How To Measure Flicker

Light fixtures are essential lighting tools in life. They are helpful for our study and work, and LED lights are also the most widely used in all kinds of light fixtures. Among them, No flicker or low-frequency stroboscopic rate is a significant factor when choosing them.

What is the flicker of light?

A flicker of light sources refers to the fluctuation characteristics of light output by AC or pulsed DC power supply. The light flux, illumination or brightness change with the periodic change of current amplitude. In other words, the intensity of light changes with time.

Influence of flicker on lighting quality

Light flicker shows the change characteristics of light intensity with time, and there are mainly two aspects:

One is the periodic characteristics of change frequency, which indicates the speed of change;

The other is the amplitude of the change-the amplitude. Frequency and amplitude have different impacts on the body. For example, When we work or live under low-frequency flicker lights (such as inductive ballast fluorescent lamps) for a long time, it will cause symptoms such as visual fatigue and even headache.

According to the report of IEEE standard P1789, the eye can identify the flicker below 70Hz, and it will cause headache, dazzle, uneasiness, or cause epilepsy. While the flicker higher than 70Hz can not be perceived, it does not mean that there is no impact on the body, it also can cause headache and visual fatigue, vision loss, even rapid heartbeat, and other reactions if long-term use.

Research shows that it is harmless to the body only when the flicker is higher than 160hz or even above 200Hz. The influence of amplitude can cause different physiological reactions with the level of flicker frequency.

In some special lighting occasions, there are special requirements for different flicker characteristics, such as occasions requiring special exposure, such as photography, low flicker will decrease the photo quality.

How do reflect flicker characteristics?

As mentioned above, many relevant organizations have studied that the flicker is closely related to the lighting quality. For example, EnergyStar, IEC, ANSI, CIE and other organizations in the United States etc. Generally, there are mainly two aspects to show flicker.

1. The periodic characteristic of flicker-frequency

As mentioned above, although many studies believe that the flicker of more than 160hz is harmless to the body, there is no conclusion. Sometimes it is also defined according to the relevant power harmonic characteristics.

2. Intensity change of flicker- amplitude

ANSI/IES defines two parameters, percent flicker (stroboscopic percentage) and flicker index (stroboscopic coefficient). Percent flicker refers to the peak value and valley value of the flicker waveform, while the flicker index divides the energy area into area1 and area2 based on the average value and the energy value of the waveform obtained by integration. Some are also characterized by fluctuation depth and modulation depth.

Flicker measurement method

Currently, there are no standard flicker measurement methods in the industry, mainly including IEEE, IEC PST, California CEC, assist and CIE standards. The measurement methods and quantitative standards of each are also different. Still, the results can be used as an essential index to measure the flicker of LED light, so we can choose which according to our actual requirement.

IEC PST measurement method

This method is based on IEC 61000-3-3, IEC 61000-4-15 and IEC TR 61547-2015 standards; it is used to evaluate the flicker of lighting fixtures.

PST = 1 means that 50% of the experimenters (probability) feel the flicker.

IEEE Std 1789 measurement method

IEEE standard quantifies the flicker percentage in a cycle- the difference between the maximum light output and the minimum light output divided by the sum of the maximum light output and the minimum light output. And the flicker index-the amount exceeding the average optical output in a cycle divided by all-optical outputs.

To normalize the level of flickers, it can divide into three aspects: no, low and high flicker risk.

CIE SVM measurement method

SVM is the parameter recommended by CIE to quantify the flicker effect. Refer to CIE IN: 006-2016 standard, and it is also recommended by lighting Europe and NEMA.

SVM < 1, flicker invisible;

SVM > 1, flicker visible.

ASSIST measurement method

It is recommended by ASSIST and LCR, aiming at the evaluation of visible flicker and flicker effect.

California CEC JA10 measurement method

JA10 of California CEC TITLE 24 standard specifically puts forward requirements for flicker of dimmable products, mainly by quantifying the light fluctuation of the lighting system and characterizing the flicker level with the percentage of modulation depth below a certain frequency.

The above five measurement methods are commonly used for flicker measurement in the industry.

How to measure the flicker of the light source?

According to the above information on flicker characterization parameters, we should follow the below conditions when making flicker measurements.

1) The photoelectric conversion detector must have a fast response speed and can sample quickly

2) The detector can simulate the light induction curve of eyes-the bright function curve and can measure the illuminance or brightness at the same time

3) With software calculation

According to the above requirements: there are two types of flicker measuring instruments in the market.

One is a photometer; it should keep between 20 ~ and 30 ℃ambient temperature during the test.

Other is a spectrometer. Although a spectrometer is cheaper, it can only measure one wavelength at a time. If you only want to know this, it is enough to measure the flicker. Because the light intensity change of the same light in the visible light band is synchronous and proportional, in a cycle, we only need to measure the luminous flux change curve of a specific wavelength, calculate the fluctuation depth according to the peak and valley values, and then calculate the average luminous flux (use the integration method to get the area s of a cycle in Figure 1, and divide it by the cycle length T to get the average luminous flux), and then integrate the Q1 region, Q2 = S – Q1, and then we can calculate the flicker index from Q1 and Q2.

In addition, if you want to know the flicker easier way, there are many kinds of flicker measuring instruments on the market which can directly read the specific data of various flicker parameters through the device, such as the Handheld flicker measuring instrument (such as SFIM-300 spectral scintillation illuminometer), desktop touch screen instrument (such as LFA-2000 flicker tester).