There is alternating current flowing through the light bulb, so why can't we see the light bulb flickering?

■ Preface

A few days ago, I saw an interesting post on the headlines, discussing why when there is alternating current flowing through an ordinary light bulb, the magnitude and direction of the current is changing all the time, but we can't see the light bulb flickering?

If I ask this question to my son who is already in college, he will laugh at it and say it is too simple. The author of the post also listed two reasons why people can't feel the light bulb flickering:

Reason 1: The frequency of the brightness change (flickering) of the light bulb should be the same as the frequency of ordinary household AC power (50Hz). Due to the persistence of vision effect, people will not be able to perceive the flickering of the light bulb.

Reason 2: The filament of a light bulb (incandescent lamp) has thermal inertia. Although the amplitude change of the alternating current causes fluctuations in electric power, it is reflected in the filament temperature more smoothly, so the actual fluctuation is small and difficult for the human eye to detect.

▲Persistence of vision effect

The human ability to perceive light flicker is not completely consistent with the phenomenon of visual persistence. The frequency of people's perception of light flicker is much higher than the frequency corresponding to visual persistence (about 1/15 second). In order to eliminate the effect of flicker, ordinary film movie players flicker twice for each frame of the picture.

Different parts of the human eye have different abilities to perceive flicker. The edge of vision (peripheral vision) can perceive flicker at a higher frequency than the center of the eye, so you can see some flicker when you look to the side, but you can't detect it when you look straight ahead.

Since persistence of vision involves human perception, it is not easy to measure. However, the intensity of light emitted by ordinary lamps varies greatly and the frequency of the changes can be measured by photoelectric sensors.

In the SP-45ML photodiode amplifier circuit and its dynamic characteristics, a light intensity measurement circuit based on the SP-45ML phototube and its dynamic characteristics are introduced. Its output voltage is linearly related to the input light intensity (light energy). The corresponding frequency is greater than 25kHz, which can be used to measure the light intensity changes of commonly seen lights.

▲ SP-45ML photodiode amplifier module and its optical fiber

Based on the measurement results, the most common lamps can be determined:

 How big is the change in light intensity?

 How often does the light intensity change?

01Several Light Bulb Brightness Tests

Next, select several types of light bulbs available in the office, and use optical fibers to guide the light into the SP-45ML photodiode amplifier module. Use an oscilloscope to observe the output voltage waveform of the measurement module to reflect the changes in light intensity.

1. Small incandescent lamp

Incandescent lamps emit heat radiation when the tungsten filament in the bulb reaches a high temperature due to the heat energy generated by electric power. The magnitude and spectrum of the heat radiation are related to the temperature of the filament. In a stable state, the input power and the radiation capacity (energy dissipated by heat conduction) are balanced. The heat energy consumed by the bulb is related to the input voltage amplitude and the filament (thermal) resistance.

▲ Brightness test of small incandescent lamp

The figure below shows the light intensity signal of the bulb (blue) and the applied voltage signal (orange, the waveform after transformer step-down coupling). It can be seen that the frequency of the light intensity change of the incandescent lamp is twice the frequency of the input AC voltage (100Hz).

▲ Alternating current and light intensity waveform

Due to the thermal inertia of the filament, the emitted light intensity fluctuates within a certain range, and the waveform is a sine wave. The fluctuation range is about 50% of the average light intensity. The following is the specific measurement data.

■ Brightness parameters

Frequency: 100Hz

Mean: 2.801, MIN = 2.133, MAX = 3.508, Max-Min = 1.375

Brightness change rate:

2. LED Lights

Below is another white LED lamp. It looks similar to an incandescent bulb, and includes an AC-DC converter, LED drive circuit, and LED lamp panel packaged together.

▲ LED light

The measured light intensity (blue) signal is basically constant. It does not change with the change of AC voltage. We know that the light intensity of LED is proportional to the current flowing through it. The LED driving circuit in this type of bulb often has a constant current function, so the overall light intensity is relatively constant.

▲ Alternating current and light intensity curve

In order to improve efficiency, the LED drive circuit uses PWM drive. So if you amplify the light intensity signal, you can see that the LED light intensity shows high-frequency fluctuations, with a frequency of about 10kHz. The frequency of light intensity changes is very high, far exceeding the frequency range that humans can perceive.

▲LED light brightness fluctuates at high frequencies

The specific range of measured light intensity fluctuations is approximately 36%.

■ Brightness fluctuation parameters

Average value: mean=0.5648

MIN=0.46575, MAX=0.672, MAX-MIN=0.20625

Brightness Fluctuation:

3. Small fluorescent lamp

This is another small fluorescent lamp in the office. It has an electronic ballast installed inside, and the high-voltage pulse generated can stimulate the fluorescent tube to light up.

▲ Changes in light intensity of small fluorescent lamps

The figure below shows the brightness change of a small fluorescent lamp. It can be seen that the brightness change amplitude is very small, and the frequency is 100Hz. From the brightness curve, it can be seen that the substrate is full-wave rectified, and the voltage waveform driven by the load after capacitor filtering.

This means that the electronic ballast in the small fluorescent lamp does not stabilize the DC voltage after full-wave rectification, and the capacitance of the filter capacitor is relatively small.

▲ Strong sunlight

The following are the specific brightness change values. The brightness change amplitude is about 21%.

Average value: mean=2.043

MIN=1.805, MAX=2.242, MAX-MIN=0.4375

Brightness Fluctuation:

4. Ordinary fluorescent lamp

Offices today still use ordinary fluorescent lamps, which are still widely used since they were invented in 1934.

Although the lamp is on the roof, it is easy to bring the light into the light intensity measurement module with the help of a long optical fiber.

▲ Ordinary fluorescent tube

The figure below shows the change of the light intensity of ordinary fluorescent lamps with input voltage. Compared with the incandescent lamps mentioned above, the light intensity fluctuation of fluorescent lamps is also 100Hz, but the fluctuation waveform is different. The light intensity fluctuation of incandescent lamps presents a sinusoidal fluctuation curve due to the thermal inertia of the filament. The light intensity of fluorescent lamps actually utilizes the afterglow phenomenon of phosphors, and the light intensity does not drop to 0 as the voltage decreases. When the voltage is raised again, the ultraviolet rays excited exceed the afterglow intensity, and the light intensity immediately rises. At this time, there is no thermal inertia, so the light intensity changes from weak to strong very quickly.

▲ Changes in the intensity of ordinary fluorescent light

By analyzing the light intensity values, we can see that the light intensity of fluorescent lamps varies greatly. The light intensity variation is close to 90% compared to the average value!

Why can’t we usually detect the large changes in the intensity of fluorescent light? The main reason is that the frequency of change (100Hz) exceeds the range of human perception. However, it is said that some people can still perceive this 100Hz flicker using peripheral vision.

Average value: mean=0.4426

MIN=0.2112, MAX=0.6019, MAX-MIN=0.3906

Brightness Fluctuation:

5. Small halogen bulb

Below is a small halogen bulb used for fill light when taking pictures of samples on the laboratory bench. It is essentially an incandescent lamp, but halogen gas is added inside to extend the life of the bulb.

▲ Small halogen bulb

Below is the intensity variation of the bulb (blue). Compared with the incandescent bulb above, its light intensity variation is smaller, only about 20%.

▲ Brightness changes of small halogen lamps

Average value: mean=3.899

MIN=3.485, MAX=4.329, MAX-MIN=0.844

Brightness Fluctuation:

6. Incandescent lamp after half-wave rectification

Since the power consumed by an incandescent lamp is proportional to the square of the input voltage, its brightness changes at twice the frequency of the input AC. If a diode is used to half-wave rectify the input AC, the brightness of the bulb should change in line with the frequency of the input AC.

▲ Half-wave rectification for incandescent lamp power supply

The figure below shows the change in brightness of an incandescent lamp after AC passes through a diode. The brightness change is consistent with the frequency of the input voltage, which is 50Hz. At the same time, the amplitude of the brightness change is greatly increased, and the ratio of the brightness change to the average value reaches about 150%.

▲ Incandescent lamp brightness after half-wave rectification

Average value: mean=2.241

MIN=1.0162, MAX=3.4912, MAX-MIN=3.375

Brightness Fluctuation:

If you observe an incandescent lamp that is driven after half-wave rectification, you can actually perceive it flickering.

※ in conclusion

By measuring the brightness changes of ordinary lamps with a simple SP-45ML photodiode, we can see several different situations:

The brightness of an ordinary incandescent lamp will change by 20% to 50%, and the change curve shows a sinusoidal fluctuation with a frequency of 100Hz.

The brightness of ordinary fluorescent lamps varies by about 90%, showing a full-wave rectified waveform with a frequency of 100Hz.