Öhman Johan, Sjödahl Mikael
Appl Opt. 2018 Jan 1;57(1):A157-A163. doi: 10.1364/AO.57.00A157.
Side scattered light from micrometer-sized particles is recorded using an off-axis digital holographic setup. From holograms, a volume is reconstructed with information about both intensity and phase. Finding particle positions is non-trivial, since poor axial resolution elongates particles in the reconstruction. To overcome this problem, the reconstructed wavefront around a particle is used to find the axial position. The method is based on the change in the sign of the curvature around the true particle position plane. The wavefront curvature is directly linked to the phase response in the reconstruction. In this paper we propose a new method of estimating the curvature based on a parametric model. The model is based on Chebyshev polynomials and is fit to the phase anomaly and compared to a plane wave in the reconstructed volume. From the model coefficients, it is possible to find particle locations. Simulated results show increased performance in the presence of noise, compared to the use of finite difference methods. The standard deviation is decreased from 3-39 μm to 6-10 μm for varying noise levels. Experimental results show a corresponding improvement where the standard deviation is decreased from 18 μm to 13 μm.
使用离轴数字全息装置记录微米级颗粒的侧向散射光。从全息图中重建出一个包含强度和相位信息的体积。由于轴向分辨率较差会使重建中的颗粒拉长，因此确定颗粒位置并非易事。为克服这一问题，利用颗粒周围重建的波前来确定轴向位置。该方法基于真实颗粒位置平面周围曲率符号的变化。波前曲率与重建中的相位响应直接相关。在本文中，我们提出了一种基于参数模型估计曲率的新方法。该模型基于切比雪夫多项式，拟合相位异常并与重建体积中的平面波进行比较。从模型系数中可以找到颗粒位置。模拟结果表明，与使用有限差分法相比，在存在噪声时性能有所提高。对于不同的噪声水平，标准差从3 - 39μm降至6 - 10μm。实验结果显示出相应的改进，标准差从18μm降至13μm。
