Conservation and lighting

There are 3 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 

Light is a form of energy which is measured and expressed in wavelengths. Natural light starts at a wavelength of 300 nanometres, anything shorter than that 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  collections. 
• Visible light. This is the spectrum of light, between 400 and 760nm, that humans can see. This spectrum is seen in the 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 high levels of ultraviolet radiation, whereas tungsten bulbs emit significant amounts of infrared. 

Reducing visible light 

Lux levels 

The intensity of visible light is measured in lux (1 lux = 1 lumen per square metre). It’s important to monitor lux levels, especially when lighting sensitive objects. You only need 50 lux to to see the shape and colour of an item, so be careful to avoid higher levels of lux when displaying valuable and fragile 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 
• Prints
• Drawings 
• Watercolours 
• Stamps 
• Manuscripts 
• Coloured and old photographs 
• Miniatures 
• Transparencies 
• Unprimed, thinly coloured paintings on canvas 

Moderately sensitive (200 lux) 

• Oil and tempera paintings 
• Lacquerware 
• Plastics 
• Wood 
• Furniture 
• Horn 
• Bone 
• Ivory 
• Undyed leather 
• Minerals 
• Modern black and white photographs 

Insensitive (300 lux) 

• Stone 
• Ceramic 
• Metal 
• Glass

Once you’ve established the correct lux levels for displaying your collection, you need to control and monitor daylight and artificial light. 

Daylight fluctuates in intensity, making it difficult to control.  However, removing it altogether could make your museum or gallery 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.

Artificial light helps to create a better atmosphere within your museum.  You have total control over the colour, warmth, intensity, and distribution of artificial lighting. 

Reduce any damage from artificial light by: 

• Using low-wattage bulbs 
• Reducing the number of lamps
• Diffusing the light 
• Using dimmer switches

Damage from light is a cumulative effect. The longer you leave an item exposed to light, the more damage it’ll do. Reducing the damage done by light means considering both intensity and the length of time exposed. For example, 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. 

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, calculated at seven hours a day for six days a week over 52 weeks. Based on this average, an item can be exposed to light 2,184 hours a year.   

Exposure hours are multiplied by recommended light intensity to calculate lux hours. Using average annual light exposure, the recommended maximum number of lux hours is: 

• 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 reduced by: 

• Putting items into storage once they’ve reached their annual recommended lux hours 
• Changing displays regularly, rotating items from storage
•  Carefully turning pages of books and illuminated manuscripts regularly 
• Fitting curtains to display cases 
• Fitting time switches to artificial lighting 
• Installing movement sensors that only switch on lights when someone’s in the room 
• Using curtains or blinds to exclude all light when the museum is closed. 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 affecting how you see exhibits. 

Both daylight and artificial light emit UV radiation. Its short wavelength is the most damaging component of light to museum artefacts. 

There are various 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 continue to monitor how effective they are. Laminated glass lasts the longest, although you should check with your supplier to see if their films and filters are appropriate for use in museums. 

Many light sources emit infrared radiation, which is invisible and generates heat.  Tungsten lights are famously inefficient: a 100 watt bulb could use 94% of its electricity for heat. 

Hot display cases and rooms can cause fluctuations in relative humidity, damaging collections in the process. Infrared radiation should be managed and controlled. 

 Tips for 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, these bring light from an external heat source,  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. 

Sometimes investing in the right lighting can come with a considerable initial cost. 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. 

For more information on collections care, see our other advice guides.  

See advice from Collections Trust, who have further information on preserving your museum’s collections.

The Victoria and Albert Museum offers a comprehensive case study of their low-energy lighting systems.

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)