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The higher brightness, the better for outdoor LED display screen ?


In the entire LED display screen industry, brightness has always been the most important indicator we use to evaluate display performance, especially for outdoor purpose application . Over times, the higher brightness , the better outdoor LED display screen . Actually this is totally misunderstanding . Because most of us are ignore another indicator that is extremely important to the display effect—contrast.

According to the definition of the industry standard SJ/T11281-2007 “Light Emitting Diode (LED) Display Test Method”, the ratio of the maximum brightness to the background brightness under a certain ambient illumination is called the highest contrast (see specific test conditions and methods This standard).For example, when the screen is in full white status, the brightness is 5000cd/m2, and the full black status is 1cd/m2, so the contrast of the screen is 4999:1. The definition of contrast actually emphasizes the visibility of the display screen during the working environment . Compared with the brightness index, the contrast index increases the brightness of the entire black screen, and the brightness of the full black screen is different from the ambient light and the screen body,Reflective attributes of LED and mask are closely related.

The right image shows the outdoor comparative examples can be seen that the screen with high brightness and low contrast does not have relatively good brightness and high contrast.

In addition, even with a constant contrast, also are not the higher brightness, the better. The screen with too high brightness may bring about the following three problems: (1) high energy consumption, (2) light pollution, and (3) reduced hardware life.

In summary, we believe that the brightness of the LED display is not as high as possible, and the contrast index is better than the height index to show the effect of the LED display screen ; and under the condition of certain contrast, also is not the higher the brightness, the better.

The larger the viewing angle of the LED display, the better?

Before discussing the view angle, we also need to clarify the concept of the view angle. Many companies in the LED display screen industry think the view angles just means the content that which we can view in any angle. Actually, this is not the case. According to the definition of the industry standard SJ/T 11281-2007 “Light Emitting Diode (LED) Display Test Method”, assuming that the brightness of the normal direction of the display screen is Lf, the brightness of the LED display screen is detected from the left and right sides of the center line of the LED display screen. When the luminance values on the left and right sides fall to Lf/2, the angle formed between the two observation lines is called the viewing angle in the horizontal direction of the LED display screen.The vertical direction is the same.

The view angle of the LED itself is also based on the same definition above. Due to the definition of the brightness test on the LED display screen, the light sensor needs to included at least 16 LED pixels, which may cause the viewing angle of the LED screen to be slightly larger than the viewing angle of the used LED (assuming that the mask will not blind the light).

So is the LED display’s viewing angle bigger, then better? Actually, this depends on different application requirements. For example, in road traffic information screens, since the audience of the display content is only a road vehicle, the viewing angle is usually only about 30 degrees. An excessively large angle of view will result in light pollution at other angles and make the energy be lower efficiency. Another example, the exterior wall of an outdoor building. The advertising display shows that the audience of the content is the passersby on the street, so the horizontal viewing angle needs to be relatively large, and the vertical viewing angle should be minimized.

The narrower of the LED wavelength range, the more consistent the color?

The uniformity of color display on LED display screens depends on the following three points: (1) the consistency of color between individual LEDs, (2) the consistency of the output current of the driver chip, and (3) the consistency of temperature. Among these, the consistency among LED individuals is the most important. Although the current point-by-point correction method based on the control system is already mature, the correction is at the expense of absolute brightness and trichromatic saturation. Therefore, the factory consistency of the LED is the still crucial point for the uniformity of the display color.

The current method in the industry is that LED suppliers use the main wavelength as the color sub-bin to improve the color consistency by continuously reducing the upper and lower limits of the dominant wavelength. However, using the dominant wavelength as the only parameter to limit the color is a completely unreasonable way.

The right picture shows the CIE1931 color space defined by the colorimetry field. All visible colors are within the horseshoe shape and can be strictly defined by (x, y) coordinates . Each color has two main wavelength and saturation properties. The dominant wavelength is defined by the line connecting the wavelength point and the equal energy point (0.333, 0.333) on the boundary. The saturation is defined by the ratio on the line segment. For example, On the left picture , we select a certain green point B as the line connecting point E and point B and extend it to point A (540 nm). The dominant wavelength of green is 540 nm; the saturation of this point is BE. /AE = 50%. All color points on the line segment AE have a dominant wavelength of 540 nm, and the difference is that the saturation is not the same.

Therefore, it is not scientific to design the color of the dominant wavelength in Bin. Since the specification of the saturation is completely lost, even a narrow wavelength range cannot precisely limit the color consistency.

In summary, because of the irrationality of defining the color itself through the dominant wavelength, it is not possible to effectively improve the color consistency by narrowing the dominant wavelength range of the LED. The correct approach is to rationally limit the main wavelength range and saturation range of the shipping products by using the color coordinates in Bin.

The larger the display pixel density, the better?

The LED display pixel density seems to be always larger , the better, because it means that the theoretical display screen will be more delicate . However, How many pixel density of the LED display do we really need? we think that need to be considered from the following three aspects.

First , the human eye’s resolving power (without considering human eye observation equipment. According to “TCO Certified Displays 7.0” requirements, the human eye needs at least 30 display pixels within one degree of viewing angle, which means a single point spacing. The perspective requirement is 1/30.

Second, the film source , the load capacity of the video processor and driver chip . If the resolution of the film source is much lower than the resolution of the display device, the display device obviously has excess capacity. For video processors and driver chips, if the capacity is kept constant, the increase in load pixels will cause a reduction in the refresh rate or grayscale. In many cases, the low refresh or low gray display is worse than the pixel density.

Third, the cost. Increase in the number of LEDs will increase with the square of the pixel resolution. PCB board costs also increase because of the more responsible circuit design per unit area. In addition, if you do not want to sacrifice the number of scans, refresh rate or grayscale, the cost of driving components will also increase dramatically.

At the same pixel pitch, the larger the LED size, the better?

This problem can be equivalent to the proportion of LED light emitting surface occupied in the total display area, the display effect is not as large as possible. The first point to emphasize is that the discussion of display effects must be based on the perfect view distance. If the view distance is too close, which leads to the discreteness of the pattern pixels, it is a problem that the pixel density is not designed properly, rather than the problem that the LED occupies an insufficient proportion.

In the case where the view distance is sufficiently large, the occupation ratio of the LED light emitting surface in the total display area will influence the contrast of the screen. Since the space between the LED and the LED is occupied by the mask, even the darkest LED indicates that the light reflectance is higher than the mask. Therefore, the proportion of occupied LED light emitting surface in the total display area is higher, then the contrast of the screen is lower.

Outdoor protection capacity = waterproof performance?

LED display screens for outdoor use need to deal with severe environmental tests, and the reliability of LED is very good. At present, many practices in the industry directly look at the waterproof performance. Obviously, the failure of LEDs on the display screen due to moisture or water ingression accounts for a high proportion of all failure modes.However, the outdoor reliability of LED is not simply equivalent to waterproof performance. We believe that in addition to the waterproof performance requirements of LEDs, at least the following three points need to be considered: (1) the influence of outdoor complex gas components on LEDs; (2) the influence of solar radiation on LEDs; (3) the harsher outdoor high and low temperatures And the effect of humidity with LEDs.

Not “ dead pixel ” = LED did not fail?

The industry usually uses the “dead pixel ” ( LED short circuit or open circuit ) to define the failure of the LED, use the “dead pixel ” rate of PPM to judge the quality of the LED.Obviously, the short circuit and open circuit of the LED will directly lead to the failure of the dead point, line or cross display on the LED display. However, we believe that LED failure cannot be defined only as an open circuit or short circuit, but should be a change in any critical specification parameter that exceeds an acceptable range. Such specification parameters should at least include: (1) Brightness – The attenuation of the LED brightness will reduce the brightness of the screen, and the inconsistent attenuation between RGB will cause the overall color float of the screen, and the inconsistent attenuation between individual LEDs will cause a “flowering screen”; 2) The color point – the change of the color point will cause the screen saturation to decrease, the trichromatic color and the white balance color float, the color point offset between LED individual elements will cause the “ mixed screen “; (3) voltage value – – The aging of the LED will cause the voltage to rise, too high LED voltage may cause the driver chip to control the problem; (4) reverse leakage current – especially for indoor small pixel pitch applications, the reverse leakage current is larger LED precursor to short circuits.

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