Bottom Line September/October 2021

October 15th, 2021 by Nathan Hobbs

Explaining Heat Rejection

By Steve DeBusk

Our industry relies on science to explain and compare window film performance, though customers rarely understand it. Still, sometimes you need to go there, especially when you’re talking about heat rejection in auto tints—but the conversation applies to architectural tints too.

To talk heat rejection without losing your customers, keep it short and to the point. Briefly explain a few basics of solar science and how they relate to commonly-used heat rejection performance measures. If you think you’re losing them, head straight for the bottom line.

The Science

Sunlight is composed of three different types of rays. Each produces wavelengths in a diverse range of nanometers (nm), and they all cause heat.

Ultraviolet (UV) rays are 300- 380 nm and 3% of sunlight. They harm the skin and contribute to the cracking or fading of a car’s interior.

Visible light rays are 380-780 nm and 44% of sunlight. They display the rainbow of colors we see in the world every day.

Infrared (IR) rays are 780-2500 nm and 53% of sunlight. We can feel them with our bare skin, and at 53%, they’re responsible for about half the sun’s heat.

The Measures

Because all the rays in sunlight cause heat, you must measure them all for an accurate evaluation of a tint’s heat-rejection performance.

• Total Solar Energy Rejected (TSER) is the International Window Film Association’s (IWFA) recommended measurement for heat rejection. It takes all the sun’s heat-causing rays into account: UV, visible light and IR.

• Infrared rejection (IRR) is a partial heat rejection measure, only for IR rays. It can be misleading. Blocking 99% of IR rays means you’re only blocking 99% of the 53% of sunlight that is IR heat.

• Even more misleading is measuring a specific range within IR rays. If an IRR measure is displayed for a range less than the entire range of IR, 780-2500 nm, it’s measuring even less than 53% of the sun’s heat—or an even smaller fraction.

The Bottom Line

Sunlight is comprised of three different types of rays. They all penetrate windows and cause the inside of a home or car to heat up. Tint helps, but not all perform equally. The number to look for is TSER. It stands for Total Solar Energy Rejected (TSER) and measures heat rejection for all the sun’s rays. The higher the TSER, the more heat a tint blocks.

IRR measures IR ray rejection. IRR doesn’t give you a complete picture of tint performance because IR rays are responsible for only about half of the sun’s heat. The number to look for and use for comparison is TSER. And if you see only IRR, or worse yet, IRR for less than the full 780-2500 nm range, then beware. The numbers may have been configured to make the performance seem more impressive.

One last tip: explaining wavelengths in nanometers (nm) for different types of rays might be more detailed than you, or anyone, wants. With heat rejection, the goal is to define just enough to clarify what’s key while ensuring your customer’s eyes don’t glaze over before you finish.

Steve DeBusk is the technical enablement manager for Eastman Performance Films, LLC.

To view the laid-in version of this article in our digital edition, CLICK HERE.

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