The sun, which radiates roughly as a black body at around 6000 °K (see figure below), emits light with various wavelengths including the ultraviolet, visible, and infrared areas of the electromagnetic spectrum. However, the solar irradiance, which is the amount of radiant energy received from the sun per unit area per unit time, is greatest at visible wavelengths (300-800 nm) and peaks in the blue-green part of the spectrum1.
To discuss the solar irradiance we must consider the change in intensity of the solar radiation as it travels from the sun to the earth. Defining power as the energy received per unit time, solar irradiance is the power per unit area outside our atmosphere received from the sun, and is also referred to as the solar intensity. Every planet receives a different intensity of radiation from the sun, because the amount of light that hits an object is inversely proportional to the distance squared. In other words, the farther away an object is from the sun, the lower the power incident to the object’s surface (as discussed in Photon Energy and Flux). Using the mean earth-sun distance, we can find the average value of solar irradiance incident to the earth’s atmosphere, which is essentially constant. This value is therefore referred to as the solar constant, or air mass zero (AM0) radiation (see Air Mass).
The solar constant is measured to be about 1.353 kW/m2, according to various pieces of equipment (i.e. satellites) that capture radiation outside our atmosphere. It can be seen from the above graph that while the spectrum of radiation from the sun is very similar to that of an ideal black body, it does deviate somewhat. This is because the sun’s own atmosphere tends to absorb some wavelengths over others, thereby affecting the amount of photons for these particular wavelengths that reach our atmosphere.
Above (Ref. 2, M.A. Green, UNSW): It is shown in this graph how similar the general shape of the sun's irradiance is to the shape of a black body's irradiance. The irregularities or dips in the solar irradiance is explained above. Notice that the maximum energy gets lower as the air mass increases (see Atmospheric Effects and Air Mass).
1. Nelson, Jenny. The Physics of Solar Cells. London: Imperial College Press, 2003.
2. Green, Martin A. Solar Cells: Operating Principles, Technology, and System Applications. Englewood Cliffs: Prentice-Hall, Inc., 1982. Full book ordering information at www.pv.unsw.edu.au.