DAYLIGHT TRANSMITTANCE: The percentage of visible light that a glazing transmits through glass.

SHADING COEFFICIENT (SC): Measures the total solar heat gain through the glazing compared to 1/8" clear glass under the same conditions.  It includes both the solar energy transmitted directly plus any absorbed solar radiation that is re-radiated as heat into the interior.  The number ranges from 0.0 to 1.0. Standard insulating glass has a SC of .81 The lower the shading coefficient, the lower the solar heat gain.

RELATIVE HEAT GAIN (RHG): A number which represents the amount of heat entering a building due to both solar radiation and conductive/convective heat gain. The total heat flow through the glass for a specific set of conditions.  This value considers indoor/outdoor temperature differences and the effect of solar radiation (Btu's/hr./ft 2 ).

SOLAR HEAT GAIN COEFFICIENT: The percentage of Solar Energy either directly transmitted or absorbed and then re-radiated into the building.

LOW EMISSIVITY COATINGS: Coatings applied which allow shortwave energy to be transmitted through glass, but reflect long wave infrared radiation.  The lower the emissivity, the lower the resultant U-value.

U-VALUE: Measures the heat loss or gain due to differences between indoor and outdoor air temperatures (Btu's/hr./ft²). The U-value equals one divided by the R-value (U=1/R).  The lower the U-value, the better the insulating performance.

R-VALUE: Measures the insulation effectiveness of the window.  The R-value equals one divided by the U-value (R=1/U).  The higher the R-value, the better the insulating  performance.

PYROLYTIC LOW-E: Glass with a coating, either visually reflective or low emissivity, applied at high temperatures and fired into the glass surface during float glass production.

SOFT COAT LOW-E: Glass with a coating applied in a secondary process at ambient temperature within a vacuum chamber.

HEAT MIRROR™: A high performance soft-coat low-e sputtered directly on an optically clear substrate then suspended within the airspace of an insulated glass unit.

A simple ratio of Light to Heat.  The higher the number, the more light per amount of heat.  A low number means that either it is a dark glass or a lot of heat gets through.  A superior, spectrally-selective glass will have a K> 1.0  This may also be called Light to Heat Ratio (LHR) or Coolness Factor.