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Purpose This study aims to propose and study a refractive index sensor based on measuring variations of the internal diffuse reflectance from a glass interface in a functional design. The device is uncomplicated to assemble with simple optical elements and it can be built as a robust and stable sensor. Design/methodology/approach This study presents a simplified theoretical model of the signal obtained with the proposed device and perform a detailed analysis of its potential resolution and merits. Findings The authors report proof-of-principle experiments with a home-made device to evaluate its performance as a refractometer and index of refraction sensor. Originality/value The main novelty of the device is the use of a diffusing surface to couple light into a glass plate with a wide range of angles of refraction, including angles larger than the critical angle with the external medium, and using the same diffusing surface to couple reflected light out of the glass plate, including light that suffered total internal reflection.