Causes, hazards, judgment criteria and solutions for strobe in LED lighting

LED semiconductor lighting network signal assessment of the impact of the light on the human eye, it is inevitable to mention stroboscopic, with the advancement of electric light source technology and people's understanding of the light biological understanding, the solution to the stroboscopic problem is undoubtedly a healthy lighting Big key.

When people are in a lighting environment, it does not seem to be noticeable. In fact, there is not much intuitive perception of the concept of strobe, except for warning-like flashing marks or lamp failures. When observing the rotating fan blades, it is found that the blades are sometimes "stationary" and sometimes "rotating", and this phenomenon can also occur in the running car tires, which can be explained as the visual persistence caused by the stroboscopic light source.

In the figure below, there is a white point on the black disc. When the disc rotates, the continuous change of graph a will occur under the condition of no stroboscopic illumination. Under the fluctuating frequency flash source, the phenomenon of graph b will appear.

a general situation b strobe situation

Strobe cause

The stroboscopic light source is essentially that the light emitted by the light source exhibits a certain frequency and periodic change with time, and changes with time between different brightness and color. If the driver of the luminaire does not have suitable electronic circuits, such as ballasts, drives or power supplies, the source will strobe, the greater the fluctuation of the output luminous flux, the more severe the strobe.

The technical mechanism of generating stroboscopic has both the factors of power supply, the performance factors of electric light source technology and the unreasonable factors of lighting design. For many lighting fixtures, the operating current of the light source must fluctuate with fluctuations in the input voltage, directly causing stroboscopic fluctuations in the light output.

Taking the incandescent lamp as an example, the driving is 50 Hz AC mains, and the light flickering frequency is 100 Hz after the negative half cycle and the positive half cycle are superimposed; the gas discharge lamp using the inductive ballast also fluctuates with the fluctuation of the power supply frequency. In addition, the instantaneous fluctuation range of the power supply grid is between 10% and 20%, which deepens the light fluctuation of the lighting fixtures.

Commonly used stroboscopic index to describe the stroboscopic degree of the light source, the integrated stroboscopic ratio and two other variables: optical waveform and duty cycle. Wherein, the stroboscopic ratio is equal to the difference between the maximum light output and the minimum light output in one switching cycle divided by the sum of the maximum light output and the minimum light output, and the stroboscopic index is equal to the amount exceeding the average light output in one switching cycle divided by the total light output. . The lower the stroboscopic ratio and the stroboscopic index, the less the stroboscopic effect caused by the flashing of the light source.

Strobe measurement of common light sources

Strobe hazard

1. Photosensitive epilepsy or scintillation-induced epilepsy:

Existing research data and industry experts point out that photosensitive epilepsy does exist. Bittencourt and Fisher et al. proposed the epithelial theory of photosensitive epilepsy in 2004 and 2005. Bittencourt pointed out that the incidence of photosensitive epilepsy is very low in the whole population, the annual incidence rate is about 0.001%, and the incidence rate in adolescents is about 0.006%. And the probability of suffering from this disease in patients with epilepsy is as high as 10%.

Fisher estimates the incidence of photosensitive epilepsy in people aged 5 to 24 by measuring abnormal brain waves of light stimulation. The incidence rate is between 0.01% and 0.025% in people aged 5 to 24, and 2% to 14% in patients with epilepsy under the stimulation of flashing light. The chance of inducing rapid epileptic seizures. The American Epilepsy Work Foundation pointed out that the factors influencing the symptoms of photosensitive epilepsy include scintillation frequency, light intensity, modulation depth, spectral composition and some factors related to the visual nervous system.

2. Vision persistence effect caused by stroboscopic

Bullough et al. used a series of waveforms and frequencies to study the stroboscopic effect of a solid-state light source and found that the drive modulation depth, frequency, duty cycle, and waveform have an impact on the visibility of the stroboscopic effect.

Bullough stroboscopic research experiment

At the same time, industry experts also agree that the stroboscopic effect is mainly due to the fact that this visual artifact will bring a series of problems. SafetyatWork mentioned that although some lights that are not recognized by the human eye strobe, it will make the field of view fly fast. The machine appears to be visually slow or static, which is very dangerous and can easily cause accidents.

Constantly bright light source and strobe light source

3. Physiological problems such as headache, migraine, nausea, and visual disturbances

The American Headache Society has pointed out that many people with migraine headaches are more sensitive to light, especially glare, bright light sources with flicker can cause migraine, and low-frequency flicker is more serious than high-frequency flicker; some flashing fluorescent lights or TV screens can cause migraine-like Symptoms, many studies have also shown that for these scintillation-induced migraine patients, their EEG is usually different from those without migraine. However, the physical discomfort caused by those factors of the scintillation source has not yet reached a definitive conclusion.

4. Use eye fatigue, vision loss

The flashing of the lighting fixtures often affects human productivity. When studying the influence of flicker on people's fatigue degree, it is found that the mental fatigue caused by flicker is very small, and the main factor causing human fatigue is the work content; non-visible flicker can affect the movement track of the eyeball, affecting reading and causing vision loss. Studies by Wilkins et al. have shown that visual correlation performance at higher frequency scintillation sources is better than low frequency sources.

Strobe criterion

IEEE published IEEEPAR1789: 2013 "Draft Risk Assessment-Potential health effects of flicker from LED lighting" in April 2013. The criteria for the determination of stroboscopic are shown in the figure below. The unaffected areas in the figure are shown in green, and the low-risk areas are shown in orange:

The relationship between the flash frequency and the fluctuation depth function of low risk and imperceptible levels of influence

The document pair stroboscopic criteria are:

(1) The frequency is lower than 9 Hz, and the undetectable fluctuation depth limit is 0.288%;

(2) The frequency is in the range of 9-3120 Hz, and the undetectable fluctuation depth limit is: frequency × 0.032%;

(3) The frequency is greater than 3120 Hz, and it is considered that there is no stroboscopic at all, and the assessment is exempted.

How to reduce stroboscopic

Analysis of the cause of stroboscopic, it can be found that the key to reduce stroboscopic is electric light source technology and drive technology. If the LED is driven by a constant, non-oscillating DC source, the stroboscopic effect can be achieved. Drivers in the market also need to consider driving costs, size, reliability, efficiency and other factors, manufacturers and industry experts also try to find a strobe level that people can accept.