Technical & Editorial

The Inner World of LCD TVs - Part II


Vertical Alignment

The other type of LCD technology used in TVs is called vertical alignment (VA), which was originally developed by Fujitsu in 1996. As the name implies, the LCD molecules are aligned vertically—that is, perpendicular to the substrates. If no voltage is applied, light passes through the LCD material but is stopped by the perpendicular polarizer at the other end. As more voltage is applied, the molecules reorient themselves horizontally, and more light passes through the second polarizer.

VA panels are less expensive to manufacture than IPS because they have only one transistor per subpixel. However, they suffer from more image degradation when viewed from off axis. In addition, the degradation is not uniform from one side to the other—in one direction, the image looks lighter and washed out, while from the other direction, it looks darker. Also, the color changes depending on the direction from which you look at the panel. This is because from one direction, you are looking at the molecules from their side (which looks lighter), while in the other direction, you are looking at them end-on (which looks darker).

To address this problem, a variation called multi-domain vertical alignment (MVA) was developed by Fujitsu in 1998. In this technology, a small pyramidal bump is placed in each cell, and the LCD molecules align themselves with the sides of the pyramid. As a voltage is applied, the molecules flatten out in a radial pattern, becoming more horizontal and allowing more light to pass through the second polarizer.

Fig.3: With simple or mono-domain VA (left), viewers from one side see a lighter image, while viewers from the other side see a darker image. With multi-domain VA (right), viewers see the same image from any direction.

With MVA, the image quality still degrades as you move off axis, but the degradation is the same no matter where you are in relation to the screen. This is because you are always looking at some of the molecules from their sides and other molecules end-on.

Variations of MVA include patterned vertical alignment (PVA), which was developed by Samsung, and Advanced Super View (ASV) developed by Sharp. All share the basic characteristics of deeper blacks but more off-axis image degradation than IPS.

Fig.4: In Sharp's ASV technology, also called Continuous Pinwheel Alignment (CPA), the liquid-crystal molecules form a circular pattern when an electric field is applied.

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