  We present a two-layer accretion disc model developed to simultaneously fit
optical long baseline visibilities and spectral energy distributions of T Tauri
accretion discs.
  Our model includes viscous heating and absorption of stellar radiation. It
analytically expresses the vertical structure of the disc using a two-layer
approach: the outer layer is heated by the star and by the inner layer, and the
inner layer by visous dissipation and by the outer layer. We compare three
prescriptions for the flaring of the irradiated surface, and conclude that a
smooth profile with r ~ r^1/8->1/7 has predictions close to those of comparable
numerical models. The observables (spectra, images and visibilities) are
numerically determined.
  We study the influence of disc parameters on the structure and observables.
Then we apply it to three stars observed in IR interferometry. For T Tau and SU
Aur we find a model fit consistent with both visibilities and spectrum but we
could not in the case of T Tau North, which might come from caveats in the flux
correction from the close South companion.
  We find that even a single interferometric measurement at one wavelength can
bring a very strong constraint on disc models. We predict that future massive
interferometric observations will provide a breakthrough in the understanding
of accretion disc physics.
