Focal depth of digital lensless Fourier transform micro-holographic system

A novel method for measuring the diffusion coefficient of liquids is introduced in this paper. A transparency capillary is used as both diffusive pool and imaging element, spatially resolving ability of the capillary in measuring refractive index of liquid is utilized to observe and record diffusive process directly, diffusion coefficient of liquid is calculated based on Fick second law and acquired experimental data. With the new method, the diffusive process of pure glycerol in water at 25 ℃ is investigated, the obtained diffusion coefficient is 0.898×10-5 cm2/s, which is of a relative error 4.47% compared with the literature value measured by the holographic interference. The influence of measurement accuracy of refractive index and capillary viscosity on measuring result is also analyzed. The method is characterized by micro-quantity sample required, faster measurement and better stability, which opens a new way to measure diffusion coefficient of liquid medium.

Measuring the diffusion coefficient of liquids by capillary imaging method: equivalent refractive index method

Li¹,

Xiao-Yun²

2013

Digital holographic microscope employing dual-Lloyd’s mirror

Yuan¹,

Chen²,

Shou-Ping³

et al. 2014

We propose a digital holographic microscopy (DHM) setup employing a configuration with two Lloyd's mirrors, which is based on self-referencing and dual-wavelength optical phase unwrapping. We use two Lloyd's mirrors to fold the beam which does not exhibit sample structure and acts as the reference beam, returning onto itself to form a dual-wavelength hologram. Two wrapped phase images for every wavelength are reconstructed by angular spectrum method. Then the wrapped phase image and the three-dimensional profile image are acquired by dual-wavelength optical unwrapping method. In the experiment, we use two lasers of different wavelengths of 532 and 632 nm to record a hologram. Numerical methods are subsequently applied to reconstruct the hologram to enable direct access to both phase and amplitude information. The quantitative experimental results with dual-wavelength DHM involve a deviation less than 5% from the calibration values. The validity of this method is demonstrated.

Study on extending the depth of field in reconstructed image for a micro digital hologram

Yang¹,

Wu²,

Ying-Chun³

et al. 2015

Digital micro holography offers an in-situ, non-contact and three-dimensional way to explore the microscopic world. However, as it is difficult to focalize the whole object in one single reconstructed image, the application of digital micro holography to cases with a large longitudinal object volume is limited by the microscopes depth of field. By extending the depth of field in reconstructed micro holograms in the wavelet domain, this paper fully takes advantage of numerical reconstruction algorithms to solve this problem. First, a recorded hologram is rebuilt using the wavelet transform approach by setting up an appropriate longitudinal interval to obtain a series of reconstructed hologram planes. Then each plane is decomposed with wavelet into its sub-images of both high and low frequencies. Furthermore, the local variance of the maximum intensity gradients of the high- and low-frequency coefficients is calculated and utilized as the focus criterion. Finally, the image planes are fused into a single one with the depth of field extended to a large extent. The feasibility and robustness of this reconstruction procedure for both continuum and particle fields are investigated. One of the demonstrations is made in an experiment of a tilted continuum:carbon fiber. It is different from most of the previous applications where the interrogated is the particles and where the area involved is parallel to the CCD. The carbon fiber gets successfully reconstructed in three dimensions, and the measurement errors of its diameter are presented together with the reconstruction distances. Another is an experiment of a dispersed particle field:micro transparent particles are generated by an ultrasonic atomizer, for which the reconstruction procedure achieves an extended depth of field. In addition, a numerical model based on generalized Lorenz-Mie theory is used to simulate the holograms of both opaque and transparent particles of 1-15 m in diameter. Variations of the longitudinal location errors with the Fraunhofer number are analyzed, and comparisons are made between the results of opaque and transparent particles. Both the experimental and simulation outcomes show that this reconstruction procedure is a reliable one to acquire an extended-depth-of-field hologram for both the continuum and the dispersed particle fields, and then to accurately measure the objects.