A method for measuring refractive index inhomogeneity through different plates of potentially different refractive indices comprising the steps of: (a) projecting projected light (R) onto a front surface of a lightguide , said lightguide including: (i) a first pair of external surfaces parallel to each other, said external surfaces including said front surface and a back surface , and (ii) a set of coated plates , said set of plates : (A) parallel to each other, (B) between said first pair of external surfaces, and (C) at a non-parallel angle relative to said first pair of external surfaces, and (iii) each of said plates having a corresponding index of refraction (n), and (b) capturing an interferogram image of an interference pattern to measure the refractive index inhomogeneity between said plates , said interference pattern between refracted/reflected light rays (R') and externally reflected light (Re'), (i) said refracted/reflected light rays (R') being the result of said projected light (R) traversing, reflecting internally, and then exiting said lightguide via said front surface and (ii) said externally reflected light (Re') being the result of said projected light (R) reflecting from said front surface , (c) determining said inhomogeneity from said interferogram image. (a) projecting projected light (R) onto a front surface of a lightguide , said lightguide including: (i) a first pair of external surfaces parallel to each other, said external surfaces including said front surface and a back surface , and (ii) a set of coated plates , said set of plates : (A) parallel to each other, (B) between said first pair of external surfaces, and (C) at a non-parallel angle relative to said first pair of external surfaces, and (iii) each of said plates having a corresponding index of refraction (n), and (i) a first pair of external surfaces parallel to each other, said external surfaces including said front surface and a back surface , and (ii) a set of coated plates , said set of plates : (A) parallel to each other, (B) between said first pair of external surfaces, and (C) at a non-parallel angle relative to said first pair of external surfaces, and (A) parallel to each other, (B) between said first pair of external surfaces, and (C) at a non-parallel angle relative to said first pair of external surfaces, and (iii) each of said plates having a corresponding index of refraction (n), and (b) capturing an interferogram image of an interference pattern to measure the refractive index inhomogeneity between said plates , said interference pattern between refracted/reflected light rays (R') and externally reflected light (Re'), (i) said refracted/reflected light rays (R') being the result of said projected light (R) traversing, reflecting internally, and then exiting said lightguide via said front surface and (ii) said externally reflected light (Re') being the result of said projected light (R) reflecting from said front surface , (i) said refracted/reflected light rays (R') being the result of said projected light (R) traversing, reflecting internally, and then exiting said lightguide via said front surface and (ii) said externally reflected light (Re') being the result of said projected light (R) reflecting from said front surface , (c) determining said inhomogeneity from said interferogram image. <EOS>