This is Chirk Castle, just over the border into Wales. It is owned by the National Trust. I spotted the "Light Monitoring" on a piece of furniture in the first room. Photons of light can provide enough energy to change the chemicals in some materials over time and ruin them. This is a problem in preservation. So they put blue material into a card. The material you can see is exposed to the light but there is more under the cardboard that acts as a control. The difference between the two is read by a machine. Wonderfully simple and low-tech: Physics applied in conservation.
