Filtering ultraviolet

Ultraviolet radiation carries more energy per photon than visible light and does disproportionate damage per unit received. It is also, unlike visible light, entirely unnecessary for viewing an object. That combination makes UV filtering the best value intervention in the whole of light control: it removes real damage and costs the display nothing.

What can be filtered, and how

Windows. Adhesive film applied to the inner face of the glazing is the standard retrofit. Good products block the great majority of UV while passing most visible light, so the room does not visibly darken. Application quality matters, because bubbles and lifted edges are both unsightly and a sign the film will fail early.

Framed works and glazed cases. UV-filtering acrylic sheet replaces ordinary glazing in a frame or case. It carries the filter through the material rather than as a surface coating, so it does not peel and generally lasts as long as the sheet. Acrylic scratches more easily than glass and can carry a static charge, which matters for loose pigment media such as pastel, where a non-static or glass alternative is preferred.

Lamps. Filtering sleeves for fluorescent tubes are still relevant in older stores. Modern LED sources emit essentially no UV, so lamp filtering has largely become a non-issue for new installations, which is one of the genuine advantages of the switch.

Rooflights and large glazed areas. These usually dominate the UV load in a naturally lit gallery, and they are also the hardest to reach for maintenance. Where access is difficult, filtering glazing at replacement time is worth far more than film that nobody will be able to inspect.

Verifying it, then and later

Fitting a film without measuring is an act of faith. The check takes minutes: read UV in microwatts per lumen on the inside face and compare with an unfiltered reference, either outside or an adjacent unfiltered pane. A combined lux and UV meter gives both numbers at once.

The measurement matters more later than at installation. Films degrade under the radiation they are absorbing, and performance falls gradually over years without any visible change. A film that blocked well when fitted may be doing much less a decade on, and nothing about its appearance will tell you.

The practical discipline is to record the reading at installation, put a re-measurement into the maintenance cycle, and record the film’s fitting date somewhere that survives staff changes. A film with no known age should be assumed to be past its useful life until measured.

Where filtering stops helping

The single most persistent misconception in light control is that a UV-filtered display is a safe display. It is not. The visible light that remains fades dyes and pigments exactly as it always did, and a UV-filtered case at 500 lux will fade a watercolour steadily.

Filtering removes a cheap, disproportionate fraction of the damage. Everything after that is a matter of illuminance and hours, which is the domain of the exposure budget, and for the most sensitive material the effective control is rotation rather than any filter.

Order of work

1. Measure UV content at the object, in microwatts per lumen, for each significant light source.
2. Filter what is high, starting with daylight, which is almost always the largest contributor.
3. Re-measure to confirm the filter did what it claimed.
4. Record the installation date and set a re-measurement interval.
5. Then move to visible light, illuminance and hours, which is where the remaining damage is.