Transmittance refers to the percentage of radiation that
can pass through glazing. Transmittance can be defined for different types
of light or energy, e.g., visible transmittance, UV transmittance, or
total solar energy transmittance.
Transmission of visible light determines the effectiveness of a type
of glass in providing daylight and a clear view through the window. For
example, tinted glass has a lower visible transmittance than clear glass.
While the human eye is sensitive to light at wavelengths from about 0.4
to 0.7 micrometers, its peak sensitivity is at 0.55, with lower sensitivity
at the red and blue ends of the spectrum. This is referred to as the photopic
sensitivity of the eye.
More than half of the sun's energy is invisible to the eye and reaches
us as either ultraviolet (UV) or, predominantly, as near-infrared. Thus,
total solar energy transmittance describes how the glazing responds to
a much broader part of the spectrum and is more useful in characterizing
the quantity of solar energy transmitted by the glazing.
With the recent advances in glazing technology, manufacturers can control
how glazing materials behave in these different areas of the spectrum.
The basic properties of the substrate material (glass or plastic) can
be altered, and coatings can be added to the surfaces of the substrates.
For example, a window optimized for daylighting and for reducing heat
gains should transmit an adequate amount of light in the visible portion
of the spectrum, while excluding unnecessary heat gain from the near-infrared
part of the electromagnetic spectrum.
On the other hand, a window optimized for collecting solar heat gain
in winter should transmit the maximum amount of visible light as well
as the heat from the near-infrared wavelengths in the solar spectrum,
while blocking the lower-energy radiant heat in the far-infrared range
that is an important heat loss component. These are the strategies of
various types of low-emittance coating