1 Introduction The appearance of LED display (hereinafter referred to as LED screen) has set off a new technological revolution in the field of outdoor large-screen display. Previously, the problems of low brightness, small area and high cost of outdoor display devices have been well solved. At present, LED screens are widely used in public information publishing, commercial advertising, traffic information prompts and other occasions, becoming the first and even the only choice for outdoor large screen display. The high brightness of the LED screen greatly alleviates the problem of contrast reduction caused by daylight exposure on the surface of the screen, which affects the display recognition. However, as the night falls, the ambient illumination is rapidly reduced, and the LED screen is particularly glaring and easy to form traffic. Hidden dangers, causing interference to the normal life of the surrounding residents. In recent years, with the extensive use of LED screens, its optical security issues have become increasingly prominent, and the complaints about the safety of LED screens have been concentrated. How to solve the problem of LED screen safety has become the focus of the industry and the whole society. 2 Light safety research objects and classification The research object of light safety is the adverse effects that the illuminant brings to the observer, including photobiohazard, visual resolution, physical and psychological discomfort. Light safety is mainly divided into two major categories, one is photobiosafety and the other is interference light. (1) Photobiosafety Research on ultraviolet, visible, and infrared radiation to human tissues, mainly to the human eye and skin. The main types of photobiological hazards are: photochemical UV hazard of skin and eyes, near-ultraviolet hazard of the eye, retinal blue light hazard, retinal heat hazard, infrared radiation hazard of the eye, skin heat hazard, etc. The eye is one of the most important sensory organs of the human body. Its own ability to repair is poor. It is most vulnerable to external optical radiation. Even visible light may cause damage to the human eye. For example, too strong short-wavelength blue light may cause retinal damage. Photochemical damage of photoreceptor cells and degradation of pigment epithelial function, excessive radiation can also cause thermal damage to the retina. (2) There are two main types of interference light: one is direct interference light, which means that the visual resolution caused by high-intensity illuminators in the field of view decreases (when the brightness of the illuminator is high, it occurs less) or the psychological discomfort (occurs when the brightness of the illuminant is low, which occurs more frequently); The second is indirect interference light, which means that the illuminant does not directly appear in the field of view, and interferes with the problem caused by the reflection of the surface of the light object into the human eye (such as the interference light that is projected into the residential room). Luminaires that cause interference light problems are generally much less bright than the illuminants that cause photobiosafety issues. GB/T 20145-2006 "Photobiosafety of Lamps and Lamp Systems" states that when examining photobiohazards, detailed spectral data is required for wide-band visible light only when the brightness of the source exceeds 104 cd/m2. If the brightness of the light source is less than this value, the exposure limit will not be exceeded. [1] According to the investigation of the LED panel manufacturer and the actual measurement on the spot, the maximum brightness of the LED screen is currently 6 000~8 000 cd/m2, which has not reached the minimum brightness value that may cause photobiosafety problems. Therefore, it can be considered that the current LED screen is not Any photobiological hazard to humans, including the retinal blue light hazard. The research on LED screen safety mainly includes direct interference light and indirect interference light of LED screen. The research on direct interference light is limited to the interference light that causes discomfort. 3 LED screen interference light analysis The light security problem of the LED screen mentioned above is mainly the problem of interference light. The interference light is not a pure physical quantity. It is not only related to the characteristics of the LED screen itself, the positional relationship between the observer and the LED screen, but also the psychological and physiological state of the observer. Related, the main influencing factors are as follows: (1) LED screen characteristics Maximum brightness: At present, the maximum brightness of the LED screen has reached 8 000 cd/m2. The greater the maximum brightness of the LED screen, the more serious the interference light; whether it has the brightness automatic adjustment function: if the maximum brightness of the LED screen is kept at 8 000 cd/m2, it will cause nighttime The interference light will be very serious. The LED screen has the ability to automatically adjust the maximum brightness of the screen according to the ambient illumination, which greatly reduces the interference light of the LED screen; Screen size: In general, the larger the size, the more serious the interference light problem. (2) Positional relationship between LED screen and observer Relative distance: In general, the closer the relative distance is, the greater the damage. The LED screen orientation is in line with the observer's line of sight: in general, the closer the display is to the observer's line of sight, the greater the hazard. Opposite corner: The solid angle of the LED screen for the observer is determined by the distance, screen size, etc. The bigger. (3) Environmental illumination Ambient illumination is an important factor affecting the evaluation of interference light. Everyone may have such experience. The ambient illumination during the day is very high, and the interference light of the LED screen is not obvious. When the ambient illumination begins to decline in the evening, the LED screen will be dazzled. (4) Subjective state of the observer Age: The lens transmittance of the eyeballs of infants and the elderly is different from that of adults, which may have different feelings for the interference light of the LED screen. Occupation: Different occupations have different feelings about light. Especially the occupations that often contact photosensitizers are sensitive to light. Even some occupations (such as furnace workers and welders) may cause damage to vision. Health status: Eye diseases People's feelings are different from normal people. Patients with diseases such as albinism may have photophobic reactions. Psychological state: When people are in a pleasant mood and when they are upset, they will have different feelings about the interference of LED screens. (5) Display content Display content pleasure: The LED screen is a tool for transmitting visual information, and the popularity of the displayed content affects the viewer's subjective feelings. The picture changes rapidly (flashing): rapid changes in the image can easily cause discomfort to the viewer. Color (blue, red, white, etc.): The effects of different colors are generally mainly reflected in the two aspects of photobiosafety and brightness change, but for the current LED screen, because the brightness does not reach the level of photobiosafety problem, Therefore, the consideration of color effects is limited to its contribution to brightness and its variations. (6) Place of use The nature of the use of the site generally does not affect the evaluation of the interference of the LED screen, but the level of interference light that can be accepted in different places of use (residential areas, commercial areas, hospitals, roads, etc.) will vary. (7) Environmental conditions Environmental conditions such as temperature, humidity, wind speed, noise, vibration, dust, etc.: When the environmental conditions are bad, the discomfort caused by the harsh environment will affect the judgment of the discomfort caused by the LED screen, and the subjective feeling of the person is seriously affected by the environmental conditions. Interference makes the evaluation inaccurate. 4 LED screen safety standardization requirements Although the optical safety of LED screens has received wide attention, there are currently no LED screen safety related standards in the world. At present, the optical safety evaluation method of LED screens in actual engineering is to organize personnel to observe on site. Due to the limitations of subjective evaluation itself in accuracy and the specific process methods are not standardized, the results have low reliability and poor comparability. How to accurately evaluate becomes a problem that plagues the industry. To solve the problem of LED screen safety, we must first establish a unified, scientific and effective evaluation and measurement method standard for LED screen safety, and standardize the evaluation of LED screen safety, so that the data and conclusions obtained by different inspection organizations are comparable. Based on the collection and analysis of a large amount of data, the technical requirements for LED screen safety related performance parameters are proposed, and then the design, installation and use of the LED screen are guided. 5 Standardization study of LED screen interference light 5.1 Standardization assumptions At present, in order to regulate the interference light control of the LED screen, it is urgent to carry out the following two standardization work: firstly, it is necessary to formulate an evaluation specification for the interference of the LED screen, accurately define the interference light of the LED screen, define a clear scope of application, and propose interference. The core performance parameters of the light evaluation and the specific index requirements of different application sites; secondly, the on-site measurement method of the LED screen interference light used in conjunction with the evaluation specification shall be formulated, and the specific implementation methods such as the measurement procedure, measurement conditions and measurement equipment requirements of the interference light parameters shall be specified. Through the formulation of the above two standards, in order to promote the gradual solution of the LED screen interference light problem. 5.2 Evaluation Specifications Since the current user and manufacturer are unfamiliar with the concept of LED screen interference light, in the LED screen interference light evaluation specification, the LED screen interference light should be accurately defined first, and the possible hazards should be explained to define the LED screen interference. The applicable range of light; secondly, the core performance parameters of the LED screen interference light should be specified, which should be fully considered, including but not limited to: maximum brightness, ambient illumination, diagonal angle, installation distance and orientation, night shutdown period, etc. According to the nature of the work of the site to meet the functional requirements of the site as the basic premise, the indicator value of the interference light shall be specified. For example, the residential area shall not affect the normal working and rest of the residents. It should be noted that the brightness of the LED screen and the change of the display content will cause interference light problems, but when the brightness of the LED screen is low enough, the display content changes will not cause excessive interference to the observer, plus the control of the display content of the LED screen. The operability is not strong, and the influence of the change of the display content is fully considered in the performance index, and sufficient redundancy is left, and the display content is not required to be further required (applicable to the general situation, and is considered separately for special cases). The ambient illuminance has a significant impact on the evaluation results of the disturbance light. The experiment shows that the ambient illuminance changes very much during the day. Figure 1 shows the measured urban area of ​​Guangzhou on April 23, 2011 from 20:02 to April 24, 19:01. The ambient illuminance change map, the weather is cloudy; the maximum ambient illuminance value of the day reaches 97 100 lx, which occurs at noon, the minimum ambient illuminance value is 0.1 lx, which occurs at midnight, and the ambient illuminance is at sunrise and sunset (generally two In the hour, it has become very large. At present, due to the serious problem of nighttime interference light, the ambient illumination will drop sharply in the evening, so we must pay attention to the evaluation of interference light in these two periods. 