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Stylized Text: UV Transmittance and Fading.

Picture of Fenestration Logo. Fading of interior furnishings is often attributed to ultraviolet radiation (UV) from the sun passing through windows onto interior surfaces. However, UV is not the only portion of the solar spectrum which can damage artwork or furnishings inside buildings. Virtually the whole spectrum is of concern, which is why long term exposure to solar radiation should be limited. Here we explain some of the contributions to fading and other damage of interior furnishings and materials due to solar exposure.

Supported by the outstanding energy-efficiency levels of today’s low-emissivity glasses, current architectural designs favor a large number of windows with clearer glass than ever before. Consumers also drive this trend, with their demand for large, open interior spaces flooded with natural light.

While this trend has brought more light into buildings, and in many case energy savings due to reduced electric lighting usage, another trend has, at the same time, made interior fabrics and finishes more fragile: the emergence of environmentally friendly materials.

Driven by pollution laws, fabric dyes, wood stains, paints and other coatings found in modern buildings have been formulated to have a more benign environmental impact, but may in consequence be less stable than their predecessor materials, typically, solvent-based. Today’s water-based products have a number of obvious environmental benefits, but some are more susceptible to fading over time, a significant drawback.

In addition, because of ozone depletion, higher levels of solar UV now reach the surface of the earth. This further contributes to the rate of fading. These three trends—more natural light transmittance, more fragile interior components, and a higher concentration of UV—have resulted in a greater awareness of fading.

What is Fading?

Fading is a change in color with time.  It is measured by evaluating the color of a material at two or more points in time. Often it is a loss of color or a reduction in color saturation due to bleaching.  For the purposes of this discussion, we are also interested in additional material damage due to solar exposure, such as embrittlement and cracking.

Causes of Fading

Fading has two main causes.  The first is chemical, where chemical changes in the coloring agents of a material can cause a change (or reduction) in color.  Chemical reactions that lead to fading can be influenced by many environmental factors, such as the type of coloring agent/chemical, the chemical environment of each coloring agent in the material, the ambient chemical environment of the material, and the temperature, humidity, and radiation environment. In addition, wear or abrasion can physically remove coloring agents from a material's surface.

Ultraviolet Radiation

The sun's energy is made up of three distinct spectral components: ultraviolet radiation, visible radiation, and near-infrared radiation.  What distinguishes these from one another is the wavelength ranges that characterize them, commonly measured in nanometers (nm).  A nanometer is very small.  A human hair is over 100,000 nanometers thick.

Ultraviolet radiation is invisible to the human eye and has the shortest wavelengths of the three types mentioned, from 300 to about 380 nm.  Visible light covers the approximate range from 380 to 780 nm, while the near infrared radiation (sometimes called invisible solar heat) has the longest wavelengths, from 780 to 4045 nm.
Ultraviolet radiation (UV) is the single largest contributing factor in fading of fabrics, carpets and other furnishings.  Although visible light, electric lighting, heating, humidity, age of fabrics and fabric dyes all play a part in the process, UV radiation is attributed to 40% of the damage.  Protecting against UV is not just important in hot, sunny climates.  Even in cold, cloudy climates, UV radiation can damage furnishings.
UV can also be hazardous to humans.  According to the American Academy of Dermatology, exposure to the sun and its harmful UV radiation is causing an epidemic in skin cancer cases in recent years. 

Several products are moderately effective at blocking ultraviolet radiation.  Low-emissivity coatings on glass provide additional protection from UV.  However, even the best of these coatings still transmits 26% of the UV radiation incident upon them.  One PVB interlayer supplier states that laminated architectural glass made with clear or tinted interlayer is essentially opaque to UV radiation. 

Quantifying the Effects of Fading

The most authoritative research on quantifying fading damage was done in the early 1950s by the United States National Bureau of Standards.  The U.S. Library of Congress undertook this research, in order to design a glass filter to protect the original copies of the Declaration of Independence and the Constitution.  Scientists found that blocking all of the ultraviolet radiation portion of the solar spectrum would not eliminate fade damage for most fabrics, but will slow down the rate of fading by a factor of about three. 

Because so many factors influence fading, finding the effect of one factor is extremely difficult.  All parameters except the one being studied must be held constant for the duration of an experiment, which may run for months or even years of testing.  That is why there is relatively little research in this area.  To study the effect of radiation on fading, it is important to focus on one type of material while keeping the environment constant.  Factors in the environment include chemical composition of the atmosphere, temperature, and humidity.  The known exposure to radiation, including the known spectrum, and known dose (intensity X time) must be identified.  Then there must be sufficient duration to observe the rate of color shift, or fading.
There have been studies like this of the fading and other damage effects of solar radiation but no consensus has yet emerged on which portion of the solar spectrum is most responsible nor on what spectral weighting function is appropriate for assessing in a single “UV transmittance” figure the contribution of different solar UV wavelengths to the damage.

For a while, the straight integrated spectral transmittance from 300 to 380 nm was used, given the symbol T-UV. More recently, the spectrum of interest has been extended beyond the UV portion to cover the range from 300 to 700 nm, and a different weighting function was selected for use. With this system, the human photopic visibility function, often given the symbol V(λ) and called the V-lambda weighting function, is replaced by another function purported to better represent the damaging portions of the solar spectrum.

The damage-weighting function most often used is:

Picture of equation.

The resulting damage-weighted transmittance has the symbol T-dw. The methodology is based on the work of Jurgen Krochmann in Germany and stems from his studies of the damaging affects of radiation on paintings and other museum artifacts. The Krochmann damage weighting function was incorporated into ISO/CIE publication 89/3 “On the deterioration of exhibited museum objects by optical radiation” and is referenced by NFRC [National Fenestration Rating Council] optical properties standard NFRC 300 in computing the damage-weighted transmittance T-dw.

It is extremely difficult to isolate the influence of radiation on the fading process.  The best currently available indication of a glazing’s effect on fading is the damage-weighted transmittance, T-dw, of the glazing system.  T-dw is the weighted transmittance at normal incidence for the center-of-glass region.  The wavelengths of radiation that have the most influence on fading (when all other factors are held constant) are given the most weight.  In determining the weighting curves, the type of experiment described above is repeated using different radiation sources and analyzing the results.  Each weighting curve is specific to a given material, environment, and history.

The current method for calculating T-dw is based on studies of art materials, not furnishing or construction materials, so its applicability to those types of materials is unknown.  In addition, there is a difficulty in communicating the fact that T-dw is a relative measure (ranking) for glazing systems, not something that can be used to predict the actual amount of fading in any given situation.