**4.2 Global and direct irradiation**

Global solar radiation on horizontal surfaces can be measured with a pyrometer or reference cell which is an instrument that measures global solar radiation from all directions. To calculatee the global solar radiation we use the formulate as follows.

Diffuse solar radiation (Gb).

Direct beam solar radiation (Gd).

Solar radiation on a horizontal surface is the sum of the horizontal direct and diffuse radiation.

$$\mathbf{G}\_{\rm H} = \mathbf{G}\_{\rm d} \star \mathbf{G}\_{\rm p}$$

The solar module is irradiated by permanently changing solar spectrum. To nevertheless be able to compare the performance of different solar cells technology, a standard spectrum with a relative air mass (AM) of 1.5 was defined:

$$\text{Irradiance} = \frac{Avaage\ value \left[W/m\,^\circ2\right] \* 115,3\, mV}{1000 \left[W/m\,^\circ2\right]} \tag{1}$$

