Technical Articles

The Inner World of LCD TVs - Part II



As you'll recall from Part I, the basic idea of an LCD is that nematic liquid-crystal molecules naturally align themselves with each other and with grooves in the director plates, or substrates, that form the boundaries of the liquid-crystal layer. The grooves in the front plate are perpendicular to the grooves in the back plate, causing the liquid-crystal molecules to form spiral structures, not unlike DNA molecules, between them.

Just outside the director plates are polarization plates, also at right angles to each other. Light from the backlight is polarized by the inner polarization plate, then that polarization follows the twist in the liquid-crystal molecules and emerges through the outer polarization plate. But when a voltage is applied to electrodes on either side of the LC material, an electric field is created, and the orientation of the LC molecules aligns with the field. As a result, the polarization of the light passing through the LC material is not entirely realigned with the outer polarization plate, which then blocks it. The stronger the field, the less light gets through.

The design I described in Part I, which is called twisted nematic or TN, is insufficient to make a television with acceptable picture quality. In particular, the viewing angle is narrow and asymmetrical, response time is slow, and color reproduction is poor. In the quest for better LCD TVs, two major approaches have emerged—IPS (in-plane switching) and VA (vertical alignment)—each with several variations. Go to Page 3: In-Plane Switching