Monthly Archives: September 2015

Flicker in lighting

We get a lot of questions regarding flicker. People are asking what it is and what causes it. I decided to to some research and write a post about the topic. I have very little technical understanding and therefore I try to explain this in simple manner.

What is Flicker

You all know the effect when it is strong enough. Lights go out and on again in very short breaks. This used to be the problem of fluorescent tubes and at least I have learned that this is the time to change the lamp.

Flicker refers to quick and repeated changes in light intensity, which makes the light seem unsteady and broken. This effect is caused by variations in supply voltage or in the power line voltage.

These changes can be caused by dimming or for example by welding machines using high current. Usually voltage changes are quite small and don’t cause any harm to luminaires. However, even small changes can cause flicker and have an effect for people working in offices, schools etc.

Can you see the flicker

It depends. Human eye can see the flashing of the lights up to about 50 to 60 flashes/second (50Hz-60Hz), most sensitive frequency area for human eye is the range from 10Hz to 25Hz. When the frequency is higher that 60Hz most people can’t detect the flicker anymore. Some people have been known to see the flicker up to 100Hz.

In fact, fluorescent lights using magnetic ballasts have a flicker around 100-120Hz, which most people can’t see. This is because it is powered by ballasts with a frequency of 50Hz (60Hz in the US) and lamps flicker with the double frequency. When there is a problem with fluorescent lamp or with the ballast the flicker frequency drops below 100Hz (generally to the ballasts’ frequency) and it becomes visible to human eye.

Flicker from 100Hz to 500Hz can cause a stroboscopic effects, which means that you see objects in motion as series of still images. These kinds of lights are used in discos, but can be extremely dangerous in other environments.

When the flicker reaches a frequency around 2 kilohertz, we can no longer detect it at all.

You can see the flickering in higher frequencies for example with a camera. Although this depends little on the device. The better cameras don’t necessarily show any flicker.

Flicker seen with camera

Flicker seen with camera

Does it harm you?

So this is the big question. Flickering lights have been know to effect headaches, eye strain and fatigue. In Some countries there are laws regulating the maximum flicker in certain environments. For example in Russia there are regulations for schools, offices etc.

Some countries are starting to realize these health problems and have started to regulate the maximum flicker in luminaires.

Measuring Flicker

There is no standard for measuring flicker at least not at the moment. The Illuminating Engineering Society (IES) has developed two metrics to be used to measure flicker: Flicker percent and flicker index.

MF250N Flicker meter

MF250N Flicker meter

Flicker Percent

Flicker percent is the most commonly used metric to measure flicker. This metric tells how big difference there is in one flicker cycle. So it tells that how much does the amount of light drop from the maximum. The 100% flicker means that the light goes completely off at some point and 0% flicker means completely steady light.

For example Russian standards regulate that the flicker of a luminaire must not be over 20% and in some environments the flicker must be 0%.

So Flicker percent is calculated(from the below picture): (A-B)/(A+B)x100%

Flicker Index

Waveform

Flicker index

Flicker index is a little bit more hard to explain. The index has a value between 0 and 1. One being the maximum and zero the minimum.

Index takes the average lighting output into account. Basically the index compares the area above the average light output to the area of the whole cycle.

Flicker index is therefore calculated: AREA 1/(AREA 1 + AREA 2)

The smaller the index is, the smaller the flicker is.

Devices

Luckily there are devices that can measure the flicker. For example UPRtek’s MF250N LED Meter is made for measuring all aspects of flicker. You can use it to  measure both of these metrics as well as the frequency of the flicker.

More information on this meter you can request here.

Why does LED Flicker?

The problem with LEDs is that they differ a lot from older lighting technologies. Tero wrote a post about LEDs and the basics behind this, so I won’t delve into that with much detail.

To be short, if LED is supplied with a constant current, it won’t flicker. But the current will have to be really constant.

The most common reason for LED flicker is a bad LED Driver. If the driver fails to provide constant current, the led that it powers will flicker.

As the driver converts the AC to DC, there will be some ripple, which will cause the frequency to jump typically to the double (as it was with fluorescent lamps). This means that the LEDs waveform will follow the driver’s waveform.

Flicker on drivers is called ripple. Basically it is a synonym for flicker.

Other reason for LED Flicker is dimming. If the dimmer controls the LED with lower frequency than 200Hz, it will cause significant flicker. This is due to the fact that some dimmers alternate the current that the LED is supplied with.

How to reduce flicker in LED applications

Well, I think that it’s kind of self-explanatory: get a driver with lower ripple. As I mentioned the driver is the key to the amount of flicker. You will need to think that whether the application can tolerate some flicker or not. Generally these low drivers cost more than the ones with high ripple. Its worth noticing that every driver has some kind of ripple.

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