The distribution of solar energy reaching the surface consists of ultraviolet (UV), visible (VIS) and near infrared (NIR).
All objects except the absolute temperature of 0 K degree (-273 ℃) emit energy corresponding to that temperature.
The energy emitted from the sum of surface temperature of about 6 500 K is that specific wavelengths are absorbed by ozone, oxygen, nitrogen, carbon dioxide and moisture in the atmosphere, and only the remaining energy is reached to the earth surface.
Generally, solar energy transmitting through a glass window is defined to use only a range of 300 nm to 2 500 nm.
The following Fig. is an solar spectral distribution represent in ISO 13837 norm.
When calculating the solar energy transmittance or reflectance, define the solar spectral distribution based on air mass, AM=1.0 is a vertical entry of the atmosphere, AM=1.5 is an indication of the energy distribution when the sun's light penetrates the thickness equivalent of 1.5 times that of the atmosphere and reaches to the surface.
Most characteristics are defined on the basis of AM=1.5, but some old Norm are described to be calculated on the basis of AM=1.0 or AM=2.0.
Therefore, solar characteristics calculated on the basis of solar energy such as solar energy transmittance, reflectance, total solar energy transmittance or shading coefficient should be checked of air mass(AM) values.
There are two ways in which solar energy enters the room through a glass window.
Some of the light energy of the 300 nm to 2 500 nm range is transmitted directly through glass into the room, but the rest of some energy is absorbed into the window and then re-radiated into the room.
The amount of energy that solar energy directly transmits through glass windows is called the solar radiation transmittance (Te).
The total solar energy transmittance is summation of solar direct transmittance and the re-emitted energy which is absorbed to the windows that transmitted into the room. (SHGC=Te+qi)
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