Guide to museum glazing: Shedding light on transmittance and fading

In choosing the most appropriate glass for commercial and residential projects, more architects look at the issue of fading, specifically with regard to fabrics, finishes, carpeting and artwork that will occupy the interior of their finished buildings.

In assessing the potential fading risks associated with the glass they specify, most architects look at a single measure on the performance data sheet: ultraviolet light transmittance. While useful, this measure fails to present a comprehensive view of solar-radiation risks, because light outside the ultraviolet range also causes significant fading.

Members of the construction industry slowly discover that a lesser-known measure—damage-weighted transmittance—may be a far more reliable indicator of potential fading because it considers UV light and visible light.

Fading: a growing concern
Several factors explain why people voice more concern about fading of interior components.

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, 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 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, which is a primary drawback.

In addition, because of ozone depletion, higher levels of UV light now reach the surface of the earth. This has the effect of increasing the rate of fading.

These three trends—more natural light transmittance, more fragile interior components and a higher concentration of UV light—have resulted in a greater awareness of fading issues among architects and those who ultimately occupy buildings.

A limited measure of potential fading
Since UV light represents only a small fraction of the total radiation from the sun—3 percent—this measure is extremely limited. In fact, fading of interior components can also be caused by light in the visible spectrum, accounting for a much greater portion—47 percent—of total solar radiation. The remaining 50 percent of solar radiation comes from the infrared spectrum and is associated with heat gain, not fading.

Since UV light constitutes only one of several components that cause fading and is not weighted,  assessing a glass based only on its UV transmittance level does not provide a true indication of its ability to protect against fading.

The 2003 edition of the Glass Association of North America’s Laminated Glazing Reference Manual recognizes that UV light transmittance is an insufficient measure, and advises architects to look beyond the UV spectrum:

Because of its high energy level, ultraviolet radiation—radiation below 380 nanometers wave length—is a very significant contributor to material deterioration and color fading. However, damage can also be caused by visible light … [and those specifying glass should] account for damage in the visible spectrum, as well as that caused by UV.  —The manual is available at www.glasswebsite.com.  

Despite its limitations, UV light transmittance has been the traditional way of assessing the risk of damage to interior components. But that slowly changes, as members of the architectural industry discover a readily available and more realistic assessment tool.
Damage-weighted transmittance

To account for the fading damage that can result from radiation in both the UV range and the much larger visible spectrum, German researcher Jurgen Krochmann has created a measure called damage-weighted transmittance.

Krochmann’s original measure, Tdw-K, covers the UV and visible parts of the spectrum from 300 nanometers to 500 nanometers. However, a more accurate assessment of damage-weighted transmittance can be calculated using Tdw-ISO, a function recommended by the Commission Internationale de L’Eclairage in Austria. Tdw-ISO covers the solar spectrum from 300 nanometers to 700 nanometers.

While not yet standard information on glass-performance data sheets, the damage-weighted transmittance rating for a given glass product can be requested from the manufacturer or easily calculated using Window 5.2 thermal-analysis software, provided free of charge by Lawrence Berkeley National Laboratory, www.lbl.gov. Window 5.2 allows users to calculate damage-weighted transmittance using both Tdw-K and Tdw-ISO functions. Tdw-ISO is generally considered to have greater validity, because it covers the visible range all the way to 700 nanometers.

The difference in fading potential can be dramatic when one compares the traditional UV transmittance measure to the more comprehensive damage-weighted transmittance measure.

Based on UV light transmittance alone, a laminated glass may seem the best choice to minimize interior fading. However, when damage-weighted transmittance is used to compare glass choices, it becomes evident that low-e and tinted glasses can be just as effective in preventing fading as laminated products when considering the full 300-to-700 nanometer light range.

In fact, the combination of a high-performance low-e coating with laminated interior lites results in an excellent damage-weighted transmittance of 0.44, and is an attractive glass type for many applications. This glass choice will protect against fading and reduce infrared energy to a low level to minimize solar heat gain, while still allowing high levels of natural light into a building’s interior.

Best measures lead to best glass choices
Whatever the specific concerns associated with an architectural project—and whether they center on energy efficiency, appearance, fading or all of these issues—architects must work with the best and most comprehensive performance measures available.
While several trends—an increasing number of windows, less stable fabric dyes and interior finishes, and higher levels of UV light—have made fading a growing concern, the glass industry has responded with technologies that protect against fading while maximizing other performance elements.

Low-e and tinted glasses often make outstanding choices for protecting against fading, while also offering excellent year-round energy efficiency, yet these options may be overlooked if only the UV transmittance level is considered.

Working with damage-weighted transmittance, in order to provide a look at the overall solar-radiation protection provided by a given glass configuration, architects can make informed choices that will protect fabrics, furniture and other interior design elements while meeting the spectrum of their clients’performance needs.