Lighting in museums is very complicated, we all know that. Many components come into play in the rooms: aesthetic criteria, safety criteria, conservation criteria, criteria linked to the exhibition story that is being proposed, among many others. Some of these components are subjective, and will vary from one professional to another, others cannot (or do not want to) be measured, since their convenience is judged based on the sensations or feelings that the light helps to generate in the person responsible for the care. exhibition assembly, in the artist, or in the museum director.
Not so in the restoration departments. Here we are talking about something else. These elements should not come into play in this space, since it is a work space, in which there are people carrying out a very specific task. The visual requirements of this type of work are known and documented. Here we can already talk about magnitudes, and not sensations: Lux, color temperature, glare index, color rendering index, flicker index / frequency. We can determine precisely and without giving rise to interpretations what that light should be like, and we can quantify its properties.
The first thing to point out is that you have to avoid auxiliary lighting. They are the most widespread in restoration departments, but in reality they are the worst to work with. The reason this is so is that these luminaires cause high luminances on the work surfaces in comparison to the surroundings. The watchmaker who works in the middle of darkness, but with a strong light on his table, where he manipulates those tiny pieces with his skillful and spatulated fingers, conveys a romantic image of the trade, but very inconvenient in terms of ergonomics. Every time our friend the watchmaker looks up from his small workspace, his eyes have to suddenly adjust to the darkness, only to do so again when his gaze lands again on that tiny ruby that prevents the wear of a metal shaft. . This is a fact that has been known for decades, so much so that it is reflected in EN-UNE 12464, which is the regulation on workplaces. This standard does not only talk about the illuminance that a worker must have in their task area, but also relates it to the lighting around them:
And why is it still done with auxiliary lighting? For many factors. We are talking about very critical tasks. A chromatic reintegration, or the delicate passage of a thread to restore a historical fabric, are tasks that require high illuminance, and for which errors in execution are costly. Casting a lot of light on a small task area is cheap, but providing field illumination with high illuminance and uniformity in a large room is more expensive. Sometimes certain aspects are prioritized, which as a consequence lead to more errors, and less productive workers because they reach visual fatigue after a few hours of work.
We are going to answer the question that titles this article in 4 blocks, some will be familiar to you, others not so much, but all are important and should not be overlooked if we want to adequately illuminate a restoration department:
Illuminance and uniformity
I could soon end by saying that the best way to restore is indirect sunlight. It is the richest light of all in terms of chromatic components, and its color temperature when the sky is clear and the sun is high will not distort any white; very warm tones carry their dangers. It is true that solar radiation contains ultraviolet and infrared, but the works are not permanently in the apartments, so in most circumstances it would be tolerable. Unfortunately this is sometimes not feasible, for many reasons, but possibly the most common is that restoration departments are usually in the basements of museums. But that’s okay, because today we can imitate solar radiation, or at least what interests us most about it, that it is not necessary to add infrared and ultraviolet rays to the general lighting of a restaurant department. The following is the solar spectrum, on a June day at noon on the Iberian Peninsula:
In the spectrum we can see how all the components are generously represented, these proportions allow us to adequately see all the colors within our sensitivity range. In restaurant departments there are usually several types of luminaires, some better than others, but in terms of their spectral composition they usually leave much to be desired. The most widespread are the luminaires that still use fluorescent tubes, and these may be the usual ones on the market, others that have much better quality, specific for tasks that require high visual performance. Below, you can see the spectrum of an ordinary tube like the one found in any parking lot, office, and another of the highest quality, both of which are very present in restaurant departments.
We can see the poverty of the spectrum of the tube on the left, especially in contrast with the one on the right, which has “less space” and a somewhat more generous representation of most colors. The CRI of each of the tubes in the image is 80 for the one on the left, and 91 for the one on the right.
There are those who already have LED lights, either because they have purchased new lights, or because they have bought LED tubes and have modified their old lights to make them compatible. The most widespread is this second option, T8 type LED tubes, economical and with a typical CRI 80. But these LEDs present some problems, about which we are going to go into a little more detail.
It would be somewhat contradictory to now start talking bad about that ugly blue peak that standard LEDs have, both as I started the article saying that there is nothing wrong with illuminating our restoration departments with indirect sunlight, since the exposure time is minimal. , and this problem pales next to another much more serious one: that the art restorer does not see correctly. Sunlight has a strong absolute, not relative, blue component. So the central point of my statement of reasons is not that, it does not go for what it has, but for what it does not have. LED became viable for lighting when the blue LED was discovered. Almost all LEDs used for lighting are actually blue LEDs coated with chemical components that generate a physical effect called fluorescence, which converts that blue band into small bands of longer wavelength, and therefore lower energy. This means that, from that peak of blue light, all the colors that are on the right can be “created” because they are less energetic, but not those on the left, such as violet. Therefore, to achieve a high color temperature, most LEDs have to use that blue peak, this compensates in this way for the absence of longitudinal components to the left of the blue. One solution to this problem is to use purple core LEDs. The mechanics of operation are the same as blue core LEDs, with the difference that the light is generated further to the left, in the purple region of the graph. This allows LEDs to generate a more continuous spectrum, and not have to resort to “blue peaks” to obtain high color temperatures, which are appropriate for restoration work. Here we can point out something obvious: if an LED does not have the color purple within its components, it is evident that it cannot reproduce it correctly, therefore the absence of these components of the spectrum can increase the probability of errors in chromatic reintegrations. On the other hand, this continuous spectrum has the positive point of offering a color rendering index of between 98 and 99, with the maximum possible being 100. For those most knowledgeable in the matter, note that the TM-30-18 has values less than three units of 100 in Rg and Rf, which gives us very high reliability, probably the highest possible today with artificial lighting.
Here it is clear how the radiation has a more continuous appearance, with the radiation peak being quite far from blue, close to 540nm, this shows us that it is not necessary to resort to blue peaks to have high color temperatures, like the one in the image , which is 5000K. To finish with this block, we can point out that there is scientific evidence that continuous spectra generated from purple present a lower damage factor than conventional LEDs that start from blue in some materials, which is an advantage to take into account. , also extendable to lighting in exhibition rooms (1).
Illuminance and uniformity
We are going to present these two concepts hand in hand because their relationship is direct, since they are obtained from conditions such as the photometric distribution of the luminaires to be installed, in addition to their quantity and position in the room. As with many other things, illuminance and uniformity are quantifiable and predictable, that is, they can be foreseen through a project or a lighting calculation before any installation is made.
As we pointed out in the introduction, we have regulations that regulate the luminous values that must be available in a workplace, I am referring to UNE-EN-12464. In this standard, there are a series of tables by activity that can shed some light, never better said. Art restorers often complain that they are somewhat forgotten, and they are right, and I’m not saying this because they are in the basement: the norm does not reflect their activity. But we can approximate it, we can deduce it. We already know how important color treatment and the high visual demands of the work are, so we can base ourselves on tasks with similar requirements and demands. We are going to see two tables, one from the printing sector, and another from the textile industry. .
The two activities have the same values, and can be understood as analogous tasks in terms of the demands of the art restorer. A chromatic reintegration requires a very fine inspection of colors, and a restoration of a historical textile requires that the technician perform a darning with colored threads that have previously been dyed in a laboratory to find the exact color. We therefore need a value of 1500 lux, with a Ra (or CRI) greater than 90, and a color temperature greater than 4000K. My work activity has led me to visit many restaurant departments. And I have never, I repeat, never, found one that presents this illuminance value. Or perhaps that value can be achieved by projecting a powerful auxiliary light on the Work, but that would ruin the other important value that was cited at the beginning of the article. Uniformity is essential, and can be foreseen in the lighting calculation that must precede the lighting installation. It is a relationship between the average and minimum illuminance, and its correct arrangement is a key element of visual comfort. This is a capture of a lighting calculation in which we can appreciate the projected uniformity through a recreation in false colors:
Many people will not know how to correctly define what glare is, although we all know perfectly well what it is. Let’s go to an explanation extracted directly from the standard:
With this information we can intuit some things, such as that for example auxiliary lights are not the best option in a restaurant department. The moment you move your eyes in a certain direction you will see directly the luminous surface of the luminaire, you will be more vulnerable to reflections as the light source is on a horizontal axis, and this when you are not directly the victim of a luminaire in the workplace. work of a colleague and that points to your eyes. The best thing, without a doubt, is to generate a uniform and generous field of light, which is a product of the luminaires installed on the ceiling, and which allows us to relegate the auxiliary luminaires exclusively when the process perhaps requires appreciating volumes with shadows through a light source tangential to the surface of the work, or the use of ultraviolet. Luminaires that have adequate UGR values generally have adequate shielding or optical treatments on their luminous surfaces; not just any indoor equipment is suitable.
Flickering light is an old problem that got out of hand with the invention of the fluorescent tube. Flickering sources can cause long-term health problems, or seriously aggravate existing problems. Well defined, we could say that it is an oscillation in the intensity of the luminous flux over a period of time. Fluorescent tubes do not have light inertia, this means that when there are no electrons circulating the tube is off, which happens 100 times per second because the supply network brings us an alternating 50Hz signal that passes through zero in each cycle, therefore at least when the ballasts are magnetic. Some of us perceive them, especially in peripheral vision, that flickering that you see in the corner of your eye, but as soon as we look at the source we no longer notice it. It may or may not affect you, there are people who do not perceive it, who do not notice the blinking, but nevertheless it affects them, they may suffer visual fatigue in a short time, or they may start to get a headache in minutes. I think we all know someone who experiences it, when it’s not ourselves.
LED equipment can also present flickering, this is related to the quality of its components, and a concept of electronic knowledge called output ripple in which we are not going to delve into, just point out that it is an unwanted oscillation in the auxiliary power equipment. of LED luminaires which, of course, can be limited as long as it is taken into account in the design of the equipment, which does not happen as frequently as we would like.
Flicker is also quantifiable, there are laboratory and portable instruments that allow us to measure it. Its negative effects are described, and how it is quantifiable, although it is not present in the interior lighting tables, we do have its acceptable values defined in other regulatory documents.
If we take these 4 concepts into account when lighting an art restoration department, we increase the guarantees of visual comfort that the restorer needs, which will significantly increase the effectiveness of this important work.
- (1) HUNG-WEN LUO, MING RONNIER LUO, HUNG-SHING CHEN. (2018) Museum lighting with LEDs: “Evaluation of lighting damage to contemporary photographic materials” https://www.researchgate.net/publication/323979260
- EN 12464-1:Luz y Alumbrado o Iluminación – Alumbrado de los puestos de trabajo- Parte 1: Puestos de trabajo en interiores
- COMMISSION INTERNATIONALE DE L’ECLAIRAGE. (2004). Control of Damage to Museum Objects by Optical Radiation. CIE Technical Report 157:2004. Vienna: CIE
- Voluntary California Quality Light-Emitting Diode (LED) Lamp Specification contains some flicker criteria – CEC-400-2012-016-SF, published in December 2012
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