A COMPARISON OF SELECTED UV FILTERING MATERIALS FOR THE REDUCTION OF FADING
Patricia Cox Crews
A VARIETY OF ULTRAVIOLET FILTERING MATERIALS are in use in museums. However, most filters used over windows, skylights, and fluorescent lights are made of rigid acrylic sheets or flexible films of polyester or acetate that have an ultraviolet absorber dispersed throughout the substrate. Rigid acrylic sheeting is known by a variety of trade names including Plexiglas¯, Acrylite¯, and Perspex¯, whereas flexible polyester film is often described by its tradename Mylar¯. Both rigid and flexible filters can be obtained in colorless forms as well as forms with varying amounts of a yellow tint ranging from almost colorless to an amber color. The yellow-tinted filters vary widely in the amount of visible radiation they absorb. Some with only a light yellow tint remove very little visible radiation, while others with an amber tint absorb much more visible radiation.
Thomson9 described the ideal UV absorbing filter as a colorless one that would prevent all UV radiation down to 400 nm from passing through but would not hinder the passage of any visible light. Crews3 suggested that the ideal filter should be modified based on recent research showing that colorless filters afforded some natural dyes no protection against fading. Unlike contemporary synthetic dyes that are faded primarily by ultraviolet radiation, most natural dyes are faded by both ultraviolet and visible radiation, particularly visible light in the violet and blue regions of the electromagnetic spectrum.9 Therefore, Crews3 suggested that the ideal UV filter should be tinted yellow so that it could prevent the passage of some visible radiation (i.e., violet and blue light) as well as all UV radiation. Yet museum personnel sometimes object to yellow-tinted filters because they distort color rendition in exhibit areas.
In an effort to identify a material that would provide the desired level of protection without color distortion, a UV filtering material of flexible polyester film containing a silver reflective coating was examined. Unlike filters with strong yellow tints, it causes almost no distortion of color rendition when used as a filter for fluorescent lamps; however, its effectiveness in reducing the fading of natural dyes was unknown. This research compared the effectiveness of this UV-absorbing material with a silver reflective coating to the widely used UV-absorbing filters with light yellow tints (almost colorless) and the less widely used filters with strong yellow tints (almost amber).