Spectrally Selective Windows

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Block Glass Windows
Other Names:
Spectrally Selective Glazing, Spectrally Selective Glass
A window glazing that permits some portions of the solar spectrum to enter a home while blocking others, used to reduce energy costs.

Spectrally selective glazing is a tinted or coated window glazing that has optical or solar properties which vary across the solar spectrum. These glazings typically offer a low transmission of solar gains in conjunction with a high transmission of visible light, blocking unnecessary radiation and reducing energy costs.


[edit] Spectrally Selective Windows

There are a wide range of spectrally selective window options but the newest generation of spectrally selective glazings exaggerate the differences between the visible and infra-red portions of the spectrum, two of the three components of solar radiation (visible radiation -- sunlight --, infrared radiation and ultra-violet radiation). In this way the spectrally selective windows ensure that infra-red radiation is blocked.

[edit] Description

Approximately 50% of the sun's energy transmits heat but does not transmit light. This occurs because the infrared and ultra-violet portions of the sun's light occur outside of the visible portion of the spectrum. Spectrally-selective glazings transfer a high percentage of the sunlight but screen up to 80% of the infra-red radiation. A low percentage of radiant heat transmits from the sun and reduces the need to cool building interiors.

[edit] Tints, Glass Types and Coatings

Spectrally selective glass may include different tints and types of glass.

[edit] Glass Tints

Spectrally selective glazings often feature a slight green or blue appearance. The tint is slight, however, because the visible light is transmitted relatively evenly over the visible spectrum. These tints don't affect the U-Value or thermal insulating value of the window. The most energy efficient spectrally selective replacement windows, are tinted so it is almost unnoticeable.

[edit] Tempered Glass

Large window units which have spectrally selective glass panels are often manufactured using tempered glass to strengthen the glazing unit.

[edit] Low E Coating

Low-emissivity (Low-E) coatings can be added to the spectrally- selective glazing to improve thermal performance.

[edit] Ratings

The effectiveness of spectrally selective glazings are measured by light-to-solar ratio (LSR). This is the ratio of visible light transmission divided by the solar heat gain coefficient for the glazing system. Clear un-spectrally-selective glazing units will have a value of close to 1.0 while a quality spectrally-selective glazing system will have a value of greater than 1.7. The highest possible light-to-solar ratio is approximately 2.0.

[edit] Considerations

Homeowners should review the benefits offered by spectrally selective replacement windows before making a purchase. These include the amount of visible light that the window transmits. The best spectrally selective energy efficient replacement windows transmit at least 70% of visible light, ensuring that the home benefits from proper daylighting while the windows block unwanted infrared radiation.

[edit] Selection

When choosing a spectrally selective replacement window homeowners should review the window's clarity, heat blocking rating, aesthetics and cost savings.

[edit] Clarity

A high quality spectrally selective replacement window should be clear yet able to block significant amounts of unwanted solar heat and reduce glare.

[edit] Heat Blocking Ratings

Spectrally selective windows that block acceptable amounts of heat and sunlight transmissions should display a Solar Heat Gain Coefficient of 0.51 or less and a Visible Light rating of 15% or less.

[edit] Aesthetics

A clear spectrally selective replacement window should provide a clear view of the outdoors.

[edit] Cost Savings

A spectrally selective window should, when used in combination with windows that offer sufficient U-Factor values, allow the homeowner to realize a return on the investment within four to seven years in reduced energy bills.

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