  A prominent manifestation of the solar dynamo is the 11-year activity cycle,
evident in indicators of solar activity, including solar irradiance. Although a
relationship between solar activity and the brightness of the Sun had long been
suspected, it was only directly observed after regular satellite measurements
became available with the launch of Nimbus-7 in 1978. The measurement of solar
irradiance from space is accompanied by the development of models aimed at
describing the apparent variability by the intensity excess/deficit effected by
magnetic structures in the photosphere. The more sophisticated models, termed
semi-empirical, rely on the intensity spectra of photospheric magnetic
structures generated with radiative transfer codes from semi-empirical model
atmospheres. An established example of such models is SATIRE-S (Spectral And
Total Irradiance REconstruction for the Satellite era). One key limitation of
current semi-empirical models is the fact that the radiant properties of
network and faculae are not adequately represented due to the use of
plane-parallel model atmospheres (as opposed to three-dimensional model
atmospheres). This thesis is the compilation of four publications, detailing
the results of investigations aimed at setting the groundwork necessary for the
eventual introduction of three-dimensional atmospheres into SATIRE-S and a
review of the current state of the measurement and modelling of solar
irradiance. Also presented is an update of the SATIRE-S model. We generated a
daily reconstruction of total and spectral solar irradiance, covering 1974 to
the present, that is more reliable and, in most cases, extended than similar
reconstructions from contemporary models.
