display.

One of the films used in many LCD systems is the Dual Brightness Enhancement Film (DBEF). A DBEF acts as a reflecting polarizer. It enhances the display by recycling light that the bottom polarizer would otherwise absorb. A diagram of how a DBEF does so is shown below.

This article discusses how to model a Dual Brightness Enhancement Film (DBEF) using the Dual BEF Surface object. Note that we will not be modeling the many layers of the DBEF. Rather, we will model it based on the output information (i.e. the percentage of light of a given polarization that is transmitted through the film). We then look at the effect of the DBEF on the system to make sure our model is viable.

The Dual BEF Surface Object

The Dual BEF Surface object is a rectangular surface that splits rays into transmitted and reflected components based on their polarization. In the object’s parameters, we can set the fraction of transmitted and reflected x- and y-polarized light. For this example, we will make an ideal DBEF with 100% transmission of y-polarized light and 100% reflection of x-polarized light.

Testing the DBEF

To analyze the performance of our DBEF, we construct a mock system consisting of a source, a diffusive surface, a polarizer, a reflective enclosure to prevent rays from escaping, and a detector to analyze the output power. The system is available as an attachment at the top of this article.

We model a backlight and diffuser in this test environment with a rectangle source and a rectangular volume with its material set to mirror. The mirror volume will enclose the rest of the objects to prevent rays from exiting the system. We apply a Lambertian scatter distribution to the front face of the volume to act as a diffusive surface. Now, rays reflected from the DBEF scatter into new rays with random polarization and recycled into the system. Note that this model is not an accurate model of a backlight, but will simulate what we need to analyze. We will use it in our mock system to compare the power in the system with and without the DBEF. For information on how to model an actual LCD backlight system, you might want to look at http://www.zemax.com/os/resources/learn/knowledgebase/how-to-model-an-lcd-backlight.

We model the bottom polarizer of the LCD cell using a Jones Matrix object. This object generates a surface with parameters corresponding to the real and imaginary parts of a Jones Matrix. Examples of common Jones Matrices are in the table below. We want to model a Y Analyzer to send only y-polarized light so we will set D Real to 1 and all other parameters to 0 as indicated by the corresponding matrix. For more examples and tips on using the Jones Matrix object, see http://www.zemax.com/os/resources/learn/knowledgebase/how-to-use-the-jones-matrix-surface

To analyze the DBEF, we will place the Dual BEF Surface object between the source and polarizer and put a Rectangle Detector after the polarizer. To prevent loss of energy at thresholds, we can increase the maximum intersections and segments per ray and decrease the minimum relative ray intensity. This yields a fractional loss of energy on the order of  which is enough to say that the thresholds have little effect on our system.

Analyzing the Results

To compare the system with and without the DBEF we will run a ray trace twice: once while always ignoring the Dual BEF Surface object and once while never ignoring it. The settings for ignoring the surface are in the object properties. Once we get each result, we can compare the two values of output power to determine whether the DBEF enhances the brightness of the system.

The output power of the system without the DBEF returns approximately 50% which is as we expect on average for a randomly polarized source. Comparatively, the system with the DBEF yields a total power of about 82% which shows an increase in the output brightness of the system. This corresponds to an average 64% increase in output power which is equivalent to similar products on the market.