Many intelligent terminals use the TFT LCD module as the human-machine interface (HMI) given its high performance in display quality, cost efficiency, and custom flexibility, for instance, a typical computer monitor, television screen, and mobile.

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As product managers of the terminals, or only enthusiasts who are interested in intelligent products, you should know the basic manufacturing knowledge and product concepts of LCD for making the right decision.

It also can help you to communicate with your suppliers, give ideas of what can be tailored in the interface, and make the selection.

TFT LCD manufacturing processes

From a product perspective, we can divide the TFT LCD module into three layers, from bottom to top: backlight unit, electrical system, and light-and-color control system. 

learn more about TFT-LCD: “What are active matrix LCD and TFT-LCD?“

From the manufacturing process perspective,  it starts from the light-and-color control system and then assembles all three parts.

Therefore, there are three main processes, the TFT array process, cell layer process, and LCD module assembly. 

The first two processes are about the production of the light-and-color control system, including TFT substrate, CF substrate, and LC (liquid crystal), collectively referred to as LCD panel. 

The last step is the integration and assembly of the LCD panel, electrical system, and backlight unit.

TFT array process

This process is to produce usable thin-film transistors (TFT) substrate and color filter(CF) substrate.

The TFT array process is similar to making the semiconductor. 

To get qualified products, it needs to repeat the processes of cleaning, coating, increasing photoresist, exposure, development, etching, removing the photoresist, etc. Generally, it needs to repeat 5 to 7 times.

Figure1: Step 1- TFT array process

Cell Layer process

This process takes the output from the previous process as the top and bottom substrates, drips liquid crystal in between, and then bonds them together as a whole. 

The combined glass made in this process is called an LCD panel. 

In the LCD industry, these two processes, TFT-array and cell layer, are finished by the same LCD manufacturer. In another word, those LCD factories supply LCD panels. 

After that, the big ‘glass’ can be cut into pieces as needed in different sizes.

Figure2: Step 2- Cell Layer process

LCD module (LCM) assembly

Using the cut-size LCD panel as the basic materials, LCM manufacturers proceed with the bonding of the backlight unit, and electrical components (like driver IC, FPC, etc.) onto the ‘glass’ panel, then assemble it with the outer frames (or bezel) for protection.

In the TFT display, it is common to do the custom in the LCM stage.  

The size of the screen, the backlight unit, and the electrical system provide choices to cope with the requirement of the main system of the terminals.

Figure3: Step 3- LCD module (LCM) assembly

Now you have known how the LCD and LCD module is produced in a factory in different steps.

Moreover, you still should understand some basic concepts and features of LCD, which are related to the display quality and the setting with your devices. 

Actually, in the communication and development for a display, display manufacturers will provide specifications of the LCD as a baisc for further discussion. 

Here are the key concepts in the specification:

8 Key Concepts of LCD and TFT-LCD

Size of Screen

Figure4: The common size of each monitor

Display size uses the diagonal length of the actual visible area, the unit is an inch (1 inch =2.54 cm) in common.

Aspect Ratio

The aspect ratio is the proportional relationship and ratio of the width to the height of the visible area of the LCD, also called the screen ratio. 

The aspect ratio of LCD is mainly 4:3, 5:4, 16:10, and 16:9, the ratio of 16:9 is closer to the golden section ratio, which is more comfortable for human eyes to watch. 

In the design of the TFT LCD module, H (horizontal) is usually used to represent the length, and V (vertical) represents the width.

Figure5: Aspect Ratio sample

Active Area (AA)

Figure6: Active Area(AA) sample

The active area is the total area of the TFT LCD screen that can be displayed. 

But in common, factories mark only the length and width of the area on the draw for easy understanding.

Through the display size (diagonal length) and aspect ratio of the LCD screen, the specific length and width of the effective display area can be calculated by using the Pythagorean theorem.

Pixel

Pixel, one of the small dots that make up a screen display, is used as a unit of resolution.

Pixel Pitch

PPI is the pixel density of a display, the number of pixels per inch size. 

The display providing clearer images has a high PPI value.

(X: the number of pixels in length,

Y: the number of pixels in width,

Z: the size of the screen)

Figure7: PPI calculation formula

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Resolution

Resolution is a measure of the image display, which depends on the size of the dots that make up the image. 

Usually, it is measured by the number of horizontal and vertical pixels, in the form of ‘horizontal pixels × vertical pixels‘.

For instance, a display with a resolution of 1024*768 pixels means it has 1024 pixels in the horizontal direction on the effective display area, while 768 pixels in the vertical direction. 

In a fixed plane, the higher the resolution, the more details a monitor can display.

Figure8: The common resolution size corresponding to LCD

Power Consumption

Since about 90% of the power consumption of TFT LCD is from the backlight, to adjust the power consumption of the LCD module it needs to start from the backlight. 

In theory, to achieve the same brightness, the big-size LCD consumes more power than the smaller one. In practice, it still ways to regulate.

Lifespan

How long does an LCD last? 

The lifespan of the LCD module mainly depends on its illuminance (LED strips in the backlight). When the performance of the light source drops dramatically, the display is useless.  

The industry uses Hour to value the lifespan of an LCD. And in normal, LCD and TFT LCD can last for tens of thousands of hours.

That is to say, if the LCD is used for 5 hours a day, it can last more than 10 years.

8 Key Optical Parameters of TFT LCD

Viewing Angle 

The viewing is determined for the horizontal or 3, 9 o’clock direction and the vertical or 6, 12 o’clock direction concerning the optical axis which is normal to the LCD surface.

The viewing angle is the angle from the normal to the four directions of up, down, left, and right, which is sequentially defined as U, D, L, R. 

Generally, the viewing angle of the horizontal direction L, R is larger than that of the vertical direction U, D. 

For the small and medium-sized LCD, the general viewing angle can be 30 degrees, the wide viewing can achieve 45 degrees, and the full viewing angle means it is almost close to 90 degrees.

BM-7A professional equipment measurement

Aperture Ratio

Figure10: Aperture Ratio

It is the calculated result of the effective area of each pixel that can transmit light divided by the total area of the pixel. 

The higher the aperture ratio, the screen has higher light transmittance and a brighter display.

Response Time

The response time measures the response speed of the LCD screen to the input signal, that is, the response time of the LCD from dark to bright or from bright to dark.

The unit is ms (milliseconds). 

When the screen is switched, the faster the response, the smoother the switching is.

Luminance (Brightness)

People also refer to it as brightness (even though both concepts are not the same).

It is the luminous intensity, projected on a given area and direction. 

Its unit is candela per square meter (cd/m2), which can be measured by BM-7/A. The test method is shown in the following figure.

Figure11: Luminance measurement by BM-7A

Uniformity of luminance variation is important. The higher the uniformity, the display quality of the LCD screen is more consistent.

Luminance Uniformity

Generally, in small and medium-sized LCMs, we measure the brightness of 9 points, then divide the measured minimum value by the maximum value to obtain the uniformity of the LCD. 

Figure12：Calculation method of luminance uniformity

Contrast Ratio (CR)

Figure13:Calculation formula of CR

Optically, the contrast is the ratio of brightness between complete white to complete black. 

With a higher contrast ratio, in theory, the display should produce deeper blacks with increased grayscale detail.

Color Gamut

Color gamut is the range of colors within the visible color space, for example, all the colors visible to the human eye.

The range of color gamut of an LCD is represented by the triangular area formed by the connection of R, G, and B. 

The wider the area of the triangle indicates the device can express richer in color and vice versa.

Figure14: Color gamut- visible colour spectrum vs RGB display & printing

Color Depth

Color depth indicates the number of colors that a pixel can display. The unit is “bit”.

When an LCD has more color depth (number of bits),  its intensity level (grayscale) between the brightest and the darkest is larger, it can display more numbers in a color. It means the LCD can display images very fine. 

Above are the basic knowledge and general concepts of LCD, explore more you can read below articles or contact us for any query:

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