Refractive index determination of liquids in a cuvette by quantitative phase measurement with digital off-axis holography

This paper proposes the Average Phase Difference Method (APDM) based on single-wavelength, off-axis digital holographic microscopy (DHM) for liquid refractive index measurement. Unlike existing methods, APDM determines the refractive index of liquid within a simple cuvette by analyzing the average phase difference in relation to the cuvette's geometric size. This eliminates the need for a microfluidic chip, dual-wavelength optical source, liquid-core cylindrical lens or dual-liquid setup. To verify its effectiveness, the refractive indexes of salt solutions and sucrose solutions were measured by APDM; the measuring error was less than 0.55%. The concentration of solutions was characterized by APDM at a concentration resolution of 0.1%. Expanding its utility, APDM was further applied to analyze protein hydrolysis in egg albumen, yielding results in accordance with professional references. While requiring only one frame of digital hologram, APDM demonstrates precise measurement capabilities with significant potential in ingredient or concentration analyses in the fields of chemistry and biology.