LCD Display as a Polarized Light Source
Key Concept: Liquid Crystal Displays (LCDs) are found in digital watches, cell phones, laptop computers, and flat screen televisions. This type of display consists of a material known as a liquid crystal placed between two crossed polarizing filters. Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid and those of a solid crystal. The molecular orientation in a liquid crystal, and hence the material's optical properties, can be controlled with an applied voltage.
Normally, crossed polarizing filters absorb the light passing through them. However, in an LCD, tiny liquid crystal elements rotate the plane of polarization of incident light by 90 degrees. This allows the passage of light in and out of the polarizer-liquid crystal-polarizer “sandwich.”
When a voltage is applied to the crystal, the degree to which the crystal alters the direction of polarization is changed, thus reducing the brightness of the pixel. Adjusting the voltage across the liquid crystal in each pixel results in different levels of gray, ranging from white to black (see figure below). For color displays, each pixel is divided into three sub-pixels, each of which is covered with color filters to allow for additive mixing of three basic colors.
Since liquid crystals do not emit light, a visible display requires an external light source. Small and inexpensive LCD panels, such as those found in calculator and digital watch displays, often rely on light that is reflected back toward the viewer after passing through the panel. Since the panel is not completely transparent, a substantial part of the light is absorbed during its transits of the panel. Images displayed on this type of panel may be difficult to see except under the best lighting conditions. On the other hand, LCD panels used for computer displays and video screens are typically backlit with fluorescent tubes or arrays of light-emitting diodes (LEDs) that are built into the sides or back of the panel. To provide a display with a more uniform light level, light from these sources is typically dispersed in a diffuser panel.
Try This: Examine the LCD display on a watch, calculator, or laptop computer through a single Polaroid filter. Rotate the filter and note the effect. What does this tell you about light from liquid crystal displays?
Using Your Computer LCD as a Source of Polarized Light
Key Concept: Liquid crystal displays produce polarized light.
Try This: Slowly rotate a Polaroid filter between your eye and the screen. What do you observe? Now place a sample of birefringent material, such as clear plastic, between the screen and the filter. What do you see? Rotate the filter and observe the appearance of the sample.