Agents of Deterioration: Light



Today’s agent of deterioration is guaranteed to be illuminating. It’s one that’s pretty glaring, if I do say so myself. Don’t worry; I’m sure you’re bright enough to have already figured out what it is. It’s light!


This one is filled with dilemmas because without light, there would be no way for us to see… well, anything really. But, light can also damage and degrade objects quite rapidly. So with this agent of deterioration, we have to compromise and come up with some sort of balance between seeing the artefact, and protecting it.


Now, there are different types of light than can cause various damages to objects. There’s regular light, meaning the spectrum of light that is visible, the stuff that makes us see. Then there are two other invisible forms we need to be aware of: Ultraviolet and Infrared light. We can’t see them, but they can be super damaging to artwork.


And yes, before you start writing this in the comments, visible light sources and light bulbs or you know like… the sun can also produce ultraviolet and infrared light. And there’s a lot of physics and science that go in to talking about photon energies and light particles and light waves and what have you, but I am not a light science expert so I will only be telling you about the effect various light forms have on cultural heritage objects. Capiche?


But let’s get into some introductory science first by looking at the light spectrum. Light is made up of electromagnetic radiation or waves. Different types of light have various energies. Let’s look at the light spectrum shall we? On the high end, we’ve got ultraviolet rays. Ultraviolet light is made up of short, closely packed together waves. These have more power. After that, we have all the colours of the rainbow. This is the visible light that makes up everything we can see. As the colours transition to red, the wavelengths get longer, and we have infrared light on that end. These three things are what we need to worry about when protecting cultural heritage.


These three forms of light have different damaging effects on objects. Ultraviolet rays have a very high energy and can lead to yellowing and complete breakdown of materials like adhesives and paper. Think of old painted surfaces of lawn furniture that just starts flaking off, or old yellow glue that crumbles when you touch it. Uv light just causes


Visible light, all that stuff in-between UV and infrared light causes fading or bleaching of colours. Some colours can fade in just a few hours, whereas some could take years to become noticeable. Of course if something is in direct sunlight, it will fade a lot faster in comparison to lower museum lighting. I have an example here:

Remember whenever they were changing the board decorations in elementary school and the places where pictures were over the construction paper were always darker than the uncovered spots? Prime example of fading.


After UV and visible light, we have infrared light. This type of light wave doesn’t have enough energy to cause any photochemistry to happen, so any damage caused by this form of light is due to exposure to heat. Museums don’t really use incandescent light bulbs that give off heat anymore, and they’re not so big on displaying exhibits in direct sunlight, so a lot of this risk is already diminished.

Infrared isn’t as important to think about in comparison to UV and visible light, so we won’t be focussing on it too much. When dealing with infrared, just check for the amount of warmth coming out of a light source. You don’t want to overheat your object because too much heat, or keeping an object at the incorrect temperature can also be very damaging to an object, but we’ll be getting to that in a later video.


Some objects are very sensitive to light such as newspaper; oil paints dyed fabrics and paper, whereas some are completely immune like inorganic materials such as stone, metal, and glass. So go ahead and shine all the light you want on that big unpainted statue, but you best be careful with that reed basked and painted parchment.


So if light can cause all this damage to an object, it’s obvious then that we’d want to limit the amount of light exposure to an object. For measuring visible light in museums, we use the term “lux”, which essentially means the light intensity. Lux is a unit of measure to determine how much light is hitting an object.


When displaying an object we need to keep lux as a priority, because the more light we shine on an object, the faster it will degrade. Think about it in terms of if you heat water at the highest setting on your stove, it’ll boil a lot faster in comparison to if you’re heating it up on a much lower setting. So increasing the lux means increasing the rate of decay to an object. WELP that’s an easy fix right?! Just keep the levels low. Weeeeeeellll…. Not really.


You see there is a minimum amount of lux needed for us to be able to see. A good base level to go off of is 50 lux. That’s enough for a well-functioning human eye to see full colour vision. Let’s call it reasonable visibility. 50 lux is still a bit dark for everyone’s liking, though. If we want to see dark surfaces better, you need to multiply 50 lux by a factor of three. If you want to see more contrast, add another factor of three. If you want to see fine detail, add ANOTHER factor of three. If you’re older, you guessed it- factor of 3! The thing is, not everyone has perfect eyesight, and as we age, our ability to see in low light situations. That MEANS if we also want our older populations to enjoy museums, we need to do these sorts of calculations to figure out what the most acceptable level of light is.


Again, it’s all about that balance and compromise. Something that is a big part of exhibition design and museum display. You might see people walking around with light meters to check the amount of light hitting an object. This is to make sure the lights are at the correct level in accordance to the type of object they are illuminating.


Museums calculate light exposure on a surface in something called mega lux hours by measuring the light intensity and the number of hours the object will be exposed to it. If we’re using what we’ve already learned, a higher lux will create more damage per hour than a lower lux. Given that, museums calculate what the acceptable lux would be for the maximum amount of exhibit time. This is why you see a lot of rotation in museums, because objects can only be on display for a certain amount of time before they need to go back in storage. They reach an acceptable level of fade, then are put away so they can come out again in the future. It’s also why they turn pages of books that are on display. This way each page fades the same amount so pages don’t degrade unevenly.


Another think we want to limit is direct light exposure. Indirect light is best so the beams are solely concentrated on one part of the object. Natural light is also very hard to control, so that’s why you notice that it’s limited in museums.


The actual light source is also a big factor in museums. Fluorescent lights and sunlight both give off UV rays. Fluorescent lights less so, but still, we’re talking more of a cumulative process because light degradation doesn’t happen immediately. Halogen lamps, while they don’t have UV, they do get super hot, and that’s not good either! Then what lights are we supposed to use then?! Well, LED lights are great because they’re bright but they don’t get hot and they don’t have as much UV as fluorescent lights. Fibre optic lights are the excellent for the museum environment because they produce no heat, and very minimal UV. They’re expensive, but they’ll save your object, and the planet because they reduce carbon emissions.


Another way to combat all these light rays shooting at an object is to put UV filters on your windows and exhibition cases. Or just make sure your sensitive artefacts aren’t in an area that is exposed to a lot of uncontrollable light.


Something that I’ve seen in a few museums, is light reduction using motion sensors. That means that some objects that are very sensitive to light like painted wood, are placed in a corner with low light. But, if there isn’t anyone around for a certain amount of time, the lights go off so as to not expose the objects to any more unnecessary light hours. I think it's genius, and then you feel super cool walking in to a dim corner of a museum and the lights go on just for you.

So the next time you find yourself complaining that the museum you’re in is too dark, remember there is a method to our madness and it’s all for the greater good!


Looking to find out more?


Canadian Conservation Institute- Agents of Deterioration: Light by Stefan Michalski



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