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Solar (sun) radiation travels to earth by electromagnetic waves at
the speed of light (186,286 miles per second or 300,000 kilometers
per second). The electromagnetic spectrum carries different kinds of
rays (gamma, x-ray, visible, infrared, etc.) each with a different
wavelength between 0.1 to 150 micrometers long (a micrometer, μm, or
a micron is one millionth of a meter) and each with its own
frequency. The frequency is measured in hertz/sec and it represents
the number of waves produced per second. Not all solar radiation
reaches the earth but the radiation which penetrates the atmosphere
[ultraviolet (0.29 to 0.40 μm), visible (0.40 to 0.76 μm), and near
infrared rays (0.76 to 150 μm] has a high energy content (per m2).
This energy is converted to heat energy when it is absorbed in the
roof of a building or any object.
Reflectivity is the ability to "deflect" or reject radiant energy
like a mirror reflects light. Solar reflectivity is the numerical
measurement from 0 to 100% of the total amount of incident solar
radiation that is not absorbed by the surface.
Thermal radiation is emitted from objects on earth by
electromagnetic waves (infrared rays) of low energy content
(therefore, containing less heat) due to the temperature (internal
energy) of the object.
Thermal radiation refers to wavelengths extending from 1.50 to 1000
μm. It includes:
The "longer" wavelengths from the middle infrared (1.50 to 5.60 μm),
to the far infrared (5.60 to 15.00 μm), and the extreme infrared
(15.00 to 1000 μm).
The "longer" wavelength radiation has a very low energy content
compared to shorter wavelength radiation. The lower the object's
temperature, the longer the wavelength emitted and the lower the
rate and energy emitted.
Emissivity: a body continually emits radiant energy at a rate that
is related to its temperature and the nature of its surface (E =
εσT4SURFACE).
Thermal emissivity is the rate at which a body emits radiant energy
compared to a "perfect blackbody" emitter at the same temperature.
Electromagnetic radiation that is absorbed by matter is converted
into internal energy, which can be stored, transferred by
conduction, converted back into electromagnetic radiation that is
given off (emitted) by the material itself.
The amount of radiation heat emitted should equal the amount of heat
absorbed minus that conducted (towards the heat sink) because the
laws of thermophysics demonstrate that energy is never lost (Law of
Conservation of Energy).
