When you look at a window film sample card there are all these different terms and values describing what the product does. Problem is do you know what these terms mean and how they will affect you? In this article I will define the terms and try to shed some light on what they mean.
- Total Solar Transmittance or sometimes just 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. In plain English this just means that whatever percent is shown on the sample card is the amount of the suns energy/heat that is allowed to actually pass through the window or glass with that particular product.
- Total Solar Reflectance or sometimes just 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. In other words this is the amount of the suns energy that the film reflects off the glass. Typically the shinier the film surface the higher the percentage of energy is reflected off the glass.
- Total Solar Absorption or sometimes just 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). A simpler way of saying this is that it is the amount of heat/suns energy that is absorbed and held in the glass.
- SPECIAL NOTE: The first three items we have looked at, Total Solar Transmittance, Total Solar Reflectance and Total Solar Absorption will always add up to 100. This is because these three items are telling you how the film is handing the suns energy as it hits and passes through the film. If you know any two of these values then you can easily figure out the third by adding the two known values and subtracting from 100 which will give you the missing value.
- Visible Light Transmittance (VLT): 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. More simply stated, this refers to the amount of visible light that is allowed to pass through the film. The lower the number the darker the film would be.
- Visible Light Reflectance (VLR): The percentage of total visible light to be reflected by a glazing system that can be seen visually. Otherwise known as the shiny appearance. If the film has a high Visible Light Reflectance of say 60% then the film will appear to be a highly mirrored type of finish. If the film has a low Visible Light Reflectance of say 10% then it will not appear any more reflective or mirrored then a normal window. Some films have different values for interior VLR and exterior VLR, these are commonly known as dual reflective technology films.
- 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. UV is one portion of the total solar energy spectrum which greatly contributes to fading and deterioration of fabric and furnishings. Simply put, this number refers to the amount of UV that is allowed to penetrate through the window film. UV as a general rule of thumb causes approximately 40% of the fading and sun damage to furnishings, flooring and artwork etc. When looking at films and comparing them, you want to pay attention to this value as it will make a big difference in your fading protection. Even just a 1% difference is this value is significant. Always go with the highest UV protection possible when fading is a major concern.
- Ultraviolet (UV) Rejected: This is the reciprocal of the UV transmitted. If the UV rejected number is higher than you will have better UV protection. For fading protection always go with the best UV blocking available. Many films have 99% UV blocking capabilities. When fading protection is the goal then you will also want to consider visible light and heat since these are large contributors to fading.
- U-Factor or sometimes Winter Median U Value: 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 greater the resistance to heat flow, meaning better insulation.
- R-Value: The measure of the resistance of heat flow through a material. The inverse of the U-Factor. Typically people see or refer to R-Values when talking about things such as wall insulation. Window films rarely use this number, more often than not they use the U-Factor.
- Solar Heat Gain Coefficient (SHGC): A measure of the solar radiation admitted through a glazing system. Measures how much heat from the sun is blocked. SHGC is expressed as a number between 0 and 1. The lower the SHGC, the more a product is blocking solar heat gain. Typically you will want to find a film with the lowest SHGC if you are looking for heat blocking for energy savings.
- 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. The measure of the solar heat gain coming through the glass of a window. A low shading coefficient means lower heat gain through the window.
- Total Solar Energy Rejected (TSER): The percent of incident solar energy rejected by a glazing system equals solar reflectance plus the part of the solar absorption which is re-radiated outward. This is the number that I always refer to for total heat block of a film. You will find that all films are rated for this and it is a great way to compare the total heat blocking qualities of the films.
- Light to Solar Heat Gain (LSG) or Light to Solar Gain (LSG): The ratio of Visible Light Transmission (VLT) to Solar Heat Gain Coefficient (SHGC). Calculated as LSG = VLT / SHGC. When the LSG is greater than 1.0 it means that the glass or film transmits more visible light than heat.
- Emissivity: The measure of a surface’s ability to absorb or reflect far-infrared radiation. The lower the emissivity, the higher the far-infrared reflection. Infrared radiation is what is sensed by the body as heat. The lower the emissivity rating, the better the insulating qualities of the glazing system.
I hope that having a better understanding of the numbers and terms used to evaluate window films helps you make a more informed decision.