Twisted Nematic (TN) LCD Display


The twisted nematic effect (TN-LCD) was a main technology breakthrough that made LCDs practical. TN LCD does not require a current to flow for operation and uses low operating voltages, low current suitable for use with batteries. TN-LCD displays led to the rapid expansion in the display field, quickly replacing other displays like LEDs, plasma, CRTs etc. By the 1990s, TN LCDs were widely used in portable electronics, although since then, many applications of LCDs adopted more advance technologies to the TN-effect such as in-plane switching (IPS) or vertical alignment (VA). Many monochrome digit and alphanumerical displays still use TN LCDs.

The TN display takes advantage of the ability of the nematic substance to rotate the polarization of light beams passing through it. Two polarizing filters, parallel planes of glass with their polarizing lines oriented at right angles with respect to each other, are positioned on either side of the liquid crystal. When light enters the display, it is polarized by the input filter. In the absence of an electric field, all the incoming light is transmitted. This is because the light polarization is rotated 90 degrees by the nematic liquid crystal, and the light therefore passes easily through the output filter, which is oriented to match the 90-degree shift.

With the application of a voltage, an electric field is produced in the nematic liquid crystal. Under these conditions the polarization effect is reduced. If the voltage is large enough, the polarization effect disappears altogether, and the light is blocked by the output polarizing filter.

TN LCD Structure

Fig.1 TN LCD Structure

 

The transmission of the LCD as a function of applied voltage. There is a threshold behavior for LCDs and no change in transmission occurs until a threshold voltage, Vth, is reached. Transmission then decreases as the voltage increases until saturation is reached. Threshold voltage (V90) is typically 1.0-2.5 volts, and saturation (V10) occurs at about 2.5-5 volts.

LCD Transmission vs. Applied Voltage

Fig.2 LCD Transmission vs. Applied Voltage

TN LCD Pros and Cons

Pros:  Low cost, low power, low voltage, easy to manufacture, faster response compared to other types of LCD.

Cons: Low contrast, narrow viewing angle, bad color shifting, gray scale inversion.

 

Different type TN LCD displays

  • HTN (High Twisted Nematic)

As the normal TN LCD twists 90 degrees while HTN twist 110̓ to 130 degrees. The purpose is to increase the contrast and viewing angles. HTN finds its application in multiplex LCD which TN has ghosting. HTN LCD price is a little bit higher than TN LCD and its performance is also a little bit better than TN LCD.

HTN LCD

Fig.3 HTN LCD

  • ETN (Enhanced contrast Twisted Nematic)

ETN has a dye in the LCD fluid which increases the contrast in negative mode displays giving improved background appearance.

ETN ISO Contrast Diagram ETN LCD Display
Fig.4 ETN ISO Contrast Diagram Fig.5 ETN LCD Display

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