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Terminology

Light Transmission

The percentage of external light that passes through the particular glass type, measured against 100% external light source. While all glass types including clear will reduce some light transmission, greater care should be taken when using high performance solar control glass in domestic applications as some have a major effect on light transmission into the building. Light Reflection - The percentage of light reflected by the exterior surface of a particular glass type. The degree of reflection does not differ greatly between most glass types, with the obvious exception of high performance solar control glass.

Total Solar Transmission

The percentage of solar heat - that is, the total of directly transmitted heat plus the absorbed and inwardly re-radiated heat through the glass. If cooling is paramount, the lower the total heat transmission percentage the better.

Solar Reflection

The percentage of heat reflected by the exterior surface of a particular glass type. The degree of reflection does not differ greatly between most glass types, with the obvious exception of specifically designed glass for commercial applications.

UV Transmission

The percentage of ultraviolet light that passes through a particular glass type, as measured over the 300-380 nanometre wavelength range. The lower the percentage, the better protection for furnishings and carpets.

U Value

A measurement of the rate of heat flow through a window. The lower the U Value the better its insulating performance. Glass with a low U Value is generally used to keep the warmth in a room in cold climates or at night.

Solar Heatgain Coefficient (SHGC)

The fraction of solar radiation admitted through glass or a window, both directly transmitted, absorbed and subsequently released inwards, expressed as a number between 0 and 1. If cooling is paramount, choose a glass that has a low solar heatgain rating. This glass should only be used in areas of direct sunlight.

Sound Transmission Class (STC)

A single number rating the efficiency of building elements in reducing transmission of noise. It is expressed in dB (decibels) and the larger the number the better the attenuation.