Absorption, reflection and transmission of solar radiation

When solar radiation is projected on an object, part of the radiant energy will be absorbed, part of the radiant energy will be reflected, and the rest of the radiant energy will pass through the object. According to the law of conservation of energy, there should be

Formula 1

where α—— solar absorption ratio;
ρ—— solar reflectance;
τ – solar transmittance.
If the object is opaque, that is, if the object cannot transmit solar radiation, then there is

Formula 2

Some objects, even if they are not very thick, can transmit radiation only in some specific wavelength ranges, and cannot transmit radiation in most wavelength ranges. For example, ordinary flat glazing is almost impermeable to radiation with wavelengths greater than 3 μm.

For the reflective surface of an object, there can be the following 4 different types.

(1) Mirror reflection surface
The surface of the object is very smooth and clean, and its reflection performance to projected solar radiation is like a mirror, which conforms to the law of reflection. The reflection angle is equal to the incident angle.

(2) Diffuse surface
The surface of the object is very uniform, and it reflects the projected solar radiation in all directions indiscriminately. This surface is called a “diffuse surface”, and its reflection is called “diffuse reflection”.

(3) Mirror-diffuse surface
The solid surface is dominated by specular reflection, but there is also some diffuse reflection around the mirror reflection. This surface is called a “mirror-diffuse surface”.

(4) Hybrid reflective surface
Solid surfaces that have both diffuse and specular reflections are called “hybrid reflective surfaces”.
Similar to the case of reflection, the transmission of solar radiation by solid surfaces also has four types, namely “mirror transmission surface”, “diffuse transmission surface”, “mirror diffuse transmission surface” and “hybrid transmission surface”.

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