Window units are commonly fabricated with clear, tinted, or coated glass. Variations in the tinting and coating processes influence the solar, optical, and heat absorption properties of the glass. Of most importance in this discussion is the influence of the total solar absorbance properties on the development of high thermal stresses. If the total solar absorbance of filmed annealed glass exceeds 50%-60%, thermal stresses may become critical. The only reliable method to estimate the total solar absorbance of filmed glass is through laboratory testing or by computer modeling in conjunction with lab test data.
Clear safety film can be applied to coated or tinted glass without substantially changing the risk of thermal fracture. However, as with insulating glass, the glass may be in a state which is near failure prior to the application of the film. In such a case, failures that occur after the application of the film may wrongly be associated with the film application. Therefore, if the solar absorbance of the glass without the film is near the critical point, it may be best to not apply window film.
Clear glass is manufactured with a minimum amount of non-glass constituents so that it is colorless with a high light transmission which ranges from 75% to 92%. The total solar absorbance of clear glass is dependent on its thickness and generally varies from 3% to 41%.
Tinted glass is manufactured by adding colorants to a clear glass batch to create the desired color so that the colorants become an integral part of the glass matrix. The visible light transmission of tinted glass ranges from 14% to 83%. The total solar absorbance of tinted glass depends upon the thickness of the glass, and the amount and type of colorants that are added to the glass. The total solar absorbance of tinted glass generally varies from 30% to 74 %.
Coated glass is manufactured by chemical vapor deposition during the on-line float glass process or through various vacuum deposition technologies in an off-line process. Coated glass is often referred to as reflective glass. This type of glass would include low-e coatings which reflect primarily long-wave infrared radiation. The coatings affect both the solar/optical properties of the glass as well as the aesthetics of the installation.
The value of the total solar absorbance of a particular piece of tinted or coated glass should be determined from manufacturer's literature. If the total absorbance is unknown it is not recommended that window film be applied to annealed glass.
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