Volume 9, Issue 1 - January 2008
A Better Barometer
The manufacturers of glass and other glazing materials have considered ultraviolet (UV) light transmittance to be the ultimate measure of glazing material’s ability to protect home furnishings (carpeting, drapes, furniture, etc.) from fading due to normal sunlight exposure for decades.
While it is true that exposure to UV light (280 – 380 nanometers (nm)) is a primary contributor to the fading, it is now known that visible light (380 – 780 nm) also is a significant contributor (by as much as 40 percent) to fading.
The Lawrence Berkeley National Laboratory (LBNL) Windows and Daylighting Group, which provides technical support for more energy-efficient and cost-effective residential windows, warns against using UV transmittance as the sole barometer for measuring potential fading damage. They state: “Because it (Total Ultraviolet Transmittance (TUV) is unweighted by a “detector” function, TUV has no correction for the spectral sensitivity of materials to radiation (fading) damage. It doesn’t even cover the spectral range in which damage occurs, just the UV, meaning below the visible.”
The LBNL Windows and Daylighting Group goes on to say that TUV is “believed to have little meaning” as it relates to fading damage.
As a consequence, new factors that quantify the overall effect of UV and visible light on fading have been developed. We feel that the most comprehensive is the International Standards Organization Damage Weighted Transmittance (Tdw-ISO), which many experts now use to more accurately assess the potential effects of various glazing materials on fading. This factor quantifies the ability of glass to reduce fading by measuring the effects of both transmitted UV and visible light.
The procedure used to calculate this newer, more comprehensive Damage Weighted Trans-mittance was developed by the International Standards Organi-zation (ISO) and is based on a weighting function recommended by the International Commission on Illumination (CIE), a technical organization on lighting and illumination.
The Solar Spectrum
UV light, which represents only about 3 percent of the solar spectrum, includes wavelengths from 280 to 380 nm.
The visible portion of the spectrum includes wavelengths greater than 380 nm to about 780 nm.
The Infrared (IR) portion of the spectrum encompasses wavelengths greater than 780 nm and up to approximately 4000 nm. The IR portion of the spectrum is typically associated with heat. While IR does not contribute directly to fading, the heat caused by the absorption of IR radiation can influence the fading process.
The Tdw-ISO Calculation
By comparing the damage weighted transmittance of various glass types, window manufacturers can compare their ability to protect interior components from fading more effectively.
While damage weighted transmittance is not published on most glass manufacturer data sheets currently—it is available by request only. The DWT can be calculated using the Lawrence Berkeley National Laboratory’s Window 5.2 thermal analysis software.
The chart above shows the damage weighted transmittance for several glass products from one manufacturer in relation to their UV transmittance, as well as the damage weighted transmittance. In this review, it was noted that glass products manufactured with tinted and low-E coatings have damage weighted transmittance comparable to laminated glasses, which typically have zero UV transmittance. That’s because tinted glass and low-E coatings offer reduced UV transmittance with more protection from the potential damage inflicted by visible light.
The Lawrence Berkeley Laboratory’s (LBL) 5.2 Windows software calculates total weighted damage using two methodologies. The first, Tdw-K, created by German researcher Jurgen Krochmann, covers the UV and visible parts of the spectrum up to 500 nm. Tdw-ISO is regarded to provide a more accurate assessment, however, because it includes the visible range up to 700 nm. According to the LBL, “Tdw-ISO (TDW) is weighted using a function recommended in the CIE standard which also derives from the work of Krochmann, but is considered to have more general validity.”
Paul Bush serves as manager, flat glass technical services, for PPG Performance Glazings. © AAMA Glass Material Council Newsletter, October 2007 issue. Reprinted with permission of AAMA. All rights reserved. www.aamanet.org