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Why Infrared Rejection isn’t a complete measurement of heat rejection.

Most window films demonstrate an infrared rejection measurement. We explain why you probably shouldn't rely on it as a measurement of total heat rejection.
Thermometer in the sun

Infrared heat only makes up about 53% of the total heat we receive from the sun. So with the other 47% of the sun’s heat left out of the picture, is there any point looking at the level of infrared rejection window films offer?

In theory, if you only wanted to know how much heat rejection was caused by a film’s infrared rejecting technology – and not from other factors such as its colour – then an infrared rejection measurement would be very useful. It would be particularly helpful if you are looking for a film that keeps your windows light, whilst blocking out a significant amount of heat.

However, there’s an important caveat to keep in mind when comparing different infrared rejection measurements: there isn’t really an industry standard to measure infrared rejection.

This means that the infrared rejection measurement you are reading may only demonstrate the infrared rejection achieved within a very small and favourable part of the infrared spectrum – rather than across the whole spectrum. It means that the level of infrared rejection claimed could refer only to the very small section of the solar spectrum that also happens to give the best result. So whilst technically true, such a measurement may have little value in the real world.

Solar Radiation Intensity Graph
The sun’s energy comes to us in a range of different wavelengths, which vary in intensity. As this simplified solar radiation intensity diagram shows, intensity is at its peak within the visible light portion of the spectrum.
Solar Radiation Intensity Selective Test
Testing infrared rejection only at specific wavelengths within the infrared spectrum (take the above example) may show a very high level of rejection. In the real world this doesn’t really mean much in terms of heat rejection, as it doesn’t consider all the other wavelengths of solar energy in the spectrum.

“Rejects up to 91% of infrared heat” might be the case in a very specific lab scenario that only tests against 10% of the infrared spectrum, but how does that translate to the amount of heat you feel on your skin as the sun belts through the window while you’re driving down the highway in the middle of summer?

While infrared rejection can certainly be used as a partial indicator of a film’s heat rejecting technology, a more complete demonstration of heat rejection is Total Solar Energy Rejection (TSER). TSER shows you exactly how much heat is rejected, rather than just the infrared portion (or a very specific part of the infrared portion). 

Because Total Solar Energy Rejection covers the entire solar spectrum, there’s no cherry picking the best segment of the solar spectrum to give the best sounding result. It’s by far the best and most reliable comparison between different films when it comes to heat rejection, and the comparison we recommend looking at first. 

You can definitely consider infrared rejection when comparing films, but unless you’re planning on keeping your car in a lab, make sure to also look at Total Solar Energy Rejection so you can be sure how effective your film will be in the real world.