With its rich expression and brilliant colors, LED display has become a favorite of the advertising industry in recent years. Stations, ports, docks, plazas, traffic lanes and other crowded places are filled with large and small, eye-catching LED displays. As it has increased, people have become more and more complaints. Its too high brightness and unreasonable position have a great impact on people's vision and life. At present, there is no standard for limiting the light pollution of LED displays in China, and there is no clear requirement for LED light pollution in the world.
This paper combines the "Shenzhen Outdoor Advertising Management Measures" and the algorithm of disability glare to study and discuss the brightness standard and setting position of outdoor LED display, and hope to find a way to limit the light pollution of LED display at night.
Research subjects and related standards and regulations
The most influential people with high-brightness LED displays are motor vehicle drivers. Because pedestrians can transfer their own gaze, motor vehicle drivers generally need to look ahead. The glare of the LED display at night causes the driver to be uncomfortable or directly affects the reliability of the perceived object, and is liable to cause a traffic accident. Therefore, the driver has higher requirements for glare control. If the display settings meet the requirements of the driver, they should also meet the requirements of pedestrians.
Due to the visual area, compared to the display screen of the parallel road axis, the glare of the display screen directly facing the driver's line of sight at the intersection is more affected. Therefore, this paper mainly studies the display screen for the direction of the vehicle.
Glare can be measured by two indicators: 1) disability glare threshold increment TI, 2) discomfort glare level G.
In its 1995 recommended motor vehicle traffic road lighting standard, CIE only introduced an evaluation index for glare limitation, and abandoned the requirement for G. Therefore, this paper only analyzes the depreciation. In fact, CIE 115-l995 reports that "devices designed within the recommended threshold increments are generally acceptable for their associated discomfort glare." That is, TI, meeting the requirements, G can also meet the requirements.
Article 7.0.2, paragraph 3 of the “Design Rules for Urban Night Lighting†(JGJ/T 163-2008) stipulates that the threshold increment of glare generated by non-road lighting facilities on urban roads should not exceed 15%. Therefore, the TI value generated by the LED display should be no more than 15%.
Article 5.6.2 of "Design Specification for Urban Night Lighting" (JGJ/T 163-2008) stipulates that the maximum allowable value of the average brightness of advertising and sign lighting in different environmental areas and different areas shall comply with the requirements of Table 1.
In Table 1, the E1 area is a natural dark environment area, such as a national park, a nature reserve, and an area where the observatory is located; the E2 area is a low-brightness environment area, such as a rural industrial or residential area; and the E3 area is a medium-brightness environment area, such as Suburban industrial or residential areas; E4 area is a high-brightness environmental area, such as urban centers and commercial areas.
The "Shenzhen Outdoor Advertising Management Measures" stipulates that the setting of electronic display advertisements with an area of ​​more than 10 m needs to be approved.
Combined with the above two regulations, it can be known that for Shenzhen City, the government-controlled LED display area is above 10 m2. In fact, most of the outdoor full-color LED display area is more than 10m2, and most of them are set in the prosperous area (E4 area). The maximum allowable value of the average brightness is as shown in Table 1 and should be 400cd/m2.
LED display set height calculation
Because the delta value of the disability glare threshold generated by the LED display should be no more than 15%, we can use the calculation formula to find the minimum set height of the display facing the direction of the vehicle, as shown in Figure 1.
The disability glare threshold increment TI is calculated as
Where: 1) Lav is the average brightness of the road, the applicable range is 0.05cd/m2
From this, when θ is in radians,
Now let's calculate the illuminance Eθ: The illuminance calculation stipulates that when the circular illuminant diameter is less than 1/5 of the irradiation distance, or the line source length is less than 1/4 of the irradiation distance, the illuminance can be calculated by the point source. It can be seen from Fig. 1 that the side length of the display screen is much smaller than the driver's line of sight, and the illuminance can be calculated as a point source.
In the formula, the brightness is, S is the area, and d is the distance from the point to the cursor. It can be seen from Table 1 that the maximum allowable average luminance is 400 cd/m2, so the luminance here is 400. r is the side length of the equivalent square, that is, the side length of the square equal to the rectangular area of ​​the display screen. A is the distance from the center of the display to the driver's eyes. h is the height of the display center to 1.5 meters above the ground.
Substituting formula (3) into formula (2) can be obtained
When θ=20°, the conversion to radians is 0.348; and sin20°=0.342, the difference is only 1.7%. It can be seen that when θ<20°, the radians of θ are approximately equal to sin θ. therefore,
When TI=15%, r/h=0.0578 (6)
It can be seen that r is inversely proportional to h, and the longer the side length of the display screen, the larger the area. The more you need to set up in a higher position, the disability glare does not exceed the specified value.
When the display screen area is 10m2, the side length of the equivalent square is r=3.16m, the generation formula (6) is h=54.67m, plus the driver's eye height is 1.5m, and the height of the 10m2 LED display center should be from the ground. It is 56.17m.
in conclusion
From the above research and calculations, the following conclusions can be drawn:
1) The maximum average brightness of LED displays with an area greater than 10 m2 at night should be 400 cd/m2.
2) The size of the LED display screen should be linearly inversely proportional to the height from the ground. According to formula (6), it is easy to calculate the height of the LED display of different areas from the ground. Note that the calculated h also needs to be added to the driver's eye height of 1.5 meters.
3) The height of the center of the LED display with an average brightness of more than 10 m2 of 400 cd/m2 should be greater than 56.17 m.
At present, there is no strict control on the height of LED display. There are many intersections with display heights well below 56.17m. Combined with the actual situation in China, we can appropriately relax the establishment of height conditions.
The intersection display has a long-term influence on the driver who stops at the intersection waiting for the traffic light, and the glare is the most harmful. We can refer to the assumption of CIE (the light-blocking angle of the vehicle roof is 20 degrees, the observer's eyes are at 1.5 meters high, and I keep watching. At the point 90 meters away from the road ahead, the driver is at the intersection, the bottom of the display screen is blocked by the roof of the vehicle, and no glare is generated, that is, the display is at a level of 20° or more. Therefore, when the driver's gaze distance is 90 meters, the minimum height H=34.26m from the bottom of the LED display screen can be obtained from Fig. 1.
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