  We solved the radiative transfer and statistical equilibrium equations in a
two-dimensional cross-section of a cylindrical structure oriented horizontally
and lying above the solar surface. The cylinder is filled with a mixture of
hydrogen and helium and is illuminated at a given altitude from the solar disc.
We constructed simple models made from a single thread or from an ensemble of
several threads along the line of sight. This first use of two-dimensional,
multi-thread fine structure modelling combining hydrogen and helium radiative
transfer allowed us to compute synthetic emergent spectra from cylindrical
structures and to study the effect of line-of-sight integration of an ensemble
of threads under a range of physical conditions. We analysed the effects of
variations in temperature distribution and in gas pressure. We considered the
effect of multi-thread structures within a given field of view and the effect
of peculiar velocities between the structures in a multi-thread model. We
compared these new models to the single thread model and tested them with
varying parameters. These new computations show, for the first time, the effect
of integrating the radiation emitted in H and He lines by several cylindrical
threads that are static or moving along the line of sight. They can be used to
interpret high-spatial and spectral resolutions of cylindrical structures found
in the solar atmosphere, such as cool coronal loops or prominence threads.
