Smartphone-Based Critical Angle Refractometer for Real-Time Monitoring of Brix Value

We demonstrate the first smartphone-based critical angle refractometer for monitoring refractive index and Brix value in real time. The principle design of an optical coupler is based on the total internal reflection of incident light from the phone screen. The images acquired from the front-facing camera are directly analyzed on the smartphone using a custom developed application, to detect the shift of gradient peak position corresponding to the refractive index and Brix value. Calibrations are carried out using nine water-sucrose solutions providing a linear response in the refractive index range of 1.3330–1.3575 RIU (R 2 = 0.9984) with the sensitivity and the resolution of 2778.1 pixel/RIU, and $\text {3.6}\times \text {10}^{-\text {4}}$ RIU, and a linear response in the Brix value of 0–16.25 °Brix (R 2 = 0.9968) with the sensitivity and the resolution of 4.239 pixel/°Brix and 0.24 °Brix. We also successfully determined the sugar content in six fruit juices with the absolute error from standard refractometer at less than 0.21 °Brix.