12.1 Terminology
Visible
Light Transmission
The percentage of visible light that is transmitted through the glass. The VLT is measured in the 380-780nm wavelength range perpendicular to the surface. The higher the percentage the more daylight. Also know as Tv, Tvis, LT and VT.
Visible Light Reflection
The percentage of visible light that is reflected by the glass surface, measured in the 380-780nm wavelength range perpendicular to the surface. The VLR can be given as the reflection from the external surface 1 or internal surface 2. The higher the percentage the more reflection. Also known as LR, VR and Rvis.
Ultraviolet Elimination
The percentage of ultraviolet radiation eliminated by the glass, measured over the 290-380 nm wavelength range. The higher the percentage the less UV is transmitted. This value is calculated from the percentage transmission of ultraviolet (Tuv). Therefore UV Elimination = 100 - Tuv.
Fading Reduction Coefficient
The ratio of fading reduction of a glass type when compared to the fading
protection of 3mm clear float. The FRC of 3mm clear float is by definition 1.0 and represents
the minimum fading protection offered by standard glazing. The lower the fading reduction coefficient,
the better the fading protection offered.
The FRC is derived from Damage Weighted Transmission (Tdw-K) data which is a measurement of
the fading reduction over the whole solar spectrum, not just ultraviolet. It is ‘weighted’ to
include the fact that fading damage occurs from a broad band of solar energy such as infra-red,
visible light and ultra-violet.
Solar Heat Gain Coefficient
The measure of the total solar energy transmittance entering a building through the glazing as heat gain. It is the total heat transmission of direct solar transmission and that proportion of absorbed radiation that is re-radiated into the building from the action of heat absorbing glass. The lower the SHGC the better the glass restricts heat energy transmission. The SHGC is also known as the Solar Factor (SF) or g.
Shading Coefficient
The ratio of the total solar heat gain through a particular glass compared to the total solar heat gain through 3mm clear float glass. (86%) The shading coefficient of 3mm clear float is by definition 1.0 and represents a base glass performance. The lower the shading coefficient the less heat gain and thus more shading is provided by the glass. The shading coefficient is calculated as SC = SHGC / 0.86.
Window Efficiency Rating System
The complete WERS rates the performance of windows including frames in various defined New Zealand climate zones. For this catalogue star ratings apply to centre of glass only (WERS cog) to compare glass types and a maximum rating of five stars indicates the best performance possible.
The stars have been calculated on the WERS Star Rating Approximator for Glass - Version 2. Ratings in stars and half star values are given for as follows with five stars indicating the premium performance:
Winter Heating Star rating is a measure of how easy it is to maintain an average New Zealand home at a comfortably warm living temperature through an average climate-year (although principally in winter).
Summer Cooling Star rating is a measure of how easy it is to maintain an average New Zealand home at a comfortably cool living temperature through an average climate-year (although principally in summer).
Fading Star rating is a measure of the amount of damaging solar radiation that is able to enter through the glazing, such that a choice can be made about the degree of protection that you wish to give you furnishings and fabrics. A glazing with five stars will not completely eliminate the fading of all furnishings, but significantly reduces the magnitude of the penetrating solar fading energy.
Condensation Star rating is a measure of the amount of unwanted condensation that will occur on the room side of the glazing of windows in an average New Zealand home, with 5 stars indicating premium performance with a minimum of winter condensation.
Climate Zones
Zone 1 represents the top half of the North Island, including Thames from
Greater Auckland north.
Zone 2 represents the rest of the North Island except for the Volcanic Plateau.
Zone 3 represents the South Island and the Volcanic Plateau of the Central
North Island.
U Value
The U Value is the measure of air to air heat transfer through glass due to the thermal conductance of the glazing and the difference between indoor and outdoor temperatures. It is expressed as W/m2K (Watts per m2 per 1° Kelvin) or W/ m2 °C (1 Kelvin equals 1°C). The U value is a measure of the rate of heat gain or heat loss through the glazing due to environmental differences between outdoor and indoor air. It is measured at the centre of the glass (cog).The lower the U Value the lower the heat transfer, the better the insulation.
It is also the inverse of the R Value U=1/R. U Values are determined using a set of weather conditions established by the National Fenestration Rating Council (NFRC) in America. It is possible to modify these conditions and calculate U Values for specific environmental conditions.
Coolness Factor
The Coolness Factor (or luminous efficacy) is the visible light transmission divided by the shading coefficient. CF = VLT / SC. It is a useful means of comparing different glass types in terms of the trade-off between light transmission and heat control in selecting glass. Glass types with a coolness factor of 1 transmit as much light as heat, those with a coolness factor lower than 1 transmit more heat than light and those with a coolness factor greater than 1 transmit more light than heat.
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To
the right hand side of each performance diagram is a swatch showing a photographic
sample of the glass.
R
Value (Total Thermal Resistance) 
Sound
Transmission Loss 
Sound
Transmission Class 
Weighted
Sound Reduction Index 
Perceived
Sound Reduction