UV-Blocking Window Films for Use in Museums—Revisited
by Colleen Boye, Frank Preusser, and Terry Schaeffer
Introduction
In naturally lit galleries, the radiation that constitutes daylight can present a hazard to many of the materials found in art and archival collections. Ultraviolet (UV) radiation, in particular, is invisible to the human eye but can fade colorants and damage fibers and polymers. Therefore, institutions generally use window films to block unwanted solar radiation.
Film suppliers have continually expanded their offerings and updated film technology to meet increased demand from commercial, residential, and automotive customers.
However, the needs of museums have not been addressed specifically during this expansion of the range of window film products. Museum staff must determine the efficacy of individual films and select those that best meet their requirements for completely blocking UV and reducing visible
light to the desired level without altering color values. To this end, the conservation community has been evaluating UV-blocking window films for more than two decades (1-5).
They have had a variety of goals and thus have used different types of measurements and performance criteria.
Evaluation of UV-blocking window films has been revisited
recently in a survey of the UV and visible light transmitting properties of products from several suppliers (6). After presenting a useful summary of film composition and structure,
the author tested the UV transmission of the unmounted film samples without adhesive. A UV meter with response optimized for UVB radiation (280-320 nm) was employed in this initial evaluation. Several films were rejected on the basis of these results. In the second part of the investigation,
the transmissions of the remaining films were characterized by absorption spectrophotometry.
After some consideration and discussion of the methodology and results reported, we found that we questioned the appropriateness of some of aspects of the author’s initial evaluation process, for the following reasons: in actual use the films are always applied to glass, which absorbs most
UVB radiation; the test did not accurately measure transmission of UVA (320-400 nm), which has been shown to damage many materials; adhesives contribute to the performance
of some films; and the mixed light sources present during this test were not representative of daylight. Also,absorption spectra have low precision when the absorption level is high, making the spectra difficult to interpret in the particular wavelength regions of interest.
Several 3M films were rejected by the preliminary study. If this were accurate, it would be cause for concern, as 3M products have frequently been used by the museum community
for their UV blocking properties. Because of our reservations we undertook our own study using a calibrated light source and UV-visible transmission spectrophotometry. We included several films from 3M and other manufacturers that were tested in the previous study, as well as some new
films. All the window films tested incorporated adhesive and were tested both on and off window glass. In this first part of our investigation, transmission spectra of all the samples were obtained and the data used to characterize the UV rejection and color neutrality of the films. The ageing
behavior of the films will be examined in a second study.