Evaluating and identifying pearls and their nuclei by using optical coherence tomography

Using optical coherence tomography to assess luster of pearls: technique suitability and insights

Luster is one of the vital indexes in pearl grading. To find a fast, nondestructive, and low-cost grading method, optical coherence tomography (OCT) is introduced to predict the luster grade through the texture features. After background removal, flattening, and segmentation, the speckle pattern of the region of interest is described by seven kinds of feature textures, including center-symmetric auto-correlation (CSAC), fractal dimension (FD), Gabor, gray level co-occurrence matrix (GLCM), histogram of oriented gradients (HOG), laws texture energy (LAWS), and local binary patterns (LBP). To find the relations between speckle-derived texture features and luster grades, four Four groups of pearl samples were used in the experiment to detect texture differences based on support vector machines (SVMs) and random forest classifier (RFC)) for investigating the relations between speckle-derived texture features and luster grades. The precision, recall, F1-score, and accuracy are more significant than 0.9 in several simulations, even after dimension reduction. This demonstrates that the texture feature from OCT images can be applied to class the pearl luster based on speckle changes.

Measuring Shape Parameters of Pearls in Batches Using Machine Vision: A Case Study

To solve the problem of low precision of pearl shape parameters’ measurement caused by the mutual contact of batches of pearls and the error of shape sorting, a method of contacting pearls’ segmentation based on the pit detection was proposed. Multiple pearl images were obtained by backlit imaging, the quality of the pearl images was improved through appropriate preprocessing, and the contacted pearl area was extracted by calculating the area ratio of the connected domains. Then, the contour feature of the contact area was obtained by edge tracking to establish the mathematical model of the angles between the edge contour points. By judging the angle with a threshold of 60° as the candidate concave point, a concave point matching algorithm was introduced to get the true concave point, and the Euclidean distance was adopted as a metric function to achieve the segmentation of the tangent pearls. The pearl shape parameters’ model was established through the pearl contour image information, and the shape classification standard was constructed according to the national standard. Experimental results showed that the proposed method produced a better segmentation performance than the popular watershed algorithm and morphological algorithm. The segmentation accuracy was above 95%, the average loss rate was within 4%, and the sorting accuracy based on the shape information was 94%.

Rapid detection of color-treated pearls and separation of pearl types using fluorescence analysis

Pearl identification plays a key role to maintain transparency in the gem industry by disclosing potential color treatments and classifying pearl species. Current techniques for pearl identification have been limited by expensive instrumentations and long measurement time, severely restricting their use outside of major gemological laboratories. There is a strong demand for simple and inexpensive identification instruments designed for non-specialized users and small-scale gemological laboratories. For this purpose, we demonstrate a portable fluorescence spectroscopy for pearl treatment detection and species classification based on pearl's nacre fluorescence detection. This device can be used to rapidly separate naturally colored pearls from treated colored pearls, detect potential treatments applied to white colored pearls, and separate pearls between certain species in seconds, based on their differences in nacre fluorescence intensity. The system enables noninvasive testing of loose pearls, pearl strands, and mounted pearl jewelry under normal office lighting conditions. The experimental prototype demonstrates high accuracy for automatic pearl color treatment screening, referring 100% of the treated colored pearls. Furthermore, similar protocols can be applied to evaluate popular pearl enhancements such as bleaching and to extend its application to separate akoya pearls and their South Sea counterparts.

Dual-side view optical coherence tomography for thickness measurement on opaque materials

Optical coherence tomography (OCT), as an optical interferometric imaging technique, has found wide applications in various fields. In principle, OCT is well suited for imaging layered structures, and thus, one of the typical applications is thickness measurement. However, due to the limited imaging depth resulting from light attenuation, thickness measurement by OCT is limited to non-opaque materials. In this study, we developed a novel (to the best of our knowledge) dual-side view OCT (DSV-OCT) system for thickness measurement on opaque materials. The dual-side view was achieved on a conventional swept source OCT platform by creating two symmetrical sampling arms. This allows us to image both sides of the material simultaneously and produce the surface contours of the two sides in a single C scan. Finally, the thickness of the opaque material can be calculated from the two surface contours above. We demonstrated that our DSV-OCT technique can measure the thickness of opaque material with an accuracy of about 3 µm.

Quantitative discrimination of pearls using polarization-sensitive optical coherence tomography

We propose a robust method that can quantitatively discriminate genuine pearls from imitation ones by introducing the concept of entropy in the polarization-sensitive optical coherence tomography (PS-OCT). Qualitatively, by examining the birefringence properties of the nacre region of pearls with PS-OCT, the genuine pearls can be easily discriminated. To quantify the amount of birefringence formation, however, the concept of phase retardation entropy is introduced, which is expected to have a higher value when a PS-OCT tomogram has more diverse phase retardation values in its histogram. Experimental confirmation demonstrated that the phase retardation entropy of a genuine pearl was always higher than an imitated pearl. By experimenting with various genuine and imitation pearls, we can say that the phase retardation entropy is effective as a quantitative criterion for discriminating and evaluating pearls.

Automated Internal Classification of Beadless Chinese ZhuJi Freshwater Pearls based on Optical Coherence Tomography Images

Optical coherence tomography (OCT) has been applied to inspect the internal defect of beadless Chinese ZhuJi fleshwater pearls. A novel fully automated algorithm is proposed to classify between normal and defective sub-layer in nacre layer. Our algorithm utilizes the graph segmentation approach to estimate the up and down boundaries of defect sub-layers from flattened and cropped image, and also proposes the strategy for edge and weight construction in segmentation process. The vertical gradients of boundary pixels are used to make grading decision. The algorithm is tested by typical pearl samples, and achieves 100% classification accuracy. The experiment result shows the feasibility and adaptability of the proposed approach, and proves that the OCT technique combined with proposed algorithm is a potential tool for fast and non-destructive diagnosis of internal structure of beadless pearl.

Single-step method for fiber-optic probe-based full-range spectral domain optical coherence tomography

We propose a single-step method appropriated for a fiber-optic probe-based full-range spectral domain optical coherence tomography (OCT). The fiber-optic probe was scanned over a sample with a magnetically driven actuator. In the reference arm, a phase shift of π/2 was applied during two neighbor axial scanning, from which the complex spectral interferogram was directly reconstructed. Since the complex-conjugate-free OCT image is obtained by doing just one Fourier transform on the complex interferogram, obtaining the full-range image is simple in algorithm and effective in computation time. Some full-range images of biological samples created with the proposed method are presented and the processing time is analyzed.

High-speed cross-sectional imaging of valuable documents using common-path swept-source optical coherence tomography

A common-path swept-source optical coherence tomography (SS-OCT) is a promising scheme for implementing a high-speed and stable OCT system. We investigate the capability of a common-path SS-OCT system to perform the cross-sectional imaging of valuable documents translated at high speed for the check of its security feature. The influence of transport speeds, up to 2000 mm/s, on the depth resolution and the signal intensity is experimentally evaluated using a SS-OCT system equipped with a swept source at a center wavelength of 1335 nm and with a sweep repetition rate of 50 kHz. The degradation of the measured signal is in good agreement with theory.

Multimodal analysis of pearls and pearl treatments by using optical coherence tomography and fluorescence spectroscopy

We present an integrated optical system that consists of optical coherence tomography (OCT) and laser-induced fluorescence (LIF) spectroscopy for multimodal analysis of pearls and pearl treatments. The OCT source and the LIF excitation beams were aligned together to illuminate the same spot of a pearl fixed on the sample stage that was under rotation. As a result, both OCT images and LIF spectra of the pearls were detected at the same time and also at the same place. For OCT, a 1310 nm-centered swept laser source was used. For LIF, a 405 nm laser diode was used and a lensed multimode fiber was utilized as a fluorescence probe. The tomographic investigation on the internal structure of a pearl allowed us to evaluate and categorize the pearl nondestructively as was previously reported. In addition, the measurements of fluorescence spectrum and its decaying rate helped to determine the species of mother oyster. The proposed multimodal analysis made it possible to classify the pearls and also to disclose the treatments made on the pearls.