  The solar limb darkening function is well known and is widely employed in
models of the solar atmosphere. However, there has been a lack of systematic
spectrally resolved measurements. Therefore we recently decided to start an
observing campaign with the Gregory Coud\'e Telescope at IRSOL in Locarno in
order to produce a spectral atlas obtained at 10 different heliocentric angles
$\theta$, chosen so that $\mu = \cos\theta$ covers the interval from 0.1 to 1.0
in step of 0.1. The measurements carried out till now include the spectral
range from 439 nm to 666 nm. The collected data provide information about the
anisotropy of the emergent radiation field on the solar surface, allowing a
better modeling of the Second Solar Spectrum. In addition the data give
observational constraints that should be taken into account when modeling the
solar atmosphere.
