Thinking on the safety brightness design of LED display

With the improved performance of LED lighting devices, in order to make the information highly recognizable in the case of high background brightness during the day, LED display companies continue to improve the brightness of LED display screens. The problem is that in the night background, outdoor bright LED display If the screen does not reduce the brightness, it will give the viewer a glare feeling, especially the bright display on the road at night will affect the driver's vision and normal driving; the light pollution of the advertising display around the community will affect the normal life of the residents.

In fact, unrestricted increase in the intensity of the LED output light is even enough to cause damage to the human eye, and protective measures should be taken to prevent unprotected eyes from looking directly at the LED for more than a few seconds. There are colleagues around me who have caused serious dry eye syndrome due to lack of safety protection knowledge during the test of LED display screen and no protective measures. At present, once such diseases are triggered, they are irreversible and there is no effective medical treatment.

1. LED display screen affects human eye health

In order to find a safe brightness design specification and provide a safety protection solution, we should first analyze the mechanism by which the light of the LED display screen acts on the human eye. The effect of LED display light on the eye is affected by various factors such as LED irradiance, illumination duration (irradiation energy), LED brightness, LED wavelength, flicker frequency, concentration of LED light (viewing angle distribution), ambient brightness, etc. The effect is also related to the pupil size of the viewer's eyes, the type and content of the pigment, and the refractive state of the eye.

(1) Irradiance, irradiation

The intensity of LED light is usually expressed in terms of irradiance. Irradiance (also known as power density) is the radiant power per unit area of ​​the surface, in watts per square meter (W/m2) or watts per square centimeter (W/cm2). Calculation formula: irradiance = radiation power / irradiation area

The intensity of light during a certain period of illumination is usually expressed in terms of radiation energy (Q), in units of joules (J) or millijoules (mJ). Radiant exposure (also known as energy density) is the radiant energy per unit area of ​​the surface, in joules per square meter (J/m2) or joules per square centimeter (J/cm2). Calculation formula: Irradiation = Radiation energy / Irradiation area

The relationship between the radiant energy, the radiant power and the irradiation time (T) of the LED is: radiant energy = radiant power x illuminating time. When the optical radiation power is 1 W and the irradiation time is 1 S, the optical radiation energy is 1 J.

Under the same irradiation conditions, as the dose of light irradiation increases, the incidence of eye damage increases, and the degree of damage increases. For discontinuous light, the wider the pulse width, the higher the frequency, the stronger the output power, and the greater the damage to the human eye.

(2) Light measurement (LED brightness, wavelength)

Figure 1 View function V(λ)--λ curve

As shown in Fig. 1, in the visible wavelength range, due to the physiological characteristics of human visual cells, the visible light of different wavelengths of the same energy acts on the visual cells, but the human eye feels that their intensities are different, that is, shooting When entering the human eye with the same energy and different wavelengths of visible light, the human eye feels that their brightness is different. In the visible spectrum, the human eye is most sensitive to the middle of the spectrum (yellow-green), and the closer to the ends of the spectrum, the less sensitive it is. Under the condition of causing the same visual response, the optical radiation power required for light having a wavelength of λ is P(λ), and the light having a wavelength of 555 nm (the most sensitive to the light of the human eye) is required. The optical radiation power is P (555), then V(λ)=P(555)/P(λ) is defined as a viewing function of wavelength λ. The visible function value of light having a wavelength of 555 nm is 1, the visual function value of other wavelengths is less than 1, and the visual function value of invisible light is 0.

The light emitted by the short-wavelength blue LED, even if the irradiance is the same as the bright yellow-green color, but the brightness of the human eye is not very bright, which makes the hazard of blue light difficult to detect. Blue-wavelength LED light is more likely to cause safety hazards than green light, and it is easier to unwittingly damage the eyes.

According to the principle of light wave energy, the shorter the wavelength, the stronger the photon energy. For the same number of photons, the short wavelength is greater than the long wavelength energy, that is, the same number of photons are radiated, and the short wavelength blue light is more harmful.

(3) Concentration of LED light (viewing angle distribution)

The light output power of LEDs is not evenly distributed in all directions. When viewing LED displays through different angles, different brightness perceptions (radiation energy) are obtained. Therefore, by controlling the light distribution of LEDs, they are distributed in the desired direction. , you can avoid light pollution. Evenly distributing the LED output light can also prevent the intense light from being concentrated and causing damage to the eyes.

(4) Ambient brightness

The human eye's perception of brightness is dynamic, just like the camera's automatic light-sensing device (aperture), the human eye can quickly scan the ambient brightness and adjust the pupil size to adapt to the ambient illumination. The experimental results show that it takes only a few seconds (less than one minute) to adjust and adapt to a high-brightness environment. However, it is subject to a longer process from the high-brightness environment to the low-brightness environment. The human eye adjusts the time according to the initial brightness. It takes more than 5 minutes. In order to adapt to a completely dark environment, it takes 30 minutes for the human eye to see the surrounding objects for the longest time.

When the contrast between the brightness of the LED display and its ambient brightness is relatively large, in order to adapt to the LED display and the ambient brightness at the same time, the pupil of the human eye needs to be constantly adjusted, so as to adapt to the light and dark, bright and dark changes, in order to see clearly screen. This is bound to cause eye pupil adjustment hysteresis and brightness changes, a large amount of LED light radiation energy into the fundus, causing damage to the fundus tissue.

(5) pupil size of the human eye

Different pupil sizes directly affect how much light enters the fundus. Due to the large difference in pupils of the human eye in different environments, the amount of radiation required to cause the same degree of damage to the retina can vary by several orders of magnitude. That is to say, in a bright environment, LED light can make people feel uncomfortable at most, but in a dark environment, LED light can cause irreversible damage to the eyes. This is why the brightness of the display should be strictly matched to the ambient brightness.

(6) Flashing frequency

The low-frequency flickering of the LED display causes the pupil of the eye to constantly adjust, trying to adapt to the light and dark changes during the blinking, resulting in a greater discomfort for the viewer.

At the same time, the low-frequency flicker will cause the rolling image of the video shooting image with the LED display as the background, which will affect the LED display quality.

(7) Biological characteristics of the eye

Fig. 2 Schematic diagram of ultraviolet absorption rate (%) of each layer of human eye

As shown in Fig. 2, since the ocular tissue has different reflection, absorption, and transmittance of light of different wavelengths, the portions of the light of different wavelengths are different for the eye damage. The far-field ultraviolet absorption rate of the cornea with wavelength shorter than 280 nm is close to 100%. As the wavelength increases, the absorption rate decreases and the transmittance increases. Therefore, ultraviolet light mainly causes damage to the cornea and lens.

Figure 3 Transmittance of the eye medium to light

As shown in Fig. 3, the light transmittance of the ocular medium in the 400-900 nm visible light band is as high as 80% or more, and the change is small; in the near-infrared band, the general trend is that the longer the wavelength, the smaller the transmittance. Thus LED visible light primarily damages the retina and choroid. Since these two layers of tissue contain a large amount of pigments and blood vessels, low dose light irradiation can cause heavier damage.

The working wavelength of the LED is mainly visible light. Visible light is electromagnetic radiation that can be seen by the human eye, with a wavelength range of 400-700 nm. Because the eye has different reflection, transmission and absorption rates for different wavelengths of light; the spectral absorption peaks of different pigments in the eye are also different, so in the clinical, different colors of glare LEDs will cause damage to different tissues of the human eye.

The eye tissue contains three pigments that absorb light energy. Melanin is present in the pigment epithelial layer and choroid of the retina; hemoglobin is present in the blood; lutein is present in the macular area, cataracts, and turbid vitreous. The absorption spectra of different pigments are different. The absorption peak of lutein is about 4OO nm, which absorbs the highest blue light, absorbs part of the green light, and does not absorb yellow light at all. The melanin absorption peak is around 510 nm, which has a certain absorption to the visible band, but absorbs blue and green light higher than other wavelengths. The hemoglobin absorption peak is around 570 nm, which has the highest absorption of yellow light, a certain absorption of green light, and little absorption of red light. The absorption characteristics of these various pigments determine that LEDs of different wavelengths may cause damage to different tissues. Since the pigment tissue absorbs light very easily, the more pigment content in the fundus, the more likely it is to cause damage to the retina.

(8) refractive state of the eye

For an emmetropic eye, the parallel beam passes through the natural focus of the eye, and the focus falls directly on the retina, forming a small spot that is more likely to cause fundus damage than the refractive eye.

2. LED safety display related parameter setting method and safety protection measures

(1) Adjust the brightness of the LED screen according to the ambient brightness

The main purpose of brightness adjustment is to adjust the brightness of the LED screen according to the ambient light intensity, so that the LED display looks clear and bright without glare. Most of the domestic LED display manufacturers have already realized this function, but there are not many enterprises and users who really use this technology. On the one hand, there is a lack of accurate and easy-to-use environmental brightness detection equipment on the market. On the other hand, LED The display industry lacks adjustable specifications for reference.

Figure 4 Correspondence between ambient brightness and display brightness

As shown in Fig. 4, when the human eye adapts to the ambient brightness of 800 candelas per square meter, the brightness range that can be seen is 80-8000 candelas per square meter. Any display content beyond this brightness range needs to be adjusted by the human eye to adapt and gradually see. For example, in an outdoor environment of 5,000 candelas per square meter, the brightness of the display device that is easy to view is about 500-50000 candelas per square meter. In this environment, if you want to clearly identify 60 candelas per square meter of mobile phone content, you need It is this truth that the human eye tries to adjust for a few seconds to gradually adapt.

According to this theory, it is possible to establish a design specification for the brightness adjustment method of the LED display. The gray scale range falls within a range suitable for comfortable viewing by the human eye according to the ambient brightness. Especially at night, the maximum brightness of the display should be less than 10 times the ambient brightness.

Figure 5 shows the relationship between the ambient brightness and the optimal output brightness of the display device.

Figure 5 is the ambient brightness curve from the morning to midnight, and the optimal brightness setting curve of the corresponding (subjective) LED display. It can be seen from the test results that the ambient brightness change in the day is very obvious. On a sunny day, the brightness at the brightest time and the brightness at the darkest time can reach 30,000 to 1. The corresponding LED display brightness setting changes are also beyond imagination. In the night environment, when the LED display is set to 60cd, the LED screen is bright enough, but by noon during the day, even 5000-6000cd is not bright enough. The difference between light and dark is close to 100 times.

Due to the lack of specification constraints, many outdoor LED displays show very glare at night, causing serious light pollution to the surrounding environment and invisible damage to surrounding residents. It is recommended to establish industry standards as soon as possible, constrain corporate behavior, and force enterprises to install brightness adjustment devices. During the use of the LED display, the brightness output level of the LED display should be automatically adjusted according to the industry standard according to the change of the ambient brightness, and the high brightness of the LED display is prohibited in the dark environment.

(2) Specification of the blue output of the LED display

The human eye has different light perception abilities for different wavelengths. Since the brightness is a parameter based on the perception characteristics of the human eye, the brightness cannot be accurately reflected by the brightness alone. Irradiation is used as a measure of visible energy safety, which more accurately reflects the dose of light on the eye. The measured value of the irradiance metering device should be used instead of the brightness perception of the blue light by the eye as a basis for determining whether the blue light output intensity would cause damage to the eye. LED display manufacturers and users should reduce the blue output of LED displays under the conditions of display.

(3) Specification of the illumination distribution and direction of the LED display

The rationality of the light distribution of the LED should be considered as much as possible, and the light energy outputted by the LED is evenly distributed in all directions in the visible angle range, so that the small-angle LED light is directly directed to the human eye. At the same time, the direction and range of LED light should be limited to reduce the pollution of the LED display to the surrounding environment.

(4) Standardize the display output frequency

LED display manufacturers should design the display screen in strict accordance with the requirements of the LED display specification. The output frequency of the display screen meets the requirements of the specification to avoid discomfort to the viewer due to the flickering of the screen.

(5) Explicit safety measures in the LED display manual

In the LED display user manual, note the precautions for use, explain how to properly adjust the brightness of the LED display, and the long-term direct view of the LED display may cause harm to the human eye. When the brightness auto-adjustment device fails, manual adjustment should be used or the LED display should be turned off.

When encountering a dazzling LED display in a dark environment, the self-protection measure should be, do not look directly at the LED display for a long time or carefully identify the details on the LED display, try to avoid the LED being focused on the fundus after focusing through the eye. Spot, burn the retina.

The following are the precautions noted by Nichia in the LED manual.

(6) LED display design, production process protection measures

Design and production personnel will contact the LED display more frequently than the user. In the design and production process, it is necessary to test the LED overload operation state. Therefore, the design and production personnel are easy to be exposed to the strong light of the LED, and should pay attention to and take special protection measures for the LED display design and production process.

LEDs in the production and testing of outdoor high-brightness LED displays, the relevant staff should wear black sunglasses with 4-8 times the brightness attenuation, in order to see the LED display details up close. During the production and testing of indoor LED displays, the relevant staff must wear black sunglasses with a brightness reduction of 2-4 times.

Especially for the staff who test the LED display in the dark environment, it is necessary to pay attention to safety protection. You must wear black sunglasses before you can look directly at the LED display.

summary:

Combining the author's 17 years of experience and practical experience in the LED display industry, this paper roughly puts forward the influencing factors and related safety protection measures for the safety brightness of LED display. I hope to in-depth discussion and improve the industry standard for LED display production. The company provides design basis to avoid the impact of light pollution on the environment, and avoids damage to the eyes caused by users and R&D and production personnel who do not understand the relevant safety protection knowledge.