We will now deal with the solar spectrum which is shown in these figures. The yellow curve corresponds to the black body radiation at 5,900 degrees K, yellow color. So orange curve matches a solar spectrum outside the atmosphere. So it corresponds to 1.3 kilowatts per square meter. The spectrum outside the atmosphere follow very well the Planck's Law so as the first order, the sun behaves as a black body. Then zero radiation is affected by the interaction with the atmosphere. It is revealed by the first spectrum sea level AM equal 1.5. Ultraviolet part is absorbed by the atmosphere. This is due to absorption by ozone. So ultraviolet contribution that correspond to the most energetic photons more than 3 eV represents only five percent of the spectrum. Visible orange, blue to red from 0.4 - 0.75 microns, 40 percent of the radiation. Then the spectrum as obtained is infrared which represent more than half of the first spectrum energy. The infrared portion more than 0.75 micrometer displays absorption bonds related to the absorption of photons by either the CO2 or the water vapor. Solar modules are characterized in normalized conditions given here. These radiation absorption phenomenon by gaseous molecules can be explained quite simply. Consider the case of a CO2 molecule which consists of two double C-O bonds. These bonds' vibration modes is infrared. The infrared photons at this energy will just excite these vibration modes CO, HO, and so on, and therefore, be absorbing. This is also the case for a CO2 glass, that for the same reason, absorbs infrared. This is the origin of the greenhouse effect. The glass or CO2 is transparent than the visible black body at 5,900 K but absorb infrared on beyond condition at 300K, resulting in over heating. And as I note, the cause of the phenomenon, we often mention energies, so in electron volts. However, the solar spectrum is usually described in from wavelengths. Your So wavelength is the inverse of the energy. To easily switch from one to the other, we must remember that E, electron volt, is equal to 1.24 over lambda, the wavelengths in micrometer. For example, the energy of a blue photon, 0.4 microns, correspond to three eV. For red light, wavelengths around 0.6 micron, so energy is two eV. We present here the variation of the solar spectrum as function of the air mass ratio. Out of atmosphere, AM zero. The integrity radiation is 1.3 kilowatts per square meter at AM one. So the sun at it's heat the polar density is slightly less than one kilowatt per square meter. The solar module performances are recorded at AM 1.5 curve three. On curve 4, correspond to AM two. That is to say at lower sun elevation, AM two corresponds to less than 700 watts per square meter. So comparison of the various curve shows that high energy path of the spectrum is preferably absorbed by the atmosphere. This is why when the sun at sunset high value AM appears red or reddish. So far, we have consider only the direct solar radiation. But the sensor, such as photovoltaic cell, is sensitive to all components of the radiation. Direct radiation is the one coming directly from the sun. But part of the solar spectrum that is scatter is over the atmosphere. For example, by aerosol or water vapor tablets. The so-called Rayleigh scattering which varies as lambda to the minus four, so Rayleigh scattering tends to preferentially affect high energy photons which explain the blue color of the sky. Also, so light can be scattered by other elements such as clouds and so on. As shown here, this diffusion can be more or less endothermic. Albedo corresponds to the ground reflection with an angular on spectral deponents strongly affected by the soil. For example in the case of snow, the optical concentration mirrors, lenses conceals only direct radiation and only marginally affect the scattered radiation. All the components mentioned above vary considerably depending on the time of the day on weather conditions. I represent data recorded on the campus of Ecole Polytechnique in summer. The blue curve corresponds to direct radiations, diffuse radiation in red, and the green one to the irradiance on horizontal plane downwelling. The first record, August 11, corresponds to a clear day with some clouds in the afternoon. Direct radiation almost reaching one kilowatt per square meter at noon. While the diffuse radiation is low, under 10 percent. So next, there is a cloudy day and probably rainy, especially in the afternoon. Direct radiation in blue becomes very small, and diffuse radiation in red becomes dominant close to 500 watts per square meter. Cloudy causing greatly affects the impinging solar energy. It's conversion into electricity, therefore, affect significantly the color green. We treated, during the seconds, the solar spectrum. That is to say, the available energy. We will look subsequently to the operating principle of solar cells. Thank you.
