  We present IRIS, a new generic three-dimensional (3D) spectral radiative
transfer code that generates synthetic spectra, or images. It can be used as a
diagnostic tool for comparison with astrophysical observations or laboratory
astrophysics experiments. We have developed a 3D short-characteristic solver
that works with a 3D nonuniform Cartesian grid. We have implemented a piecewise
cubic, locally monotonic, interpolation technique that dramatically reduces the
numerical diffusion effect. The code takes into account the velocity gradient
effect resulting in gradual Doppler shifts of photon frequencies and subsequent
alterations of spectral line profiles. It can also handle periodic boundary
conditions. This first version of the code assumes Local Thermodynamic
Equilibrium (LTE) and no scattering. The opacities and source functions are
specified by the user. In the near future, the capabilities of IRIS will be
extended to allow for non-LTE and scattering modeling. IRIS has been validated
through a number of tests. We provide the results for the most relevant ones,
in particular a searchlight beam test, a comparison with a 1D plane-parallel
model, and a test of the velocity gradient effect. IRIS is a generic code to
address a wide variety of astrophysical issues applied to different objects or
structures, such as accretion shocks, jets in young stellar objects, stellar
atmospheres, exoplanet atmospheres, accretion disks, rotating stellar winds,
cosmological structures. It can also be applied to model laboratory
astrophysics experiments, such as radiative shocks produced with high power
lasers.

Instrumentation and Methods for Astrophysics