LED full color display screen light distribution solution
Abstract: This article mainly introduces a full set of light distribution schemes for LED full-color display screens, including K-factor management of production chips, control of packaging processes, adjustment of white balance, and design of LED optical Lens, representing the industry's advanced light distribution solutions idea.
The LED display screen industry in mainland China first started around 1987. After ten years of joint development, it has begun to take shape. At present, there are more and more manufacturers of LED display screens, and there are many excellent enterprises among them. They have jointly prospered this emerging high-tech industry. The 21st century is an era of flat panel display. As one of the mainstream products of flat panel display, LED display screen will also have greater prospects.
LED display screen is a kind of electronic display combined with high technology such as computer technology, information processing technology, electronic technology, optics and colorimetry. Generally speaking, LED display screen is divided into monochrome screen, dual color screen and full color screen . The so-called monochrome screen, as the name suggests, the display is composed of a single LED of a certain color. The display screen using red and green LEDs as a picture element is called a two-color screen or a color screen. The display screen that puts the three LEDs of red, green and blue together as a picture element is called a three-color screen or a full-color screen. As the brightness of green chips continues to increase, full-color screens are gaining popularity and promotion at an unprecedented rate. Especially the success of Beijing's bid for the Olympic Games, it put forward the call of "Green Olympics", making the market prospect of LED full-color screen more attractive.
LED full-color screen is divided into indoor screen (Indoor) and outdoor screen (Outdoor), we first introduce a few common terms of full-color screen. Pixel: The abbreviation of the primitive point, generally composed of R / G / B three-color LED. According to the different ways of composition, it is divided into different combination methods such as 1R / 1G / 1B, 2R / 1G / 1B, 2R / 2G / 1B, etc. It is the basic imaging unit of a full-color screen.
Pitch: The distance between the center points of the primitives of the display screen, generally expressed in mm, which determines the number of primitives of a full-color screen. Gray Scale: The fineness of the brightness adjustment of the display screen is called the gray scale of the display screen. Generally speaking, the higher the gray scale, the richer the display color. There are generally two ways to drive LEDs to change colors. One is to change the LED current. In general, the working current of LED is best set below 20mA to ensure the resistance of LED to attenuation. The other is to use the visual inertia of the human eye and use pulse modulation to achieve grayscale control. A person's perception of the average brightness of a primitive can depend on its decay / destruction (duty cycle), that is, the duty cycle is changed periodically. As long as the period is short enough, the human eye cannot perceive its decay / destruction.
Generally speaking, the pitch of indoor screens (Pitch) is 2-12mm, of which 2-8mm indoor screens generally use the TOP type patch method or the currently developed sub-surface patch method. The pitch of the outdoor screen is 14-26mm. With the current green LED brightness level, the display screen with a pitch of 20mm is the most mainstream product. Domestic display screen manufacturers have made rapid progress after more than ten years of development, but compared with developed countries such as Europe, America and Japan, there is still a certain gap, especially the most important display device LED packaging technology and design level There are still many shortcomings. Shenzhen Lehman Optoelectronics Technology Co., Ltd., as one of the members of the China Optical Optoelectronics Industry Association LED display screen branch, has long been focusing on the light distribution research of LEDs for display screens. The company has brought together a group of elites who have long been committed to LED research and invested a lot of equipment And manpower to conduct in-depth research on the products of leading foreign peers, and combine their own characteristics to summarize a set of solutions for LED light distribution and color matching, which are now shared with you to jointly improve the process and light distribution level of each LED packaging factory. Do your part to improve the technology of China's LED full-color display screen. The LED light distribution method is a complete set of technology, which includes K-factor management of the wafers put into production, control of the packaging process, white balance adjustment, and optical design of LED Lens. The statements are as follows:
1. K-factor management of wafers Lehman Optoelectronics strictly controls the wafers of each mass-produced product. We have created our own file for each product. From this file, we can clearly know the chip data of a certain production order, and we will archive and analyze all the graded data, so that we have the K of all the products of the company. -factor details, so that the products are included in the control from the source, we can accurately calculate the specifications of the wafers we need according to the specific needs of customers. Especially for the production of products for full-color display screens, our control of wafers is particularly strict. It is not only enough to meet the brightness requirements of customers. We have to stand on the perspective of customers and solve the white balance adjustment for them. In addition to meeting the customer ’s wavelength requirements and brightness requirements, the following two details should be particularly noticed. First, the brightness of the finished product must be normally distributed, and the phenomenon of brightness discontinuity must not occur. The requirements are also the same, and special attention must be paid to the average wavelength details of the wafers thrown and the number of wafers per board when feeding the wafers. To be analyzed on a computer, a histogram can also be made to clarify the wavelength of the wafers Whether the distribution is normally distributed. If the above details are not paid attention to, when the display screen manufacturer adjusts the white balance, because the brightness and wavelength of the LED are not normally distributed, it is particularly difficult to adjust.
Second, the control of the packaging process The LED packaging process for full-color display screens has its own characteristics. The first thing we have to do is to control the raw materials, because the outdoor full-color screen is used in a harsh environment, either long-term working at high temperatures or long-term working at low temperatures, and long-term corrosion by rain, such as LED reliability is not very good, it is easy Blind spots appear, so we pay attention to the control of the quality of raw materials. The company has a complete set of advanced equipment for material inspection, which can help us to strictly control the raw materials. Our full-color LED products use high-quality copper brackets with high thermal and electrical conductivity, which can greatly reduce the thermal resistance of LEDs. In addition, we also made special regulations on the coating of the bracket to ensure our quality requirements. In order to improve the brightness of the finished product, we also made a matte treatment of the stand bowl. In the selection of epoxy resin, we specially selected high Tg glue that can resist UV according to the working characteristics of the display screen. In the baking process, we use a slow cooling and hot baking process. This baking process can improve the Tg point of the glue after a lot of experiments, and the most important thing is that it can effectively reduce the glue inside. Stress, which greatly improves the reliability of the product.
3. White balance design According to the needs of customers, we have summarized a set of our own calculation methods. As long as the customer tells us the brightness you need and the dot pitch of the display screen, we can serve the customer in all directions and make a perfect light distribution plan for the customer's display screen. To facilitate the following introduction, I first introduce a few terms and definitions. White balance: The process of blending three colors into one imagined white. The deployment can be divided into two contents:
Color and brightness.
1: Set fixed conditions: temperature, voltage, and select power cord.
2: Set the color target and range (tolerance), generally refers to the X and Y values ​​of the target color.
3: Perform white brightness adjustment. The total brightness of the mixed color is equal to the total brightness of the various colors that make up the mixed color is called the law of brightness addition. Lights with the same color appearance, regardless of whether they have the same spectral composition, have the same effect in color mixing, that is, all visually identical colors are equivalent, and the law can derive the substitution law of colors. If the color matching equation uses (C) to represent the unit of the matched color, (R) (G) (B) represents the unit of the red, green, and blue primary colors that produce the mixed color, R, G, B, and C represent the red and green, respectively , Blue and the number of matched colors, when a match is reached:
C (C) ≡R (R) ≡G (G) ≡B (B)
≡ means visually equal, R, G, B are representative quantities, which can be negative values.
Please refer to the figure below: Let us introduce a law of gravity to everyone. In the picture, B is color 1, G is color 2, R is color 3, C is the mixed color of G + R, and W is the mixed color of B + C. In fact, it is a schematic diagram of white balance. We can use C as a mixed color of green and red. As can be seen from the figure, it is located in the yellow area. C is then mixed with blue. It can pass through our white area. White light can be mixed. Where B = X1 + Y1 + Z1C = X2 + Y2 + Z2
X1, Y1, Z1; X2, Y2, Z2 are the tristimulus values ​​of blue light and mixed color C. The law of center of gravity is: CM / BM = B / C = (X1 + Y1 + Z1) / (X2 + Y2 + Z2)
That is, the distance of CM is inversely proportional to C, that is, the larger the proportion of C in the mixed color, the shorter the distance of CM. This is the same as everyone's experience when making white lights. The more the amount of phosphor, the light color The more yellowish (ie, the shorter the CM). Here we introduce our calculation method: 1. First, we must convert the surface light source to a point light source to get the brightness value of point W. We know that the unit of the surface light source is Nit, that is cd / m2, and the brightness of our LED is mcd , We pay attention to transfer the unit to mcd. Divide the total brightness of the display screen by the number of dots per square meter.
2. Find the coordinates after matching the colors of R and G (the four-point coordinates of R, G, B and W are known to calculate the intersection point of the extension line of the BW straight line to get the C point coordinate).
3. According to the law of the center of gravity, the IV value at point C can be calculated (the IV value at point W is known).
4. According to the principle of complementary colors, the IV values ​​of B, G and R can be obtained.
5. Calculate the required current value based on the required IV value (provided that the 20 mA IV value is known, and the IV is approximately proportional to the current). Note: 1. This method can calculate the number of LEDs required for R / G / B.
2. Approximately calculate the current value required by R / G / B during use. The current here can only be approximate, for the following reasons:
â‘ IV and current are not necessarily in direct proportion.
â‘¡ The known IV and WD of the LED are an average. Therefore, when adjusting the white balance, it should be slightly adjusted according to the actual situation, and it cannot be adjusted according to this value. Maybe you think the above method is too professional and cumbersome, then we have a relatively simple method for your reference. For the convenience of explanation, we will introduce it by way of example. We know that the R / G / B ratio of mixing a white light is roughly 3: 6: 1, and we can easily calculate the brightness value required for each color LED based on this knowledge. For example, a customer needs an outdoor full-color screen with a pitch of 16mm and a brightness of 5000Nit, so how should we calculate how high the brightness of each color of our LED is. 1. First calculate the number of pixels per square meter = 1 m2 / (16mm X16mm X10-6) = 3906 pcs / m2
2. The brightness of each Pixel = 5000Nit / 3906 pcs / m2 = 1.28cd / m2
3. If the picture element composition method is 1: 1: 1, it can be calculated as follows
Red = 1.28 X 30% X 1000 = 384mcd
Green = 1.28 X 60% X 1000 = 768mcd
Blue = 1.28 X 10% X 1000 = 128mcd
So that we will know the answer quickly, is it convenient? However, the above calculation method can only be calculated very roughly, and one thing to pay special attention to, because the LED is installed on the screen, a layer of black glue should be sealed, which will reduce the brightness of the LED, plus various LEDs are aging There are different degrees of attenuation afterwards. According to our many years of experience, in order to make the display screen reach the necessary 5000Nit for the image to be clear in the sun, it is best to increase the design value by 30%, which is safer at 6500Nit. Fourth, Lens's optical design LED is a very precise optical system. It is a good optical design that the bowl, wafer, and Lens must be properly matched. Generally speaking, most of our packaging factories did not pay much attention to this point, which is also a very important reason why our display screen technology can not keep up with developed countries such as Europe and the United States. After a lot of analysis, we found that the red wafers and the green and blue wafers with double pads have great differences in the material of the wafers, the thickness of each structural layer, the refractive index, and their bare crystal spots, and their physical dimensions There is also a big difference. If you want to keep the brightness change of the three colors of R / G / B LEDs in the range of 90 degrees to the left and right, you must use different optical Lens.
Lehman Optoelectronics invested in the development of two optical Lens (mold strips), one for red wafers and the other for green and blue. In this way, we have successfully solved this difficult problem for many years. Below we compare the R / G / B three-color light distribution diagram of Lehman LEDs with the R / G / B three-color light distribution diagram of common LEDs on the market, so that everyone can be more clear. The above two pictures are the light distribution effect of LED products using optical lenses developed by Lehman Optoelectronics, which are the horizontal and vertical brightness and angle change curves. The next two pictures are the light distribution effects of common LED products on the market. It can be seen from the figure that the LED products on the market may be reddish at an angle of about 50 degrees, and it is difficult to adjust to an ideal effect. The products developed by Lehman Optoelectronics take into account the customer's needs. The light distribution curves of the three colors of red, green and blue in the range of 90 degrees to the left and right are smooth, the consistency is good, and the ideal level is reached. Regulation provides strong support. The above is Lehman Optoelectronics' experience in LED packaging, light distribution and color matching. Take this opportunity to share with you. We would like to work together with all LED display screen companies to make joint efforts to improve the overall technical level of LED display screens in China.
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