TN Phone Screens: How They Work

TN Phone Screens: How They Work TN — Twisted Nematic — is one of the oldest and most straightforward LCD technologies. It was the first liquid crystal alignment mode to be commercialised for flat panel displays, and though it has largely been displaced in smartphones by IPS and OLED, understanding TN reveals the core operating principle…

TN Phone Screens: How They Work

TN — Twisted Nematic — is one of the oldest and most straightforward LCD technologies. It was the first liquid crystal alignment mode to be commercialised for flat panel displays, and though it has largely been displaced in smartphones by IPS and OLED, understanding TN reveals the core operating principle underlying all LCD types.

The Twisted Nematic Structure

In a TN cell, liquid crystal molecules are anchored to the upper and lower substrate surfaces in orientations that differ by 90 degrees. In the absence of any electric field, the molecules form a continuous spiral twist from one surface to the other — the "twisted nematic" arrangement that gives the technology its name.

This twisted structure acts as a waveguide for polarised light. Linearly polarised light entering from the backlight through the first polarising filter follows the twist of the molecules and exits with its polarisation rotated 90 degrees. The second polarising filter, oriented to accept this rotated light, passes it through and the pixel appears bright.

Applying Voltage to Darken

When a voltage is applied across the liquid crystal cell, the electric field causes the molecules to align parallel to the field — collapsing the twist. Light passing through the now-untwisted crystal layer is no longer rotated, arrives at the second polarising filter at the wrong orientation, and is blocked. The pixel goes dark.

Intermediate voltages produce partial twist, partial rotation, and shades of grey — the foundation of greyscale and colour reproduction when combined with red, green, and blue colour filters.

Limitations in Smartphones

TN panels have fast pixel response times and are inexpensive to produce, which made them attractive in early mobile devices. Their central weakness is viewing angle performance. Because the molecules tilt vertically through the cell depth, the apparent contrast and colour shift noticeably when the screen is viewed from any angle other than directly head-on. For a handheld device held at varying angles throughout daily use, this is a significant disadvantage.

TN also produces less accurate colour than IPS and offers a narrower colour gamut. These shortcomings led smartphone manufacturers to transition to IPS variants, and TN is now rarely found in phones outside of the lowest-cost devices.