Design tips for automotive LCD panels
Since the advent of TFT screen car navigation in the 1990s, after 20 years of development, it has become a digital electronic machine that can be seen everywhere in life. More than 70% of new cars in Japan are equipped with navigation.
According to the survey, most Japanese people want to be able to watch TV in the car, especially the hard disk navigator that supports terrestrial digital TV, digital music and other functions, has become the main factor driving the willingness of the navigator to buy. The number of cars equipped with a car navigation system in Europe is less than 10%. It is very different from Japanese cars with various entertainment functions built in the car navigation system. European cars treat human-machine interfaces (Human Machine InteRFace / Man Machine Interface; HMI / MMI) as As a driving support interface, there are many models equipped with LCD monitors centered on high-end cars.
The situation in the United States is different. Because the road planning in the United States is very systematic, as long as you can reach the destination according to the street name and number, the demand for car navigation is not familiar, but instead when all parents travel out, In the back seat, the real-time entertainment use of the LCD monitor to watch DVD and TV entertainment is very developed.
Car LCDs, which are responsible for path navigation, driving support, and game and entertainment development, are undergoing major application transformations under the harsh environmental conditions of automobiles, with the popularization and promotion of car navigation systems. Recently, they have even crossed car navigation systems and become cars. Instrument warning and driving aid display.
The reliability and quality requirements of automobile manufacturers are strict, and they are not comparable to consumer industries such as general household appliances, computers, digital products, etc. It involves the development of high-reliability LCD displays, quality management systems, and daily production activities. Operating system.
The most basic required characteristic of automotive LCDs is environmental resistance. Table 1 is the reliability test specifications for automotive LCDs. Because of different car manufacturers, more stringent environmental resistance tests are required, such as high temperature +95 â„ƒ, low temperature -40 â„ƒ, high temperature and high humidity 65 â„ƒ, humidity 90%, test time 2000 hours, etc. Randomly sample the samples for testing to confirm that the characteristics fully meet the requirements of the environmental resistance test in practice.
The above are test items related to reliability. In fact, liquid crystal displays are also used in such a temperature range. Therefore, various electrical, mechanical, and optical characteristics are required to operate normally in such a temperature range, but all characteristics are in this temperature range. It is not easy to achieve balanced results within, including the optical characteristics at low temperatures described below, self-heating at high temperatures, and the limits of reliability. At present, the performance cannot fully meet the requirements of automobile manufacturers.
Table 1 Reliability test specifications for automotive LCDs
The characteristics of automotive LCD monitors that are very different from other uses are the first to highlight the screen brightness. The main reason is that automotive LCD monitors require strong image visibility under strong outdoor sunlight. Generally, mobile phones can be exposed to strong sunlight indoors or outdoors Under use, the mobile phone can change the direction or use the hands and body to cover the sun. However, the liquid crystal display fixed in the car cannot change the direction arbitrarily. Therefore, the screen brightness must exceed the sunlight.
A touch panel similar to a car navigator is pasted on the screen. The touch panel will cause a decrease in the transmittance. In addition, the surface of the touch panel will reflect external light, resulting in reduced visibility of the LCD. Although Sharp Corporation of Japan manufactures touch sensors around thin film transistors in an attempt to improve the light transmittance and reflection problems of touch panels, it will make the production process and cost more complicated and expensive. The brightness of automotive LCD monitors is usually very high. Some LCD monitors have built-in (In-panel) protection acrylic plates with a penetration rate of less than 50%. Advanced car models also cooperate with the overall design. The LCD screen when the engine is not started It is completely black. In this case, a high-brightness LCD is especially needed.
Generally, the brightness of 450 ~ 500cd / m2 is regarded as the standard value for LCD monitors. In some cases, the brightness of 450 ~ 500cd / m2 or more is required, and even exceeds the specification of more than 1000cd / m2. Therefore, it is necessary to greatly increase the brightness of the backlight illumination module.
Compared with mobile phones, NBs and PCs, another difference between automotive LCDs is the angular characteristics of brightness, because the light distribution characteristics of automotive LCDs are very special. Mobile phones, NBs, PCs, and other personal electronic products mostly use the display screen as a front. There is no problem as long as the front brightness is maintained as a product. However, in automotive applications, the LCD is mostly fixed to the side of the dashboard. The viewing angle of the assistant's seat is about 30 degrees, so the light distribution characteristics of automobiles and mobile phones are very different, but these characteristics can be obtained by combining the optical films of the backlight module components.
Fig. 1 is the light distribution characteristics of liquid crystal displays for automobiles and mobile phones, (1) and (2) in the figure are examples of light distribution characteristics of crystal displays for vehicles, and (4) are the light distribution characteristics of liquid crystal displays for mobile phones Examples.
The LCD of the mobile phone itself has a very low brightness, but it uses an optical diaphragm to converge the light in the front direction, even if the front of the same light source can also change very bright. (3) It uses the same optical film as the mobile phone to construct the light source structure of the vehicle LCD.
From this, it can be seen that when the same optical diaphragm structure is used as a mobile phone, even if the light source is the same as that of a general car, the front brightness is about 1.4 times, but the brightness at a 30-degree angle is reduced by 40%. Based on the above phenomenon, if the LCD for car is compared with the mobile phone, even if the same screen area, the brightness of the light source needs to be:
The ratio of the front brightness is about 2 times Ã— the ratio of the light distribution characteristics is about 1.4 times = 2.8 times
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