Typically, diffraction theory is treated from the perspective of the diffractive element. That is, depending on the size of the diffractive element, we describe light-matter interaction in a way or another. Typically, a vectorial electromagnetic description is needed for subwavelength objects. Whereas objects that are greater than 5 times the wavelength can usually be described using scalar electromagnetic theory [1].
Figure 1: Realm of validity of different diffraction models as a function of the ratio between the smallest grating period in the diffractive element (Λ) and the reconstruction wavelength (λ).