Now that we have knowledge of the global parameters of the Sun, we are in a position to turn to an interpretive study of the photons which are the principal means by which information comes to us from the Sun. If we can make certain measurements on the photons from the Sun, such as their distribution in wavelength, and the integrated flux of radiant energy, we may hope to extract information about the temperature and other physical quantities in the region from which the photons originated. The photons originate mainly in a region which can be considered roughly as “the (visible) surface of the Sun”: a more precise definition of this region will emerge subsequently from a discussion of radiative transfer.