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Is infrared rejection the same as heat rejection?

Find out the difference between infrared heat rejection and total heat rejection.

The sun’s infrared radiation is invisible to the human eye, but we can still feel it as heat. The good news is, several window films on the market claim an infrared rejection of well over 90%. So if infrared radiation is solar energy we feel as heat, does that mean that these films block out over 90% of the sun’s heat?

Although we humans experience infrared radiation as heat, it only accounts for about 53% of the total heat we receive from the sun. 

Both visible light (44%) and ultraviolet light (3%) make up the remaining 47% of the sun’s heat that reaches us here on Earth. So while it’s all good and well to say a film rejects 90% of infrared, it still leaves us in the dark about the how the film deals with the remaining 47% of the sun’s heat.

Diagram showing sources of solar heat
The source of the sun’s heat broken down into Ultraviolet Light, Visible Light and Infrared Light.

Let’s have a look at both ultraviolet (UV) and visible light and fill in the missing pieces of the heat rejection pie.

UV radiation is invisible, destructive radiation that causes our car and home interiors to fade, our skin to burn, and in cases of extended exposure, skin cancer. Along with these damaging properties, UV radiation accounts for around 3% of the heat we receive from the sun. 

Fortunately, any quality window film will reject a minimum 99% of UV radiation. This means that as well as blocking out all of that harmful radiation, pretty much all of the heat caused by UV is blocked out too. 

That leaves the remaining 44% of heat that comes with visible light. A simplified way to understand heat from visible light is: more light = more heat. While there are some other factors at play, it makes sense that a very dark window film will reject more heat than a very light window film – as long as the two film technologies are exactly the same in every way other than colour.

Crystalline 70% and 40%
Here is a dark and a light shade of the same 3M™ Window Film. Their infrared and UV rejecting technology is exactly the same, the only difference is their colour. In this case the darker film will have a higher level of heat rejection because it blocks out more visible light.

Just as there’s more to heat rejection than infrared rejection alone, there’s more to heat rejection than the darkness of the film alone. Just because a film is darker doesn’t necessarily mean it will block out more heat – visible light is only one piece of the pie, remember? 

Ultimately, a combination of factors including a film’s technology will determine the amount of heat that it rejects. For instance, a very light multi-layer optical film can reject more heat than carbon film that’s twice as dark

So with all of these different factors to consider, wouldn’t we be better off comparing films with a heat rejection measurement that takes into account the heat from UV, visible light and infrared? 

That’s why the best way to understand a film’s level of heat rejection is to look at its Total Solar Energy Rejection (TSER). This measurement takes into consideration heat from the whole solar spectrum, covering infrared, visible light and UV. Put simply, if a film has 50% TSER, it rejects 50% of the sun’s heat. Clear, simple, useful.

While technically true statements like “rejects up to 90% of infrared heat” might be great marketing, they’re not exactly telling the full story when it comes to heat rejection. If you do your research and compare films with a reliable measurement like TSER, you can be confident of the amount of heat rejection you will achieve and avoid disappointment.