LCD (Liquid crystal display) is a type of display panel, that is widely used in health sectors, gadgets, etc. It is the most popular form of the display panel that has completely replaced CR tubes.
Components of LCD
1. Light Source
It is an unpolarised backlight that you supply to the liquid crystal display. Some of them are as follows.
- Cold cathode fluorescent lamp (CCFL)
- External electrode fluorescent lamp (EEFL)
- Hot cathode fluorescent lamp (HCFL)
- Flat fluorescent lamp (FFL)
- Light-emitting diode (LED)
- Ambient light
Among all these, the most popular light source is the FFL.
A polarizer is a special type of light filter that passes a polarized light vibrating in a specific plane. It also absorbs the remaining light vibrating in different planes. Two of the polarizers are present in the LCD and are aligned in an opposite direction.
3. Thin Film Transistor (TFT)
Thin film transistors (TFT) are enclosed within the glass surface. It is transparent in nature. You need to supply gate voltage to the transistor to turn it ON. For every pixel, there is a unique TFT assigned to it.
4. Liquid Crystal Display (LCD)
As the name suggests, a liquid crystal consists of crystals of liquid which shows the property of the solid crystal. In a normal condition, the cylindrical crystal molecules align themselves in a random direction. In this case, the light will twist itself when it comes in contact with the crystal. When it gets a certain voltage, the molecules align themselves in the same direction. In such a case, the propagating light will not change its direction.
The other side of the liquid crystal consists of a common voltage/ ground.
Thus, the liquid crystal is enclosed between two glass plates from two opposite sides.
5. Color Filter
The color filter consists of units of red, green, and blue colors. This unit of RGB is known as a pixel. Every unit of pixel consists of three sub-pixels (red, green, and blue). This is the source of color. For example, the red sub-pixel will only allow the red light to pass through it.
The pixel is the reason for the resolution of the images/videos. We will talk about it at the end of our topic.
If the light passes through the second polarizer, a certain color will be displayed. Else, the display will be black.
First of all, the light source transmits the light towards the polarizer. The polarizer then converts the white unpolarized light to the polarized form, vibrating in the same direction as that of the alignment of the polarizer.
After that, the polarized light passes through the TFT layer enclosed within the glass plane. Then it passes to the liquid crystal and finally to the second polarizer. When you supply gate voltage to the TFT, it gets turned ON. This will allow source code to flow through the crystal. On receiving this voltage, the crystals will align themselves in the same direction. The light then passes through the crystal as it is. This light then passes through the color filter and then to the second polarizer. Since the second polarizer is acting in opposite direction to that of the first polarizer, it will block the path of color. In this case, you will see black color on the display.
Similarly, if you do not supply voltage to the crystal, it will twist the incoming light. For example, the light propagating in the horizontal direction will start to vibrate in the vertical direction. Thus the color will be able to pass through the second polarizer. Thus, in this case, you will be able to see the colors.
By controlling the intensity of these three colors, many more colors can be produced.
Types of LCD
1. Based on the activation of pixels
- TFT LCD:- About 80% of the current market of LCDs is occupied by the thin film transistor (TFT) technique.
- Non-TFT LCD:- LCDs that do not consist of TFT, such as the thin-film diode (TFD) occupy the remaining minor portion of the market.
The reason for TFT LCD, being so popular is because it provides a brighter and better quality display.
2. Based on the mode of transmission of light
- Reflective:-The ambient light gets reflected after striking a mirror. The light then goes towards the display. This is a traditional technique. The positive aspect of this technique is that you do not need to supply external power for the supply of light. It is cheap. However, the drawback is that you will not be able to operate it in a dark place. For example, some traditional calculators operate only in the presence of light.
- Transmissive:- It consists of a light source such as LED or fluorescence lamp. This light then transmits towards the display. It is the most popular one among all three given ones. Some of the examples include medical instruments, gadgets, etc.
- Transflective:- It is a combination of both reflective and transmissive light at the same time. It helps to reduce the use of external power consumption. For example, advanced watches may use this mode of light transmission.
3. Based on the orientation of the molecules of the crystal on applying the voltage
- Twisted Nematic (TN) Panel:- These are the oldest ones among all the other LCD panels. They have poor view angle, color precision, and color quality. So, they are not suitable for applications such as graphic designs. However, they have the best response time and refresh rate. Thus they are suitable for gaming purposes. They are also cheap to purchase.
- In-Plane Switching (IPS) Panel:- These panels have a better viewing angle, color precision, and color quality. So, they are useful for applications such as graphic designing. However, they have low response time than the TN panel. The other minor problem is the “IPS glow.” It’s when you see the display’s backlight shining through it, for the most extreme viewing angles. Also, the cost price is more than TN and VA panel.
- Vertical Alignment (VA) Panel:- The features lie between TN and IPS panels.
Advantages of LCD
- The power consumption is low in comparison to CRT and LED.
- They are relatively cheaper than other techniques.
- LCDs provide dull but natural colors.
- They provide great brightness and contrast.
- They are thinner in comparison to CR tubes.
Disadvantages of LCD
- The speed, response time, contrast, and brightness of LCD are not as good as a LED display.
- The LCD seems less likely to burn out in comparison to the LED display.
- The black color in the display does not seem to be pure black. Rather it looks gray.
- You need an additional light source for the operation. Thus, it makes the display quite thick in comparison to the LED display.
Applications of LCD
- Use for gadgets such as mobile phones, calculators, etc.
- Digital cameras, watches, etc.
- Display panel for numerous medical equipment such as a biochemistry analyzer, freeze, etc.
Maintenance of LCD
If you want to clean the LCD then make sure that you turn off the machine. After that, take a cotton cloth or tissue paper. Soak it in water and clean it gently. Make sure that you do not use any concentrated chemicals such as acids or alcohol.
If the display board stops working, make sure that you check the following things.
- If your display board is completely black, then it can be due to two main reasons. One of the reasons is that the circuit board which supplies power to the display board has been damaged. The other reason can be because the light source has been damaged. You can check the DC voltage of the input of the display board with the help multimeter.
- If your display displays irrelevant colors, then it means that it is getting the needed power for the activation. But, the components of the LCD have been burned out.
In conclusion, you may not be able to repair the display on your own. So, it will be better to replace it with a new one.
Resolution and PPI
We have already stated in the topic of color filter that three sub-pixels (red, green, and blue) make a pixel.
Let’s say that you are given a resolution of 1080 x 1920 pixels, in the specification of the machine. It means that it can handle HD images/videos of 1080 pixels. The term 1080 x 1920 pixels means that there are 1080 pixels height-wise and 1920 pixels lengthwise. So, in total, you have got 1080 X 1920 = 2073600 pixels (nearly 2 megapixels).
From the above statements, we can conclude that for two different displays with the same resolution but with different display sizes, the quality of the video/images will differ.
Due to this reason, we need to consider PPI for our analysis. PPI (Pixel per inch) refers to the density of pixels per inch of the display. Mathematically, PPI is the ratio of no. of pixels diagonal of the display to the length of diagonal in an inch. Higher is the value of PPI, the higher will be the image quality and vice versa. In general, the PPI value ranging from 350 to 450 is considered OK for use.