Technical & Editorial

The Inner World of LCD TVs - Part I

ARTICLE INDEX

How Can a Crystal be Liquid?

The very name of this technology is mysterious. Liquid crystal? What the heck is that? It's a type of material that exhibits characteristics of both a liquid and a crystal. This material was first discovered in the late 1880s, but it wasn't until the mid-1960s that liquid crystal was discovered to change its optical properties—that is, how light passes through it—according to an applied electric field. This is the feature that allows liquid crystal to be used in visual displays of many types, including televisions—hence the term "liquid crystal display" or LCD.


Fig. 2: Nematic liquid-crystal molecules (Source: Sharp Electronics)

Basically, liquid-crystal materials consist of rod-shaped polymer molecules that naturally form thin layers, unlike normal liquids in which the molecules are distributed much more randomly. And the molecules in certain types of this material, called nematic liquid crystal, align themselves in roughly the same orientation from one layer to the next, as shown on the left in Figure 2 above. Even more interestingly, if nematic liquid-crystal material is placed against a solid surface etched with microscopic grooves, the molecules next to the surface align themselves with the grooves, as shown in the center and on the right of Figure 2, and the molecules in the layers above align themselves with the ones below, forming a crystal-like lattice.

The actual display panel in an LCD TV—commonly called "the glass"—consists of several layers (not to be confused with the molecular layers in the liquid-crystal material itself). At the core of the panel is a thin film of nematic liquid crystal (a total of less than 5cc in a 42-inch TV) sandwiched between two transparent plates called alignment layers or directors, which are etched with microscopic, parallel grooves.


Fig. 3: Nematic liquid-crystal molecules twisting between alignment layers (Source: Sharp Electronics)

As illustrated in Figure 3, on one director plate, the grooves are horizontal, while on the other plate, they are vertical. As a result, the nematic liquid-crystal molecules next to the plate with horizontal grooves align themselves horizontally, while the molecules next to the plate with vertical grooves align themselves vertically. The molecules between them are forced to form a spiral as they try to align themselves with their neighbors. As a result, this configuration is called "twisted nematic."

The director/liquid-crystal/director assembly is further sandwiched between two more plates that polarize light passing through them. Before I explain why this is important and how it relates to the spiral formation of the liquid-crystal molecules, I need to make sure you understand the concept of polarization.