Volume 15, Issue 3 - May-June 2011

feature


Back to Basics: A Lesson in Light
Members of the Window Film Industry Share the Science Behind Window Film
by Katie Hodge

This is the first in a series of articles that explains the basics of why window film works the way it does and how it saves energy and protects interiors.

How does window film keep heat out? How does it protect against fading and unwanted glare? How does window film control light? The industry works to sell the product every day, but does everyone really know how and why it works?

Back to Basics
Let’s start with a discussion of light, energy, heat, reflection, transmission and absorption. Within the electromagnetic spectrum is a spectrum of energy in the form of waves. Different wavelengths exist and three of them play a major part in our discussion. Right in the middle of the spectrum is visible light and it sits in the range of 400 to 700 billionths of a meter (nanometers). The energy in each wave is proportional its wavelength (which is the distance between any two corresponding points on successive waves) so if it is a shorter wavelength than it will have a higher amount of energy; a longer wavelength a lower amount of energy. Ultraviolet light (UV) is a part of the electromagnetic spectrum that has a frequency greater than visible light and only accounts for about 3 percent of solar energy. It’s also the most dangerous portion and can damage parts of the eye even though it is not visible to the human eye. Infrared light, which has a frequency less than visible light, makes up about 53 percent of solar energy and is also not visible to the human eye.

These components of solar energy—ultraviolet light, visible light and infrared light—react in three ways when they hit a surface. They are either reflected off of the surface, transmitted through the surface or absorbed into the surface.

“[Light can] be transmitted through like a piece of glass, absorbed into it like a black car on a hot day or it can be reflected like a mirror,” says Ron Jones, architectural programs manager for Sarasota, Fla.-based Madico.

“Light can complete a combination of these actions, but they always have to equal 100 percent,” says Jones. “The mirror will reflect almost 100 percent. The black car will absorb nearly 100 percent and the glass will transmit almost 100 percent.”

“Being able to explain what each performance value is or how it is determined really goes a long way in the consumer’s eyes in terms of professionalism.”
—Jon Mitchell, Solar Gard

Defining Light
• Visible light- The part of the electromagnetic spectrum that humans perceive. The infrared and ultraviolet bands precede and follow visible light.
• Ultraviolet light (UV)- Relating to electromagnetic radiation having frequencies higher than those of visible light but lower than those of x-rays, approximately 1015-1016 hertz. Some animals, such as bees, are capable of seeing ultraviolet radiation invisible to the human eye.
• Infrared light- Relating to the invisible part of the electromagnetic spectrum with wavelengths longer than those of visible red light but shorter than those of microwaves.
• Absorption- The taking up and storing of energy, such as radiation, light, or sound, without it being reflected or transmitted. During absorption, the energy may change from one form into another. When radiation strikes the electrons in an atom, the electrons move to a higher orbit or state of excitement by absorption of the radiation’s energy.
• Transmission- To cause (light, heat, sound, etc.) to pass through air or some other medium.
• Reflection- The change in direction of a wave, such as a light or sound wave, away from a boundary the wave encounters. Reflected waves remain in their original medium rather than entering the medium they encounter.


Enter Window Film
Taking all of that science into consideration window film attempts to regulate the amount of energy that is absorbed or transmitted in order to reduce energy consumption, fading and glare.

“Obviously, the goal is to reduce transmission so that leaves window film the opportunity to either absorb solar energy or reflect it,” says Jon Mitchell, technical services representative for Solar Gard in San Diego.

“The more window film can reflect, the more energy-efficient it is,” adds Jones.
Most films are built with ultraviolet inhibitors in them to protect against UV light.

“[Window films] have to have UV inhibitors built into them because the polyester base that the film is made out of is destroyed by ultraviolet light,” says Jones. “That is why nearly all window films block nearly 100% of the ultraviolet light because they are protecting themselves. The extra benefit is that [film protects against] the ultraviolet light is the most damaging portion of the solar energy spectrum to fade, skin, eyes, etc.” because the polyester base that the film is made out of is destroyed by ultraviolet light,” says Jones. “That is why nearly all window films block nearly 100% of the ultraviolet light because they are protecting themselves. The extra benefit is that [film protects against] the ultraviolet light is the most damaging portion of the solar energy spectrum to fade, skin, eyes, etc.”

To understand film as a product you have to understand its origins. Dyed polyester hasn’t always been the material used to make film. It’s compositions and materials have changed dramatically over the years.

“The original window films were just vapor deposition aluminum or silver films. People think its all dyed polyester, but that actually came after the silver films because the silver was too shiny,” recalls Jones. “The silver was very efficient at blocking a lot of solar energy and they realized the more they put on there, the darker it got, the more efficient it was. It blocked more heat because it was reflecting a lot of the visible light spectrum.”

Understanding the importance of the original silver films and the benefits to that type of a film can help installers and dealers better understand newer products liked dyed films.

“The original silver 20 ends up looking like a mirror, but it’s very effective because it reflects energy away from the window,” says Mitchell. “Any energy that is reflected doesn’t need to be absorbed. It’s important because the energy that is absorbed has to be released since objects want to become equal temperatures with their surroundings. So a portion of that absorbed energy gets released inward and contributes to heat gain.”

“Consumers are looking at specifications more closely and they are more widely available especially with retrofit windows.”
—Jon Mitchell, Solar Gard

Light and Bright
Times have changed since the days of all silver films and now the emphasis seems to be on letting as much natural light into the desired space. The architectural community refers to this as natural daylighting. The window film industry has faced some misconceptions about its ability to allow natural light in (see article on page 34 for more on misconceptions about window film).


“The new direction of things is light and bright. Everybody seems to want lighter and lighter film,” says Jones. “They want you to not affect the visible light as much, but still block as much of the infrared as you can.”

Being able to allow natural light in, but also keeping the heat out becomes vital to creating comfortable and energy-efficient buildings.

“People only see visible light, but the other two portions are also contributing to heat gain,” says Mitchell. “So when an architect or a specifier is looking for a product that doesn’t transmit heat but does transmit visible light they would be looking at a ratio between a solar heat gain coefficient (which encompasses all of the heat coming in—ultraviolet, visible and infrared) compared to just how much visible light people see. In the industry it’s called the light-to-solar-gain-ratio. It’s pretty simple to understand. A ratio of “People only see visible light, but the other two portions are also contributing to heat gain,” says Mitchell. “So when an architect or a specifier is looking for a product that doesn’t transmit heat but does transmit visible light they would be looking at a ratio between a solar heat gain coefficient (which encompasses all of the heat coming in—ultraviolet, visible and infrared) compared to just how much visible light people see. In the industry it’s called the light-to-solar-gain-ratio. It’s pretty simple to understand. A ratio of one means that you are transmitting equal amounts of heat and light. The higher over one you go the more light you are transmitting compared to solar heat. And that is what we call spectrally selective.”

There are misconceptions when it comes to the capabilities of window film and what is noticeable to the human eye though.

“Typically there really is an over-abundance of light. On a shiny bright day you are going to want to knock off about 65 percent of visible light. It’s so much better for the human eyes,” explains Jones. “When your eye gets over-exposed it is relaxed and then outside your eye gets bombarded with light your brain wants to shut it down. It’s too much information.”

“The human eye has a natural ability to be able to open up to let in more light when it gets darker so unless two different films were side by side you probably wouldn’t be able to see a 10 percent difference,” adds Mitchell.

If an architect or a customer wants to allow visible light they are not going to get 100 percent blockage by film. The clearer the film the more visible light is coming into the desired space.

“The best they are going to get in 56 percent solar energy rejection,” says Jones. “That 44 percent of visible light is there. If you have clear film then it means that visible light is coming through.”

That doesn’t mean that film can’t provide a controlled amount of natural daylight. Jones says he always thinks of film as a way to manage light. He says that with the right film you won’t even notice that the film is there.

“With the new spectrally selective films you really can’t even notice it. The samples at my house of the 60 percent film are practically invisible. If you put a 70 percent film on a window it literally disappears,” says Jones.

Understanding Why
Being able to understand how window film works and the science behind it can be valuable in many ways. Consumers want to be able to trust that they are making the right decision when they hire their local window film
company. Continuing self-education can only improve skills and confidence when it comes to the jobsite.

“Consumers are looking at specifications closer and they are more widely available especially with retrofit windows. They usually have questions because it can be complex to understand,” comments Mitchell. “Being able to explain what each performance value is or how it is determined really goes a long way in the
consumer’s eyes in terms of
professionalism.”

As technology changes consumers and those within the industry will need to stay educated on what is new and what gives consumers the benefits they are looking for. Understanding the basics of how a product works can make the difference between success and failure or a signed contract and a lost job.

Katie Hodge is the editor of Window Film magazine.


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