Glass Terminology Glossary

The following is a glossary of common glass terms used in the glass manufacturing industry. If you have questions or need more clarification on any of the terms used in connection with our architectural glass products, contact us.

Annealed Glass

The most common form of glass found in residential applications, annealed glass is a standard sheet of float glass that is easily broken and produces large dangerous shards when shattered. On its own it provides very little insulation and acoustical qualities.


The measure of a surface’s ability to emit long-wave infrared radiation.

Heat-Strengthened Glass

Flat or bent glass that has been heat-treated to have a surface compression between 3,500 and 7,500 psi (24 to 52 MPa) and meet the requirements of ASTM C 1048, kind HS. Heat-strengthened glass is not a safety glazing material and will not meet the requirements of ANSI Z97.1 or CPSC 16 CFR 1201. View Heat Treated Glass Products

Insulating Glass Unit

Two or more lites of glass spaced apart by a desiccant filled spacer bar and then hermetically sealed to form a dual-glazed unit with an air space between each lite. (Commonly called IG units.) View Insulated Glass Products


The measure of the thermal conductivity values of glazing materials. K-value is expressed as W/m2.K. The lower the K-value, the higher the performance of the product.

Laminated Glass

Two or more lites of glass permanently bonded together with one or more interlayers. View Laminated Glass Products

Light to Solar Gain (LSG)

The ratio of visible light transmittance to solar hear gain coefficient. A high LSG represents a relative efficiency of a glazing materials and its ability to transmit daylight white blocking heat gain.

Low-Emissivity (or Low-E)

A low rate of emitting (radiating) absorbed radiant energy. The radiant energy (heat), i.e. long wave infrared, is in effect, re-radiated back toward its source.

Monolithic Glass

A single sheet of float glass.

PVB Plastic Interlayer

A vinyl-based, pliable, clear plastic material that is used to bond glass together.


The thermal resistance of a glazing system expressed ft2/hr/oF/Btu (m2/W/oC). The R-value is the reciprocal of the U-value. The higher the R-value, the less heat is transmitted throughout the glazing material.

RAT Equation

When combined UV, visible light and infra-red (solar energy) strikes glass, it is reflected (R), absorbed (A), and transmitted (T) in different proportions, depending on the type of glass involved. This gives us the RAT Equation which accounts for 100% of solar energy.

Relative Heat Gain

The amount of heat gain through a glass product taking into consideration the effects of solar heat gain (shading coefficient) and conductive heat gain (U-value). The value is expressed in Btu/hr/ft2 (W/m2).

The relative heat gain is calculated as RHG = (Summer U-value x 14oF) + (Shading Coefficient x 200). The Lower the relative heat gain, the more the glass product restricts heat gain.

Shading Coefficient

The ratio of the solar heat gain through a specific glass product to the solar heat gain through a lite of 1/8” (3mm) clear glass. Glass of 1/8” (3mm) thickness is given a value of 1.0; therefore, the shading coefficient of a glass product is calculated as follows:

Solar Heat Gain of the Glass in Question
Solar Heat Gain of 1/8” Clear Glass

Solar Control Glass

Tinted and/or coated glass that reduces the amount of solar heat gain transmitted through a glazed product.

Solar Energy Reflectance

In the solar spectrum, the percentage of solar energy that is reflected from the glass surface(s).

Solar Energy Transmittance

The percentage of ultraviolet, visible and near infrared energy within the solar spectrum (300 to 2100 nanometers) that is transmitted through the glass.

Solar Heat Gain Coefficient (SHGC)

The portion of directly transmitted and absorbed solar energy that enters into the buildings interior. SHGC is typically 86% of the shading coefficient. The lower the solar hear gain coefficient, the better the performance.

Solar Reflective Coatings

Solar reflective coatings reduce solar heat gain through high reflection and absorption. The glass appears almost mirror-like. Typically, the coating reflects and absorbs high amounts of visible and infra-red portions of the solar spectrum. As a result, heat gain is drastically reduced, but the so is the amount of visible light transmission.

Solar Spectrum

The sun radiates solar energy or sunlight by electromagnetic waves over a range of wavelengths known as the solar spectrum (290-2500 nanometers, where 1 nanometere = 1/1,000,000,000 of a meter).

The solar spectrum is divided into three bands. They are:
Ultra-violet light (UV) 290nm-380nm
Visible light 380nm-780nm
Infra-red 780nm-2500nm

The energy distribution within the solar spectrum is approximately 2% UV, 47% visible, and 51% infra-red. Only the visible light is seen by the human eye.

The shorter the wavelength, the higher the energy associated with the radiation. For example high energy UV light causes sunburns, fabrics to fade and plastics to deteriorate. While the longer wavelengths, and low radiation produce the visible light.

STC (Sound Transmission Class)

A single number rating derived from individual transmission losses at specified test frequencies (for more information see ASTM E 90 and ASTM E 413). It is used for interior walls, ceilings and floors and in the past was also used for preliminary comparison of the performance of various glazing materials. Learn About the High Transmission Class of SonicGuard™ Laminated Glass

Tempered Glass

Flat or bent glass that has been heat-treated to have either a minimum surface compression of 10,000 psi (69 MPa) or an edge compression not less than 9,700 psi (67 MPa) in accordance with the requirements of ASTM C 1048, kind FT or meet the requirements of ANSI Z97.1 or CPSC 16 CFR 1201. Outside of North America, sometimes called “toughened glass.” Learn About Vitrum’s Premium TrueForm™ Tempered glass

Thermal Heat Transfer

Heat is transferred either by convection (upward warm air currents), conduction (passing form one object to another), or radiation (where heat passes through space to an object where it is reflected, absorbed, or transmitted). The absorbed portion of the energy is subsequently dissipated by radiations (or emission) to both the outside and inside, in varying proportions, dependent on the type of glass and external weather conditions.


A measure of air-to-air heat transmission (loss or gain) due to the thermal conductance and the difference in indoor and outdoor temperatures. As the U-value decreases, so does the amount of heat that is transferred through the glazing material. The lower the U-value, the more restrictive the fenestration product is to heat transfer. Reciprocal of R-value.

Visible Light Reflectance

The percentage of visible light (390 to 770 nanometers) within the solar spectrum that is reflected from the glass surface.

Visible Light Transmittance

The percentage of visible light (390 to 770 nanometers) within the solar spectrum that is transmitted through glass.