STRUCTURAL GLASS FACADES - USC School of Architecture

Solar Spectrum
Sunlight is comprised of 2% ultraviolet light (UV), 47%
visible light and 51% infrared (IR). Wavelength is a
measure of the solar spectrum; a nanometer (nm) is a unit
of length where 1nm = 10¯9m . Visible is in the middle of the
spectrum in the range of ~380 – 780 nm. UV is in the
range of ~300 – 380, and can have damaging effects on
everything from skin to plastic and upholstery. IR is in the
range of ~ 780 – 3000 nm and can have a problematic
heat effect; short-wave IR converts to heat energy when
absorbed by an object.
Figure 2.31 Chart of solar spectrum
(Viracon 2006, p.4.)
Solar Energy and the RAT Equation
When solar energy meets glass, portions of it are reflected, absorbed or transmitted – giving
you the RAT equation – which accounts for 100% of solar energy, which is equal to the sum
of solar reflectance, absorption and transmittance. Example: a single pane of 1/8 in (3mm)
clear glass transmits 83%, reflects 8% and absorbs 9%. Absorbed energy is emitted back
partially to the interior and partially to the exterior. (See Figure 2.32 and Figure 2.33
following.)
Figure 2.32 RAT equation (Viracon
2006).
Figure 2.33 RAT equation example
(Viracon 2006).
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
113