Solar radiation must pass through Earth's atmosphere before it reaches Earth's surface. Earth's atmosphere is composed primarily of nitrogen ( \( 78.084\% \) by volume), oxygen ( \( 20.946\% \)), argon ( \( 0.934\% \)), carbon dioxide ( \( 0.0408\% \)), and water vapor, the content of which fluctuates. In addition to these components, methane, hydrogen, nitrogen oxides, sulfur compounds, ozone, radon, iodine, ammonia, and atmospheric aerosols (0.05-0.35 \( \mu m \) in diameter) are present in trace amounts. The atmosphere causes radiation to be absorbed and scattered on its way to the Earth's surface.

Solar radiation undergoes absorption and diffusion ( Fig. 1 ). In addition to the direct radiation from the Sun, there is also reflected light and a scattering component that reach the surface of the PV panel [1].

Solar radiation incident on the Earth's surface is composed of direct \( I_{b} \) (beam radiation) and scattered \( I_{d} \) (diffuse radiation), sometimes called diffuse or sky radiation ( Fig. 2 ). Diffuse radiation is radiation from the Sun that has scattered in the Earth's atmosphere ( 1 ).

If solar radiation falls on a plane deflected from Earth level, the radiation consists of direct radiation \( I_{b} \) scattered \( I_{d} \) and reflected radiation \( I_{od} \) from the surface of the surrounding ground ( 2 ).

On Fig. 2 marks \( I_{b} \) - direct radiation, \( I_{d} \) - diffuse radiation, \( I_{\alpha b} \) - direct incident radiation at an angle \( \alpha \), \( I_{\alpha o} \) - radiation reflected from the surface of the surrounding ground.

The radiation reaches the Earth's surface through different thicknesses of air layers depending on the location on the globe, so different intensities of solar radiation are observed ( Fig. 3 ).

The change in the path length of a solar ray S through the Earth's atmosphere when the incidence angle ϕ changes is shown in Fig. 3.

where the angle ϕ is between the direction the Sun is facing and the direction of the zenith.

This causes a change in the power of solar radiation reaching the Earth's surface. This illustrates the fact that different locations on Earth have significantly different amounts of solar energy for use in conversion to electricity.

The ratio of the path length traveled by the Sun's rays when incident radiation is at a certain angle to the path length traveled at a right angle is called the air mass number AMm (Air Mass m). If the air mass number is m=1, then the Sun's rays reach the earth at right angles. A simplified scheme for determining the air mass number of the atmosphere is shown in Fig. 4. Radiation above the Earth's atmosphere is m=0 and is denoted AM0; radiation at sea level when the Sun is at zenith is m=1 and is denoted AM1. The air mass number is approximated by the formula:

where the angle ϕ is between the direction in which the Sun is located and the direction of the zenith ( Fig. 4 ).

If there is a need to determine the air mass number with greater accuracy, one should know the atmospheric pressure and then use the following formula ( 5 ) (own elaboration) [2].

where \( p_{0} \)=1013 hPa, \( p_{} \) – current atmospheric pressure, hPa=100 Pa.

For Poland, an air mass number of AM1.5 was assumed (latitude of approximately \( 48_{}^{o}\textrm{} \), see Fig. 5 ).

Differences in lighting conditions at different locations on the Earth's surface have forced the standardization of conducting tests on all photovoltaic panels in exactly [2] the same environmental conditions. Standard Test Conditions (STC) have been introduced. They define the radiation spectrum (AM1.5), illuminance - the amount of incident energy per 1 \( m^{2} \) within 1S (1000 \( \frac{W}{m^{2}} \)) and the temperature at which the \( (25{}^{o}\textrm{C}) \) tests are conducted.