Window film is a thin polyester laminate applied to the inside of glass. Depending on how it's built, it absorbs or reflects parts of the sunlight hitting the window: visible light (brightness and glare), infrared (heat) and ultraviolet (fading and skin damage).
Three kinds of sunlight
- Visible light is what you see. Blocking some of it reduces glare and adds privacy, but also darkens the view.
- Infrared (IR) is felt as heat. It's roughly half of the sun's energy.
- Ultraviolet (UV) fades furniture and dashboards and damages skin. Nearly all quality films block 99% of it, even clear ones.
The best films are selective: they block a lot of infrared and ultraviolet while letting through plenty of visible light. That's why a light ceramic film can keep a room or car cooler than a dark, cheap one.
How to read a spec sheet
| Measurement | What it means | What you want |
|---|---|---|
| VLT (visible light transmission) | Percentage of visible light that gets through. Lower is darker. | As dark as you like and your state allows |
| VLR (visible light reflectance) | How mirror-like the film looks, inside or outside. | Low for a natural look; higher for daytime privacy |
| TSER (total solar energy rejected) | Share of the sun's total energy kept out. | Higher is cooler |
| IR rejection | Share of infrared heat blocked. | Higher is cooler |
| UV rejection | Share of UV blocked. | 99% or more |
| SHGC (solar heat gain coefficient) | Home films: share of solar heat that gets in, from 0 to 1. | Lower is cooler |
Unfilmed car side glass lets through about 90% of visible light. When your state limits tint to, for example, 35% VLT, that usually means the glass and film together, so the film alone has to be a bit lighter. Check the limits for your state on our tint laws page.
Absorbing vs reflecting
Dyed and carbon films absorb heat into the film, and the glass re-radiates much of it outdoors. Metalized and reflective films bounce heat away before it's absorbed, which is why reflective home films reach the highest heat rejection. Ceramic films use non-metallic particles that absorb infrared selectively without a mirror look. Our guide to types of window film compares them.
Night driving and visibility
Very dark tint does reduce what you see at night, which is one reason states limit front side windows. A lighter ceramic film gives most of the heat benefit without the loss of night visibility.
Home windows: winter as well as summer
Low-emissivity (low-E) films also reflect heat back into the room in winter, improving the window's insulation. Solar Gard Low-E Silver AG 25 is one example.
Questions about a specific window? Contact us and we'll help you choose.
Photo: Wykymania, CC BY-SA 3.0, via Wikimedia Commons, cropped.
