As described on the previous page, the solar constant S is a measure for the total radiation energy.
You also need to know how the energy of sunlight is distributed over wavelengths or, equivalently, photon energy,
because one type of solar cell may have a good performance at short wavelengths, while another type of cell may be
better at long wavelength. Therefore, knowing the total radiation energy alone is insufficient for characterizing a solar cell.
The energy of sunlight is called intensity or irradiance. The Figure below  shows the intensity of
sunlight over wavelength (left) or photon energy (right), measured above the top regions of the earth’s atmosphere (black line).
The spectral colours are also indicated.
The spectrum is called the AM0 spectrum. This acronym stands for air mass zero, meaning that the spectrum
was measured with no air between the sun and the receiver.
Note that the shape of the two representations of the solar spectrum is different because the photons with long wavelength
carry a smaller amount of energy than photons with a short wavelength. At which color do these two representations of the AM0
spectrum have their maximum?