This article is an answer to several common technical problems in the LED display industry by Zhao Yun, senior application engineer of OSRAM Opto Semiconductors, and Liang Zechun, marketing manager, and OSRAM Opto Semiconductors’ corresponding product solutions for different technical aspects
The higher the brightness of the outdoor display, the better
In the domestic LED display industry, brightness has always been the most important indicator for everyone to evaluate the display effect, especially for outdoor screens. Over time, it has formed a misunderstanding that the higher the brightness, the better, but it ignores another indicator that is extremely important to the display effect-contrast
According to the definition of industry standard SJ/T11281-2007 “Light Emitting Diode (LED) Display Screen Test Method”, under a certain ambient illumination, the ratio of the maximum brightness of the display screen to the background brightness is called the highest contrast ratio (for specific test conditions and methods see the standard). For example, the brightness of a screen is 5000cd/m2 in full white state, and 1cd/m2 in full black state, so the contrast ratio of the screen is 4999:1. The definition of contrast actually emphasizes the degree of recognizability of the display screen in the use environment. Compared with the brightness index, the contrast index increases the brightness of the full black screen state, and the brightness of the full black screen state is related to the ambient light and the screen body ( The reflective properties of LEDs and masks) are closely related. The following outdoor comparison examples show that a screen with high brightness and low contrast is not as effective as a screen with relatively low brightness and high contrast
Figure 1 Comparison of the effects of screens with different brightness and contrast ratio
In order to improve the contrast of LED displays, OSRAM Opto Semiconductors has been focusing on reducing the reflectance of LED displays since it launched the world’s first black-body surface-mounted three-in-one LED (LRTB GFUG) in 2007. The current main product LRTB GRUG (Fig. 2) It is one of the LEDs with the highest surface blackness on the market. Not only does it use a black body shell, but the bracket around the chip is also treated with black body. With the special driver and mask design, this product can achieve the highest contrast ratio up to 8000:1. Figure 2 LRTB GRUG of Osram Opto Semiconductors product
In addition, even under certain conditions of contrast, the higher the brightness, the better. A screen with too much brightness may cause the following three problems: (1) high energy consumption; (2) light pollution; (3) reduced hardware life
To sum up, we believe that the brightness of the LED display is not as high as possible, and the contrast index can reflect the quality of the display effect better than the height index; and under the condition of a certain contrast, the higher the brightness, the better
The larger the viewing angle of the led display, the better
Before discussing the size of the viewing angle, we also need to clarify the concept of the viewing angle. Many manufacturers in the industry do not set the viewing angle specifications accurately enough, thinking that the angles from which the displayed content can be seen are all viewing angles, but this is not the case. According to the definition of the industry standard SJ/T 11281-2007 “Light Emitting Diode (LED) Display Screen Test Method”, assuming that the brightness of the normal direction of the display screen is Lf, the brightness of the display screen is detected from the left and right sides of the normal line of the display screen center, when When the luminance values on the left and right sides drop to Lf/2, the angle formed between the two observation lines is called the viewing angle in the horizontal direction of the display screen. The same is true for vertical viewing angles
The viewing angle of the LED itself is also based on the same definition above. Due to the definition of the brightness test on the display, the light probe needs to replace at least 16 LED pixels, which will cause the viewing angle of the screen to be slightly larger than that of the LEDs used (assuming that the mask has no light blocking). As shown in Figure 3, the LED display made of 100-degree viewing angle LEDs has a viewing angle of 134 degrees
Figure 3 Relative light intensity comparison between Foldable LED Mesh Curtain and LED displa
So is the viewing angle of the LED display bigger the better? In fact, it depends on different application requirements. For example, the road traffic information screen, because the audience of the displayed content is only road vehicles, so the viewing angle usually only needs to be about 30 degrees. Excessive viewing angles will cause light pollution at other angles and reduce energy efficiency; another example is the exterior wall of an outdoor building. Advertising display screens are intended for passers-by on the street, so the horizontal viewing angle needs to be relatively large, while the vertical viewing angle should be minimized
For different application needs, OSRAM Opto Semiconductors has a series of LED products with different viewing angle specifications, and three of them are listed in Figure 4
Figure 4 Display application LEDs with different viewing angle specification
The narrower the LED wavelength range, the more consistent the color must be
The uniformity of the color displayed by the led display depends on the following three points: (1) the consistency of the color between the individual LEDs, (2) the consistency of the output current of the driver chip, and (3) the consistency of the temperature. Among them, the consistency between LED individuals is the most important. Although the current point-by-point correction method based on the control system is very mature, the correction is at the cost of sacrificing the absolute brightness and the saturation of the three primary colors. Therefore, the factory consistency of the LED is still crucial to the uniformity of the displayed color
The current mainstream practice in the industry is that LED suppliers use the dominant wavelength for color binning, and continuously narrow the upper and lower limits of the dominant wavelength to improve color consistency. However, using the dominant wavelength as the only parameter limiting color is a completely unreasonable way
Figure 5 shows the CIE1931 color space defined in the field of colorimetry. All visible light colors are located within the horseshoe shape and can be strictly defined by (x, y) coordinates. Each color has two properties of dominant wavelength and saturation. The dominant wavelength is defined by the wavelength point on the boundary and the line before the point of equal energy (0.333, 0.333), and the saturation is defined by the ratio on this line segment. For example, in Figure 5, we select a certain green point B, make a straight line connecting point E and point B and extend it to the boundary point A (540nm), then the dominant wavelength of the green color is 540nm; and the saturation of this point is BE /AE = 50%. All the color points on the line segment AE have a dominant wavelength of 540nm, and the difference is that the saturation is not the same
Figure 5 CIE1931 color spac
Therefore, the color binning method of the dominant wavelength is not scientific. Due to the complete lack of saturation specification limits, even the narrowest wavelength range cannot precisely limit the color consistency
For all LED display products, OSRAM Opto Semiconductors adopts the color coordinate bin method based on the CIE1931 color space, and simultaneously limits the specifications of the dominant wavelength and saturation of the LEDs shipped. The intuitive comparison with other manufacturers’ main wavelength bins is shown in Figure 6. The yellow quadrilateral in the figure is the color area of the No. 5 bin of OSRAM LED LRTB GRUG green light (for more other bin areas, please refer to the product specification). And assuming that another LED supplier takes the dominant wavelength range of 526nm to 531nm as the shipping specification, then the color range corresponding to this specification in Figure 6 is the red triangle area, which is almost 5 times the size of the yellow quadrilateral. Even if the dominant wavelength range is narrowed down to 2nm width from 527nm to 529nm, the color discrete region is still twice the size of our color index quadrilatera
Figure 6 Comparison of the color coordinate bin of OSRAM Opto Semiconductors and the main wavelength bin of other manufacture
To sum up, due to the irrationality of defining the color itself by the dominant wavelength, the color consistency cannot be effectively improved by narrowing the dominant wavelength range of LEDs. The correct approach is to reasonably limit the dominant wavelength range and saturation range of the shipped products at the same time through the color coordinate bin method. Osram Opto Semiconductors adopts the color coordinate binning method for all LED display products. Through accurate online binning equipment and material process precision control, we can not only ensure our own high enough production rate but also provide LED display applications with LEDs with high color consistenc
The higher the display pixel density, the bette
Display pixel density always seems to be the bigger the better, because it means that in theory the display screen will be finer. However, how big the pixel density is, we think it needs to be considered from the following three aspect
First, the resolution ability of the human eye (observation equipment other than the human eye is not considered for the time being. According to the requirements of “TCO Certified Displays 7.0”, at least 30 display pixels are required within a 1-degree viewing angle of the human eye, which means that a single pixel pitch For the viewing angle requirement is 1/
The limit to which the human eye can distinguish two point light sources is that the two point light sources and the human eye form an arc angle of 1/60, then the distance between the two points (Dpp) and the observation distance (L) have the following relationshi
When the observation distance (L) is 10 meters, the limit of point spacing (Dpp) is 5.8mm. A higher pixel density would be pointles
Figure 7 Point spacing and observation distan
Second, the load capacity of film source, video processor and driver chip. If the resolution of the film source is much lower than that of the display device, the display device is obviously overcapacitated. For video processors and driver chips, if the capabilities remain constant, the increase in load pixels will result in a decrease in refresh rate or gray scale. In many cases, the display effect brought by low refresh or low gray is worse than low pixel densit
Third, cost. First of all, the increase in the number of LEDs will increase with the square of the pixel resolution. The cost of the PCB board will also increase due to more responsible circuit design per unit area. In addition, if you don’t want to sacrifice line scan, refresh rate or gray scale, the cost of driving components will increase dramaticall
Under the same dot pitch, the larger the LED size, the bette
This question can be equivalent to the proportion of the LED light-emitting surface in the total display area, whether the display effect is bigger or better. The first point that needs to be emphasized is that to discuss the display effect, it is necessary to be at a suitable viewing distance. If the observation distance is too short, resulting in discrete pattern pixels, it is a problem of inappropriate pixel density design, rather than a problem of insufficient proportion of LED
When the viewing distance is large enough, the proportion of the LED light-emitting surface in the total display area will affect the contrast of the screen. Since the space between the LED and the LED will be occupied by the mask, and even the blackest LED, the reflectance rate will be higher than that of the mask, so the higher the proportion of the LED light-emitting surface in the total display area, the higher the contrast of the screen. lower. Figure 8 is a schematic comparison of displays with the same dot pitch and different LED size
Figure 8 Schematic comparison of displays with the same dot pitch and different LED siz
In response to this theory, OSRAM Opto Semiconductors has launched a high-brightness LED (model LRTB R48G) in a 1010-size package, which is suitable for display designs with a dot pitch of 3-5mm. Compared with traditional designs based on 2020 or similar size LEDs, LRTB R48G can provide higher display contrast while ensuring brightnes
Outdoor protection ability = waterproof performanc
LED displays for outdoor use need to cope with harsh environmental tests, and the reliability requirements of LEDs are very good. At present, many practices in the industry directly look at the waterproof performance. It is true that the failure of LEDs on displays due to moisture or water ingress accounts for a high proportion of all the various failure modes. However, outdoor reliability of LEDs is by no means simply equivalent to waterproof performance. We believe that in addition to the requirements for waterproof performance of LEDs, at least the following three points need to be considered: (1) the impact of outdoor complex gas components on LEDs; (2) the impact of solar radiation on LEDs; (3) more severe outdoor high and low temperature and the effect of humidity with LED
OSRAM Opto Semiconductors strictly stipulates the following 7 categories and 22 reliability test requirements for LEDs used in outdoor LED displays. In terms of waterproof performance, the international general IP level test specification is adopted. For the other three points mentioned above, we add corrosive gas test and salt spray test to ensure that the product is applied in various harsh outdoor environments (such as industrial or automobile exhaust environment, humid seaside environment, etc.); solar/ultraviolet radiation test can ensure The performance reliability of the product under direct sunlight outdoors; more comprehensive high and low temperature life tests with humidity, and high and low temperature impact tests to deal with drastic changes in outdoor temperature and humidity. For specific test conditions and results, please refer to the reliability report documents of different products of OSRAM Opto Semiconductor
Figure 9 Osram Opto Semiconductors’ test specifications for LED products used in outdoor display scre
s
Does not “dead light” mean that the LED has not fail
?
The industry usually uses “dead light” (LED short circuit or open circuit) to define the failure of an LED, and uses the “dead light” rate ppm to judge the quality of an LED. It is true that the short circuit and open circuit of the LED will directly lead to the failure of the dead pixels, linear or cross display on the LED display. However, in our opinion, failure of an LED cannot be defined as just an open or short circuit, but should be a change in any key specification beyond an acceptable range. Such specification parameters should at least include: (1) Brightness – the attenuation of LED brightness will reduce the brightness of the screen, the inconsistent attenuation between RGB will cause the overall color drift of the screen, and the inconsistent attenuation between individual LEDs will cause “flower screen”; ( 2) Color point – the change of color point will cause the decrease of screen saturation, the color drift of the three primary colors and white balance, and the inconsistent color point offset between LED individuals will cause “flower screen”; (3) Voltage value – – The aging of LED will cause the voltage value to increase, and too high LED voltage may cause problems in the control of the driver chip; (4) Reverse leakage current – especially for indoor small-pitch applications, the increase in reverse leakage current is precisely Symptoms of LED short circu
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OSRAM Opto Semiconductors can provide detailed reliability test reports for all LED products used in LED displays. The content of the report is not only to provide the “dead light” rate, but also through the curve of key LED specifications changing with time in each test (as shown in Figure 10, the product LRTB GRUG in the high temperature and high humidity 1000-hour test, brightness, voltage and color point change curves with time).