Gihyeon Min

Counterfeit Detection Using Characterization of Safety Feature on Banknote with Full-field Optical Coherence Tomography

We report an application of full-field optical coherence tomography (FF-OCT) for identifying counterfeit bank notes. The depth-resolved imaging capability of FF-OCT was used for tomographic identification of superficially-identical objects. By retrieving the internal structures of the security feature (cash hologram) of an original banknote, we could demonstrate the feasibility of FF-OCT to identify counterfeit money. The FF-OCT images showed that the hologram consisted of micron scale multi-coated layers including an air gap. Therefore, it is expected that FF-OCT has potential as a new non-invasive tool to discern imitation of currency, and it would find applications in a wide field of counterfeit sciences.

Refractive index measurements of multiple layers using numerical refocusing in FF-OCT

We propose and demonstrate the novel method of refractive index (RI) measurement for each layer of multilayered samples, which is based on numerical refocusing in full field optical coherence tomography (FF-OCT). The en-face FF-OCT image on an inner layer boundary of a multilayered sample is unintentionally blurred or defocused due to the RI of the sample itself, but can be numerically refocused. The refocusing is performed by numerically shifting the image sensor plane of the system, in general. However, by calculating the corresponding sample shift and then compared it with the actual sample shifting distance, we could extract the average RI of the layer between any two layer boundaries within the multilayered sample. In addition, the thickness of that particular layer could be derived at the same time. For the idea proof, several samples were prepared by stacking, for each sample, two transparent plates with a gap in between. While changing the material of the plate and filling the gap with oil, the RIs of the plate and the oil were measured. For oils of various RIs, from 1.2977 to 1.3857, the measured RIs were well matched with the reported ones within 0.205%. Moreover, even with a stack of various and multiple plates in front of the same oil layer, the oil RI and the physical thickness of the oil layer were extracted with average errors of only 0.065% and 0.990%, respectively.

Numerical correction of distorted images in full-field optical coherence tomography

We propose a numerical method which can numerically correct the distorted en face images obtained with a full field optical coherence tomography (FF-OCT) system. It is shown that the FF-OCT image of the deep region of a biological sample is easily blurred or degraded because the sample has a refractive index (RI) much higher than its surrounding medium in general. It is analyzed that the focal plane of the imaging system is segregated from the imaging plane of the coherence-gated system due to the RI mismatch. This image-blurring phenomenon is experimentally confirmed by imaging the chrome pattern of a resolution test target through its glass substrate in water. Moreover, we demonstrate that the blurred image can be appreciably corrected by using the numerical correction process based on the Fresnel–Kirchhoff diffraction theory. The proposed correction method is applied to enhance the image of a human hair, which permits the distinct identification of the melanin granules inside the cortex layer of the hair shaft.

The refractive index measurement technique based on the defocus correction method in full-field optical coherence tomography

We propose and demonstrate a novel refractive index (RI) measurement by using the numerical-sample-motion based the defocus correction method in full field optical coherence tomography (FF-OCT). Overcoming the general problem in FFOCT that is the position of the focal plane is separated from the position of the image plane when imaging a deep region inside a sample, we measure the separation distance from the position of the focal plane to the position of the image plane. The RI is determined from the separation distance that is obtained by the numerically adjusted distance of a sample position. With the proposed method, the depth resolved RIs of double layer materials are determined.

Qualitative investigation of fresh human scalp hair with full-field optical coherence tomography

We have investigated depth-resolved cellular structures of unmodified fresh human scalp hairs with ultrahigh-resolution full-field optical coherence tomography (FF-OCT). The Linnik-type white light interference microscope has been home-implemented to observe the micro-internal layers of human hairs in their natural environment. In hair shafts, FF-OCT has qualitatively revealed the cellular hair compartments of cuticle and cortex layers involved in keratin filaments and melanin granules. No significant difference between black and white hair shafts was observed except for absence of only the melanin granules in the white hair, reflecting that the density of the melanin granules directly affects the hair color. Anatomical description of plucked hair bulbs was also obtained with the FF-OCT in three-dimensions. We expect this approach will be useful for evaluating cellular alteration of natural hairs on cosmetic assessment or diagnosis of hair diseases.

Correction of defocused images in full-field optical coherence tomography using digital holography

We propose the digital holographic technique that can mathematically reconstruct the distorted two dimensional en-face images obtained with full-field optical coherence tomography (FF-OCT). As a powerful biomedical imaging modality, FF-OCT provides inner microstructure images of a biological sample noninvasively but with a submicron depth resolution. The main advantage of the FF-OCT over other OCT techniques is that, it requires only depth scanning (C-scan) without any transverse mechanical scanning (B-scan). In a FF-OCT system based on a Michelson interferometer, not only the length of the reference arm should be matched with the length to the imaging plane in the sample arms, but also the focal plane of the system should be matched with the imaging plane. When the sample has a very high refractive index than the surrounding medium, in which the reference mirror is immersed, the mismatch between the imaging plane and the focusing plane becomes a severe problem and results in degradation of OCT image. In this study, we confirm the existence of the imaging and focusing plane mismatch problem in the FF-OCT system, and propose the method that can retrieve the focused image from a defocused image with the help of digital holography. One of the major advantages of the proposed technique is that it does not require any mechanical movement for refocusing. Only numerical calculation based on the Fresnel diffraction theory is enough. The performance is demonstrated with the image of the USAF resolution target. The image of the chromium coated pattern on the target was blurred with the existence of the glass substrate, when the OCT image was taken through the substrate. The blurred image was digitally corrected to get on the focused clear image of the pattern.

Tomographic imaging system using digital holographic technique based on integrating four buckets phase shifting interferometry

We present a tomographic imaging system that is applied a continuous phase shifting interferometry scheme to digital holographic microscopy (DHM). The proposed scheme achieves en-face tomographic image from digitally recorded original hologram by 2-D sensor array. Although images obtained at out-of-focus position, the application of integrating four bucket technique to digital hologram produces refocused en-face image with showing clear field of view. The proposed technique has advantages such as reduced phase errors and faster acquisition speed when it compared with conventional discrete phase stepping method. The performance of the system is demonstrated with presenting of the images on a scratched mirror surface and of an USAF resolution target. The reconstructed images are compared with conventional microscopic images, which reveal good aggrements. We believe that the proposed method enables tomographic imaging of biological samples with providing reduced noise level and improved imaging speed.