Diffraction by nearly two-dimensional objects

Nearly two-dimensional (2D) objects are commonplace in electromagnetics and in optics: for example, the wing of an aeroplane, many biological objects and some kinds of photonic crystals are all nearly 2D objects. This paper i s intended to show that the results of the classical method which reduces t he 3D problem of scattering by such an object to a 2D one by neglecting the variation of the cross section of the object can be significantly improved without using a theory dealing with 3D objects. We present and compare dif ferent new theories which are able to solve with precision the problem of s cattering from this kind of object using codes devoted to 2D objects, witho ut neglecting the variation of section. Numerical examples are given for pe rfectly conducting crossed gratings close to 1D gratings.