\subsection{Solar spectral irradiance measurements}
\label{hrspectrometer}
Irradiance ($E$) is defined as the radiant power incident on a small element of surface containing the point under consideration divided by the area of the element. Spectral irradiance ($E_{\lambda}$) is the derivative from $E$ with respect to wavelength (${\lambda}$) and is expressed in $W\cdot m^{{–}2}\cdot nm^{–1}$. The photon irradiance ($E_{n,p}$) is the number of \textit{einstein} (mol of photons) per time interval (photon flux, $q_{n,p}$), incident on a small element of surface containing the point under consideration divided by the area of the element \cite{Braslavsky2007}. Spectral photon irradiance ($E_{n,p,\lambda}$) is the derivative from ($E_{n,p}$) with respect to ${\lambda}$ and is expressed in $einstein\cdot m^{{–}2}\cdot s^{-1}\cdot nm^{–1}$. $E_{\lambda}$ can be converted into $E_{n,p,\lambda}$ by means of the Planck relation:

\begin{equation}
\label{eq:elam}
E_{n,p,\lambda}=E_{\lambda}\cdot \left( \frac{\lambda }{N_{A}\cdot hc}\right)
\end{equation}

where $N_{A}$ is the Avogadro constant, $h$ is the Planck constant and $c$ is the speed of light.

Systematic measurements of solar spectral irradiance ($E^{S}_{\lambda}$) were carried out with a high-resolution Avantes spectrometer (AvaSpec-ULS3648 model). The equipment is executed with an option of Irradiance Mode, using dark and reference data. Avantes spectrometer includes a fiber optic sensor and computer connection through USB2 interface cable and all devices are portable. It also has a 3648 pixel CCD detector linear array, grating (200-1100 nm) and slit-25. The instrument works in several integration times (from 10 microseconds to 10 minutes) and its signal extends up to 59000 counts for 16bit ADC. The fiber optic sensor was placed on a flat, horizontal position to perform data acquisition. During the measurement routine, software was configured to the interval 247-812 nm with an integration time of 19 $\mu$s to achieve good stability and resolution. Solar spectral photon irradiance ($E^{S}_{n,p,\lambda}$) was calculated from $E^{S}_{\lambda}$ (measured \textit{in situ}) using Equation \ref{eq:elam}.