Light is important for us to be able to see but it is also one of the most damaging causes of deterioration of collections. This damage is cumulative and causes permanent damage to objects. Ultra Violet (UV) radiation is the most damaging spectrum of light and causes a photochemical change to happen within the material.
It is therefore very important to monitor the light levels and ensure that the needs of viewing and using the collection are balanced with the fact that the light exposure will be gradually deteriorating the material.
Light is measured in Lux = one light unit (Lumen) per square meter and UV is measured in microwatts per lumen = proportional measurement of the light falling on an object
Not all materials react to light in the same way so different light levels will be appropriate for different objects. The table below shows which material types are light sensitive and the recommended lux level at which they should permanently be displayed.
|Highly light sensitive (<50 Lux)||watercolours, pastels, prints, drawings, dyed leather, fur, feathers, textiles, paper, photographs, miniatures, some plastics|
|Moderately light sensitive (<200-250 Lux)||oil paintings, lacquerware, wood, furniture, horn, bone, ivory, undyed leather, minerals, most plastics, painted wood and sculpture|
|Light stable||Metals, glass, ceramics, stone|
There are various methods in which to measure the light levels falling on your collection:
Blue wool dosimeters / fading strips – these are strips of wool, mounted on card which have been dyed with a blue dye which is known to fade at a specific rate. Part of the wool is covered so one part is permanently in darkness and the other is exposed to the light
- Low cost
- Easy to install
- Good for monitoring light fall on a specific object
- Only show cumulative affect of light on the object after a specific period
- Does not give accurate numerical readings
- Only shows results “after the fact”
Handheld Lux and/or Lux and UV meters – these take electronic measurements and can be used to take spot readings
- Give instant numerical readings
- Adjustments can be made almost instantly
- Lux only meters are not too expensive
- Combined Lux and UV meter can be expensive
- Spot readings do not show what happens in the museum when people are not there
Continuous monitoring Lux and UV devices – these take continuous readings of the light levels within a space or that falling on a specific object
- Give 24 hour monitoring so you can see what happens out of hours
- Software generally easy to use and produces graphs which make it easier to see patterns
- Less time consuming than spot reading
- Monitors and software can be expensive
- Could potentially be intrusive in a display
Using light monitors
- Place the monitor next to the object you would like to measure light levels for
- Ensure the sensor is pointing the same way as the object being measured
- Make sure there is no shadow between the sensor and the light source
- If taking spot readings, they should be taken as regularly as possible, ideally everyday at the same time
- If using a continuous monitoring device make sure they are downloaded regularly so they do not become full and stop recording
There are three ways to reduce the rate of deterioration by light:
- Reduction of UV radiation
- Reduction of the duration of illumination
- Reduction of light levels
Reduction of UV radiation
The recommended level of UV is less than 75 microwatts per lumen but levels should be kept as low as possible..
UV light levels can be controlled using a UV absorbing film or varnish applied to the inside of window glazing. Care should be taken if applying to historic window glass which is fragile and may be broken during replacement of the film. It is therefore advised to always seek specialist advice in this instance.
Reduction of the duration of illumination
When objects are not being viewed they should be kept in blackout conditions. This can be achieved by:
- Use of blackout curtains, blinds or shutters at the windows.
- Open blackout as late as possible before visitors arrive and shut as soon as the space closes.
- Individual case covers made out of black out materials. There may be a risk of developing a microclimate inside the cover so specialist advice should be taken.
- If there is more than one of a collection consider rotating displays and showing only one at a time.
- For extremely light sensitive objects e.g. photographs, facsimiles can be displayed and the original kept in conservation standard storage.
Reduction of light levels
A balance must always be achieved between controlling the light levels and allowing enough light for visitors to see and enjoy the objects. The aim is to get the average level in a room to be 150 lux.
- The human eye needs time to adjust to lower light levels and therefore avoid visitor routes leading from very brightly lit spaces to dark spaces, if possible allow for a gradual lowering of the levels and avoid glare.
- Direct sunlight can give levels of around 50,000 lux and therefore should be prevented from falling on objects. It can also generate heat – solar gain – which can cause damage.
- Sun blinds or curtains should be installed which can be adjusted according to times of the day, year and weather conditions to prevent direct sunlight falling on objects whilst still allowing light into the room.
- Consider the position of light sensitive objects in the space and if possible move away from light sources.
Artificial light will also cause damage to objects and should be counted when looking at cumulative light exposure. Some lights will also emit UV and heat and this is important when choosing lights and displays.
Conservation lighting is a term that means an artificial light will provide good quality light, without UV radiation, generating the smallest possible temperature rise on the surface of an object at the lowest illumination levels to enable good visual acuity and colour discrimination.
There is also now an aim in most museums to reduce their carbon footprint and the the type of artificial lighting selected can have a big impact on this. Lighting technology is developing all the time and it is likely that yet more efficient lamps will become available in the next few years.
Comparison of low energy light alternatives to traditional tungsten lighting
|Tungsten Halogen||Compact Fluorescent (CFL)||Light emitting Diode (LED)|
|Light quality||Pure white light||Warm white light||Available in pure white and warm white|
|Colour rendering||Excellent||Good||Very good|
|UV output||Acceptable||Requires UV filter||None|
|Heat output||Very high||Low||Low|
|Lifetime||2,000 hours||8,000-10,000 hours||15,000-30,000 hours|
|Recommended usage||Use where traditional lamp appearance is required and white light is acceptable||Use where traditional lamp appearance is required and UV output is not an issue||Best currently available for replacement of general lighting|