Understanding Light As An Agent Of Deterioration For Collections

by | Jan 31, 2023 | Blog

Light is one of the trickiest Agents of Deterioration: it’s all around us, even when we can’t see it, and it’s what lets us appreciate everything around us visually. We need it and use it all the time so it seems almost unfair that it can also be really damaging.

Light comes in different forms

There’s actually much more to light than we see with our eyes. Visible light is part of the electromagnetic spectrum. On either side of it are two kinds of invisible light that we worry about in preservation:

  • Infrared radiation (IR), and
  • Ultraviolet light (UV)

We can’t see these but they have a big impact.

What light does

Ever had a photo fade on your noticeboard? This is one of the biggest problems with visible light: it causes fading and colour changes.

UV meanwhile disintegrates organic substrates, chewing up our objects. It weakens materials, and causes cracking, embrittlement, and changes in appearance like yellowing, chalking of surfaces, and darkening of adhesives.

What about IR? This culprit relates most closely to our foe Incorrect Temperature as it causes thermal ageing, turning things brittle or warping them.

Light damage is cumulative. There are no take-backsies with light. It builds over time and cannot be undone.

Materials at risk

What we call organic materials (based on cellulose or proteins) are very sensitive to light. This includes: paper, cotton, linen, wood, parchment, leather, silk, wool, feathers, dyes, paints, resins, lacquers, and many more.

Plastics and synthetic dyes are also sensitive: it’s wise to treat all plastics as at risk. While some have additives to make them more resistant to UV damage these delay symptoms rather than stop them.

We no longer say anything is impervious because we keep re-evaluating what effect light has on even very tough materials (e.g. stone, metal, glass, and ceramics).

Monitoring light

Monitoring can help you understand where and what the issues are. It can nip problems in the bud before you have to do costly interventions later, or being told an object is beyond saving. That’s a win-win!

In collections care we measure light in lux (or foot-candles) and it’s a measure of illuminance: how much light falls onto a surface. UV is measured in microwatts per lumen.

There are two main types of monitoring: spot checks, and cumulative.

Spot checks are done with light meters (AKA lux meters). You can get these stand-alone, or combined with UV and even IR. These meters are simple to use: a button push tells you everything immediately.

Cumulative monitoring can be high or low tech. A popular choice is something called a blue wool card. It has stripes of blue fabric that fade in a predictable way, giving you an idea of how much light the adjacent object has been exposed to.

Finally there are data loggers: these gather data over time on the visible light and UV levels via sensors and give you a fuller picture of what’s happening throughout the day.

Common sources of light

Light sources can be natural or artificial.

Our natural source is daylight, which is actually two different things: direct sunlight, and the general light that we associate with the sky even if we can’t see the sun (skylight).

In the artificial camp we find incandescent lamps, halogen bulbs, fluorescent lamps, and light-emitting diodes (LEDs). LEDs are definitely becoming the norm and have the added benefit of emitting little or no UV and no IR.

How much light is too much?

For UV we want none at all.

We don’t need UV to see something, so we can only win when we take UV out of the equation for our collections.

For visible light it’s a little trickier.

Over the years we’ve wrestled with level recommendations. Often the cited numbers are 50 lux for highly sensitive things, 150-200 lux for moderately tetchy materials, and 300 lux for robust ones. We try not to think about it like that anymore though.

Partly this is because no light level is ‘safe’ (they’ll all alter collections) and partly because we’re talking more about access. Many conservators will still tell you that 50 lux is a good benchmark for ‘low light levels’ for very sensitive materials.

When considering light levels it’s better to first ask who you would like to see it, and secondly what it’s made of, so that you can make a holistic decision.

Some research suggests that it’s only at 500 lux that most people of a broad range of ages can see objects well, but there’s no reason to keep light levels that high all the time.

You can read more about these lines of thinking in Stefan Michalski’s article for the Canadian Conservation Institute and David Saunders’ book on museum lighting.

Light hacks and control methods

Start by looking at your light sources.

Daylight via windows can be mitigated by:

  • UV blocking films (remember to replace these regularly)
  • UV blocking laminate windows
  • Blinds or shutters
  • Electrochromatic glass

Your artificial light sources can be improved by:

  • Changing older bulbs to zero UV type LEDs
  • Using filters or diffusers in front of your lights
  • Using a dimmer system
  • Limiting the amount of time they stay on, e.g.:
    • Turn them off at night or outside visitor hours
    • Only turn them on at set times per day (creating a viewing time)
    • Have them be motion activated

Other things we can do include:

  • Using covers: let visitors lift a flap, remove a lid, or open a drawer
  • Rotating objects: switch objects, turn pages, and change it up
  • Moving collections: if you can’t change the window, can you change what hangs in front of it?

Light at the end of the tunnel

Light is necessary to enjoy our collections and there’s loads we can do about its impact. We can monitor light levels and that helps prevent problems and make informed decisions. If we think about who we want to see our collections we can choose light levels that allow for both use and longevity.

Hear more from Jenny Mathiasson, objects conservator, collections care consultant in Wales in the UK, and the producer and voice of The C Word The Conservators Podcast, in the Conserv Community.

If you have any questions about environmental monitoring, integrated pest management, or just want to talk about preventative conservation, please reach out to us! Don’t forget to check out our blog or join our community of collections care professionals where you can discuss hot topics, connect with other conservators or even take a course to get familiar with the Conserv platform.

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