Flicker is a somewhat thorny topic that has been little discussed from the point of view of museography. And the truth is that a museum has several peculiarities that make it a place especially sensitive to the problem of flickering lights.
I usually start this type of articles in this order: defining, pointing out the causes, the measurement parameters, the effects it produces, and finally a solution. But keeping my potential reader in mind, we are going to start almost at the end, we will define it and go on to point out the problems it causes in museums, which are not few. From there we enter the soporific part of causes, metrics, and possible solutions.
Flicker is an oscillation in the intensity of light over a certain period of time. This means that, in a very short period of time, say 5 milliseconds, the light source experiences a rapid change in the light intensity it emits. A strober (disco flash in Spain) is a flicker machine.
This device that the Devil carries, what it does is turn a light source on and off rapidly, causing that sudden blinking effect. When it flashes very quickly, it is because it is increasing the flashing frequency, turning on and off several times per second. If the strober flashes on and off quickly, but only once per second, it would be operating at a frequency of 1 hertz (Hz), and if it flashed 5 times per second, it would be operating at 5Hz.
Light sources can suffer a similar phenomenon, some turn on and off completely many times per second such as fluorescent tubes with magnetic ballast, and others suffer significant changes in light intensity, such as LED luminaires.
THE PROBLEMS IT CAUSES
The problems depend on the flicker values (rate and frequency) and also on the observer. Not all human beings are affected equally, although first we will talk about what can happen with video cameras.
Museums are spaces of culture, beauty, activism, creation. It is quite common to have recording devices, from the cell phones that we all carry in our pockets, to professional video cameras.
A common problem these devices have is the need for adequate light. Some museums have no problems with light. Some modern art institutions have generous sources of natural and/or artificial light, but art galleries that safeguard older works, for example, tend to have much stricter limitations in terms of lighting, since preventive conservation sets the parameters. maximum that can be influenced on the works. The lack of light is already beginning to be a problem for many devices, which begin to show noise in the dark areas of the image. A realism museum in which the works are at 50lx has even less lighting on the rest of the surfaces, which is a serious problem for most recording devices.
If we add flicker to this, the quality of the recording will be greatly compromised. In the following video, you will see three experiments with light. In the first, an LED spotlight with adequate electronics that does not cause flickering, then a fluorescent tube, and finally an adjustable LED luminaire whose design has not taken flicker into account.
In the following video, which is the central axis of the article, we can see the behavior of different light sources, the first two have optimal flicker control, the others do not.
Here, what is not visible to our eyes becomes a true chiaroscuro party that spoils any recording. What the instrument shows is the oscillation of light over time, it is graphically representing the behavior of the electronic signal, which then affects the light signal emitted by the LED.
Apart from those flickers and bars that go around the screen, we have a small fan on the screen with the idea of showing another collateral damage of the flicker: the strobe effect. This problem, contrary to the previous ones, does directly affect human vision, since it gives us an erroneous reading of the speed of an object. It is true that in museums it does not represent a great threat, but not in industry, where flicker has made a rotating machine appear immobile, posing a high risk to workers.
As we had mentioned before, what happens with humans is very diverse, and of course it has to do with the values of the flicker, it is obvious that we will all perceive a flicker of 5Hz frequency (with some horror) but at typical values of lighting, from around 100Hz upwards, and all things being equal, many things can happen:
That we perceive it, but that it does not bother us or affect us.
That we do not perceive it, but that it affects us.
That we do not perceive it, and that it does not affect us.
That we perceive it, and it also affects us.
The conditions are of various kinds. The most common problems generated by the flicker are documented depending on its frequency. I put a graph below:
Within the range 3-70Hz, it produces an immediate effect on health, and can cause serious damage such as epileptic seizures.
Between 70-200Hz, it produces an effect through long-term exposure, including visual fatigue, headache, reduced concentration level, etc.
Above 200Hz, not perceptible to most people, there are no known negative effects on health, but visual efficiency is reduced, especially the ability to recognize details, causing visual fatigue.
As we can see, the known problems are numerous, the more serious the lower the blinking frequency. We dealt with the subject of visual fatigue in another article that analyzes the lighting in another area of the museum, no less important than the room: the restoration department.
In a museum room, in addition to the cameras that one day may come to leave in a few minutes, we have workers, who sometimes we forget. There are room attendants, security staff, cleaning staff, maintenance staff, and let’s not forget the visitors of course. All those mentioned are the users of the room, not only the last ones, so the lighting design must attend to all the edges that can affect the health and comfort of all the users of the room.
Exposing a room assistant to high levels of flicker can negatively affect their health, or worse, trigger more serious disorders due to previous conditions, as occurs with people who suffer from epilepsy.
WHAT CAUSES IT?
Flicker occurs due to the nature of the signals that provide power to the lamps and LEDs. In fluorescent lighting, this problem occurred because the tube works through a magnetic ballast that does not modify the mains frequency, which is alternating, so these oscillations pass directly to the tube, which, having no light inertia, turns off and on. as many times per second as the alternating signal passes through zero, in the case of Europe whose network frequency is 50Hz we are talking about 100 cycles per second.
In current LED luminaires, the flicker is caused by what is known in our professional jargon as a “driver”, which is nothing more than the power electronics that almost all LED equipment has. The signal that this device sends to the LED is not usually perfectly linear, but rather presents some ripple, which when not paid attention can generate very high flicker rates. This is very common in retrofit elements: tubes, LED bulbs, LED halogens; What they have in common is their low price and little space to arrange the electronics, which is usually an obstacle to installing more sophisticated elements that put a stop to this problem.
In LED luminaires like the one with which we have done this experiment, the flicker can appear when the light intensity of the source is regulated. And this is a problem that we have encountered in museums that, having luminaires from top brands, and with recognized prestige in the market, usually present flicker problems when being regulated, a problem that comes to light when a television production company visits the room.
The most widespread way of measuring flicker is through two parameters, index and frequency. With this criterion we have the advantage that there are studies that have parameterized acceptable flicker magnitudes, so it is much more useful than other measurement criteria, such as percentage.
Thus, we have tables that relate the two concepts, so that based on our measurement we can determine if the luminaire is acceptable or not.
Low power LED lamps and luminaires usually have a frequency of 100Hz, so whether or not they have acceptable flicker values depends on the output ripple, and of course when they are regulated, a very high frequency square wave is generated. that neither people nor recording equipment can be detected.
From here, the higher the output frequency of the electrical signal, the fewer problems the light source will present.
How to avoid it?
The short answer could be that you don’t complicate things and buy the lights from our company, but we are going to be a little more rigorous.
Once the luminaire is installed, nothing can be done that does not involve intervention on the luminaire itself. Some museums have sent me third-party luminaires to replace the electronics with one better prepared to work in a room, but sometimes this is not possible because some luminaire manufacturers also design their own electronics, and what we find is that There is no space to put a device that we can acquire on the market and that meets the required characteristics.
Exceptionally, it has happened that the flicker is caused by a poor choice of regulator (if it is due to phase cut) of the luminaires. It may also happen that the regulator requires a “calibration” that is done by balancing the current load with a rotary control that many devices have built-in. This is increasingly rare, as most museums opt for dimmers integrated into the luminaire, or DALI, Casambi controls, etc.
There are flicker levels that are not scandalous, but can nonetheless generate signals that will be perceived by the cameras. If this happens, it is unlikely that the provider will take charge of replacing the luminaires or part of them, because with the standard in hand, this problem cannot be considered as a design error, or an undesirable feature that must be corrected at the vendor’s expense.
The fundamental thing is that, if you find yourself in a situation of having to buy room lights, the flicker is one of the parameters that you take into account when choosing your provider, indicating the frequencies and/or minimum rates that they must have the luminaires, regardless of their dimming level.
Do you have problems with flickering light sources in your museum?
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Our company Matisse Lighting S.L. will attend the CM Málaga 2023 Culture & Museums International Tech Forum to be held in June