IR Rejection Part2 –The Slight of Bandwidth

I might catch a bunch of grief over this blog but I feel it is something that needs to be addressed. We have long used BTU Meters in our industry as an easy way to illustrate window film’s performance to a consumer. An Infrared heat lamp is placed in front of a BTU meter and a measurement is then noted. Next a piece of window film is placed between the meter and the lamp and WOW! The meter almost bottoms out to zero. What just happened?

Before I address that question let me just say that this type of demonstration does have its place in a sales presentation. The heat lamp demonstration is a way to allow a consumer to “feel” the difference between a filmed and unfilmed piece of glass. I agree that this is a good way to demonstrate that window film works and that it is a viable way to increase the performance of glass. Here is the part that I take issue with; Improperly trained sales persons will use this IR lamp demonstration and compare the BTU measurement between different window films. The way that they present the results leads a consumer to conclude that one film’s superior BTU reading means that it overall outperforms the other films that it is being compared too. Note the following demonstration found on You Tube: [Update See Why This Video Is Unavailable] [Another Example Can Be Found Here]

In my opinion this is a deceptive practice. What you are seeing is a lamp that throws out over 90% of it’s energy as infra red radiation and a meter that is designed to measure the amount of infrared that it is being bombarded with. There is no way to account for the energy in the Visible Light Spectrum which makes up a sizable portion of the total solar energy because Infrared Light Bulbs are not designed to produce visible light. Remember too, in a previous blog we established that IR rejection does not translate to overall film performance. Nonetheless, let’s break this demonstration down by asking ourselves some questions.

What wavelength does this meter sample at? Well a little research shows that it is between 900-1050nm. Is this the entire Solar Spectrum that is being sampled? No, the solar spectrum starts at around 300nm and extends past 2500nm.* It becomes clear that BTU meters like the one in the demonstration are not capable of measuring total solar performance past 1050nm.

What wavelengths of the IR spectrum is the film rejecting energy? Some films are designed to reject IR at 900-1000nm. You can find these references as footnotes added to IR rejection specifications. If you refer back to the solar spectrum chart in our previous blog you will see that this particular wavelength range is one of the lowest intensity areas in the IR spectrum. That just so happens to be where beam splitters sample IR rejection (950nm) and it is also part of the “sweet spot” that some BTU meters sample at. So if a particular film performs best at that specific wavelength then it is naturally going to produce a better reading on a BTU meter than other films, but this does not necessarily mean that its overall energy rejection will be better. Why? As we have established, Total Solar Energy Rejection is an entirely different, and more accurate, measurement that IR rejection. Total Solar Energy Rejection (TSER) and the Solar Heat Gain Coefficient (SHGC) are total performance measurements that take in the entire solar spectrum rather than select wavelengths. Due to the publishing of single region measurements that can be marketed to resemble total performance, such as IR rejection, the International Window Film Association went so far as to publish a clarification on the difference between IR rejection and Solar Heat Gain Coefficient measurements. SHGC measurements can be used to accurately compare the total performance of one window film to another whereas single region measurements cannot.

Therefore to imply that BTU readings are an accurate test to determine overall performance from one film to the next has no basis according to the facts. It is a deceptive sales presentation to say the least and in my opinion it should cease to be used in this manner. Third party performance testing by a reputable organization such as the NFRC is a much more accurate and reliable way to gather and publish data so that you can compare the performance of one window film to the next. Ask your sales professional to provide you with NFRC test results rather than hyped up marketing demonstrations designed to capitalize on a consumers lack of understanding.


*ASHRAE 2005 Handbook Fundamentals– 31.14


4 Responses to “IR Rejection Part2 –The Slight of Bandwidth”

  1. 1 Dexter Kinsley July 5, 2008 at 2:21 pm

    Not to mention, heat lamps emit far more infrared than visible light when compared to the sun, so if a film selectively absorbs or blocks more infrared between 780 & 1050nm, the BTU meter’s reading will certainly be overstated and not representative of total solar energy rejected.

    BTU meters measure all UV (granted UVA), visible light range and as mentioned only a small portion of the infrared band. It in no way compares to the accuracy found in lab photo spectrometers.

    Quote “No, the solar spectrum starts at around 300nm and extends past 2500nm.” Unquote

    The solar spectrum begins at ‘0’. 300nm is in the UVA band leading downward through UVB and UVC until arriving at 0.

  2. 2 vclimber July 5, 2008 at 2:57 pm

    Thanks, another good comment!

    Let me just address this part:

    The solar spectrum begins at ‘0′. 300nm is in the UVA band leading downward through UVB and UVC until arriving at 0.”

    Yes it does Dexter, but when we are dealing with film we only deal with the wavelengths that reach the earth’s surface. So when we factor in the atmosphere that blocks the smaller wavelengths, the UVC band (100-280nm) and glass that blocks almost all the UVB (280-320nm)that leaves us at about 300nm plus that actually transmits through glass. Thus when we test film we do not necessarily have to account for what is already filtered out by atmosphere and glass so therefore I did not account for it in this article. If I did though, someone would come along and probably tell me “why account for what is already filter out?” -lol


  3. 3 Dexter Kinsley July 6, 2008 at 4:50 am

    No, Thank you for clearing this up! For the life of me I could not wrap my head around the precise points where UVC & UVB are found in respect to the 300nm start point you had given.

    I just believed it was important to address the entire UV portion so those that have knowledge of the EMS, but not solar films, would come away from your reading a bit less perplexed.


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