WINDOW TECHNOLOGIES: Properties Primer

Solar Radiation The basic properties of glazing that affect radiant energy transfer.

Emittance

When solar energy is absorbed by glass, it is either convected away by moving air or reradiated by the glass surface. This ability of a material to radiate energy is called its emissivity. Window glass, along with all other objects, typically emit, or radiate, heat in the form of long-wave far-infrared energy. The wavelength of the long-wave far-infrared energy varies with the temperature of the surface. This emission of radiant heat is one of the important heat transfer pathways for a window. Thus, reducing the window's emission of heat can greatly improve its insulating properties.

Standard clear glass has an emittance of 0.84 over the long-wave infrared portion of the spectrum, meaning that it emits 84% of the energy possible for an object at room temperature. It also means that for long-wave radiation striking the surface of the glass, 84% is absorbed and only 16% is reflected. By comparison, low-E glass coatings have an emittance as low as 0.04. This glazing would emit only 4% of the energy possible at its temperature, and thus reflect 96% of the incident long-wave infrared radiation.

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