  The reconstruction of the solar spectral irradiance (SSI) on various time
scales is essential for the understanding of the Earth's climate response to
the SSI variability. The driver of the SSI variability is understood to be the
intensity contrast of magnetic features present on the Sun with respect to the
largely non-magnetic quiet Sun. However, different spectral synthesis codes
lead to diverging projections of SSI variability. In this study we compare
three different radiative transfer codes and carry out a detailed analysis of
their performance. We perform the spectral synthesis at the continuum
wavelength of 665 nm with the Code for Solar Irradiance (COSI), and the
Rybicki-Hummer (RH), and Max Planck University of Chicago Radiative MHD (MURaM)
codes for three 3D MHD simulations snapshots, a non-magnetic case, and MHD
simulations with 100 G, and 200 G magnetic field strength. We determine the
intensity distributions, the intensity differences and ratios for the spectral
synthesis codes. We identify that the largest discrepancies originate in the
intergranular lanes where the most field concentration occurs. Overall, the
applied radiative transfer codes give consistent intensity distributions. Also,
the intensity variation as a function of magnetic field strength for the
particular 100 G and 200 G snapshots agree within the 2-3% range.
