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Preliminary Solution of LED display "mosaic" Phenomenon

Preliminary Solution of LED display "mosaic" Phenomenon

LED display manufacturer has been plagued by "Mosaic" phenomenon problem. Phenomenally, LED display "mosaic" phenomenon manifested as a display surface luminance inconsistencies in the subregion, namely poor uniformity. Full-color LED display poor uniformity including poor brightness uniformity and poor color uniformity both sides. Poor uniformity, "piebald" or "mosaic" phenomenon seriously affected the image viewing. In the same piece of the display tube using the same wavelength, the color uniformity performance uniformity and color fidelity in two ways. Color uniformity referring to the same screen, each pixel color display with a color, between the module and the module; color fidelity refers to the color image and the source image on the display or the source scene between reproduction degree of agreement. Related to the causes and metering LED color, in order to improve color uniformity and color fidelity, the need to reduce chromatic aberration, which has an article involving also choose the appropriate device to correct. However, the brightness inconsistency is the most common, is purely caused by the manufacturing process, which belongs to the mechanical properties, also contributed to the "mosaic" The fundamental reason is that the device can not be used post-correction correction, the study of this problem is the main task of LED screen manufacturers.

Manufacturing process causes include screen manufacturing and tube manufacturing process, because according to the current electronic technology, as long as the errors of electrical characteristics can be improved in the late correction, mechanical properties can not improved by later correction only can be achieve by manufacturing process itself.

1. Cause of "mosaic" and solutions

As described above, the root causes of mosaics are consistency of the lamp itself and consistency when used.

1.1 The reasons of LED lamp itself.

Consistency of the lamp itself including brightness uniformity, color consistency, view point consistency; now, after years of development of the LED display, for the first two points, people have a deep understanding, there have been a variety of ways to solve it. The latter is often overlooked. The author many years of practice, the inconsistency of angle is the most easily overlooked, there are four areas for the following reasons:

(1) LED lamp up and down, left and right angle size inconsistency, shown in Figure 1;

As we all know, the conventional lamp type of LED display lamp is usually used as an elliptical tube, LED early luminous efficiency is low, to get a better viewing experience, lamp manufacturers use optical means of using the human eye habits, increase angle of left and right, narrow angle of up and down, effective use of the light volume, the result is to bring large left and right angle, typically 110 °, up and down a small angle, generally only 45 °. If the lamp vertical Display Screen, you can use an angle no greater than 22.5 °. For example: display high 8m, from the ground along the high 3m, viewing distance 50m under the circumstances, it will result in the loss of 8 ° angle, as shown in Figure 1. From Figure 1and Figure 2 , it can be seen that the angle is substantially linearly changes with the brightness, i.e. luminance corresponding to 50% of 22.5 °. A variety of tests proved that in case of high brightness, 10% error can be tolerated, in other words, more than 10% can be found; further concluded that the error must be less than 4.2 °; if the tube distortion 1 °, reflect the display is 6 °, as shown in Figure 4.

(2) the inconsistency of LED lamp vertical angle , left and right angle.

UP and down angle of LED lamp and lamp, the angle of the left and right are differences, the author has tested a variety of lamp light pattern, wherein NICHIA and CREE lamp in line with specifications parameters, most products there are differences in point of view , e.g., with a 110 ° angle may not symmetrical about the angle, shown in Figure 5, such as + 30 ~ 80 °, the same is true up and down, it is likely Up 30 °, down 15 °, if the selected light does have this situation, there will be "dark spots" and "spotted" phenomenon.

(3) The inconsistency of LED lamp angle and screen angle.

Often, people put the lamp angle directly understood as the display angle of view, this is misunderstanding, first display are larger, and lamp direction is also inconsistent with the screen direction, the actual on Figure 3, the angle range of the lamp falls outside the scope of most human observation. Eye view from the principle point of view, a suitable observation distance should be 2,000 times of the pixel pitch or 5 times of the diagonal distance, select greater of them.

(4)The inconsistency of different color lamp angle.  

From the manufacturing craft of LED lamp, the light shape of red, green, and blue lamp is inconsistent. In fact, we found that only the current date light of CREE submit of the compliance curve provided, part of CREE original factory products can be. Therefore, if you are using a three-color light-shaped lamp with poor compliance, there will be "dirty spots" and "spotted" phenomenon.

2, Cause of LED lights usage

Cause LED lights use is a matter of LED display manufacturers are most concerned about, is the main tasks of manufacturers, but also reflects the main basis for manufacturers technical level, management level, the scale of production and marketing.

LED use should be noted that there is uniformity of LED selection, angle selection, the correction of tube direction, calibration kit design, cabinet design and outer structural design.
(1) LED lamp Selection

Try to use the same brand, same model, same grade LED lamp. The better consistency , the more expensive price, you must first choose a lamp with good consistency angle.

(2) LED angle selection

Try to choose large angle, especially large vertical angle of LED. LED angle selection usually based on the actual use of the LED display to angle requirement. small angle, brightness is relatively high. In the production of the display, some companies in order to achieve positive brightness requirements, have reduced left and right, even up and down angle, so have to choose the angle of LED according to actual situation, such as road traffic screen, choose small angle screen, or small left and right angle but large up and down angle lamp. For Square, remote viewing screen, choose large left and right angle, but small vertical angle lamp, if the scene is similar to "watch across the river" situation, you can choose small left and right & up and down angle screen. For high altitude, and close viewing can choose the circular tube of large angle or select SMD lamps.

(3) LED light correction

After the lamp and lamp angle determination, here the LED light correction that have to ensure of LED lamp and the screen surface is vertical or appropriate angle. To do this, you must take the necessary effective measures before welding, welding, after welding. Before welding the LED tube to be mixed into, after lamp plug in unit board card, required fixtures position, jig pattern as shown in Figure 6. The welding process can not be loose, fall off, remove the light board after soldering operation to light until the module is attached to the interior, do not touch the lamp, but also check the angle of the lamp meets the requirements of the lamp again before potting.

(4) Design Kits

Kits commonly refers to the display of various types of modules, including bottom shell and mask. On the bottom shell required have to be flatness, strength and durability. Cost pressures in the current PC plastic package will be incorporated a lot of filler, there are certain characteristics of the loss, where the most critical is the flatness. It required flatness less than 1: 200, in front of the reasons already mentioned. Therefore, we must strictly consider the PCB into the bottom shell in the level of bias and the module fixed to the casing, since there is waterproof pad between the pan and the tank can cause the fastening screws due to the different power modules and injustice . Otherwise it will appear in the module units "mosaic" phenomenon. For the mask, it is required to be in close contact with the LED tube bore tube, play a further role in fixed; itself must have a certain strength, can protect the lamp from impact; must have good optical design, making the minimum solar deflection , can increase the contrast; hat to guarantee a certain height, the lamp and protect effect of preventing sunlight, highly brim and limit the use of the site by, for example: on the display the direction and latitude regions which, in principle, not to let the light between the lamp and the lamp, do not affect each other for the purpose; thermal deformation as small as possible, including self-heating and the influence of sunlight.

"Design Kits" means a kit used must be carefully taken into account of the above factors, and suitable for the lamp and other additional requirements of this screen. For the widespread use of "public version" module on the market, leading to the lamp and module mask serious mismatch. Public version of the module is suitable for all kinds of tubes, must be large enough, it will inevitably lead to the lamp easy "crooked" result once use it. If the problem does not occur purely coincidental, but most did not consider binding housing needs.

Accordingly, once determining the using method of display, requiring special design special modules, success stories such as: Beijing Pangu Building LED display taken by Kang Jia, its panel module is dedicated project design;

(4)module Glue

Outdoor screen lamp plate mounted to modules need to be added waterproof sealants, which are usually black potting, due to the level of influence the final film surface brightness, if uneven, it will further affect the resulting "mosaic" phenomenon, typically, the adhesive surface between the module error is less than 0.3mm, the level of better than one thousandth.

In the module structure shown in Figure 7, the rubber surface when gluing adhesive surface height requirements overshadowed all the metal parts of the lamp foot; typically Glue height not exceeding module mask pillar (hereinafter referred to as the cover column) as a reference under plastic for offset rubber surface height from the column to cover no more than 0.3mm.

(6) Case-related design and production

Case involving flatness and requirements to ensure flatness is from beginning to end, we must ensure that the box front and the box is fixed parallel to the surface, requiring the display cabinet after the completion of the flatness of better than five thousandths.

Currently used mostly for housing is sheet metal, apply cold-rolled steel oraluminum, in order to achieve the above process requirements, must ensure that the cabinet panel and casing deformation during use. Therefore, we must pay attention to cabinet design back muscle: between modules must be equipped with ribs, in cross section as shown in Figure 8, wherein the width W must be greater than 15mm, height change with the ribs length, as shown in Table 1. The data in Table 1 is the result of structural calculations and the actual tested.

Each contact must ensure the reliability of solder, solder 3 joints each corner on each side, a total of 36 joints, and not missing. Bars and welding parts of each panel must be spaced at least three joints.

Through the above reinforced ways to ensure the tank surface load 100kg cabinet on the 300 × 300mm area is not deformed. Currently sheet metal cabinet design generally "muscle no bone", unless required lifting of the box, do not set the box frame, the outer structure design and installation need to be considered, it must not allow the cabinet laminated layer by layer.

(7) power supply design

Low voltage power supply unit design display cabinets is to ensure a constant LED current from the electrical characteristics. For full-color display is concerned, the majority of constant current driver chips. Constant current characteristic constant current chip is stable and effective control, determines the ability to adapt to changes in load. For a certain constant current chip, different constant current, will have one enter a constant voltage value, if the control chip output current at 20mA, will have to ensure that VDS is greater than 0.5V, 30mA to ensure that more than 0.7V. Changes in the load and the power supply may cause the chip into the enlarged area, loss of constant current action. For example: two red full color tube used in tandem use, when RGB full brightness, the wire load current is increased, the pressure drop increases, lower operating voltage power supply board, resulting in constant current driver chip into the enlarged area, the current decline, decrease in brightness, the screen color cast. Some chip actual may be different with specifications, need to verify the actual use in order to avoid this situation.

For wire wiring inside the box too long, it is likely to form an excessive pressure drop, in the case of low voltage 5V, the electrician is not in accordance with the safe current use. For example, 2.5 SQm wire, resistance per meter 7.44mΩ, the electrical safety current value of about 28A, if with two 10A modules, consider the case of only one meter wire, a positive and negative terminals of each voltage drop 0.3V, 4V voltage required if the lamp series, it is essential that the cell plate voltage not less than 4.8V, it is very harsh conditions, if the wire is further extended, the problem will be more serious. Low performance when the display brightness is no problem, the display will be some color deviation module all white, the general is a real point two lights used in tandem full color, all white display when the red lights up.

(8) module is installed

Module during installation must ensure that: the lamp module from external shocks and modules must be evenly flat attached to the cabinet, it is necessary to complete the installation process in a dedicated tooling mold, shown in Figure 9, the fixed screw required using a torque wrench must be consistent with the tightening torque measurement tool needs special testing after installation, does not meet the requirements to be adjusted to a standard level.

(9) Transport packaging of matrix screen

Monitor production from the manufacturer to complete the installation site is easily overlooked aspect of many factors mentioned its purpose is to ensure smooth vertical display lamp, after the display unit cabinet assembly to be properly packed, make sure the box vertically, the box should indeed play a protective role, avoid the screen by the impact of hard objects deformation. Transport conditions should be set tooling frame shown in Figure 10.

For laminated box in transit, must deal with the overlying load, must not allow direct compression box, change the shape of the box. I found many times in practice, after a good test in the factory screen after installing the transit effect is worse phenomenon, in addition to the box for their own reasons, transit negligence is one of the important reasons.

(10) outside the structure design and installation

Finally, the display must be installed onto the steel structure to form a whole, the design and construction of the outer structure, the same screen installed to keep the plane stable and flat, while the mounting structure is designed to ensure that a single box does not sag, deformation, not pressure should be left between the gap and down the box, the box itself directly to the steel structure bearing weight.

3. Conclusion

In summary, LED display "mosaic" phenomenon is a problem integrated system, all aspects of the lamp and screen production to ensure flatness of the lamp is the most fundamental solution, any subsequent "brightness, color" correction the premise is in accordance with standard produce an acceptable display.

                                                Translator: Eva
                                                 8th, Sept. 2016

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