Not all displays are designed for the same applications. Here's a quick explanation of some of the different TFT (thin-film-transistor) displays you may come across and which situations they're best suited for.
In this article, we will go over the difference between two common types of TFT displays, twisted nematic and multi-domain vertical alignment, as well as the effect of Brightness Enhancing Films.
TFT Display Basics
TFTs, in general, are composed of semiconductor (usually silicon), dielectric, and metallic layers. Their most common usage is in TFT displays, which are a subset of active-matrix LCDs.
In the simplest terms, TFT displays are composed of several layers over a backlight (usually white LEDs): polarizing filters, glass substrates, and electrodes. The differing types of TFT displays change the makeup and/or characteristics of these layers.
Brightness and viewing angles are often the major factors considered when choosing a TFT screen for an application. The brightness of a TFT screen can be determined by several factors, including the brightness of the backlights behind the display. However, it can also be affected by the structure of the polarizing glass and the effect of voltage on the electrode layers, as explained below.
TN (Twisted Nematic) Displays
TN (twisted nematic) displays are the most common type of TFT display. The differentiator with TN displays is the presence of “twisted” electrode layers of liquid crystals. When voltage is applied, it twists the electrode layer, which turns pixels on and off.
Image courtesy of M. Schadt [CC-BY 3.0]
As voltage levels change, so does the amount of “twist” in the crystals. The amount of “twist” determines how much polarized light is allowed through to the viewer.
TN Display Applications
While this is a versatile display type, it has limitations when it comes to color reproductions and viewing angles. For example, TN displays would not do well outdoors and must be viewed straight-on for best results. These limitations are why TN displays are no longer common in devices like smartphones.
They’re therefore best suited for applications that don’t require much beyond basic functional display capabilities in controlled environments. If you’re looking for a display that can handle complex and vibrant imagery, for example, a TN display is likely not your best choice.
That said, a TN display would be perfectly at home in many applications, from displays on household appliances to basic interfaces for smaller projects.
MVA (Multi-Domain Vertical Alignment) Displays
MVA (multi-domain vertical alignment) displays are a subset of VA (vertical alignment) displays. The larger term “VA displays” denotes that the liquid crystals are vertically aligned with the glass layer, moving into a perpendicular state when voltage is applied.
The “multi-domain” part of MVA comes from their polarizing glass layer. In MVA displays, the glass layer has ridges that create randomization in the crystal alignment. Where “mono-domain” displays have a unified angle for all of the liquid crystals, “multi-domain” displays have varied angles.
Image courtesy of Kent State.
When voltage is applied, the randomized crystal alignment throws light in many different directions, which improves viewing angles.
MVA Display Applications
Given that MVA displays have an increased viewing angle, they’re better suited for applications in which a screen may require extended interaction. However, because they’re not bright enough to compete with sunlight, they’re also better suited for use indoors.
Honestly, most displays can benefit from improved flexible viewing angles, especially those for devices that are responsive to voice activation from fair distances. From medical devices to home robotics, MVA screens are bright enough for in-home use while allowing a wide enough viewing angle for interaction from various distances and positions.
BEF (Brightness Enhancement Film) Displays
If you need your display to be readable in direct sunlight, you'll need to find an option that offers more brightness. One way to increase brightness is by changing the nature of the backlight of the screen.
However, light can also be maximized with the use of BEF (Brightness Enhancement Film). BEF refracts light emitted outside the conical viewing angle back through the backlight until it can exit directed towards the viewer. This allows up to 60% more brightness (or more if a second sheet is crossed at a 90° angle).
Image courtesy of American Polarizers, Inc.
BEF Display Applications
Because BEF displays are designed to be visible even in bright sunlight, they’re useful for devices designed for use outdoors.
Such displays often find use in automotive applications, especially center console screens that need to be viewable from multiple seats, even in the glare of sunlight. They’re also not uncommon in handheld devices that require exceptional viewing angles and color, such as smartphones and tablets. For many generic applications, however, BEF displays may be overkill.
Size and Cost
Off-the-shelf displays are preferable to custom displays when it comes to cost. This is why so many manufacturers choose to produce their displays in multiple sizes in addition to multiple performance types—they aim to suit the needs of many different applications so designers won't need to order custom displays.
The displays used in this video are all 800x480 transmissive-type TFTs with capacitive touch capabilities and I2C interface. There are, however, myriad displays available and this information should prove helpful for choosing between them.
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