Technical Articles

The Inner World of LCD TVs - Part II


In-Plane Switching

IPS panels are very similar to TN with one major difference—each subpixel has two TFTs (thin-film transistors) on the same surface as shown in Fig. 1, so the electric field used to realign the liquid-crystal molecules is parallel to the substrates rather than perpendicular to them as in TN panels. As a result, the liquid-crystal molecules remain parallel to the plane of the plates, which is why this technique is called in-plane switching.

Fig. 1: In IPS panels, two transistors or electrodes are placed on the same substrate in each subpixel cell, creating an electric field that is parallel to the substrate. The stronger the field, the more light passes through the cell.

Another difference between IPS and TN is that, while the grooves in the director plates are perpendicular to each other, both polarization plates are aligned in the same direction. So when no electric field is present, very little light gets through the panel assembly. As the electric field grows stronger, the liquid-crystal molecules become less twisted, allowing more light to pass through the second polarizer plate.

Because there are two transistors on the same substrate for each subpixel, they take up more space than the transistors of TN panels, which reduces contrast and brightness. (IGZO transistors mitigate this problem; more about this technology shortly.) Plus, they are more expensive to manufacture. However, IPS panels exhibit fairly consistent color and contrast from a wide range of viewing angles.

Hitachi developed IPS in 1996, and has made some improvements since then. In particular, Super IPS (S-IPS) improves refresh rates, and Advanced Super IPS (AS-IPS) improves the contrast ratio. IPS Pro—sometimes called IPS Alpha—provides a wider color gamut and even better contrast. This technology was acquired by Panasonic in 2010 and is used in LCD TVs from Panasonic, Hitachi, and Toshiba.

Fig. 2: Viewed from perpendicular to the panel, S-IPS and AS-IPS use chevron-shaped electrodes to improve transmittance. IPS Pro (aka IPS Alpha) also uses angled electrodes within a more rectangular cell.

Meanwhile, LG has been developing its own variations of IPS since 2007. These include Horizontal IPS (H-IPS), which improves contrast by twisting the electrode plane layout, and Enhanced IPS (E-IPS), which widens the light aperture to allow the use of lower-power backlights as well as improving the diagonal viewing angle and reducing the response time. H-IPS is used in LCD TVs from LG, Sony, Philips, and BenQ. Go to Page 4: Vertical Alignment