Monitoring light and UV radiation

Monitoring light and ultraviolet radiation in your museum is an important part of environmental management.

Successful monitoring will:

• Protect light-sensitive collections and materials
• Reduce the need for remedial conservation treatments
• Help your team understand best practice
• Support funding applications and loans
• Inform improvement projects, for example fitting blinds or upgrading lighting

Visible light is measured in terms of its concentration falling on a surface. This is expressed in Lux: one light unit (Lumen) per square metre (m²).

UV radiation can be measured in microwatts, or expressed as a proportion of the light. In museums, it is normal to use the proportional measurement, expressed in microwatts per Lumen (μW/Lm).

When you monitor light depends on several factors. Monitoring can be part of your daily data recording, spot checks for exhibitions, or a wider refurbishment of gallery or storage spaces. Key factors include:

• The light-sensitivity of your collections
• Your maximum light allowance for display requirements
• How you light your museum for audience needs

How you monitor light depends on what you are trying to find out. You might be mapping the pattern of light in a display space to help plan an exhibition, or checking the light falling on a particular object to make sure it is not at risk from cumulative exposure.

Place your meter in the best position to collect accurate readings. Check that nothing is blocking or shadowing it, and that it is pointing in the same direction as the object.

If you are taking spot readings to understand how light levels change in a space over time, always take readings from the same positions.

A range of light and UV monitoring equipment is available. Prices vary depending on accuracy and quality. It may be more cost-effective to bring in specialists to carry out monitoring, who can share findings and advise on improvements.

Spot readings versus cumulative measurement

Spot readings are still the most common way to monitor light in museums. However, museums are increasingly using instruments that log total light exposure over time, such as dosimeters.

The effects of light are cumulative. This means it is more important to consider the total light exposure an object receives than the light level at any one moment. For more information, read our guide to lighting your museum with conservation in mind.

Instruments for spot readings

• Light meter: contains a photosensitive cell that matches how the human eye perceives light. The cell converts light energy to electrical energy, read off a scale or shown digitally.
• Ultraviolet meter: works in the same way, using a UV-sensitive cell to convert UV radiation to electrical energy.
• Combined Lux and UV meter: combines both functions in one instrument using the same technology.

Instruments for cumulative measurement

• Dosimeters: work on the principle that light causes measurable fading of organic material, usually dyes, over time. The blue wool scale is the most well-known system. Developed to test the light-fastness of dyes, blue wool dosimeters fade to a known degree in light conditions. By comparing a faded dosimeter with the scale, you can find out how much light an object has been exposed to.
• Data-loggers, telemetric sensors, and hard-wired systems: use photo and UV-sensitive cells to take frequent, repeated readings, making it possible to chart cumulative light and UV over time. Data is sent to a computer for easy presentation and analysis

All instruments have some degree of error. Manufacturers and suppliers can tell you the accuracy of any instrument you are considering. Many instruments are most accurate within a restricted range.

Check the specifications before you buy. You will likely want an instrument that is most accurate within the 50-200 Lux range. Some UV meters struggle to accurately measure UV content at the low Lux levels typically used in museum displays.

To limit errors:

• Read instruments carefully and maintain them properly
• Keep photo and UV-sensitive cells clean and dust-free
• Each time you use a meter, cover the cell completely with your hand and check the reading is zero
• Send light and UV meters for a calibration check every couple of years

You can adjust basic light meters to read zero. Others cannot be adjusted as easily. If a meter is reading much too high, send it for repair or replace it.

Your recording system will depend on how you take readings and what the monitoring is for.

Standard approaches in the museum sector include:

• Marking light readings on a plan of the space
• Keeping a table of daily changes for known spots or objects in a space
• Taking readings in the same way each time

Always include notes and relevant information to help colleagues understand the context. Record:

• Date and time of reading
• Equipment used
• Weather conditions
• Whether windows were covered or lights turned off
• Any changes to the lighting set-up

If you are using a data-logger, make a separate note of relevant information you can refer to when reviewing the data. Make sure nothing is blocking the sensor and that it points toward the light source for accurate readings.

Changing light levels is much easier than changing temperature or relative humidity. Light monitoring often leads to immediate improvements for collections.

Light can be filtered, removed, redirected, and diffused using simple and effective technology. A quick survey of your museum’s light levels can quickly reveal problem areas and identify where conditions are safer for light-sensitive collections

Reporting your findings

A clear report makes it easier to achieve long-lasting changes that benefit your collections and build a persuasive case for improvements.

Include the following in your report:

• Recorded findings
• A list of possible options for improvements
• Pros and cons of each option
• Green and sustainable benefits of the proposal
• A preferred solution
• An argument for removing, rotating, or replacing an object where relevant

This gives stakeholders what they need to make informed budget and spending decisions.

The Collections Trust website collates an extensive list of advice guides on collections care and conservation.

The Institute of Conservation ICON (ICON) have extensive collections care guides for different materials.

Museum Development South East Conservation have advice on environmental monitoring for light.