Why there are black spots or bright spots on the TFT LCD display
Palm-sized DVDs, portable color TVs and portable computers mostly use true color TFT LCD screens to display images. Its bright and vivid colors, thin and light portable size, soft light and no radiation does not hurt the eyes, and it is deeply favored by consumers. However, attentive users sometimes find that there are individual black spots on the white image on the TFT LCD screen, and individual white spots (including red, green and other color points) appear on the black image. This makes people suspect that the machine or display is defective or defective. In fact, this is a very common phenomenon of active matrix TFT LCD displays. It is not a malfunction of the machine or the display, nor does it indicate that they are defective products. This phenomenon, whether it is a domestic machine or imported brand-name machine, also exists.
To explain this phenomenon, we have to start with the structure and working principle of the LCD screen. The most advanced and mature method for displaying images on a large-area liquid crystal display is to use an active matrix thin-film transistor (TFT) liquid crystal display technology with color filters. Figure 1 is a composition diagram of an active matrix circuit. It consists of a matrix of X-direction scan electrodes and a Y-direction signal electrode, as shown in Figure 2. At the intersection of the X and Y electrodes, a pixel unit is formed by a liquid crystal device and a thin film transistor (TFT) switch. The gate pulse voltage is sequentially applied to the scan electrodes to excite each TFT in the row to turn on. The row signal drive circuit applies signal pulses to the signal electrodes in synchronization with the scan strobe pulses. The signal electrode is connected to the drain of the TFT, so that the signal pulse charges the liquid crystal capacitor through the turned-on TFT. By the next row period, the scan pulse moves to the scan electrode of the next row, and all the TFTs of the original row are turned off, so that the charge (voltage) on the liquid crystal capacitor is protected until the next time it is selected. This is the so-called "row sequential scan drive", and its waveform is shown in Fig. 3. Since the amplitude of the signal pulse on the signal electrode is modulated by the image signal, grayscale display of the image is realized. In order to realize the color display of the image, a color filter covering three primary colors (RGB) is generally used on the liquid crystal display. The color filters are used to display the three primary colors of red, green and blue, and the liquid crystal is used as a light valve to control the intensity of light passing through the primary color filter, so that the red, green and blue primary colors of different intensities can be added and mixed to achieve full color display. This is how active matrix thin film transistor (TFT) liquid crystal displays achieve full color display. ?
To use a TFT liquid crystal display to display large-format fine color images, the entire display requires up to hundreds of thousands of pixels. Take the world-famous manufacturer, Japan's AIWA (AIWA) company's palm-sized DVD player XD? DW1 as an example, it uses a 5? 8 inch TFT color LCD display to display images, the number of pixels is 1200 (X) × 234 ( Y) = 280,800 (280,000 pixels). As long as there is an open circuit for the 280,000 TFT tubes, this pixel will become a black dot; if there is a short circuit, this pixel will become a white dot (or red, blue and other color dots). To 280,000 TFT tubes (the same number of liquid crystal devices), no one is bad. At the current level of process technology, no TFT liquid crystal production plant in the world can achieve it. Of course, in a large number of TFT liquid crystal displays, there are very few devices that do not have a broken TFT tube, but the price of the LCD screen obtained by "picking one out of one" is unacceptable to ordinary consumers. Therefore, all TFT liquid crystal display manufacturers in the world, including the world-famous Japanese companies such as Toshiba, Panasonic, Hitachi, Sharp, etc., generally stipulate a black point that is reasonably permitted and does not affect image quality and viewing effects , The number of bright spots, as a quality inspection standard. The following is a stipulated number of bright spots and black spots in a 280,000-pixel TFT LCD display of 280,000 pixels from a famous Japanese manufacturer:?
In the effective display area of ​​the entire TFT LCD screen, the total number of bright spots and black spots does not exceed 12.
At the same time, the close distance between the defect points is also made as follows:
Within φ5mm, no more than 2 bright spots or 2 black spots. ?
In order to avoid consumers' doubts, some complete machine manufacturers specifically explain this in the product manuals. For example, Japan Aihua Company has such a description text in the manual of its XD? DW1 DVD player:?
A few bright or dark spots may appear on the LCD screen. This is a very common phenomenon in active matrix display technology and does not indicate a malfunction or defect. ?
It can be seen that for all kinds of DVDs, color TVs and computers using the most advanced and mature active matrix TFT LCD display, there are a few bright spots or black spots on the LCD display, as long as they are within the range specified by the quality of the LCD display. Neither will affect the image quality and viewing effect, nor does it mean that the machine or display is faulty or defective. Of course, if there are more black spots or bright spots that exceed the requirements of the display acceptance standard, which affects the image quality and viewing effect, consumers should negotiate with the relevant manufacturers to protect their due rights and interests.
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