The Problem: Damaging Ultraviolet Radiation

Furniture. Carpets. Draperies. Artwork. All can be severely damaged by fading. Industry experts estimate that in the United States alone, fading causes hundreds of millions of dollars of damage to fabrics and furnishings every year.

But what exactly causes fading? While many people would answer "sunlight", most fading damage is caused by only a small part of the sun's energy- the portion called ultraviolet radiation. Though ultraviolet (UV) comprises only 2% of the sun's energy, it accounts for an estimated 60% of the fading damage to fabrics and furnishings.

Windows and Fading

Sunlight through windows has long been associated with fabric fading. To suppress fading damage, homeowners often install lined draperies and curtains, shutters, tinted or reflective glass, or dark, "stick-on" window films.

While all of these "solutions" reduce fading, they also prevent light from passing freely through the window, negating much of the value of having windows in the first place. Draperies, curtains, and shutters all require the resident to operate them faithfully, while tinted glass and films dramatically alter the appearance of windows from both inside and outside.

Ideally, a window would block the damaging UV rays that cause fading, while letting light pass through. Ordinary clear window glass lets in about 70% of the sun's UV radiation. New varieties of high-performance clear window glass, commonly called "low-e" glazings, provide some protection from UV radiation. Even the best of these, however, still transmit 26% of the damaging UV rays.

The Introduction of XUV Fading Protection

After several years a major new benefit for Heat Mirror insulating glass product was developed: the ability to block more than 99.5% of the damaging ultraviolet radiation, while still looking clear and colorless. For the first time, significant fading protection is available in clear-glass residential windows.

How XUV Fading Protection Works

The Solar Spectrum

The sun's energy is made up of three distinct parts, as shown in Figure 1: ultraviolet radiation, visible radiation, and near-infrared radiation. What make these types of radiation (or energy) different from one another are the wavelengths that characterize them, much like TV and radio stations use signals with different wavelengths. [These wavelengths are commonly measured in nanometers (nm). A nanometer is very small-even something as thin as a human hair is over 100,000 nanometers thick.]

Ultraviolet radiation, which is invisible to the human eye, has the shortest wavelengths of the three types of solar radiation, from 300 to about 380 nm. The next-shortest wavelengths are those of visible light from about 380 to 780 nm, while the near-infrared radiation (sometimes called invisible solar heat) has the longest wavelengths, from 780 to 4045 nm.

Return to top

Relative Damage of Different Solar Wavelengths

Work by numerous scientists, including Albert Einstein, has proven that the shorter the wavelength of solar radiation, the greater the fading damage potential. Therefore, ultraviolet radiation is the most damaging, followed by the shorter-wavelength visible light. Visible light at wavelengths above about 600 nm as well as near-infrared radiation seems to cause very little fading.

The most authoritative research on quantifying fading damage was done in the early 1950's by the United States National Bureau of Standards (NBS). This research was undertaken for the U.S. Library of Congress, in order to design a glass filter to protect the original copies of the Declaration of Independence and the Constitution. The NBS found the relationship between the wavelength of the radiation and the relative damage to be as indicated in Figure 2.

Using this relationship, scientists have calculated that blocking all of the ultraviolet radiation portion of the solar spectrum would not eliminate fading damage for most fabrics, but will slow down the rate of fading by a factor of about three. That is, a fabric that will fade by a certain amount in 3 years under normal solar exposure could take about 10 years to fade to the same point if the ultraviolet radiation is eliminated.

Heat Mirror with XUV-A Breakthrough in Fading Protection

Heat Mirror with XUV is the only glass for residential windows that blocks over 99.5% of damaging ultraviolet radiation below 380 nm, while maintaining a clear, colorless appearance.

The core of Heat Mirror technology is a clear, colorless, transparent film that is permanently sealed between the panes of glass in a window. (See Figure 3 for a cross-section diagram.) This film is coated with a very thin metallic coating, only hundreds of atoms thick, by Southwall Technologies in Palo Alto, California. (The coating is patented by the Massachusetts Institute of Technology and produced by Southwall under exclusive license from MIT.)

The combination of the film material and the coating gives Heat Mirror with XUV its unique performance capabilities. The key to Heat Mirrors performance, both for energy control and now for fading protection, is that it is a "selectively transmitting" material. Heat Mirror with XUV has been engineered to transmit parts of the sun's spectrum, such as light while blocking others, such as UV radiation. Heat Mirror with XUV is the first glass available for the residential window market with this ideal combination of fading protection and superior energy performance.

Return to top

Heat Mirror with XUV
Compared to Other Products

Performance Comparison

Ordinary clear window glass lets in up to 70% of the sun's UV rays. New varieties of high-performance window glass, commonly called "low-e" glazings, provide protection from UV radiation, as shown in Figure 4. Even the best of these, however, still transmits 11% - 26% of the damaging UV radiation. (Results shown are from independent testing done by Schott Glass Technologies in Duryea, Pennsylvania on glass from actual windows purchased in May, 1989. A test report summarizing these results is available from Southwall Technologies on request)

Since UV radiation is invisible to the eye, and is the major cause of fading for most fabrics, it should ideally be eliminated. Heat Mirror with XUV transmits less than one half of 1% of UV radiation -50 times less than competitive "Low-e" window products and more than 100 times less than ordinary double-pane glass -while maintaining a clear, colorless appearance

Why Heat Mirror with XUV Outperforms the Competition

Heat Mirror with XUV achieves superior blockage of UV radiation through a specially formulated polyester film used in the Heat Mirror insulating glass system. A unique additive in the polyester film, called XUV, provides almost total blockage of UV radiation.

Questions and Answers about XUV

Does Heat Mirror with XUV prevent fabric fading? Are my drapes, furniture, and carpets guaranteed not to fade?

Many factors contribute to fading in addition to ultraviolet radiation: portions of the visible light spectrum, humidity, heat and air quality. Although ultraviolet radiation accounts for an estimated 60% of the fading damage, these other factors will fade fabrics even if all of the ultraviolet radiation is blocked. Hence, Heat Mirror with XUV, which blocks over 99.5% of the ultraviolet radiation, will delay, but not eliminate fading. It is important also to remember that not all fabrics fade at the same rate. Fading rates are affected by the type of dye, fiber, finish, and the method in which the dye was applied to the fabric. Although fading will continue to occur with Heat Mirror with XUV, fabric life can be significantly prolonged.

Will Heat Mirror with XUV keep fine woods from bleaching and paintings from fading?

When exposed to the sun's radiation, unfinished wood typically bleaches and wood finishes become yellow or darken. Ultraviolet radiation plays a key role in this, but the importance of its role is different for every wood/ finish combination. Near elimination of UV radiation will delay, but not eliminate wood discoloration. Similarly, paintings are affected by UV to different degrees depending on the media used. Watercolors, for example, are highly susceptible to UV degradation. Eliminating UV will extend the life of paintings, but care should be taken to be sure that owners of artwork realize that fading is caused by many factors besides UV radiation.

Don't plants need UV radiation to grow?

Many studies have shown that red and blue Visible light are the portions of the solar spectrum most effective in increasing photosynthesis, and hence plant growth. Therefore, eliminating ultraviolet radiation should not affect the growth of house plants. Furthermore, since Heat Mirror insulating glass also filters out much of the infrared radiation, which can scorch leaves, and reduces temperature swings, which can shock plants, windows with Heat Mirror with XUV should improve the growth of most plants.

Other window products claim to block UV - what makes Heat Mirror with XUV different?

While other "low-e" window products do block some of the damaging ultraviolet radiation, most of these still transmit 11% - 26% or more of the damaging UV rays. Heat Mirror with XUV transmits less than one half of 1% of the ultraviolet radiation, while still maintaining a colorless appearance.

Isn't UV protection just important in hot, sunny climates?

Even in cold, cloudy climates, UV radiation causes damage to furnishings. Clouds tend to block visible and near-infrared energy more completely than ultraviolet radiation. So, even if you can't feel the warmth of the sun, UV radiation is still getting to your furnishings. Just as one can get a sunburn on a cloudy day, furnishings can suffer damage during cloudy conditions.

Do you need Heat Mirror with XUV for north facing windows?

Yes. The diffused light that comes in windows that face the north contains about 70% of the damaging UV radiation that comes in windows with direct sun exposure.

Will the performance of Heat Mirror with XUV change over time?

Southwall Technologies has developed a high degree of confidence in the Heat Mirror insulating glass system through extensive long-term durability testing. In field testing, accelerated testing, and industry standard testing, there has been no discoloration, no film sagging or wrinkling, no degradation of performance, and no evidence which would suggest that changes in appearance or performance will occur during the estimated life of a sealed insulating glass unit.

Does Heat Mirror with XUV look like ordinary glass?

The Heat Mirror film containing XUV is clear and colorless. When tightly suspended between the panes of glass in an insulating glass unit, the film is basically invisible. Windows with Heat Mirror insulating glass look like ordinary residential windows.

Return to top