1, the basic concept of flashing

It has been found that reading books under fluorescent light for a long time will be uncomfortable, although the brightness is the same as during the day. Why is this? After research, it turned out that this is because the light intensity of the fluorescent lamp changes with time. That is, the flashing of the light source.

So what is the light source flashing? The flashing of the light source is a rapid and repeated change of the light emitted by the light source over time, making the light source jump and unstable. Of course, the human eye does not easily perceive the flicker of the light source, which depends mainly on the frequency of the flicker. Generally, at a blinking frequency of 50 Hz or less, the human eye can perceive the flicker of the light source. At 8.8 Hz, the human eye is the most sensitive. At the same time, the maximum flicker frequency (critical flicker frequency) that can be perceived changes with the intensity of the light emitted by the light source. As the light intensity increases, the critical flicker frequency also increases. When the light intensity reaches a maximum, it gradually decreases. The critical flicker frequency is also reduced accordingly. When the light intensity changes at a frequency greater than 50 Hz, most people can't tell if the light source is flashing. At this time, the light source emits a steady, continuous light, because the reflection of the human eye can no longer keep up with the change of the light source. It is. For example, the human eye does not notice the flicker of a fluorescent lamp of 100 blinks per second (100 Hz).

Light source flicker can be confirmed by the stroboscopic effect. Sometimes we find a phenomenon in which when an object moves or rotates very quickly, they appear to move more slowly than they actually do, even when the object moves or rotates at the same frequency as the source (or an integer) When the relationship is doubled, the object seems to be stationary. This is the reason for the stroboscopic light, which is a phenomenon that we do not want to happen in everyday lighting. In fact, people have the illusion of stroboscopic effects, and it is very dangerous to think of some fast-moving devices as slow or even stationary.

There are many kinds of flashing lights, and the following are common:

1. Periodic flashing of illumination: such as stroboscopic flashing of AC fluorescent lamps and flickering of light source due to fluctuations in power supply voltage;

2. Illumination non-periodic flashing: If the AC fluorescent lamp starts to flash, various faults flash;

3. Display device flicker: such as flashing caused by scanning display of TV, computer, electronic game machine, advertising screen, etc.

4, art light flashes: such as neon lights, lights and lasers flashing.

2, the cause of the flashing of the light source

The flicker of the light source is essentially the subjective feeling of the reaction chain from the voltage fluctuation - the flux fluctuation - the eye to the human brain. The technical mechanism of stroboscopic generation has both the factors of power supply, the factors of electric light source performance, and the unreasonable factors of lighting design. In the following, general technical analysis will be carried out only from the perspective of 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 this stage, most of the 50Hz power frequency sinusoidal AC power directly supplies the illuminant of the electric light source. The stroboscopic frequency is 50 Hz*2=100 Hz, and it fluctuates in a sine wave pattern.

(2) The light source voltage fluctuates greatly. At present, the power frequency AC power supply in China has a transient voltage value fluctuation range of 10% to 20%, which greatly increases the stroboscopic depth with a frequency of 100 Hz and a sinusoidal wave.

(3) Electric light source performance. The incandescent lamp is a heat radiating light source that directly heats the filament, and the luminous power of the illuminating body fluctuates with a sine wave law in accordance with the frequency of the power supply. High-pressure mercury (sodium) lamp, straight tube type (inductive) fluorescent lamp, although it is an electric light source for gas discharge illumination. However, due to its start-up and ignition, inductive ballasts are used (the AC-DC-AC frequency conversion function is not available). Therefore, the discharge power of the gas discharge illuminator necessarily fluctuates with the frequency fluctuation of the power supply.

The effects of voltage fluctuations can be quantified using two concepts: the visual sensitivity coefficient curve and the flicker voltage limit curve.

The light source that can be perceived by the human eye flickers, and the sensation characteristic of the human eye to the light source flicker can be measured by a statistical method. Figure 1 is a plot of the visual sensitivity coefficient of the human eye for voltage fluctuations at different frequencies. It can be seen from the curve that the most sensitive frequency is 8.8 Hz, and after deviating from this frequency, the sensitivity decreases with frequency. When the flicker frequency is above 40 Hz, the feeling is not sensitive; the flicker above 50 Hz has no feeling at all.

Figure 2 is a flicker voltage limit curve given by IEC. It is mapped to the visual sensitivity coefficient curve, and the visually sensitive frequency is 8.8 Hz. At this time, the voltage variation d is the smallest, which is 0.29%. The curve is obtained under the periodic rectangular (or step) voltage.

The voltage variation d represents the difference between the two adjacent extreme voltages, and is given as a percentage; the voltage variation frequency r represents the number of times of voltage fluctuation per unit time, in units of min-1.