The Ultraviolet Threat!

Over the last few weeks Sarah has spent sunny days carrying out the annual check of the Ultraviolet (UV) filters.  This is an adhesive film applied to each individual pane of glass in the show room windows that filters the UV out of the natural light. It is clear and transparent therefore not affecting the view to the outside.

Taking a UV reading in the Brown Gallery, a north facing window

The whole spectrum of light (UV, visible, Infra Red (IR)) can cause damage to a variety of materials, in particularly textiles, which Knole has a lot of!  UV as a high energy and short wavelength light is the most damaging part of the light spectrum.  It causes oxidation and photochemical reactions which discolour historic objects, and with textiles the deterioration process culminates in a physical break down of the fibres causing textiles to split and eventually turn to dust!

A reading of 3 microwatts per lumen. The UV film on this window is in good condition


The UV film has an approximate 10 year life span.  So we check it annually with a lux and UV monitor.  If the film is still working effectively reading should be less than 75 microwatts per lumen.  Each years readings are compared to find out if and how much deterioration of the film has occurred. We take the readings in bright sunny days so that the film is being tested in the harshest circumstances.  Although even in cloudy days UV levels can still be extremely high. 

The film is also becomes ineffective if the adhesive fails and it begins to peel off the glass, letting the UV in once more.This is one of the windows in the Spangled Bedroom.

We keep a record for when film is renewed with the annual readings so that where are able to monitor how long the film is lasting to the standards we require to.  So far a lot of the readings have come back with better results than we expected.  On average the UV filter costs £30 per square metre.  So when it comes to renewing a lot of windows at once it is very expensive.

The photo below shows you how well it does filter out the UV.  So although expensive it is a very good investment, considering how much more damage would occur without it.

The beads in the frame have been coated in a UV reactant chemical. The left half of the frame has a UV filter on it, the right side is just clear perspex. The frame was taken outside to get as much UV as possible on to the beads



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