Two of the most marketed films today are Infra-Red absorbing dye films (sometimes referred too as simply ‘IR Films’) and ceramic window films. Most IR absorbing dye films use “organic” dye that absorbs “near-infrared” radiation at specific wavelengths. It is not uncommon to see advertisements that will claim 93, 97, and even 99% IR rejection! It seems like impressive figures that will translate into huge heat rejection performance. Not surprisingly, consumers with the help of an uneducated salesperson can be led to believe that 99% IR rejection is in fact 99% heat rejection.
CAN A WINDOW FILM REJECT 99% OF THE HEAT?
Well as much as we would wish that a window film could reject 99% of the heat, in reality there is no solar control window film that can do that. Therefore the question, what does the 99% IR rejection claim actually mean? To answer this question we first have to look at what contributes to thermal energy.
The pie chart in Fig 1.1 shows the 3 regions that make up heat or the total amount of solar energy. Note that Infrared only contributes about half of the energy and the other half comes from the Visible Light and Ultra Violet regions of the solar spectrum.
Assuming we were to filter out 99% of the infrared radiation between 780-2500nm, we would only be able to to reject about 49% of the total solar energy. That is far from 99% heat rejection! High IR in window film is not high solar or heat reflectance.
To make matters more confusing, not every window film manufacture measures their product’s infra-red rejection the same way. The “near-infrared spectrum” (NIR) stretched from 780-2500nm and accounts for 48.93% of the total solar energy. Very few window film manufactures average out the IR rejection across the entire NIR spectrum.What is most common is for a manufacture to sample specific wavelengths while omitting the majority of the NIR spectrum. This particular manufacture measured their film’s ability to reject infrared in this very narrow region only and then they market that their film rejects 97% of the infra-red. So why don’t they measure the entire range between 780-2500nm? Well, if you were to average this figure out over the entire NIR spectrum then it would be only about 86-87% infrared rejection, far lower than 97% and far less marketable.
This is just one example of the confusion that is created in the marketplace. New automotive IR rejecting films have hit the US market with claims of very IR rejection. When investigated closely you will find that these companies employ the same marketing tactic of implied high heat rejection by using infrared rejection sampled from two or three narrow and low intensity regions in the NIR spectrum. The key to understanding how these films really perform is knowing what IR rejection is, and what it is not.
WHAT IS INFRARED REJECTION
Basically, IR rejection is a measurement that is only used in marketing. It is a regional measurement that is not indicative of a film’s total performance. In fact, IR rejection is never used in determining energy efficiency or lack thereof. It is not used by noteworthy institutions such as the Department of Energy, the National Fenestration Ratings Council (NFRC), ASHRAE, nor is it used in any building modeling software designed to calculate energy usage. Why? Simply, you cannot determine total energy rejection or transmission with just the IR rejection measurement.
There are however some measurements that will tell you a film’s overall energy rejection and transmission. First is a measurement called the “solar heat gain coefficient” or better known as the “SHGC.” The SHGC tells you how much energy is being allowed to transmit through the glazing with film applied. So if your window film has an SHGC of .30 then it means that 30% of the total solar energy is transmitting through the film into the interior. The opposite of this would be the total performance measurement known as the “Total Solar Energy Rejected” or “TSER.” TSER is the opposite of the SHGC, so then your .30 SHGC equates to 70% Total Solar Energy “Rejected” by the window film.
A reputable window film manufacture will report performance for both automotive window films and architectural window films in total performance measurements so that consumers can clearly determine the overall performance of a particular window film. Architectural films have another check and balance to protect consumers called the “NFRC Performance Certification.” This is where manufactures submit their product performance data to be verified as accurate by a neutral 3rd party agency called the “National Fenestration Ratings Council” (NFRC).