Technical Articles

The Inner World of LCD TVs - Part I

ARTICLE INDEX

Polarize This!

Okay, let's get back to the two polarizer plates in LCD TVs. One of them is a horizontal polarizer, while the other one is a vertical polarizer. It would seem that all the light from the backlight would be blocked from passing through the entire structure, but it isn't. Why not?


Fig. 6: Light's polarization follows the liquid-crystal molecules' spiral. (Source: Sharp Electronics)

Because a light wave's polarization is affected by the orientation of liquid-crystal molecules. As shown in Figure 6, as the polarized light passes through the liquid crystal, the polarization follows the molecules' spiral orientation, and it ends up in the opposite polarization by the time it gets to the second plate, and so passes through that plate. In other words, light passes through the polarizer/liquid-crystal/polarizer sandwich, which it wouldn't if the liquid crystal wasn't there.

Now, recall that liquid crystal changes its optical properties when it's exposed to an electric field. Specifically, the molecules align themselves with the electric field instead of the grooves in the plates. If the field is weak, the molecular realignment is slight, but as the field gets stronger, the realignment becomes more pronounced.

In the absence of an electric field, imagine that the liquid-crystal molecules form little ladders that have been twisted, forming a double helix much like DNA. When an electric field is applied, the rungs in the middle of the ladders become more vertical, while the rungs next to the alignment plates remain fixed in the grooves. The stronger the electric field, the more the molecular rungs move into a vertical orientation.


Fig. 7: Without an electric field, light passes through; with an electric field, the molecules reorient, blocking the light. (Source: Sharp Electronics)

The more vertical the molecules are, the less the incoming light changes its polarization, which means that less light passes through the second polarizer. With no electric field, all the light passes through, as shown on the left in Figure 7; with a strong field, no light gets through, as shown on the right. This allows the liquid crystal to act like a shutter, letting more or less light pass through the entire sandwich depending on the strength of the electric field.

For all this to work, there must be a light source behind the polarizer/liquid-crystal/polarizer sandwich—the backlight. I'll get into the different types of backlights more deeply in the next installment. For now, all you need to know is that a large, flat, (hopefully) uniform light source sits behind the polarizer/liquid-crystal/polarizer sandwich, and more or less light from that source passes through the sandwich according to the strength of the electric field applied to it.