(c) 2018 Crown Copyright. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TMECH.2018.2840426, IEEE/ASME Transactions on Mechatronics A B 1 Abstract—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. Index Terms— liquid concentration, real-time measurement, refractive index, robust line fit, structured laser beam displacement, triangulation. I. INTRODUCTION EAL-TIME, noninvasive measurements of a refractive index (RI) is demanded across vast fields of fundamental research and industrial applications, such as growing of new optical materials, 3D refractive-index-matching, design of tunable optics and optofluidics, dynamic process control that includes chemical reactions, purification processes, and many others [1-3]. Historically, the minimum deviation angle [4], the critical angle measurement [5], and Abbe refractometers [6] are recognized as the classical tools in the field. Unfortunately, all of these methods utilize either prisms or V- shaped groove materials, for which the RI at the working wavelength must be precisely known in advance. Moreover, in practice a precise measurement of the deviation angle of prisms tends to be more difficult than anticipated. H. Nasibov is with the National Metrology Institute (UME), Scientific and Technological Research Council of Turkey (TUBITAK), Ulusal Metroloji Enstitüsü, PK 54, Gebze, 41470, Turkey (e-mail: humbet.nasibli@tubitak.gov.tr ; humbat.nasibov@gmail.com ). A. Kholmatov (e-mail: alisher.kholmatov@maximintegrated.com ) and I.Mehmetbeyli (e-mail: izmirm@ttmail.com ) were with the National Research Institute of Electronics and Cryptology of TUBITAK, PK 74, Gebze, 41470, Turkey. A. Kholmatov is now with the Maxim Integrated, Istanbul, Turkey. Interferometric laser refractometers [7, 8] are another class of RI measurement devices, whose measurement principles are based on the assessment of the change in the optical path that is induced by the sample under the test. The refractometers are known to achieve the most precise RI measurements amongst available techniques. However, these techniques are very sensitive to environmental conditions and their implementations are complex. A detailed review of the associated RI measurement methods can be found in [9]. In recent years, various optical fiber based RI sensing techniques were proposed [10-12] that utilize evanescent field refractive index fiber sensors, which have a miniature size and the high sensitivity. However, an intrusive nature of the sensing approach reduces their utilization in most of the industrial applications. Besides, none of these methods can be used for the long-term monitoring, since the intrusive probes may cause an undesired contamination of liquid samples if they are not properly washed and cleaned away after the use [13]. An adulteration of various liquids of common use and the environmental pollution are escalating, hence fast and nonintrusive RI measurements techniques still keep their importance, especially those that are simple in use, not bulky and are rather cost effective. Recently, Nemoto [14] proposed a moderately simple but time consuming RI measurement method that is based on the assessment of the displacement between the propagation axes of the transmitted and the incident laser beams impinging obliquely on a transparent slab material or on a rectangular cell filled with a liquid under the test. Later, an automated version of this refractometer was proposed, where the knife edge technique (used for the beam displacement measurements in [13]) was replaced with position sensitive detectors [15, 16]. Alternatively, in [17] the beam displacement was assessed with a four-quadrant silicon detector that was mounted on a translation stage, and the RI of the most widely used solvents were measured with an accuracy of up to 10  . More recently, a method that employs two single element detectors, one for a reference signal and the other for the laser beam displacement measurement was proposed in [13]. In the current study, we present an alternative method for the RI measurement of transparent materials. The method has a strong potential of application in industrial and academic environments. Specifically, this work was largely motivated by a deficiency of the practical solution to the real-time concentration control systems that are highly demanded in industrial environments. A Method for a Precise and Instantaneous Measurement of a Refractive Index Humbat Nasibov, Alisher Kholmatov and Izmir Mehmetbeyli R