Window Tint Measurements

Thanks, Mike, for a very thorough investigation! This is really helpful. You measured visible light, it would be interesting if one could measure infrared light. I believe some window films claim to reduce wavelengths other than visible. I have no idea what kind of a meter you'd need to do that.

 

<div class='quotetop'>QUOTE(Bill Merchant @ Aug 28 2006, 02:08 PM) [snapback]310734[/snapback]</div>

Well like, duh, it wouldn't be any cooler inside the car if the tint didn't block infrared. In fact, that's V-Kool's whole claim to fame, some of their films pass only a tiny amount of infrared despite having high VLT.

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I have no idea what kind of a meter you'd need to do that.
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Infrared spectrophotometer.

 

<div class='quotetop'>QUOTE(priusenvy @ Aug 28 2006, 03:50 PM) [snapback]310779[/snapback]</div>

V-Kool is metallic tint compared to some of the others discusses which are either dye or ceramic. Metallic has the highest heat-rejection of any tint material (the astronauts use it), and should be fairly consistent across manufacturers.

I too would be interested in IR rejection numbers because that's where you can really cut heat without cutting visiblity (VLT) because I personally dislike vehicles blocking the view of the driver/road/vechicle past them because of their tint; safety reasons.

Sometimes, you can measure it by using a heat lamp and passing various films in front of it, and using your hand to off-the-cuff compare them.

If there was a way to verify the claims of 99% UV blocking on most tints, that would be cool too.

 

<div class='quotetop'>QUOTE(NuShrike @ Aug 28 2006, 04:37 PM) [snapback]310807[/snapback]</div>

You can use a photochromatic sunglass lens to detect UV, since the color change is triggered by UV. That only tells you about the wavelengths that trigger the color change, but it's still a useful and easily found tool.

The glass in car windows already blocks out most UV. If it didn't, interior fabrics would fade quickly. Think about how quickly newsprint degrades when left exposed to the sun, but doesn't when it's sitting on your car seat or dash.

 

Duh, higher wavelengths go to lower wavelengths go to heat. Isn't the high iron content of the Prius glass supposed to cut IR coming in?

 

Eh, i've V-Kool. It's day and night in terms of heat reduction. Even compared to the Limo Tint (5%) I have on the back. All i can say is 'WoW'

 

<div class='quotetop'>QUOTE(Bill Merchant @ Aug 29 2006, 12:46 AM) [snapback]311016[/snapback]</div>

Sir, you assumed ignorance about ...

 

<div class='quotetop'>QUOTE(Bill Merchant @ Aug 28 2006, 05:08 PM) [snapback]310734[/snapback]</div>

<div class='quotetop'>QUOTE(priusenvy @ Aug 28 2006, 06:50 PM) [snapback]310779[/snapback]</div>

Actually, something that totally blocks infrared will make the car hotter.

Confused you yet?

Earth's atmosphere is opaque to almost all infrared radiation (see here), so there's very little direct infrared radiation from the Sun to hit the Earth. HOWEVER, visibile light (and probably UV) is absorpted by materials on Earth and re-radiates in the infrared. Since the atmosphere doesn't allow a lot of infrared radiation (i.e. "heat") to pass through it, the Earth heats up (i.e. the "greenhouse effect"). The exact same effect is what causes the car to heat up. VISIBLE light passes through the glass, is absorbed by your seats (or your dashboard, or whatever), which then re-radiate the energy as infrared radiation, which then CAN'T pass back through the glass. Therefore, your car heats up.

What all of this means is that you're MOST interested in blocking optical light, so measuring how much the tint reduces optical transmission is the right thing to do.

 

<div class='quotetop'>QUOTE(Betelgeuse @ Aug 29 2006, 02:40 PM) [snapback]311305[/snapback]</div>

Read the graph a little closer there buddy.

The part of the IR spectrum that is meaningful for heating is from 1000 to 6000nm, which is that area from 10^3 to 10^4 on that graph, NOT the area under the word "Infrared". You can see that there are many regions of high transmissivity - all those "fingers" next to the continuous block that represents the visible spectrum.

My understanding is that majority of solar heat gain through a window is from IR (about 55%) and a smaller portion is from visible light being absorbed by objects inside and then re-emitted in the IR spectrum (something in the low 40% range). This is based on research I've done in purchasing energy efficient windows for my primary and secondary residences and vacation homes, which are all located in very different climates.

For example, in a heating-dominant climate, you want a window that passes IR from the outside to maximize solar heat gain, but you want it to reflect IR from the inside to keep the heat trapped inside the house. In a cooling-dominant area, you want to block as much IR as possible to reduce solar heat gain. The whole energy-efficient window industry is based on these premises, so I'd be very surprised to hear that this is completely wrong.

 

<div class='quotetop'>QUOTE(Bill Merchant @ Aug 29 2006, 12:46 AM) [snapback]311016[/snapback]</div>

 

Well, if it looks good, dose it really matter. If you're happy, I'm happy. Thanks for the information.

Happy Motoring !