Measurement technique for refractive index inhomogeneity between plates of a lightguide optical element (LOE)

A method for measuring refractive index inhomogeneity comprising:

• (a) projecting projected light 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, and
• (b) capturing fringes of an interferogram image of an interference pattern to measure the refractive index inhomogeneity between said plates, said interference pattern between refracted/reflected light rays and externally reflected light,
• (i) said refracted/reflected light rays being a result of said projected light traversing through difference plates having same or different values of refractive index, reflecting internally from the back surface, and then exiting said lightguide via said front surface and
• (ii) said externally reflected light being a result of said projected light reflecting from said front surface.