Window Films are capable of rejecting Infrared (IR) but oftentimes this can be mistaken for Total Solar Energy Rejection. Improperly educated window film dealers, crafty marketing on the part of manufactures, and a lack of standardized testing can propagate misinformation leading a consumer to conclude that a window film will reject up to 98% of the heat! This just isn’t true at all. Let’s take a few paragraphs and learn what infrared is, why it is listed on some specification cards, and how IR rejection should be interpreted.
Infrared –What is it?
Energy created by the sun travels to earth in different wavelengths. These wavelengths make up what is known as the “Electro Magnetic Spectrum” or EM spectrum for our acronym dependent crowd out there. The diagram above illustrates how differing wavelengths affect us in different ways. When it comes to our windows and energy control we are affected primarily by what is known as the “Solar Spectrum” which is a band of frequencies made up of Ultra Violet, Visible, and Infrared wavelengths. These three areas are responsible for what we feel as heat and the total solar energy that results from solar spectrum can be regulated by window film.
The chart above shows that energy is distributed across the solar spectrum in 3 distinct bands. Infrared frequencies extend from 780 to 2500nm and beyond. 53% of the sun’s energy comes to us in this band whereas 44% of the sun’s energy is in the form of visible light and 3% in the form of UV radiation. (Note: See Updated Understanding of Energy Distribution Across the Solar Spectrum.) As you can see infrared radiation is a band of electromagnetic wavelengths that account for 53% of the sun’s energy. Clearly, this band cannot be interpreted as Total Solar Energy (Heat) nor can it be used as a total performance specification to differentiate one window film’s ability to regulate the total amount of energy that enters through glass over another film. So why do some manufacturers publish this specification in their literature?
Why List IR Rejection Percentages?
We mentioned earlier how window film can be designed to regulate these three bands of the Solar Spectrum. Certain materials that have spectrally selective properties can be incorporated into film substrates. There is one family of films that primarily focuses on IR rejection while allowing more of the visible light to transmit through the film. These films are sometimes referred to as Spectrally Select Window Films because they regulate specific wavelengths of the EM spectrum. With the advent of Spectrally Select Films eventually came the justification to list the performance of the primary regulated band in the solar spectrum namely, the infrared rejection. As a result, you will find some manufacturers listing their product’s IR rejection in varying percentages.
How Should IR be Interpreted?
Even if these specifications came about with the most noble of intentions, the reality is that IR rejection specifications can certainly be capitalized on with a little crafty marketing or simply from a salesperson that lacks the proper training and education. If we simply keep in mind that any given IR rejection percentage is simply telling you that the film is rejecting xx% of the IR band which makes up 53% of the total solar energy then it becomes easy to interpret performance in the context it belongs in.
One note of caution must be mentioned. Marketing departments can report IR rejection any way that they want because there has never been an industry standard for reporting IR rejection. One strategy for reporting extraordinarily high IR rejection numbers is to sample specific wavelengths in the IR band and report rejection. For example, a film’s specification card may say that it rejects 97% of the IR. Wow that is amazing! Or so it looks that way, however when you do some further investigation you find that they are only reporting this IR rejection percentage between 900-1000nm. Is that the entire near IR band which extends from at least 780-2500nm? Hardly! So is this film really rejecting 97% of the IR? No, it is actually closer to 86% when you average it out over 780-2500nm. We will go into more detail in future articles but suffice it to say, buyer beware. Standardized performance measurements such as the Solar Heat Gain Coefficient (SHGC) which is recognized by the NFRC the only non-profit independent rating agency for window and applied film performance is a much more accurate total performance specification that can be relied upon to compare one film’s performance to another film. Fortunately, there are many well-trained professionals in the window film industry that can help you to find the right product for your needs but it certainly does not hurt to do a little research and educate yourself along the way.