Monitoring light and UV radiation
Monitoring light and ultraviolet radiation in your museum is an important part of environmental management.
Successful monitoring will:
- Protect collections
- Reduce the need for remedial conservation treatments
- Help the museum’s workforce to understand best practice
- Support funding applications
- Inform improvement projects
Units of measurement
In museums, light is measured in terms of its concentration. This concentration is expressed in Lux, which is 1 light unit (Lumen) per square metre (m2).
Ultraviolet radiation can be measured in microwatts or expressed as a proportion of the light. In museums it’s normal to use the proportional measurement, which is expressed in microwatts per Lumen (μW/Lm).
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’re trying to find out. You may be trying to map the pattern of light in a display space to help with the planning of exhibitions. Alternatively, you may want to check the amount of light falling on a particular object to ensure it’s not at risk of being damaged.
Make sure that you place the meter you’re using in the optimum position to collect accurate readings. Check that it’s not shadowed or blocked in any way.
If you’re taking spot readings to understand how light levels change in a particular space over time, it’s vital that you always take readings from the same positions.
There’s a range of light and UV monitoring equipment available. Prices vary depending on accuracy and quality.
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.
The effects of light are cumulative, which means it’s more important to consider the total light exposure that an object receives rather than knowing 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, 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 noticeable 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’s possible to find out how much light an 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 light and UV levels, repeating the readings so frequently that it’s possible to chart 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’re buying. Many instruments are more accurate within a restricted range.
It’s worth checking the specifications of the instrument you’re purchasing as you will likely want to buy one which is most accurate 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 which are typically used to illuminate museum displays.
You can limit errors by reading instruments accurately and making sure that they’re 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’s also important to send light and UV meters for a calibration check every couple of years.
It’s possible to adjust basic light meters to read zero. Other meters can’t be adjusted so easily. If you’re 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 museum 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
- If windows were covered
- Any changes to the lighting set up
If you’re using a data-logger to measure the cumulative light falling on an object, then make a separate note of relevant information which you can refer to when looking at the data.
Apply your data
Changing the light and lighting of a space is much easier than changing its temperature and relative humidity. This means it’s 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 can immediately reveal where the problem areas are – and where light levels are safe.
Reporting makes it easier to make long-lasting changes that benefit your collections. Lighting reports can make a persuasive case for the implementation of improvements.
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.