How it works

 

Our products work by blocking out the InfraRed rays - they are the primary source of heat gain. Below is an explanation about Sunlight, how heat is transfered and what the key terms are when choosing a window film product.

What is sunlight and what are its effects?

The Electromagnetic Radiation Spectrum

The electromagnetic radiation spectrum ranges in wavelength from the shortest, gamma rays (.003 nanometers), to longest, 60-Hertz electric power (5000 kilometers). The spectrum consists of ultra short wavelength gamma rays followed in order of increasing wavelength by x-rays, ultraviolet, visible light, infrared, microwaves, radio waves, and ending with very long wavelength 60-Hertz electric power. Between the extremes lies the solar spectrum (radiation from the sun) in a narrow band of wavelengths from 200 to 2400 nanometers (nm). We are continuously bathed in a sea of particles or waves– most of them from the sun. These tiny particles travel at the speed of light and each carries a different amount of energy determined by its “wavelength”. The particle energy varies inversely with wavelength - that is, the shortest wavelength photons carry the greatest energy.

 

 

Solar Radiation

The sun provides more than 99% of the earth’s energy (starlight contributes but is so miniscule as to be inconsequential; geothermal contributes a tiny fraction of 1%). Solar energy thus powers virtually all of life on the planet. This energy is contained in what is termed the “solar radiation spectrum”, which is a small subsection of the wider electromagnetic radiation spectrum of the universe around us. Radiation from the sun is composed of three wavelength bands: ultraviolet (UV), visible (VIS) and infrared (IR).

 

The UV band (200-400 nm) and (IR) band (780-2400 nm) are not visible to the human eye. The visible band (400- 780 nm) is present between these two invisible bands. We perceive the invisible UV and IR rays differently. We sense IR as heat but cannot see it, while we neither feel nor see UV even though it can burn and damage our skin. Visible light we perceive both by its ability to render objects optically visible, and as heat when it is absorbed by a non-reflective surface.

 

We often refer to all solar radiation as “light”, recognizing that the “light” from the sun consists of both visible light and invisible (UV and IR) rays.

 

The infrared spectrum of our world extends far beyond the 2400 nm upper limit of the solar spectrum, hence solar IR is also known as near infrared (NIR). Non-solar longer wavelength radiation, termed far infrared (FIR), is emitted by all warm objects (stoves, fireplaces, furnaces, incandescent light bulbs, the human body, etc.)

 

The ultraviolet band (100-400 nm) is divided into radiation of moderate energy UV-A (320-400nm), medium energy UV-B (290-320 nm) and high energy UV-C (100-290 nm). UV-B and UV-C carry sufficient energy in each particle of light (photon) to cause damage to exposed skin, causing premature aging and wrinkling, and leading to cancer. Public health data indicate an annual incidence of skin cancer nationwide of over 17,000 new cases of malignant melanoma and over 800,000 cases of basal cell and squamous cancer. There are over 2,000 deaths from melanoma each year. Medical studies show a clear linkage between severity of exposure and development of cancer; even one episode of severe sunburn is associated with a significant increase in the likelihood of cancer.

 

The earth’s atmosphere and its ozone layer filter out the UV-C and some of the UV-B (the main cause of sunburn) but there is evidence that this protective filter is diminishing; continent-sized thin spots and holes are developing in the ozone layer. The diminished ability of the ozone layer to filter out damaging UV rays has already resulted in increased cancer rates in Australia, where a large ozone hole has developed in the atmosphere. Satellite observations show evidence of ozone thinning over the North American continent, causing concern among atmospheric scientists that our natural protection from damaging UV is being reduced.

 

Besides its potential for injuring skin UV also contributes significantly to accelerated aging, deterioration and fading of material objects such as wood, most plastics, paint finishes, fabrics, artwork, tapestries and so forth.

 

Solar control films are designed to mitigate the undesirable effects of the sun’s rays. UV absorbers are incorporated to block entry of all three types of UV rays, and eliminate over 99% of incident UV. Infrared reflecting substances are built-in to retain heat in winter and reject entry of heat-carrying IR in summer.

 

 

Heat Transfer – Heat Always Moves From Hot to Cold

Understanding solar control film requires an understanding of the way heat moves around. Aside from the special cases of air-conditioning and evaporation, heat is transferred in three ways: radiation, conduction and convection.

 

Radiation occurs when a warm object “radiates” IR rays toward a cooler object. Examples are the warming of your body by the heat lamp in your bathroom, or by sitting in the sun’s rays on a summer day.

 

Conduction occurs when two objects having differing temperatures come in direct physical contact – a saucepan sitting on an electric hotplate, sun-warmed window glass transferring heat to its framing system.

 

Convection occurs when a moving fluid, or gas, contacts a solid surface having a different temperature – cold winter wind blowing against your windowpane, an ice cube swirling in a glass of iced tea.

 

In all three transfer mechanisms heat always moves from the warmer object to the cooler.

Air-conditioning differs from natural heat transfer in that it requires expenditure of energy to perform the work necessary to remove heat from a space. Evaporative cooling results from an atomic property of matter known as specific heat of vaporization and is not an important aspect of thermal control film.

 

 

How is heat measured?

The standard unit of heat measurement is the British Thermal Unit (BTU). One BTU is the amount of heat necessary to raise one pound of water one degree Fahrenheit. Air-conditioning systems are rated in “tons of air-conditioning”, one ton being the equivalent of one kilowatt (kw) of electric power. Moving the air requires an additional ¼ kw of power, so that 1 ton of air-conditioning requires a total power of 1.25 kw. These figures can be used to calculate the savings achieved through the use of window film. Savings in air-conditioning costs are achieved in two ways: reduced heat load allows installation of a smaller, cheaper AC system, and reduced heat flow results in shorter AC operating times thereby reducing ongoing electric costs.

 

 

Glossary

 

Insulation Measurement

Two inversely related factors are in use, U-value and R-value. U = 1/R. U value is the number of Btu’s of heat transferred through 1 square foot of window glass (or other construction material) per degree F. temperature difference per hour. R = 1/U. U refers to transmission of heat and R to heat rejection, or insulating ability. Note that both U and R are independent of thickness of the material tested.

 

Total Solar Energy - TSE

TSE is the sum of the percentages of solar energy transmitted, reflected and absorbed and is always equal to 100%. For standard 1/8 inch clear glass without film the figures reported by IWFA are: Transmitted – 89%; Reflected – 6%; Absorbed – 5%.

 

Visible Light Transmission – VLT

VLT is the percentage of incident natural visible daylight passing through a window system. For 1/8 inch clear untreated glass, VLT = 89%. In general, window films will reduce the VLT along with their rejection of IR heat, but spectrally selective films are available which minimize reduction of VLT.

 

Total Solar Energy Rejection - TSER

TSER is the percentage of incident total solar radiation rejected. Typically solar control films will reduce energy transmission from the 89% value of clear glass to 32%, resulting in a rejection (TSER) of 65%. Higher is better.

 

Visible Light Reflectance – VLR

VLR is the percentage of incident visible light that is reflected by the window system. Without film the VLR of window glass is about 8%. With film the VLR is greater but varies, depending on the shade of film installed.

 

Ultraviolet (UV) Absorption

The percentage of UV absorbed by the film layer. Untreated clear glass transmits about 67% of UV. Almost all of this is absorbed by window films, resulting in a UV rejection by absorption of 99% or greater. UV rejection is the more commonly used term in the window film industry.

 

Shading Coefficient - SC

SC is the ratio of solar heat gain through a filmed window to the heat gain through unfilmed double strength glass. It is expressed as a ratio, not a percentage. Lower is better.

 

Luminous Efficacy - LE

LE measures the ability of a window to transmit visible light in preference to heat gain (chiefly by infra-red). It is expressed as the ratio of visible light transmission to solar heat transmission in a window system. It is calculated as VLT/SC. It should be greater than 1 (the LE of untreated window glass is 1.0). A high number indicates a preponderance of visible light compared to heat gain.

 

Emissivity - E

A measure of a material’s ability to reflect far infrared (FIR). FIR is the long wavelength infrared emitted from non-solar local sources. Reflection of FIR prevents heat gain from external sources in hot weather and prevents FIR heat loss from the interior of a structure in cold weather. Lower numbers are better, signifying better insulation against FIR transmission.

 

Confused? Our sales staff will be able to assist you with any further questions on all our products. Contact Us