Visible Light Transmittance
The percentage of visible light (380 - 780 nm) that is transmitted through the glass.

Solar Transmittance
The percentage of ultraviolet, visible and near infrared energy (300 - 3000 nm) that is transmitted through the glass.

Visible Light Reflectance
The percentage of light that is reflected from the glass surface(s).

Solar Reflectance
The percentage of solar energy that is reflected from the glass surface(s).

NFRC U-Value
A measure of heat gain or heat loss through glass due to the differences between indoor and outdoor temperatures. These are center pane values based on ASHRAE standard winter nighttime and summer daytime conditions.

U-values are given in BTU/(hr*ft2*°F) for the English system. Metric U-values are given in W/(m2*°K)*.
*Note: To convert from English to metric, multiply the English U-value by 5.6783.

Winter nighttime U-values are based on an outdoor temperature of 0°F (-17.8°C), an indoor temperature of 70°F (21°C) and a 15 mph (24 km/h) outdoor air velocity.

Summer daytime U-values are based on an outdoor temperature of 89°F (32°C), an indoor temperature of 75°F (24°C), a 7.5 mph (12 km/h) outdoor air velocity and a solar intensity of 248 BTU/(hr*ft2*°F) (782 W/m2).

R-Value
Thermal resistance is expressed in ft2*hr*°F/BTU). It is the reciprocal of U-value. The higher the R-value, the less heat is transmitted through the glazing material.

Shading Coefficient
Shading coefficient is the ratio of solar heat gain through a specific type of glass that is relative to the solar heat gain through a 1/8" (3 mm) ply of clear glass under identical conditions (see Figure 1). As the shading coefficient number decreases, heat gain is reduced, which means a better performing product.

Relative Heat Gain (RHG)
The amount of heat gained through glass taking into consideration U-value and shading coefficient. Using the ASHRAE standard, relative heat gain is calculated as follows:
English System:
RHG = Summer U-value x 14°F + shading coefficient x 200.
Metric System:
RHG = Summer U-value x 7.8°C + shading coefficient x 630.

Solar Heat Gain Coefficient (SHGC)
The portion of directly transmitted and absorbed solar energy that enters into the building's interior. The higher the SHGC, the higher the heat gain.

Light to Solar Gain Ratio (LSG)
The ratio is equal to the Visible Light Transmittance divided by the Solar Heat Gain Coefficient. The Department of Energy's Federal Technology Alert publication of the Federal Energy Management Program (FEMP) views an LSG of 1.25 or greater to be Green Glazing/Spectrally Selective Glazing.

European U-Value (formerly K-Value)
Based on ISO-DP10292 draft standard conditions. It is based on an outdoor temperature of 5.5°C, an indoor temperature of 20.5°C and a 4.8 m/s outdoor air velocity.