TFT Technology

TFTs are common in the display market and have been established as a great design choice for Medical, Consumer, Industrial, White Goods, and POS Systems. TFT stands for Thin-Film-Transistor and describes the control elements that actively modulate the individual pixels and allow for different color and intensity of light to be shown. The pixels in a TFT are arranged into columns and rows denoted by the resolution of the panel Horizontal by Vertical (H x V). Each pixel is then broken down into RGB subpixels, meaning a Full-HD resolution display (1080 x 1920) actually has 1080 x 1920 x 3(RGB)= 6,220,800 pixels in the glass panel. 


In theory, TFTs are quite simple. There is a backlight that is constantly on and the individual pixels act like a shutter or door blocking the light. Some panels are naturally white or black. So when the pixels are in a rest state with no voltage being applied, the backlight is either blocked or not allowed through until it receives voltage and the liquid crystals twist, changing the orientation of the light. This light manipulation with polarizer films placed perpendicular allow for the panel to block or for light to pass. Color filters are applied to each sub pixel turning the backlight into red, green, and blue light.


TFTs have a few qualities that need attention when selecting a right choice for a project:

• Viewing Direction– As light passes through the TFT stack, the light has a certain directional property. In most Twisted Nematic (TN) displays this is a vertical orientation that can be changed depending on the Liquid Crystal Rubbing Direction and Polarizer orientation. The optimum viewing direction is notated in TFT specs and is labeled as 12 o’clock, 3 o’clock, 6 o’clock, and 9 o’clock as represented in the image below. This is the optimum viewing angle and will provide the best image. The opposite direction is where the grey inversion is located and will be where the worst image is seen. The viewing direction image below shows an example of Grey Inversion on the left side, and the right side having been treated with an optical film to improve image quality. Newer technologies have been created that have removed the directional property of the TFT light by modifying the liquid crystal and polarizer structure (In-Plane-Switching [IPS], Multi-Domain Vertical Alignment [MVA], etc.). There are also optical films that can be applied to improve viewing angles and direction seen in the image above.

• Viewing Angles– These are the optimum viewing angles of the display and can be seen in the optimum viewing direction image above notated as θ . Typically this is notated as degrees in the up, down, left, and right directions.
• Contrast Ratio– This is derived from the max and min brightness of the panel when it is on. In a luminance measurement of a black and white pixel, the greater the difference the better. The backlight is always on and the liquid crystal has to block the light to attain a black color. It is not possible to block all of the light and some will always seep through, resulting in a lower contrast ratio. Contrast Ratio and Brightness are measured using the following image:

• Brightness– This is the actual measurement of light passing through the TFT to the end user. With the polarizers and color filters, a small percentage of the backlight ends up visually seen by the user. The brightness is measured in NITS (1 candela/ meter²[ cd/m²]) and typically the standard value is 100 to 300 cd/m². For outside applications we can increase the brightness to 1000 or 1500 nits to help mitigate the light saturation from the sun. There are other films we can apply or change the polarizer to help with outside viewing.

• Response Time– This represents the time needed for the pixels to update and takes in to account the materials and components used in the display. A design has limitations and will limit the capabilities of the module. Typical value is 20 to 30 milliseconds but there are faster options available. This is measured at our factory looking at the time it takes for a pixel to change from white to black.

• Backlight Lifetime– The time is represented in Hours and typically the value is between 30,000 and 50,000 hours. Some panels can be more or less depending on the LEDs chosen for the backlight. This value does not represent time until failure, but rather the amount of time until the backlight is half of the original brightness. The display will still function but will be half as bright. LEDs degrade over time and this is an unavoidable factor in TFT selection.