Monitoring of light and ultraviolet radiation in a museum is an important part of environmental management.
Successful monitoring will:
- Protect collections
- Reduce the need for remedial conservation treatments
- Train staff and volunteers in essential museum skills
- Support funding applications
- Inform improvement projects
Units of measurement
In museums, light is measured in terms of its concentration and expressed in Lux, which is 1 light unit (Lumen) per square metre (m2).
Ultraviolet radiation can be measured as an absolute, in microwatts, or expressed as a proportion of the light. In museums, it is normal to use the proportional measurement, which is expressed in microwatts per Lumen (μW/Lm).
Where and when to monitor
When to monitor
Deciding when to monitor light depends on several factors, including
- Light-sensitivity of the collections
- Your display requirements
- How you light your museum
Where to monitor
How you choose to monitor light will depend on what you are trying to find out. You may be trying to map the pattern of light in a display space to help with planning of exhibitions. Alternatively, you may want to check that the cumulative light falling on a particular object remains within the safety limits that you have specified for it.
Make sure that the meter you use is placed to give you the most representative reading. Position the sensor in the right plane and location, and check that it is not shadowed or blocked in any way.
If you are taking spot readings in order to build up a picture of the changing light in a space over time and season it is vital that you always take readings from the same positions.
There is a range of light and UV monitoring equipment available. Prices vary depending on accuracy and sophistication.
Spot reading versus cumulative measurement
Spot readings are still the most common way to monitor light in museums. However, museums are increasingly turning to instruments that log total light exposure over time, such as dosimeters, to monitor the effects of light.
As the effects of light are cumulative, it is more important to consider the total light exposure that an object may be receiving than to know the light level at any one time. For more information, read our guide to lighting your museum with conservation in mind.
Instruments for spot reading
The light meter contains a photosensitive cell which is able to match the way the human eye perceives light. The photocell converts light energy to electrical energy which in turn is read off a scale or represented digitally.
This meter works in the same way as a light meter, again using a UV-sensitive cell to convert UV-radiation to electrical energy.
Combined Lux and UV meter
There are some instruments available that combine light and UV measurement in one instrument, but use the same technology as described above.
Instruments for cumulative measurement (or continuous measurement)
Dosimeters work on the principle that light will cause a perceptible amount of fading of organic material (usually dyes) over time.
The blue wool scale is the most well-known dosimeter system - it was developed for testing the light-fastness of dyes. Blue wool dosimeters fade in light conditions to a known degree. This means that, by comparing a faded dosimeter with the scale, it is possible to find out how much light the adjacent object has been exposed to.
A more recent development is the LightCheck dosimeter which has been specifically developed for museums and galleries.
Data-loggers, telemetric sensors and hard-wired systems
These instruments use photo and UV-sensitive cells to measure the light and UV levels and repeat the readings so frequently that it is possible to build up a picture of the cumulative light and UV. The data is relayed to a computer for easy presentation and manipulation.
All instruments are subject to some degree of error. Manufacturers and suppliers of light and UV monitors will be able to tell you the accuracy of the instrument you are buying. Many instruments are more accurate within a restricted range and so it is worth checking that the instrument you want to buy is at its most accurate at the low light levels.
This is because you are likely to want to measure most accurately within the 50-200 Lux range. Some UV meters find it difficult to accurately measure the UV content of light at the low Lux levels typically used to illuminate museum displays.
Because you cannot prevent the error that is present in all instruments, make sure that you keep error to this minimum by reading instruments accurately and making sure that they are maintained properly. This means ensuring that the photo and UV-sensitive cells are kept clean and dust-free.
Check that your meters are reading correctly every time you use them. Cover the photo or UV-sensitive cell completely with your hand and check that the reading is zero. It is also important to send light and UV meters for a calibration check every couple of years.
With some basic light meters, it is possible to adjust the meter to read zero. Many more meters cannot be adjusted easily. If you are finding that the meter is reading much too high, you should send it away for repair or replace it.
Record and apply your data
Recording your data
Your recording system for light and UV will depend on how you take the readings and the purpose of the monitoring.
Standard recording systems in the sector include:
- Marking light readings on a plan of the space
- Keeping a table of daily changes
Your records should always include notes and relevant information to help your colleagues understand the context.
This should include:
- Date and time of reading
- Windows covered or not
- Any changes to the lighting scheme made
If you are measuring the cumulative light falling on an object using a data-logger then make a separate note of relevant information, which you will be able to refer to when looking at the data.
Apply your data
It is relatively straightforward to manipulate light and lighting, far more than is possible with temperature and relative humidity. It is therefore common for light monitoring to result in immediate improvements for collections.
Light can be filtered, removed, redirected and diffused using simple and effective technology. A quick survey of the light levels in your museum might immediately reveal where the problem areas are - and where light levels are safe for even the most sensitive object.
Reporting makes it easier to make long-lasting changes for the benefit of the collections. Lighting reports can serve as persuasive stakeholder communications tools.
In your report, try including:
- Recorded findings
- A list of possible options for improvements
- Pros and cons of each option
- A preferred solution
This will help stakeholders make budget and spending decisions.
The Collections Trust provides an extensive range of advice on all aspects of collections care online.
Contact us for any further questions.