Museums wouldn't be any good without lighting. People visit to see your collections, so it wouldn't be much use if everything was in the dark. Lighting can create the atmosphere of a museum or gallery and it can draw attention to some of the most intriguing items on display there.
In a museum, however, light also means damage. Dyes and pigments fade when exposed to light, even at low levels, and the effects are cumulative. Control the amount of light in your museum to conserve the original appearance of your artefacts.
Reducing light damage
There are three key ways you can limit the amount of light damage done to a collection:
- Reduce the amount of visible light or light intensity that an item receives.
- Reduce the time an object is exposed to visible light to counter cumulative damage.
- Eliminate unnecessary invisible radiation.
Visible and invisible light
As light is a form of energy, it is measured and expressed in wavelengths. Natural light starts at a wavelength of 300 nanometres. Anything shorter can't make it through the atmosphere.
The light spectrum is divided into three main groups:
- Ultraviolet (UV) radiation. Wavelengths shorter than 400nm and invisible to the human eye. UV radiation is the most damaging form of light to museum collections.
- Visible light. This is the spectrum of light, between 400 and 760nm, that humans can see, separated into the different colours of the rainbow.
- Infrared radiation. Humans also can't see infrared, which is anything longer than 760nm, but they can feel it as heat.
Most types of light contain all three components to varying levels. Daylight has lots of ultraviolet radiation, whereas tungsten bulbs emit significant amounts of infrared.
Reducing visible light
The intensity of visible light is measured in lux (1 lux = 1 lumen per square metre). Everything to do with lighting your museum well depends on ensuring the right kind of lux levels, especially for more sensitive objects. You only need 50 lux to be able to see the shape and colour of an item, so don't exceed that for the most valuable items.
The following guidelines indicate the sensitivity of different materials and their appropriate light levels.
Very sensitive (50 lux)
- Costumes and other textiles
- Fur and feathers
- Dyed leather
- Coloured and old photographs
- Unprimed thinly coloured paintings on canvas
Moderately sensitive (200 lux)
- Oil and tempera paintings
- Lacquer ware
- Undyed leather
- Modern black and white photographs
Insensitive (300 lux)
Once you have established the light levels you wish to maintain for your exhibits, you need to control both daylight and artificial light to ensure consistency of light intensity.
Daylight fluctuates in intensity, making it difficult to properly modulate. Cutting it out altogether risks making your museum or gallery too gloomy and unwelcoming.
There are several options for managing levels of daylight in your building:
- Eliminate all direct sunlight.
- Keep light-sensitive objects away from windows.
- Apply solar control film, which has a tinting effect, to all windows and skylights.
- Use net curtains, Venetian blinds or calico blinds, which are useful if tinting adversely affects the appearance of old buildings.
- Block off light using black-out blinds.
Controlling artificial light
Artificial light helps to create a better atmosphere within your museum if you are restricting daylight into the building. You have total control over the colour, warmth, intensity and diffusion of any artificial lighting in your gallery.
Reduce any damage from artificial light by:
- Using low-wattage bulbs
- Reducing the number of lamps
- Diffusing the light
- Using dimmer switches
Limiting the time of exposure
Damage from light is a cumulative effect - the longer you leave an item exposed to light, the more damage it will do. Reducing the damage done by light means considering both intensity and the length of exposure. So a delicate watercolour exposed to 50 lux for 100 hours will experience the same damage as if it was exposed to 100 lux for 50 hours.
As such, when measuring light in a museum, use annual light exposure levels for the most accurate assessment.
Recommended annual exposure
Annual light exposure is measured by the opening hours of a standard museum, calculating seven hours a day for six days a week over 52 weeks. An item can therefore be exposed to light 2,184 hours a year, based on museum averages. Lux hours are the unit created when exposure hours are multiplied by recommended light intensity.
The average annual light exposure then creates a recommended maximum number of lux hours:
- For very sensitive items, 100,000 lux hours
- For moderately sensitive items, 450,000 lux hours
Reducing the length of exposure
If you cannot reduce the light intensity enough to its recommended spot check levels, then you need to reduce the number of hours exposed to meet these recommended annual limits.
Exposure time can be lessened by:
- Putting items into storage once they have reached their annual recommended lux hours
- Changing displays regularly, rotating items from storage
- Turning pages of books and illuminated manuscripts regularly
- Fitting curtains to display cases
- Fitting time switches to artificial lighting
- Installing movement sensors that switch lights on only when someone is in the room
- Excluding all light when the museum is closed using curtains or blinds. This is particularly relevant in summer when daylight hours are much longer and could expose your items for far longer than is necessary
Eliminating non-visible radiation
Ultraviolet (UV) radiation is invisible, so it needs to be carefully monitored. This also means that it can be almost eliminated in museums without it affecting how you see exhibits.
Given the advances in anti-UV technology, items can be affected by as little as 10 microwatts of ultraviolet per one lumen of light. Both daylight and artificial light emit UV radiation. Its short wavelength is the most damaging component of light to museum artefacts.
There are many tools museums can employ to combat this invisible threat, including some of the following materials:
- Laminated glass, self-adhesive film and other UV-absorbing materials such as varnish and acrylics for windows, skylights and display cases
- UV-absorbing sleeves and filters for artificial light sources
- Lamps and tubes with a low ultraviolet emission
- White paints, based on titanium dioxide or zinc oxide. Light reflected by a white painted wall contains less than 20% of its original amount of UV radiation. Whitewash (chalk) is not effective
Each of these materials has varying life expectancy, so keep monitoring how effective they are. Laminated glass lasts the longest, while you should check with your seller to see if their films and filters are appropriate for use museums.
Many light sources emit infrared radiation, which we cannot see but we feel in the form of heat. Tungsten lights are famously inefficient. A 100 watt bulb could use 94% of its electricity for heat.
Hot display cases or rooms can cause fluctuations in relative humidity and damage collections in the process. Such radiation should, therefore, be managed and controlled efficiently.
Here are some tips on controlling heat from light:
- Mount lights at a safe distance from museum objects, preferably outside display cases.
- Use 'cool-beam' lamps, that reflect heat back but allow visible light through.
- Install fibre-optic lights, which bring light from an external heat source, thus automatically filtering out both UV and infrared radiation.
Planning for infrared radiation is essential when designing a lighting system for museums and galleries. Consult professional museum designers and conservators before investing in lights. Ensure that every lighting system meets museum standards.
Economy and environment
Sometimes, investing in the right lighting can come with a considerable initial cost. You may have read about all the different aspects to consider when lighting a museum and looked ruefully at your museum's budget.
However, choosing your lighting carefully can lead to long term economic gains. Lighting galleries efficiently prevents damage that would require expensive restoration work. Limiting light exposure also comes with the added bonus of lower electricity bills.
A proper lighting management system also protects the environment and makes your museum more sustainable. Switching lights off regularly to protect your museum's collections also lowers the building's carbon footprint.
Museums Galleries Scotland offers a wide array of advice guides that cover several different aspects of running a museum. Browse through them to learn about other important topics such as monitoring humidity and caring for paintings.
The Victoria and Albert Museum offers a comprehensive case study of their low-energy lighting systems. For more elements of collections care, read the guides provided by the Collections Trust and the Museums Association.
Several books offer in-depth insights into running museums, including:
- Environmental Management - Guidelines for Museums and Galleries (Cassar, M. 1995, Museums & Galleries Commission / Routledge, ISBN 0-415-10559-5)
- The Museum Environment 2nd edition (Thomson, G. 1986, Buttwerworth)
- The National Trust Manual of Housekeeping (Butterworth-Heinemann, 2006, ISBN 0750655291)
For any more questions on lighting, collections care and running a museum, contact Museums Galleries Scotland