This work presents a compact refractometric system based on In-Line Digital Holography (ILDH) for the non-invasive characterization of transparent media, encompassing both liquids and high-refractive-index optical glasses. The core of the system is a cost-effective, lensless setup in which a 532 nm laser source and a microscope objective generate a divergent spherical wavefront that illuminates a 10 μm aluminum particle. The resulting diffraction pattern, modulated by samples in the optical path, is recorded by a CMOS sensor. The refractive index of the sample is determined by numerically locating the axial position of the particle-reconstructed image, which directly corresponds to the optical path difference introduced by the test medium. The optimal reconstruction plane is objectively located using an autofocus algorithm based on the Kurtosis metric, which identifies the sharpest image. The system successfully characterizes media across a broad refractive index range from 1.33 to 1.78, yielding linear calibration curves for both liquid and solid samples. The instrument achieves an axial reconstruction resolution of 30 μm and a refractive index precision of ±0.01 RIU. This ILDH approach offers a highly portable, cost-effective, and non-contact solution for refractive index measurement, demonstrating significant potential for industrial quality control and high-throughput point-of-care applications.

Accurate axial localization of microparticles is a key requirement in in-line digital holography (ILDH), particularly under noisy conditions and for weakly scattered objects. This work presents experimental and simulated benchmarking of three widely used focus metrics: maximum intensity, complex amplitude, and Kurtosis. Experimental holograms of microparticles with different diameters were recorded using a compact ILDH system, while simulated holograms of a 10 µm particle were generated. Numerical reconstruction was performed using a Fresnel convolution approach with FFT-based propagation over a range of axial distances. The performance of each focus metric was evaluated based on peak definition, robustness to coherent noise, and consistency across particle sizes and configurations. The results show that both maximum intensity and Kurtosis provide consistent and reliable axial localization, with very similar behavior across all cases. In contrast, the complex amplitude metric is more sensitive to noise and exhibits larger fluctuations in the axial response. These results indicate that simple intensity-based metrics can achieve accurate localization under moderate signal-to-noise conditions, while higher-order statistical metrics improve robustness in more challenging scenarios. This work provides practical guidelines for selecting autofocus criteria in ILDH systems for particle imaging and holographic metrology.