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Light relief, part 2: the case of the mysterious reflection

In a previous post, I talked about some of the ways that we conservators monitor the amount of light coming into our galleries. That post showed a typical week’s light and UV data from a showcase:

light_eltek

This data comes from our Arctic clothing case and shows what we are looking for ideally: low(ish) light levels generally, no light during the two days that the museum is closed, and no UV at all.

Sometimes, however, your data looks a bit more like this:

light_riley_2012

This is a graph of light and UV data from the same Arctic clothing case in August 2012. The light levels in this case are far from perfect: the light (the green line) is too high overall and there is some UV (the purple line) registering on one of the days. The thing that really concerned me, however, was the sudden spikes that happen each day in the early evening. For half an hour only, the light levels shoot up so high that they are literally off the scale.

To see what’s going on, we can look at the data for just a single day, Sunday 5 August 2012 (click on the image below to see a larger version):

light_riley_2012_detail2

Our transmitters are set to monitor every half hour, and between 6.30pm and 7pm, you can see the light levels jump from 131 lux to 1245 lux … and then at 7.30pm drop down again to 176 lux.

After a bit of investigation, we realised the (unwitting) culprits: our neighbours on Lensfield Road! As the sun sets, it gets low enough to hit the windows of the three-storey buildings opposite the museum. One of these windows is in exactly the right position to reflect a shaft of light through the museum windows and into the Arctic clothing showcase. This only happens between late May and late August, and only in the early evenings. On 5 August (the day shown in the graph above), the sun typically sets at 7.40pm, which is why the spikes in light levels were seen about half an hour before that.

Sometimes, simple solutions are the best. I made a curtain from blackout lining material, and we put it over the showcase when the museum closes at 4pm. The reflections from the buildings opposite all happen after opening hours, so it doesn’t matter that we’ve hidden some of our nicest objects behind a curtain! We’ve been doing this for two years now: here you can see Willow taking the curtain down just before the museum opens in the morning (you can also see the transmitter that measures light at the bottom of the case).

Willow and curtain

The graphs below show data from August 2012 (left) and August 2014 (right). It’s still not perfect, but you can see that it’s a lot better than it was. However, remembering to put up and take down the curtain is not always convenient, so we’re currently looking into other ways to block light from coming into the gallery and hitting this showcase – I’ll write about some of these next time!

light_riley_aug12 light_riley_aug14

Christina

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