The LED display business , that started with around 1987 , has become the one with scale after decades’ development . Currentlly , the LED display manufacturers are becoming more and more with some outstanding ones . Just by their efforts , the LED dislplay business flourished . The 21th century is the century fulled with panel display products . LED display , as one of the main products , will be bound to have a prosperous future . LED display is a electronic monitor combined with many hi-tech technologies , such as computer technology , data processing technology , electron technology and colourimetry. Generally speaking , LED display can be devided to single color display , dual color display and full color display .
Single color display is the one that constituted by single color LED lamps , only one single color can be showing . The one that constituted by red and green LED lamps is called dual display . The one that constituted by red , green and blue LED lamps is called full color or tri-color display . As the continuously increase of the green LED lamp brightness , full color LED display is popularized and promoted with a fast space . Especially the success of the application of Beijing for Olympic Games , which sponsored “Environmental Olympic” , promoted the development of full color LED display . Full color LED display also can be sub-divided to Indoor and Outdoor , now let’s introduce some common used phrases in full color LED display :
Pixel: It is composed by three color LED lamps , they are red , green and blue . Based on different combination method , it also includes the following kinds , 1R1G1B , 2R1G1B and 2R2G1B and so on , which is the basic unit for a full color LED display to show contents . Gray Scale: It is the brightness fineness that a LED display can be adjusting . Generally speaking , the color will be more rich while the gray scale is higher . There are two driving methods for a LED display to change the color , one is to change the current . Generally speaking , in order to guarranty the LED’s ability of anti-attenuation , it will be better to set the display’s working current to be under 20mA . Another method to control the gray scale is to use human eyes’visual inert. Human eyes’ senseness to the average brightness is depends on the duty cycle , that is to say as soon as the period of duty cycle is short enough , huma eyes can not distinguish the difference between attenuation and extinguish.
Generally speaking , the pixel pitch for indoor used LED display is 2-12mm , between them 2-8mm are the ones with TOP SMD technology or the newly developed sub-table paste technology . The pixel pitch for outdoor used LED display is 14-26mm , between them 20mm display is the most mainstream one based on the development of green LED lamp brightness. After decades’ development , domestic LED display manufacturers has gotten a great progress in LED display making . But there is still a big gap comparing with the developed countries in Europe and North American , especially the most important parts , such as LED encapsulation and design . Our company , Dicolor, has been devoted our mind to study LED lamps’ light . Now we have a professional team fulled with elites engaged in LED searching for a long time , they summed some solutions for LED light and color matching . Now we would like to share it with all of you and hope it will be helpful to the development of the full color LED display technology .
LED light matching is a set of technology which includes the chip’s K-factory management , the control of encapsulation technology , the adjustment of white balance and the optical design of LED lens . Now please see the following statement separately , The companies for chip’s K-factory management controlled strictly for the mass produced chips , and set the specific data base for every item , from which we can see the detailed information about every produced item . Also all the data will be filed and analyzed systematically , thus we will have the detailed bills for all the chips . So we can control the chip production from the beginning , then calculate out intensively the specification of the chips what we need , especially the production of the parts for full color LED display , the control on chip is much more strict as we need to not only meet the customers’ requirement on brightness , but also to solve , from the viewer of customers , the questions possible appeared during white balance adjustment . Under this way, we need to not only meet the requirements on wavelength and brightness , but also pay more attention to the following two points : one is the brightness of the produced displays need to be normal distributed , the color should not be interrupted , the wavelength should be the same , the chip’s average wavelength and the chip quantity of every board should be listed out when they are beginning to be production , also should be analyzed by PC , for example using a column diagram , to make sure the distribution of the chip’s wavelength is normally . If the attention to the upper processes is not enough , it will be very difficult to adjust the white balance just because of the un-normal distribution of LED chip’s brightness and wavelength .
The control of the encapsulation technology . There are many unique features for full color LED display’s LED chip encapsulation. The first thing we need to do is to control the raw materials strictly. If the LED quality is not so good , it will be very easy for the LED lamps to go to broken just because of the environment that the display is working in is always very bad , eight constant high temperature or constant very low temperature , as well as the long-term corrosion from the rain . Now we have a set of advanced devices to inspect the raw materials to meet the requirement of controlling the raw materials strictly. The good quality copper pins , with the ability of good ability of heat and electricity conducting , can increase in a great degree the LED lamp’s thermal resistance . In addition , we also have a special regulation on the pin’s clad layer to make sure the product quality. In order to increase the brightness , we processed the LED lamp pins with matt finishing technology . On the question of epoxide resin , we choose especially the high Tg glue which can resistant UV . On the technology of toasting , after many many times’ experiments , we increased the Tg value of the glue , so the reliability of the products will be increased greatly .
White balance design . According to the customers’ requirement , we summarized a set of calculation methods . As soon as the customers can provide us the needed brightness and pixel pitch , we can provide them a perfect solution for light matching . At first , I would like to introduce several terms and definitions . White balance : It is the process to adjust the three colors(red , green and blue) to be a supposed white .
Two parts need to be adjusted , color and brightness . 1: Setting the regular terms : temperature , voltage and power supply cable . 2: Setting the color’s target effect and scale(tolerance) , which normally means the color’s X and Y value . 3: Adjusting the white balance. The brightness value of the mixed colors is equal to the total values of every single color brightness, which called Brithtness Adding Law . The lights with the same outside appearance , no matter if their spectrum elements are the same or not , have the same effect after mixed . That is to say , all the colors that have the same visual effect have the same value . In this way , another law can be summarized , that is Color Substitution Law . If we use (C) to represent the the unit of the matched color , (R)(G)(B) for the units of the single red , green and blue color before mixing, R, G, B, C for the quantity of the single red , green and blue color , as well as the matched color , when the match is met.
C(C) R(R) G(G) B(B) means the visual equal, R, G, B are the represented qualtity , they can be negative value .
Please see the below diagram: Let’s introduce Center of Balance Law. In the picture , B means color 1, G means color 2, R means color 3 , C means the mixed color of G and R, W means the mixed color of B and C . In fact , it is just a white balance diagram , by thinking C as the mixed color of green and red , we will find it locates approximately in the yellow area . If we re-mix C and blue color, we can get white color.
Please see the formula: B=X1+Y1+Z1C=X2+Y2+Z2
X1, Y1, Z1; X2, Y2, Z2 mean the values of blue and C mixed . So the Center of Balance Law formula is : CM/BM=B/C=(X1+Y1+Z1) /(X2+Y2+Z2)
That is to say , it is a inverse ratio between CM distance and C value , if the C value is larger , the CM distance will be shorter . It is the same as making incandescent lamp , the color will be yellower if the fluorescent powder is more . Please see the below calculation method : 1. At first we need to change the area lights to point light to get the brightness value of W point . We know the unit of light is Nit (cd/m2) , and that of the brightness is mcd.
1. Find the coordinates value after R and G colors mixed (Knowing the coordinates value of R, G, B, W points, we can calculate out the coordinates value of C point by calculating the coordinate of the line extended by the BW straight line).
2. The IV value of C point can be calculated out according to the Center of Balance Law (the IV value of W point is given).
3. The IV value of B, G, R points can be calculated out according to the law of complementary colors.
1. The needed currency value of R/G/B lamps during working can be calculated out , but it is an approximate one because of the below factors:
a. The ratio between IV and currency is not absolutely direct .
b. The known IV and WD values are average ones , so when we are adjusting the white balance , we can not following this value absolutely , we need to do a bit adjustment according to the real situation . Maybe you will think the above method is too professional and complicated , we still have another more simple method for your reference . As we know , in order to get a mixed white color, the color matching rate of R/G/B is 3:6:1. We can calculate out the brightness value for every single color’s LED lamps according to this ratio . For example , one customer need a P16mm outdoor full color LED display with the brightness of 5000Nit , then we can calculate the needed brightness value for every color’s LED lamp .
c. Calculating the pixel quantity of every square meter as below : 1m2/(16mmX16mmX10-6)= 3906pcs/©O
2. The brightness value for every pixel = 5000Nit/3906pcs/m2=1.28cd/m2
3. If the color constitution ratio is 1:1:1, the brightness value for every pixel can be calculated by the following method :
So we got the answer so quickly , isn¡¯t it fast ? But we can just get an estimated value by the upper method , and we need to pay more attention to one point , that is it is better for us to increase the brightness value by 30% . The basic requirement on the brightness to show a clear picture under sunlight is 5000Nit , after 30% increasing, it will be 6500Nit . The reason is that after inserting the LED lamp on the PCB board , a layer of black glue will be added on the face of the board , pulsing the effect of the attenuation of the brightness after aging test , the LED brightness will be lowered .
The optical design of Lens. LED is a very accurate optical system. It will be a perfect design only with accurate match between LED cup , chip and Lens . Generally speaking , many LED encapsulation factories did not notice the importance of this point , it is also one of the most important reasons that our LED display technology can not compete against the ones from the countries in Europe and North American . After long-term’s analysis, we realize that , in order to make sure the every color LED lamp’s brightness scale to be ±90 degree absolutely, it is necessary to adopt different optical Lens with the reason of differences of chip materials , the thickness of every layer , index of refraction and the facula of the bare chip between red chip and green or blue chip .