TFT (Thin Film Transistor) LCD (Liquid Crystal Display) technology is currently dominant in the display world right now due to its lightweight, low power consumption, low manufacturing cost, etc. But LCDs do have several generic drawbacks. A narrow viewing angle is one of the main issues compared with other display technologies, such as OLED (Organic Light Emitting Diodes) , CRT(Cathode-Ray Tube), Plasma, VFD(Vacuum Fluorescent Display), and most recent Samsung MicroLED displays. They also don’t have a response time issue.
LCD scientists and engineers took more than 30 years of effort to improve the TFT LCD viewing angle which has made TFT screens widely applicable in different applications ranging from automotive, home appliances, medical, military, industrial, consumer, etc. The following is a summary of the different TFT wide viewing angle technologies.
1) Why TFT LCD’s Viewing Angle is Narrow.
When we talk about TFT LCD, we normally mean TN LCD. The twisted nematic effect (TN-LCD) 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. Refer to Fig.1
Fig.1 TN LCD Structure
Fig.2 Liquid Crystal Material Birefringence
In this case, when the display is observed from a vertical direction to the substrate, the display shows a dark state, because the optical axis of the liquid crystal is perpendicular to the substrate. However, when the display is observed from the tilted direction, the display does not show completely a dark state because light leaks due to the birefringence. The birefringence becomes predominant with an increase in the voltage. At different viewing angles, the birefringence is different and the transmission is different in each direction. This is the reason for the poor viewing angle of TFT LCDs. See Fig.3.
Fig.3 TFT LCD Display Viewing Angle
2) TFT Wide Viewing Angle Technologies
- Wide Viewing Angle Polarizer
The most cost-effective and common way is to use a wide viewing angle polarizer. Of course, the improved viewing angle is not wide enough for some applications. Another limitation is that the wide viewing angle polarizer will not be able to change the Gray Scale Inversion shown at the bottom photo in Fig.3.
|Viewing Angle||Contrast||Normal Polarizer||Wide Viewing Angle Polarizer||O Film||IPS|
Fig.3 Viewing Angle Comparison
O-Film is an optical film that is applied to the TN TFT LCD and redirects light, providing all round viewing angles to any TN TFT LCD to which it is applied. It is easy to apply and relatively low cost.
The best advantage of O Film is to eliminate grayscale inversion. The drawback is to reduce the contrast and brightness. With the wider adoption of IPS, O Film gradually fell out of favor with the market.
The multi-domain method gives each pixel two or more domains with a different orientational state. In this technology, the rubbing process of conventional TN mode was avoided. The molecular long axis of liquid crystal molecules are parallel to the substrate and the direction of the molecular long axis is different in each domain. Therefore, the difference of the viewing direction can be reduced since the standing up direction of the molecule is random when the electric field is applied.
Fig.4 Multi Domain TFT LCD
The IPS (In-Plane Switching) is a method to switch the liquid crystal molecules by using a lateral electric field (Fig. 5). Because the liquid crystal molecules do not stand up diagonally the dependence of the viewing angle on optical characteristics is small, thus realizing a wide viewing angle. With cost gradually decreasing, IPS has grown to become the dominant technology for wide viewing angle application.
Fig.5 IPS LCD
The OCB (Optically Compensated Birefringence) mode consists of orientation and optical compensation film as shown in Fig. 6. It requires to apply an electric field to a homogeneous orientation with a pre-tilt angle. The method is no longer used commercially now.
Fig.6 OCB LCD
The VA (Vertically Aligned) method combines the vertically aligning film and nematic liquid crystal with negative dielectric anisotropy. Contrary to the conventional TN method, a very high
contrast ratio can be obtained due to the almost perpendicular orientation of the molecular long axis of liquid crystal molecule to the substrate. Because of the lower cost of IPS, VA is gradually phased out of the color display market but it is popular in the monochrome display market.
Fig.7 VA LCD
This article is an original piece of content written by the engineering and technical support team at Orient Display. We are an LCD and display technology provider with over two decades of industry experience in delivering cutting edge display solutions. Please browse our knowledge base if you would like to learn more about LCDs!