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The coordinate system when the focal plane and the imaging plane are separated. L: lens, CCD plane: the plane set on the CCD, correction plane: the plane on which the object at the imaging plane is focused, Z: the separation between two planes.
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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 me...
Citations
... TD-FF-OCT can yield a complex signal that comprises not only the amplitude but also the phase information of the sample that is stable across the whole field of view. This capability has previously been leveraged to computationally correct for blur in TD-FF-OCT images that arises due to mismatches in refractive indices of an immersion medium and a sample, leading to the separation of coherence and confocal gates [8]. It has been demonstrated that such blur, observed when imaging a USAF resolution target through glass, can be effectively corrected in this manner. ...
Time-domain full-field optical coherence tomography (TD-FF-OCT) is an interferometric technique capable of acquiring high-resolution images deep within the biomedical tissue, utilizing a spatially and temporally incoherent light source. However, optical aberrations, such as sample defocus, can degrade the image quality, thereby limiting the achievable imaging depth. Here we demonstrate that the sample defocus within a highly scattering medium can be digitally corrected over a wide defocus range if the optical path lengths in the sample and reference arms are matched. We showcase the application of digital defocus correction on both reflective and scattering samples, effectively compensating digitally for up to 1 mm of defocus.
... Recently defocusing correction methods [18,19] were proposed for the RI measurement, which used the image defocus phenomenon happening in a FF-OCT system. In general, the focal plane (FP) of an internal en-face image of FF-OCT is separated from the image plane (IP) of the system due to the RI mismatch of the sample from its surrounding medium [18][19][20][21][22]. By mechanically adjusting the path length of the reference and the sample arms, the image can be focused; and from which the RI of the sample can be extracted. ...
... In this paper, we propose the numerical correction method that can give the axial RI profile of a multilayered sample. Similar to our previous work [22], the proposing method utilizes the typical digital refocusing technique based on the Fresnel diffraction theory, in which the image sensor location is numerically moved so as to be placed at the conjugate focal plane of the sample while checking its focusing status with the amplitude analysis method (AMP) [22,23]. Expanding this method, we are able to simultaneously measure the RI and the thickness of a specific layer within a multilayered sample without using additional equipment or extra scanning. ...
... In this paper, we propose the numerical correction method that can give the axial RI profile of a multilayered sample. Similar to our previous work [22], the proposing method utilizes the typical digital refocusing technique based on the Fresnel diffraction theory, in which the image sensor location is numerically moved so as to be placed at the conjugate focal plane of the sample while checking its focusing status with the amplitude analysis method (AMP) [22,23]. Expanding this method, we are able to simultaneously measure the RI and the thickness of a specific layer within a multilayered sample without using additional equipment or extra scanning. ...
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.
... Scanning White Light Interferometry (SWLI) is commonly used for non destructive testing of microelectronics, as it allows fast imaging of relatively large areas with high vertical precision [1]. SWLI can also be used for sub surface imaging [2][3][4][5]. This would allow non destructive inspection of parts hidden under the top layer which is impossible using most nm resolution measurement methods. ...
... Normally when measuring in air the interference fringes appear when both in the sample and the reference mirror are in focus of the objective. When imaging through thick layers with refractive index higher than 1, the focal length of the sample arm of the interferometer objective is increased [3,5] (Fig.1, equation 1). The shift of focal length ∆h when measuring through layer with thickness and refractive index n using an objective with numerical aperture α can be expressed as ...
Microfluidic devices allow experimentation in smaller space using small
amounts of liquid, resulting in improved reaction rates, cheaper
equipment, reduced amount of expensive reagents. Very precise channel
shape measurements are needed to assure the designed flow pattern.
Several 3D imaging devices provide the necessary precision but typically
they cannot image inside closed devices. Hence it is difficult to
measure the shape of a microfluidic channel without destroying it. We
fabricated and investigated samples with different microchannels.
Several types of microfluidic channels were prepared in silicon wafer
with a subsequent covering by bonding glass wafer on top. Microchannels
in polymer have been done using epoxy-type photoresist SU-8. The
internal geometry of the channels was measured using a Scanning White
Light Interferometer (SWLI) equipped with optics that compensates for
the effects of the top glass of the channels. The geometry of the
interior of the channels can be measured with a precision similar to
surface layer SWLI measurements without destroying the channels.
... Recently, Min et al. has proposed a numerical correction method to rejuvenate the degraded OCT images. 41 The method uses the phase-shifting digital holographic technique 42 based on the Fresnel-Kirchhoff diffraction theory, which numerically relocates the defocused sample at the virtual focal plane. Ideally, a fully focused OCT image can be constructed regardless of the degree of optical distortion along the depth of the sample. ...
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.
The numerical refocusing feature of digital holography enables the reconstruction of a well-focused image from a digital hologram captured at an arbitrary out-of-focus plane without the supervision of end users. However, in general, the autofocusing process for getting a highly focused image requires a considerable computational cost. In this study, to reconstruct a better-focused image, we propose the zero padding technique implemented in the frequency domain. Zero padding in the frequency domain enhances the visibility or numerical resolution of the image, which allows one to measure the degree of focus with more accuracy. A coarse-to-fine search algorithm is used to reduce the computing load, and a graphics processing unit (GPU) is employed to accelerate the process. The performance of the proposed scheme is evaluated with simulation and experiment, and the possibility of obtaining a well-refocused image with an enhanced accuracy and speed are presented.
This paper presents an analysis of the influence of illumination with wide temporal spectrum on the properties of a defocused interference signal and numerically focused imaging in interference microscopy. It is shown that the differences in defocus influence on different spectral components of a signal with wide temporal spectrum may lead to degradation of the images of defocused sample parts, in spite of the use of numerical focusing. The magnitude of these effects depends on the temporal spectrum width, the numerical aperture of the imaging system and the amount of defocus. The influence of these effects on the properties of numerically focused imaging in Fourier domain optical coherence tomography/microscopy is considered. © 2016, Institution of Russian Academy of Sciences. All rights reserved.
We present geometric optics-based refocusing applied to a novel off-axis line-field parallel swept source imaging (LPSI) system. LPSI is an imaging modality based on line-field swept source optical coherence tomography, which permits 3-D imaging at acquisition speeds of up to 1 MHz. The digital refocusing algorithm applies a defocus-correcting phase term to the Fourier representation of complex-valued interferometric image data, which is based on the geometrical optics information of the LPSI system. We introduce the off-axis LPSI system configuration, the digital refocusing algorithm and demonstrate the effectiveness of our method for refocusing volumetric images of technical and biological samples. An increase of effective in-focus depth range from 255 μm to 4.7 mm is achieved. The recovery of the full in-focus depth range might be especially valuable for future high-speed and high-resolution diagnostic applications of LPSI in ophthalmology.
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.
