Flicker effect of light source and flicker measurement method

I. Overview 

1. Basic concept of flicker 

People have found that reading under fluorescent lamps for a long time will make people feel uncomfortable, although the brightness is the same as that of daytime. Why is this? After research, it turns out that this is because the light intensity of fluorescent lamps changes over time. That is, it is caused by the flicker of the light source. 

So what is light source flicker? Light source flicker is the rapid and repeated changes of the light emitted by the light source over time, making the light source jump and unstable. Of course, the human eye cannot easily perceive the flicker of the light source, which mainly depends on the flicker frequency. Generally, the human eye can perceive the flicker of the light source when the flicker frequency is below 50Hz. At 8.8Hz, the human eye is most sensitive. At the same time, the maximum flicker frequency (critical flicker frequency) that can be perceived changes with the light intensity emitted by the light source. As the light intensity increases, the critical flicker frequency also increases. When the light intensity reaches the maximum and then gradually decreases, the critical flicker frequency also decreases accordingly. When the light intensity changes at a frequency greater than 50Hz, most people cannot tell whether the light source is flickering. At this time, the light source emits stable and continuous light, because the human eye can no longer keep up with the changes in the light source. For example, the human eye cannot detect the flicker of a fluorescent lamp that flickers 100 times per second (100Hz). 

Light source flicker can be confirmed by the stroboscopic effect. Sometimes we find such a phenomenon that when an object moves or rotates very quickly, it seems to move slower than its actual speed, or even when the object's movement or rotation frequency is the same as the light source flicker frequency (or an integer multiple), the object seems to be stationary. This is the reason for the occurrence of stroboscopic light, which is a phenomenon we do not want to happen in daily lighting. In fact, due to the stroboscopic effect, people have an illusion that some fast-moving equipment is running slowly or even stationary, which is very dangerous. 

There are many kinds of light source flicker, and the following are common ones: 

1. Periodic flicker of lighting: such as the stroboscopic flicker of AC fluorescent lamps and the flicker of light sources caused  by power supply voltage

fluctuations; 2. Non-periodic flicker of lighting: such as the start-up flicker of AC fluorescent lamps and flicker of various faults;  3. Flicker of display devices: such as flicker caused by scanning displays 

such as televisions, computers, electronic

game consoles, and advertising screens; 4. Flicker of artistic lighting: such as flicker of neon lights, colored lights, and lasers. 

2. Causes of light source flicker 

Light source flicker is essentially a subjective feeling of the reaction chain from voltage change - luminous flux fluctuation - eyes to human brain. The technical mechanism of flickering includes factors of power supply, electric light source performance, and unreasonable lighting design. The following is a general technical analysis from the perspective of the technical performance of commonly used electric light sources.  

(1) The power supply frequency of the light source is low. Incandescent lamps, high-pressure mercury (sodium) lamps, straight tube ( inductive ) fluorescent lamps. At present, most of them are powered directly by 50Hz industrial frequency sinusoidal alternating current to the light source. Its flicker frequency is 50Hz*2=100Hz, and it fluctuates in a sinusoidal wave pattern.  

(2) The voltage fluctuation of the light source is large. At present, the instantaneous voltage value fluctuation range of China's industrial frequency AC power supply is between 10% and 20%, which greatly aggravates the flicker depth of the 100Hz frequency and sinusoidal wave fluctuation.  

(3) Performance of electric light source. Incandescent lamp is a thermal radiation light source with direct heating of the filament. The luminous power of the luminous body must fluctuate in a sinusoidal pattern with the frequency of the power supply. High-pressure mercury (sodium) lamps and straight tube (inductive) fluorescent lamps are electric light sources that emit light by gas discharge. However, since their starting and ignition both use inductive ballasts (without AC-DC-AC frequency conversion function), the discharge power of the gas discharge luminous body must also fluctuate with the frequency fluctuation of the power supply. 

The impact of voltage fluctuations can be quantified by the two concepts of visual sensitivity coefficient curve and flicker voltage limit curve. 

The flicker of the light source that can be perceived by the human eye can be measured by statistical methods to obtain the human eye's sensory characteristics of the light source flicker. Figure 1 is a curve of the visual sensitivity coefficient of the human eye caused by voltage fluctuations of different frequencies. It can be seen from the curve that the most sensitive frequency is 8.8Hz. After deviating from this frequency, the sensitivity decreases with the frequency. When the flicker frequency is above 40Hz, the sensation is not sensitive; flicker above 50Hz is completely insensitive.  

Figure 2 It is the flicker voltage limit curve given by IEC. It is a reflection of the visual sensitivity coefficient curve. The visual sensitivity frequency is 8.8Hz, and the voltage change d is the smallest at this time, which is 0.29%. This curve is obtained under the action of periodic rectangular (or step) voltage.



The voltage change d represents the difference between two adjacent extreme voltages, given in percentage form; the voltage change frequency r represents the number of voltage changes per unit time, in units of min-1. 

2. The harm of flicker to human eyes 

The flicker of light sources stimulates the human visual system and produces an uncomfortable feeling. People who work or live under flickering light for a long time may also affect the physiological hygiene of the visual system. Health and mental health. The severity of this stimulation or impact is related to the intensity, frequency, duration and long-term nature of the light source flicker. This impact is often slow, so it has not attracted people's attention for a long time. However, with the rapid development of electronics, people work under AC fluorescent lamps and in front of monitor screens during the day; and live under AC fluorescent lamps and in front of TV screens at night, even accompanied by dazzling artistic lighting. Therefore, the impact of light source flicker on the visual system has become increasingly prominent. 

In particular, the stroboscopic effect of electric light sources has caused serious harm to human production, daily life, and physical and mental health. It is mainly manifested in the following aspects:  

1. Illusion causes industrial accidents: When the stroboscopic frequency of the electric light source is an integer multiple of the speed (rotation speed) of the moving (rotating) object, the moving (rotating) state of the moving (rotating) object will produce false vision of stillness, reversal, slow movement (rotation), and the periodic repetition of the above three states, causing industrial accidents. For example, machine tool operators in the machining industry have the illusion that the turning tool rotating in the forward direction is reversing. They perform emergency reversal operations, damaging workpieces and tools, and even causing casualties. 

2. Harm to health and affect work: The stroboscopic effect can cause visual fatigue and migraines. This is especially obvious in the lighting occasions where the machinery industry uses high-pressure mercury (sodium) lamps, and the light industry, food, printing, electronics, textiles and other industries generally use straight tube (inductive) fluorescent lamps. For example, plug-in operators on the assembly line are prone to migraines due to visual fatigue and dizziness. It makes positioning difficult and production efficiency low. 

3. Harm the eyes of young people and cause myopia. After the 1980s, straight tube (inductive) fluorescent lamps were widely used in homes, schools, libraries, etc. in China. Growing primary and secondary school students suffered greatly, their eyesight decreased significantly, and myopia increased significantly. [page]

III. Evaluation method of flicker The 

parameters of flicker intensity of light sources are characterized by fluctuation depth, flicker index, and flicker percentage. Figure 3 is a typical flicker waveform.



In the figure, Φm is the flicker peak value, Φn is the flicker valley value, Φ0 is the average luminous flux, Q1 is the area under the curve above Φ0, Q2 is the area under the curve below Φ0, and T is the period. The 

luminous flux fluctuation depth is a quantification of the flicker amplitude of the light source. Light source flicker is the external manifestation of luminous flux fluctuation, and luminous flux fluctuation is the direct cause of light source flicker.  


IV. Measurement of flicker 

In the experiment of this paper, we use the transient photometer PR-110 to measure the luminous flux emitted by various light sources, and calculate the fluctuation depth to compare the flicker intensity of various light sources. 

PR-110 is a continuous and fast sampling transient photometer. Combined with photometric measuring devices of different structures, it can measure photometric parameters such as instantaneous illuminance, brightness, luminous flux or light intensity. 

In order to accurately measure the luminous flux, the light source is placed in an integrating sphere. The photodetector calibrated by V (λ) receives the light signal, and its output electrical signal is converted into a larger voltage signal through the amplification conversion circuit , and the AD converter is used for sampling. Since the single-chip microcomputer reads and stores data slowly, a dual-port RAM and an address generation circuit are used to enable the photometer to automatically store data when it is used. After all sampling processes are completed, the single-chip microcomputer reads data from the dual-port RAM and sends it to the microcomputer for display processing. The system structure diagram and physical connection diagram are as follows. V. Flicker results of typical light sources  In the experiment, we used a transient photometer to measure the luminous flux change waveforms of four typical lighting sources, including 26W spiral electronic energy-saving lamps, 60W ordinary straight-tube fluorescent lamps, 60W ordinary incandescent lamps, and single red LEDs , and calculated the fluctuation depth. In PR110, we set the sampling interval to 0.1ms, and all lamps were powered by 50Hz AC. The measurement results are shown in Table 1.  Table 1 Flicker parameters of various light sources According to the fluctuation depth, the flicker of the above light sources from weak to strong are energy-saving lamps, incandescent lamps, LED lamp cups, fluorescent lamps, and single LEDs. There will be no obvious flicker when the fluctuation depth is below 25%, so from this perspective, electronic energy-saving lamps and incandescent lamps are relatively healthy light sources. In addition, we found that due to the fast response of LEDs, the fluctuation depth of a single LED lamp is large, but the fluctuation depth of an LED lamp cup composed of multiple LED lamp tubes is significantly reduced, which is also a relatively healthy light source. Ordinary fluorescent lamps have a large fluctuation depth, and working under their irradiation for a long time is harmful to the human body.