Selling Window Film 101
Which Film for What?
There are now hundreds of colors and types of window film, and choosing the right window tint shade for your client is the goal of the window film professional. Sun Control window films block parts of the solar energy spectrum. Different window tint shades block different parts of this spectrum and give each one its specific properties. By understanding the parts of the solar spectrum that each sun control window film rejects and in what quantity, we can propose the most effective window tint shade for a client’s individual situation.
VLT (Visible Light Transmitted)
The visible light transmission is how much “visible” light actually passes through the sun control window film and into the structure. In simple terms, it’s how dark the window tint shade looks. Visible light is the light we see. The smaller the number, the less visible light transmitted. Most sun control window films will usually reflect this in their part number or item name. So, a 5% window tint shade doesn’t let in as much light, as say, a 50% film. For example: APG35SV has a VLT of 35%. Often, manufacturers will round the number up or down simply for aesthetics. A sun control window film with a VLT of 9% is usually called a “10%” film, a 21% will be called a 20%, etc. For the most part though, you can assume the value represented in the window tint shade’s part number or name is close to the actual VLT.
Solar Transmittance, Solar Reflectance, and Solar Absorption
I’ve grouped these specifications together because they are intimately linked. These numbers let you know where the solar energy from the sun is going and at what levels. If you add up these numbers from a specific sun control window film, the will always add up to 100.
Solar Transmittance indicates how much solar energy is “transferred” through the glass. The lower the number, the better that film is at keeping out the solar energy. It’s a very good indicator of how a film performs.
Solar Reflectance works just like it sounds: this is the solar energy that is “reflected” off the film and never enters the structure. Most films achieve this by having a thin layer of metal throughout the film. The type and thickness of the metal layer determines how much heat is reflected. These films can reject high levels of heat, but also reduce the amount of visible light that comes through the glass.
The last few years has seen a new class of films developed that use little or no metal at all to reflect heat. These films are called spectrally selective or IR films. They are constructed using either ceramic or specialized polymer coatings that reject the infra-red rays (heat), but allow most of the visible light to come through. Although many of them do an excellent job, the high cost of spectrally selective films keeps them from being used widely.
Solar Absorption is the remainder of the infra-red energy that is neither transmitted through the glass nor reflected. As the solar rays pass through the film and glass, this leftover energy is absorbed into the glass and film causing them to heat up. Once it’s absorbed this energy doesn’t just stay in the glass. It then radiates in the form of heat into and out of the structure. Films that are especially dark will normally absorb more heat than lighter ones, and those that are reflective. Absorption is an important variable to consider when choosing films for glass, as too high of an absorption percentage can cause some glass to break. (See Glass to Film Chart)
Ultra-Violet Light Rejected
This type of light is invisible to the naked eye, but it has an enormous effect on what it touches. In order to protect themselves, most window films block 99% of the ultra-violet rays from the sun. If they didn’t, the window tint shades would fall apart within a few months of being installed. This number is a good selling point for customers with fading issues, or illness that are affected by UV.
Total Solar Energy Rejected
This number takes into account the amount of solar energy transmitted, reflected, absorbed, and it adds in the amount of energy that was absorb into the glass and radiated back into the structure. This number is also a good indicator of the performance of a film.
All the Rest
The other spec numbers listed are used primarily to determine the films insulative factors in specific calculations and are used when computing window film energy audits on large commercial projects. The typical residential client will not be concerned with them.