Measuring the refractive index and particle size of microspheres using dual-wavelength micro-off-axis digital holographic microscopy

Abstract To improve the accuracy and efficiency of measuring the refractive index and particle size of microspheres by digital holography, a dual-wavelength micro-off-axis digital holographic microscopy technique is proposed, and two holograms at different wavelengths within the same field of view were captured simultaneously in this work. Theoretically, the refractive index difference between the particle and the surrounding medium were calculated by fitting the transmission phase distribution pattern of the particles at dual wavelengths. Experimentally, the dual-Wavelength Micro-Off-Axis digital holographic microscopy of poly(methyl methacrylate) microspheres were collected clearly at wavelength 632.8 nm and 532 nm. Based on the phase distribution of the microspheres, the refractive index differences between the object and the medium are measured to be 0.0155 and 0.0159, respectively. Furthermore, the measuring results were confirmed again by using the cross-validation method. This technique is suitable for phase measurements of samples with large thickness gradients and enables real-time, high-precision measurement of the refractive index and particle size of spherical objects.