Coal powder measurement by digital holography with expanded measurement area

Abstract

The field of view of digital in-line holography for flow field diagnostics is restricted to a small volume due to the finite size and the low spatial resolution of the available CCD. Expansion of the measurement cross section of digital holographic particle image velocimetry was investigated with a lens-based holography configuration. By sampling the chirp signal in the center lobe completely and undersampling the chirp signal in the second- and higher-order lobes by a magnified virtual recording plane produced by an imaging camera lens, the field of view is expanded. Simulation results show that the three-dimensional (3D) location and size of the relatively large particle can be reconstructed with good accuracy. A digital holographic particle image velocimetry system was established for coal particle flow field diagnostics. Compared with the lensless configuration, the field of view of the digital holography system was enlarged 1.9 times, up to 2.78 cm × 2.78 cm × 3 cm. The 3D location, size distribution, and the 3D vector field of coal powder were obtained. The results show that the application of digital in-line holography to measure large particle flow field is feasible.

Direct particle depth displacement measurement in DHPTV using spatial correlation of focus metric curves

“…Then the synthesis EFI can be binarized by applying an adaptive threshold algorithm to separate the particles. The detected particle's transverse centroid can be calculated with a gray intensity weighted algorithm [12]. The detected particles are paired using the matching probability method [32], yielding the transverse displacements.…”

“…Holography, as a real 3D imaging technique, is capable of reviving the 3D information of the tracer particles, and thus has been widely applied to particle field diagnostics [2,3]. Besides the size and 3D position measurement [4][5][6], digital holography has also been extended to measure the 3D velocity in a manner of particle tracking/image velocimetry (PTV/ PIV) [7][8][9][10][11][12][13][15][16][17][18]. Digital holographic PTV (DHPTV) shows great potential in measurements of 3D velocity field.…”

“…In DHPTV, we usually record a pair [8,12,13,16,17] (or a series [10,11]) of holograms or a hologram with multi-exposures [14,15], and then reconstruct the 3D optical particle fields to retrieve the 3D velocity field. The existing algorithms for 3D velocity calculation fall into two categories: cross-correlation (3D [9] or 2D [17]) and tracking particle one by one [7,8,[10][11][12][13][15][16][17].…”

“…The existing algorithms for 3D velocity calculation fall into two categories: cross-correlation (3D [9] or 2D [17]) and tracking particle one by one [7,8,[10][11][12][13][15][16][17]. The 3D crosscorrelation method takes the maximum point position in the 3D cross correlation coefficients of two optical particle volumes as the velocity.…”

“…Thus, various focus detection methods have been proposed to improve the accuracy of depth position [13,[24][25][26][27], and each claimed it can accurately locate the particles with a small error of z err , especially referring to the relative depth error z z / err . In the Gabor inline holography configuration, the depth position uncertainty z err usually equals to several times of the particle diameter and increases with the particle size, ranging from dozens of up to thousands of micrometers [12,20,21]. This uncertainty could be acceptable for 3D position determination in some applications.…”