Technical Window Film Terms Defined
There’s a lot of data available when it comes to solar window film. There’s basic data such as VLT, or “Solar Transmittance” but more complicated data including the amount of infrared rejection, solar absorption and more. So what does it all mean? Here are some terms and definitions to help you make an informed solar window film decision.
VLT (Visible Light Transmission): The total visible light that passes through the window and/or window film. This is shown as a percentage, and the lower the percentage the darker the film. For example a 5%VLT window film is very dark, while a 50% window film is relatively light. A 90% VLT window film, such as Clear UV Window Film is virtually clear. Usually the darker the window film, the more heat it rejects. No matter how dark you go, all of our solar window films block 99% of UV rays.
Visible Light Reflectance (VLR): The amount of light that is visibly reflected by the window. The higher the value, the more the window will look reflective and have a mirrored appearance.
Ultraviolet (UV) Radiation Rejection: This is the total amount of ultraviolet radiation that is prevented from passing through your window. These high energy wavelengths are the primary cause of fading, discoloration and skin damage. Our solar window films black 99% of UV rays protecting your family from harmful UV rays and slowing fading of furniture and carpeting.
Infrared Rejection: The amount of solar infrared radiation that is prevented from passing through the window film. Infrared radiation is most commonly known as “heat” or “heat radiation”. IR light from the sun accounts for 49% of the energy that heats the earth.
Solar Energy Transmittance: The amount of solar energy that passes directly through the window, with window film installed. The lower the number, the less amount of solar energy that comes through the glass.
Solar Energy Absorbed: The amount of solar energy that is absorbed into the glass. The lower the number, the less amount of solar energy absorbed into the glass. Higher absorption rates on double paned glass can cause the glass to break. Before purchasing your solar window film, review the “Restrictions Section” to ensure the solar window film is approved for double paned glass.
Solar Energy Reflectance: The amount of solar energy that is directly reflected away from the window, after the window film is installed. This energy does not enter your home and is not absorbed by the glass. The higher the solar energy reflectance, the more heat is simply reflected away from your windows by the window film – saving you money on summer cooling bills!
Total Solar Energy Rejected: The total amount of solar energy (heat) rejected by the window film when installed. The higher the number, the more heat that is simply rejected away. When there’s less heat entering your home, your air-conditioning doesn’t need to struggle as much during the summer. The more heat rejected, the more effect you’ll see on lowering your summer cooling costs.
Solar Heat Reduction: This is a ratio of the difference in total solar energy passing through glass before and after window film was installed. A great indication of what a particular solar window film can do for you.
Emissivity: This is the “e” in Low-E Glass. Emissivity is a measure of the ability of a product to reflect radiant energy in a room. The lower the emissivity (e-rating) the higher the ability of the window film to retain the room’s heat. This is important for keeping heat in during the winter, and helping to lower your heating bills.
U-Factor: Even more important than emissivity for retaining winter heat within your home, is the U-Factor of your unit (the entire window). U-Factor is a measure of the rate of heat conductivity of a glazing system, and is independent of solar radiation. When multiplied by the difference between indoor and outdoor temperature in Fahrenheit, it gives the amount of heat conducted in BTUs/hour/square foot of glazing: The lower the U-Factor, the better the insulating factor of the window. To put this in perspective, when you want to insulate your attic you use R-13 Insulation. Mostly all window film has an R-Rating of “1”. R-Value= 1/(U-Value)
Shading Coefficient: A ratio of solar heat gain through a window with this window film installed compared to the heat gain on a standard glass pane under the same testing conditions, also known as the sun control capability of the film. The lower the shading coefficient the more efficient the window film in rejecting solar energy.
Luminous Efficacy: Percentage is a ratio of the light transmission and shading coefficient.