Tdw-ISO: A Better Way to Measure Fade Resistance
For decades, ultraviolet (UV) light transmittance was considered the primary indicator of how well a glazing product could protect interior furnishings from fading. Today, however, researchers recognize that fading is caused by more than UV radiation alone. Visible light also plays a significant role, accounting for as much as 40% of the damage caused by sunlight exposure. As a result, the industry has adopted a more comprehensive metric known as ISO Damage Weighted Transmittance (Tdw-ISO) to evaluate a glazing system's ability to reduce fading.
The Solar Energy Spectrum
To understand why Tdw-ISO provides a more accurate assessment, it's important to first understand the solar energy spectrum and how different types of light interact with glass and coatings. Solar energy reaches the Earth in the form of short-wave radiation, including:
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Ultraviolet (UV) Light – approximately 3% of the solar energy spectrum and a primary contributor to fading
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Visible Light – approximately 44% of the solar energy spectrum and responsible for daylight, views and a significant portion of fading damage
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Infrared (IR) Light – approximately 53% of the solar energy spectrum and primarily associated with heat gain
Because both UV and visible light contribute to fading, evaluating UV transmittance alone does not provide a complete picture of a glazing product's fade-reduction capabilities. Tdw-ISO addresses this limitation by considering the combined effects of transmitted UV and visible light, making it one of the most comprehensive measures of fade resistance available today.
Understanding the Causes of Fading
Fading occurs when sunlight alters the chemical structure of dyes, pigments and fibers found in materials such as carpet, upholstery, artwork, wood finishes and fabrics. While UV radiation remains a primary contributor, visible light also plays a significant role. Additionally, heat generated by infrared (IR) radiation can accelerate the fading process, even though IR does not directly cause discoloration. Because fading results from a combination of these factors, evaluating UV transmittance alone does not provide a complete picture of a glazing system's protective capabilities.
Why UV Transmittance Isn't Enough
Historically, glazing products were often compared based on their ultraviolet transmittance values. However, experts at Lawrence Berkeley National Laboratory (LBNL) have cautioned against relying solely on UV measurements when assessing fading potential. According to LBNL, UV transmittance does not account for the varying sensitivity of materials to different wavelengths of light and ignores the contributions of visible light to fading damage. As researchers gained a deeper understanding of the science behind fading, the need for a more accurate measurement became apparent.
What Is Tdw-ISO?
ISO Damage Weighted Transmittance (Tdw-ISO) was developed to provide a more realistic assessment of a glazing material's ability to reduce fading. Unlike UV transmittance alone, Tdw-ISO considers both ultraviolet and visible light. Tdw-ISO provides architects, designers and building owners with a more meaningful way to compare glazing products and their fade-reduction capabilities.
Why Tdw-ISO Matters
For projects where preserving interior finishes, furnishings, artwork or merchandise is important, selecting glazing based on Tdw-ISO values can provide greater confidence in long-term performance. By considering the combined effects of UV and visible light, designers can make more informed decisions about glazing systems and their ability to help protect valuable interior assets.
A More Complete Measure of Fade Protection
As understanding of solar radiation has evolved, so too have the tools used to evaluate glazing performance. Today, Tdw-ISO is widely regarded as one of the most accurate indicators of a glazing product's ability to reduce fading. Rather than focusing on UV transmittance alone, it reflects a broader understanding of how sunlight affects materials over time, providing a clearer picture of how glazing systems help protect the spaces and belongings inside a building.
