# Terms

 Total Solar Transmittance The ratio of the amount of total solar energy in the full solar wavelength range (300-2,100 nanometers) that is allowed to pass through a glazing system to the amount of total solar energy falling on that glazing system. Value is usually expressed as a percent. Total Solar Reflectance The ratio of total solar energy which is reflected outward by the glazing system to the amount of total solar energy falling on the glazing system. On filmed windows this reflectance is a function of the side of the film facing the window surface. Value is usually expressed as a percent. Total Solar Absorption The ratio of the amount of total solar energy absorbed by a glazing system to the amount of total solar energy falling on the glazing system. Solar absorption is that portion of total solar energy neither transmitted nor reflected. Since solar transmittance and solar reflectance are measured directly, the following equation should be used in calculating solar absorption. Solar absorption =1.00 – (solar transmittance) – (solar reflectance) Visible Light Transmittance The ratio of the amount of total visible solar energy (380-780 nanometers) that is allowed to pass through a glazing system to the amount of total visible solar energy falling on the glazing system. Value is usually expressed as a percent. Glare is influenced by visible light transmittance through a glazing system. Visible Light Reflectance The percent of total visible light to be reflected by a glazing system that can be seen visually. Ultraviolet (UV) Transmittance The ratio of the amount of total UV solar energy (300-380 nanometers) that is allowed to pass through a glazing system to the amount of total UV solar energy falling on the glazing system. Ultra-violet is one portion of the total solar energy spectrum which greatly contributes to fading and deterioration of fabric and furnishings. U-Factor The overall heat transfer coefficient of the glazing system, U -Factor is a measure of the heat transfer that occurs through the glazing system, and its outer and inner surfaces. This value is a function of temperature, and is expressed in BTU per square foot per hour per degree Fahrenheit (BTU/ sq. ft./hr. degrees F). The lower the U- Factor, the better the insulation qualities of the glazing system. Shading Coefficient (SC) The ratio of the solar heat gain through a given glazing system to the solar heat gain under the same conditions for clear, unshaded double strength window glass (DSA). Shading coefficient defines the sun control capability or efficiency of the glazing system. Total Solar Energy Rejected The percent of incident solar energy rejected by a glazing system equals solar reflectance plus the part of solar absorption which is reradiated outward. BTU-British Thermal Unit The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. 1 BTU = 252 Calories (CAL) TON-HR One ton of air conditioning = 12,000 BTU/Hour (3.52kW)
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