A Method for a Precise and Instantaneous Measurement of a Refractive Index

A noninvasive machine vision-based system aimed at the measurement of a refractive index (RI) of slab materials and liquids inside a cell is presented. The principle of the proposed method is based on the relaying laser stripe displacements, which occur due to the difference of the refraction across mediums, to the corresponding RI. Two of the main advantages of the proposed method are that the RI of a liquid inside a cell can be measured instantaneously and without knowing any of the optical parameters of the cell walls, respectively. Other available methods however are based on a two-step measurement (first for an empty and then for the filled cell, respectively) procedure. The RI of several well-studied liquids and aqueous solutions of NaCl was measured and the results were confirmed with the corresponding reference values. The precision of up to 10-4 in RI measurement is achieved due to the ability of localizing laser stripe displacements with the subpixel accuracy and robust image processing algorithms. The uncertainty components of the proposed method are thoroughly analyzed, and it is shown that the precision can be further increased subject to the corresponding application and budget.