KEYWORDS: Phase measurement, Imaging systems, Reflection, Microlens, Light sources and illumination, Microlens array, Collimation, Phase distribution, LED lighting, Chemical elements
We present a reflection quantitative phase measurement system via transport of intensity equation (TIE). The proposed system includes an illumination collimation module and a microscopic imaging module. LED illumination is used in illumination module to avoid the effect of speckle noise. In imaging module, the 4f imaging system is formed by a long working distance objective and a lens. The constructed system was calibrated and the actual magnification of the system was 5x for 10x objective imaging. By moving the camera, two images at different defocused distances can be recorded and used for solving TIE to retrieve the phase of tested sample. Finally, the characterization of microlenses demonstrate the effectiveness of the proposed system.
The liquid refractive index is one of the important optical parameters reflecting the properties of solution. A simple scheme based on shading effect for measuring liquid refractive index is presented. When a light beam passes through the liquid, the total reflection occurs at the interface with the air to produce a shading effect, forming a circular shading pattern, and its radius is related to the refractive index of the liquid. Based on this principle, a liquid refractive index measurement system based on smartphone is built. A rectangle colorimetric dish with tested liquid is illuminated by He-Ne laser. A white stickable label paper is attached to one side of colorimetric dish. Then the circular shading pattern displayed on label paper can be recorded by smartphone. A smartphone data processing program was developed to calculate the liquid refractive index from the image taken by smartphone. System calibration and smartphone shooting parameters are discussed in detail. The refractive index measurement experiments of sucrose solution and sodium chloride solution with different concentrations demonstrate the feasibility and effectiveness of the proposed method.
We present a simple and stable liquid refractive index measurement technique via electrically tunable lens (ETL). A focal point is produced when the parallel light passes through a convex lens. If a parallel plate with tested liquid is placed behind the lens, the position of focal point will be changed, which reflect the change of liquid refractive index. The axial displacement of focus point can be obtained by moving the camera and used to calculate the liquid refractive index. To avoid the mechanical movement of system, the convex lens is replaced by an electrically tunable lens. By exactly controlling the external currents of the ETL, the position of focal point can remain unchanged after placing the parallel plate with tested liquid. The change of focal length can be obtained by the change of external currents, which equal to the axial displacement of focus point. Thus, the liquid refractive index can be measured with the change of focal length. Some experiments of water, blend oil and 75% alcohol for measuring the liquid refractive index demonstrate the feasibility and effectiveness of the proposed method.
Based on the refraction characteristic of uniform and transparent liquid, the measurement error of the liquid refractive index using the internal standard method is analyzed, and an improved laser speckle method is proposed. This method designs a double-cell structure composed of two identical rectangular parallelepiped glass cells with one cell containing the liquid to be measured and the other one being empty. This double-cell structure is placed in the built optical system based on laser speckle, and then the refractive indices of sodium chloride solution and sucrose solution with various concentrations are measured by the improved method. Compared with the measurement values of the Abbe refractometer and the internal standard method, the results show that the improved method can significantly reduce the measurement error and improve the measurement accuracy.
We present a simple and stable quantitative phase imaging technique via Fresnel biprism-based digital holographic microscopy. A Fresnel biprism is used to divide the incoming beam and generate self-referencing common-path configuration. So, high contrast hologram is produced in overlapping area. To evaluate the performance of Fresnel biprism with different refringence angles, the system magnification and length are discussed in detail. In addition, different sources of illumination, such as laser and LED, are used in proposed system for studying the feasibility of quantitative phase imaging. The temporal stability of system is also illustrated by comparing with the Mach-Zehnder-based off-axis digital holographic setup. It is shown that the proposed scheme has sub-nanometer temporal stability (~0.109𝑛𝑚) due to the common-path geometry. The experiments on micro-lens array, biological cell and water droplet are reported demonstrating its application both for static and dynamic samples. The refractive index measurement results of polyester fiber also further demonstrate the effectiveness of the proposed system.
We present a simple and effective method for dual-wavelength phase imaging in Lensless Reflection Digital Holographic Microscopy (LRDHM). A filter is inserted between beam splitter and mirror in LRDHM. By adjusting the filter and mirror, the propagation directions of two reference beams of different wavelengths can be separated, and thus two off axis holograms with different fringe directions are simultaneously captured by a monochrome camera. Our scheme is available for real-time dual-wavelength phase imaging but requires minimum optical element and system modification for LRDHM. A series of simulations on wavefront interference analysis for using a filter is discussed in detail. We demonstrate the validity of proposed method with a step target.
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