Sunscreen Pty Ltd
Window Tinting
Licensed 3M Film Installer
Wholesale 3M Film Distributor
48 HOTHAM PARADE
ARTARMON NSW 2064
AUSTRALIA
Phone: 02 9439 5999
Fax: 02 9777 8790
sunscreen@windowtinting.com.au
Spectral Selective Facts
The Below questions and answers try to help you understand how these 'Special' films stop so much heat with out changing the view.
Q1) How does the sun's energy make it through the window? The same way that it travels those millions of miles to get to the earth. Radiation. Simply put, this is energy movement by electromagnetic waves. Just like the waves from a tossed pebble ripple out in a pond, the sun's energy moves via waves to get from one place to the other. Now this solar energy does not travel in just one big wave, but instead in many waves that differ from each other by wavelength. Here wavelength means the spacing, or length, between the waves. Back to our pond analogy, ripples that are close to each other have a short wavelength while those that are spread further apart have a longer wavelength. Why is this important? Because different solar wavelengths give us different challenges to address with window film. The shortest wavelengths from the sun are called ultraviolet radiation/energy, or UV, the middle wavelengths from the sun are called visible light, and the longest wavelengths are called infrared heat/energy, or IR. Please see the diagram below for a descriptive summary:
Q2) Is the solar energy the same for all of these wavelengths? No, the amount of energy changes across the solar spectrum. Think about it in terms of your radio. When you turn the dial, you are moving from one wavelength to the other in the radio portion of the electromagnetic spectrum. Some stations come in very clearly, i.e. have a lot of energy behind the broadcast, while others are very faint. The same thing applies with the solar spectrum. Some wavelengths are not produced with as much energy from the sun or are absorbed in the atmosphere, like UV, while other wavelengths come through quite strong. Here is a plot that shows the amount of energy versus wavelength for the sun. The wavelength is given in dimensions nanometers (nm), which is one billionth of a meter. Pretty small indeed!
Q3) Okay, now I understand the solar spectrum better, but how does this relate to window film performance? Let's start out with the one of the most important performance numbers, Total Solar Energy Rejected (TSER). This metric describes the total amount of solar energy (UV + visible + IR) that is blocked, or rejected, from passing through the window. Let's look again at the solar spectra chart shown above. Since the solar energy sort of looks like piles of white sand in this plot, a TSER of 50% would mean that you shoveled away, or eliminated, half of the sand. This doesn't tell you which pile of sand you shoveled, but simply the total amount of sand that is now gone. Same with TSER. It doesn't tell you which wavelengths are being rejected, but it does tell you the total solar energy performance. Another way to describe the total solar energy performance is with the Solar Heat Gain Coefficient (SHGC), defined in the NFRC 200 - 2004 standard. This is a term common in the glass business and will be used more and more with window film in the future. The connection between SHGC and TSER is very simple: TSER = (1 - SHGC) * 100%. So if you have a TSER = 40%, the SHGC = 0.6. The TSER can be thought of as the percentage of solar energy rejected (higher the better) while the SHGC is the fractional amount of solar energy that passes through (lower the better). In the next edition of 3M Window Film's technical newsletter, I will address the issue of why 3M started publishing TSER at a 60 degree angle.
Q4) So are you saying that a film with a higher TSER is always the better performing film? No, definitely not, because you can always increase the TSER number by simply making a film darker or more reflective. Remember that TSER looks at UV, visible and IR. Very important point to remember! To make an apples to apples comparison, you need to look at films with the same visible light performance. As a quick example, here are two charts that show glazing systems with TSER = 50%, However one of these systems has a visible light transmittance (VLT) of 80% while the other has a VLT of 20%. These look identical from TSER, but the 80% film is a much higher performing film because it is spectrally selective, i.e. blocks out more IR than visible. If we took the 80% VLT system and tinted it down to the same 20% level, without rejecting any additional UV or IR energy, then the TSER number would increase to approximately 80%! Luminous Efficacy was created to help people make this sort of comparison because it is defined as the VLT divided by the shading coefficient. [More on shading coefficient below]. Higher the Luminous Efficacy, the better you are doing at rejecting the IR heat and letting the visible light through. Important since studies show that 30 - 40% of the total electricity bill for commercial buildings is interior lighting. I am sure we will start to hear more and more about daylighting in the future.
Q5) You mentioned visible light transmission/transmittance -- is that just the level of tint? Yes, you can get a pretty good idea of the VLT simply by looking at how dark the film makes the window. Visible light transmittance is defined within NFRC 200 - 2004. Simply put, VLT is the total amount of the visible energy, i.e sunlight (380 - 780 nm), that makes it through your filmed window.
Q6) Can you make a window film with high VLT and good glare reduction? No, these two performance metrics are exactly opposite each other. Improving one always lowers the other. Glare reduction is defined as the percentage of VLT reduced when film is added to the window, i.e. 100% * [VLT(glass) - VLT(filmed glass)] / VLT(glass). It simply tells you the percentage of tint your film adds to the window.
Q7) Sometimes customers ask me how much of a difference window film makes in improving the solar performance of their window? Is shading coefficient the best way to measure this? It is true that simply looking at TSER or SHGC can be a little bit deceiving because an average performing film on a great window can get a better number than a great performing film on a poor window. They really don't tell you how much of an improvement the window film made. Shading coefficient helps out a little bit because it compares the the SHGC for your filmed window to the case of the SHGC for 1/8" clear glass (= 0.87). But what if your window isn't 1/8" clear glass? This is why I like to use the Heat Gain Reduction to answer the question because, analogous to glare reduction, it tells you the percent improvement of solar energy rejection when window film is added. It is defined as 100%*[SHGC(glass) - SHGC(filmed glass)] / SHGC(glass). A Heat Gain Reduction of 20% means you are reducing 20% more of the solar energy when film is added. That simple.
Q8) I see other companies publish numbers for Total Solar Energy Absorbed, Total Solar Energy Reflected and Total Solar Energy Transmitted. Why don't you do this? The reason I don't care for these numbers is because they only tell a part of the story and can be misleading. Let's take Total Solar Energy Transmitted -- this must mean the total solar energy transmitted through the glass, right? Wrong. It is actually defined as the total solar energy that is directly transmitted through the glass, but energy can also indirectly pass through when it is absorbed within the film and then re-radiated into the room. Total Solar Energy Reflected plus Total Solar Energy Absorbed doesn't equal the TSER, even though you would think so since energy can only be reflected or absorbed, because again some of the energy that is absorbed re-radiates into the room. The only number out of here that tells you something more about the film is Total Solar Energy Absorbed because it helps you understand how much the glass may be heating up from the film absorbing energy. This gives you an indication of potential glass breakage. But we calculate the exact center of glass temperature increase with film added to the glass and use this number as a part of our whole Glass Checklist program when issuing warranties. Therefore we don't publish any of these numbers.
Q9) How can I tell whether adding window film will make the glass look more like a mirror? Glass by itself has a reflection of visible light around 8%. Whether or not this means you can see a reflection off the window depends on if the light is brighter outside or inside. Traditionally, adding a window film increased the visible reflectivity of the glass. The way this is defined is straightforward -- a visible light reflection of 10% from the interior (Rint) means the glass is slightly more reflective inside and therefore you will see a little bit more of a reflection at night. A visible light reflection of 50% from the exterior (Rext) means the glass will really have a commercial mirror-like appearance during the day. Notice at night, however, that this goes away. Again, it all depends on whether it is brighter inside or out.
Q10) Is UV Rejection just the amount of UV energy blocked? Yes, it is that simple. Add up all the energy from 300 - 380 nm, see how much of this gets through the film, and therefore you know how much is blocked.