How to Light

How to Light: ASK THE DOCTOR: Flicker and why it hertz

Flickering lights trigger headaches and can alter our perception of movement. For this reason it is important to understand why it occurs and how to prevent it

Q: What is flicker and how do I avoid it?

Flicker is a discomforting artefact of poor lighting and consists of a rapid variation in the intensity of light, which occurs in a cyclical manner.

Some frequency ranges are more noticeable to people than others, and there is a certain frequency above which the eye cannot detect the changes in the light level. This is referred to as the flicker fusion threshold or persistence limit. However, this limit is different for different people and noticeability of flicker relates also to the amount of variation of the limit, known as the flicker index.

How noticeable the flicker will be also depends on the part of your eye that is viewing the light. Seeing the light out of the corner of your eye will give a different flicker perceptibility to the centre of your visual field. If your eye is moving – or adapting to the amount of background light – this will also have an effect.

And there are well known extreme issues with flicker. About one in 4,000 people is known to have photosensitive epilepsy. Repetitive flashing lights may induce seizures in these people. Changes within the frequency range 3-70Hz are a potential risk and the likelihood of triggering epilepsy is greatest for frequencies in the range 15-20Hz. Studies show that lamps operating at 100Hz and above are thought to be safe. This is borne out by the experience of millions of people who work under these lights every day.

The perception of flicker through stationary direct viewing is dramatically reduced for most people when the frequency of modulation of the light is above 100Hz. Experiments show that less than 10 per cent of people experience the flicker under these conditions.

Stroboscopic effects

However, another issue with flicker is that of stroboscopic effects that become apparent if you move your eye while observing a flickering light. This can result in a variety of other effects that can cause visual discomfort, such as affecting the way we perceive motion. For instance, if a revolving object such as a wheel is lit by a light that flickers at the same rate at which it revolves, it will appear stationary.

At other times it can appear to rotate at a different speed, or in the opposite direction. This fact is the principle behind a strobe light but it is not the desired effect in general lighting. In fact, it could be a safety hazard if someone mistakenly thought that some equipment was stationary or was moving slowly. Studies have shown that the ability of people to localise objects is impaired bythe presence of flicker. Stroboscopic effects make it possibleto observe flicker at higher frequencies. Studies have shown that more than half of people observed the effects of flicker through stroboscopic effects at frequencies of light up to 300Hz.

Invisible flicker is also a phenomenon that is talked about widely. It is understood that the retina can detect frequencies above 100 Hz, even if the person does not notice such effects. Although humans cannot see lights flicker, the sensory system in some individuals can somehow detect the flicker.

Ever since fluorescent lighting was introduced in workplaces, there have been complaints about headaches, eye strain and general eye discomfort. These complaints have been associated with the light flicker from fluorescent lights. When compared with regular fluorescent lights with magnetic ballasts, fluorescent lights with high-frequency electronic ballasts (20,000Hz or higher) cut the number of complaints of eye strain and headaches in half. There tended to be fewer complaints of headaches among workers on higher floors compared to those closer to ground level; that is, workers exposed to more daylight experienced fewer health effects.

The measurement of flicker is important to devise a consistent way to assess the comfort level of lighting in a reliable manner. This will be discussed in a future column (and at Lux’s Lighting Fixture Design conference on 5-6 June).

Reasons for flicker

Lights can flicker for a number of reasons – poor electrical supply, electromagnetic interference from other electrical products, the circuitry in the lighting product causing oscillations in the current to the light source, and mains dimming of poorly designed LED lamps. The last is a key problem at present.

Flicker can be an elusive beast, it can occur more at certain mains voltages, at certain operating temperatures or as the light is dimmed. So, not only is it important to have a well-established way to measure the flicker, there is also a need for the industry to ensure interoperability and consistent product design testing, particularly for new entrant LED lighting products.

The emergence of LED lighting is creating a huge range of light sources that will be sensitive to changes in the current and, because of the pulse-width modulation dimming method, can have very large modulation depths and long duty cycles. This can give rise to complex flicker issues. This is compounded by the fact that there are a large number of companies that design and manufacture products that have a limited history of testing for use in the wide range of lighting environments.

The IEEE has set up a working group – PAR1789 – that is tasked with ‘recommending practices for modulating current in high-brightness LEDs for mitigating health risks to viewers’. Visit for more information.

Gareth Jones is the CEO of LUX-TSI laboratories, based in the UK and Malaysia.

Picture: Shuttershock