absorbed and hence generate electron-hole pairs in a semiconductor material. Therefore, it
is important to know the spectral distribution of the solar radiation, i.e. the number of
photons of a particular energy as a function of the wavelength λ. Two quantities are used
to describe the solar radiation spectrum, namely the spectral irradiance I eλ and the spectral
photon flux Φ ph (λ). We defined these quantities in Section 5.2.
The surface temperature of the Sun is about 6,000 K. If it was a perfect blackbody, it
would emit a spectrum as described by Eqs. (5.18), which give the spectral radiance. To
calculate the spectral irradiance a blackbody with the size and position of the Sun would
have on Earth, we have to multiply the spectral radiance with the solid angle of the Sun as
seen from Earth,
We can calculate Ω Sun with
Using R Sun = 696,000 km, an astronomical unit AU = 149,600,000 km, and R Earth = 6,370
km, we find
The blackbody spectrum is illustrated in Figure 5.5. The spectrum outside the atmosphere
of Earth is already very different. It is called the AM0 spectrum, because no (or “zero”)
atmosphere is traversed. AM0 also is shown in Figure 5.5. The irradiance at AM0 is I e
(AM0) = 1361 Wm
−2
.
When solar radiation passes through the atmosphere of Earth, it is attenuated. The
most important parameter that determines the solar irradiance under clear sky conditions is
the distance that the sunlight has to travel through the atmosphere. This distance is the
shortest when the Sun is at the zenith, i.e. directly overhead. The ratio of an actual path
length of the sunlight to this minimal distance is known as the optical air mass. When the
Sun is at its zenith the optical air mass is unity and the spectrum is called the air mass 1
(AM1) spectrum. When the Sun is at an angle θ with the zenith, the air mass is given by
For example, when the Sun is 60° from the zenith, i.e. 30° above the horizon, we
receive an AM2 spectrum. Depending on the position on the Earth and the position of the
Sun in the sky, terrestrial solar radiation varies both in intensity and spectral distribution.
The attenuation of solar radiation is due to scattering and absorption by air molecules, dust
particles and/or aerosols in the atmosphere. Especially, water vapour (H 2 O), oxygen (O 2 )
and carbon dioxide (CO 2 ) cause absorption. Since this absorption is wavelength-selective,
is important to know the spectral distribution of the solar radiation, i.e. the number of
photons of a particular energy as a function of the wavelength λ. Two quantities are used
to describe the solar radiation spectrum, namely the spectral irradiance I eλ and the spectral
photon flux Φ ph (λ). We defined these quantities in Section 5.2.
The surface temperature of the Sun is about 6,000 K. If it was a perfect blackbody, it
would emit a spectrum as described by Eqs. (5.18), which give the spectral radiance. To
calculate the spectral irradiance a blackbody with the size and position of the Sun would
have on Earth, we have to multiply the spectral radiance with the solid angle of the Sun as
seen from Earth,
We can calculate Ω Sun with
Using R Sun = 696,000 km, an astronomical unit AU = 149,600,000 km, and R Earth = 6,370
km, we find
The blackbody spectrum is illustrated in Figure 5.5. The spectrum outside the atmosphere
of Earth is already very different. It is called the AM0 spectrum, because no (or “zero”)
atmosphere is traversed. AM0 also is shown in Figure 5.5. The irradiance at AM0 is I e
(AM0) = 1361 Wm
−2
.
When solar radiation passes through the atmosphere of Earth, it is attenuated. The
most important parameter that determines the solar irradiance under clear sky conditions is
the distance that the sunlight has to travel through the atmosphere. This distance is the
shortest when the Sun is at the zenith, i.e. directly overhead. The ratio of an actual path
length of the sunlight to this minimal distance is known as the optical air mass. When the
Sun is at its zenith the optical air mass is unity and the spectrum is called the air mass 1
(AM1) spectrum. When the Sun is at an angle θ with the zenith, the air mass is given by
For example, when the Sun is 60° from the zenith, i.e. 30° above the horizon, we
receive an AM2 spectrum. Depending on the position on the Earth and the position of the
Sun in the sky, terrestrial solar radiation varies both in intensity and spectral distribution.
The attenuation of solar radiation is due to scattering and absorption by air molecules, dust
particles and/or aerosols in the atmosphere. Especially, water vapour (H 2 O), oxygen (O 2 )
and carbon dioxide (CO 2 ) cause absorption. Since this absorption is wavelength-selective,