The optical constants should signify a specificity for the material. That should not vary when the form (bulk thin film) of the sample varies.
That is why people are starting using optical and dielectric "coefficients" instead of "constants".
Check what IUPAC is saying about that.
I would imagine it's just historical nomenclature. In EM textbooks, a lot of expressions for optical constants are derived in the single-wavelength approximation, but most optical constants are dispersive with wavelength.
Optical constants are quantities characteristic of the optical behavior of a substance (as the refractive index, absorption coefficient, or reflectivity for a specified wavelength).
In particular, the optical properties of the substance are characterized by a refractive index of light, which by its nature is related to the polarizability of the molecules and the individual atomic groups under influence of the electric vector of the electromagnetic wave. In an isotropic medium, the refractive index is the same in all directions, but in an anisotropic refractive index depends on the direction of the light beam and the direction of the electric vector in the electromagnetic wave.
The optical properties of the medium associated with its electromagnetic properties. The optical properties of the medium are also characterized by complex tensor permittivity and magnetic permeability. The real parts of these tensors determine the phase velocity of the light wave, which may change both due to the electric and magnetic fields, and under the action of elastic stresses.
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