We propose an ultrasonic-assisted point-one-shot mid-infrared Fourier spectroscopy device involving an ultrasonic liquidcell and point-one-shot Fourier spectroscopy (diameter: 6 mm, thickness: 5 mm) for ear clip type non-invasive blood glucose sensors, at a low cost (approximately 1,000 EUR). As the prevalence of diabetes increases over time, methods for non-invasive monitoring of blood glucose concentration in daily life are increasingly important for diabetes control and prevention. However, there are currently no sensors for non-invasively detecting biological components in daily environments. In the current study, we propose a point-one-shot Fourier spectroscopy method comprising one objective lens and a camera. The objective lens is a spherical lens at the front side and a dual-axis wedge prism inclined along the horizontal and vertical axes. An objective beam is collimated by the objective lens, with a difference in optical path length caused by the wedge prism. This device can be used to obtain two-dimensional spatial fringe patterns. Conventional oneshot Fourier spectroscopy produces one-dimensional fringe patterns in the horizontal direction of the array device. Thus, the maximum optical path length is limited by the number of pixels in only the horizontal line of the array device. However, our proposed point-one-shot Fourier spectroscopy device can produce an interferogram with longer maximum optical path lengths compared with conventional devices because horizontal lines at difference rows are connected by the same phase pixel. Thus, spectroscopic imaging devices can be produced at a low cost because low-resolution cameras (e.g., 80 × 80 pixels) can be used for spectroscopy.
A mid-infrared spectroscopic imager needs to be portable and tough for the identification of dyestuffs used for murals in ancient tombs during archaeological on-site analysis. Meanwhile, an extremely compact and tough hyperspectral camera with mass less than 2 kg is required for mounting on drones to observe nutritive components like nitrogen and phosphorus. We proposed a near-common-path wavefront-division phase-shift interferometer as an imaging-type twodimensional Fourier spectrometer. Because the proposed interferometer has strong robustness against mechanical vibrations, a palm-sized Fourier spectroscopic imager can be realized without an anti-mechanical vibration system. We developed a palm-sized (80-mm cube weighing 0.5 kg) and tough hyperspectral camera for mid-infrared light (wavelength of 8–14 μm) that can be operated using only a notebook personal computer. Furthermore, the field of view of a conventional hyperspectral camera is narrow (e.g., 6.4 deg × 5.1 deg). However, employing a proposed field angle correlation method and using a fisheye lens as the objective lens, the field of view can be expanded to 180 deg. The total price of the mid-infrared two-dimensional spectroscopic imager is no more than several thousand USD because a lowprice microbolometer (Vision Sensing, VIM-80G2, wavelength range: 8-14 μm, 80 × 80 pixels, price: 300 USD) is used. Additionally, a long-stroke (10 mm) and high-resolution (Optical encoder resolution: 100 nm) impact-drive actuator (Technohands XCWT70-10 weighing 30 g) is introduced as a low-price (1000 USD) and tough phase-shift stage with cross-roller linear-motion guides.
High-frequency spectroscopic observation methods using small satellites and drones for monitoring of plankton in the ocean and vegetation activity have recently attracted considerable attention. However, in multi-directional spectroscopic imaging, the spectroscopic characteristics vary depending on the observation and illumination angles. Therefore, huge quantities of spectroscopic data were previously required for every conceivable combination of zenith and azimuth angles to identify plant species. The method proposed here can identify any plant species from near-surface and internal reflectance spectroscopic data, regardless of the zenith and azimuth angles. We assume that the observed spectral intensity can be calculated as a linear sum of the near-surface spectral reflectivity and the internal diffusion spectral reflectivity multiplied by the light-source spectral intensity and the reflection correction coefficients a and b. We acquire the near-surface and internal reflected light as basic spectroscopic data using the orthogonal polarized light illumination method. The coefficients a and b can be calculated from basic spectroscopic data. We obtain m-sets (ai, bi) (i =1-m) using combinations of the numbers of λ1…λn. If the reflection correction coefficient of the m-sets (ai, bi) is close to one, we identify the observed plant as a plant species contained in the basic data. If the two species are different, the m-sets (ai, bi) have uncorrelated values and the m-sets (ai, bi) reflection correction coefficient decreases towards zero. In this work, we performed feasibility demonstrations using two types of plant and successfully determined from the basic data that the observed plant is the correct plant species.
When monitoring a moist sample using mid-infrared spectroscopy, its thickness must be <100 μm to avoid light absorption from the water. Therefore, we propose an ultrasonic-assisted mid-infrared spectroscopic imaging method that can generate a reflection plane at a depth of 100 μm from the surface of the sample by creating an ultrasonic standing wave. A frequency of 10 MHz is required to obtain an optical path length of 100 μm in biological samples. However, because biological samples generally have high compressibility, attenuation of ultrasonic waves at this frequency is significant. We use agar as a biological phantom and observe that a reflection plane is generated inside by ultrasonic standing waves using optical coherence tomography. It is found that when the sample is vibrated with an 800-kHz ultrasonic wave, a reflection plane is generated at a depth shallower than the theoretically predicted value. We believe that the reflection plane is generated by parametric standing waves, which are based on parametric effect. We detect the waveform distortion using an acoustic emission sensor and confirm the higher harmonics that generate the observed reflection plane using a fast Fourier transform.
For monitoring and prevention of lifestyle diseases, we have investigated development of non-invasive ear-clip type blood glucose sensors that can be used by individuals in daily life. Because mid-infrared light (λ = 8–14 μm) is absorbed strongly by water, it is difficult to detect transmitted light from human bodies. We propose that an ultrasonic-assisted method, which can actively produce a reflection plane at a blood vessel (depth of around 100 μm), could be used to detect internal reflected light from biological tissues. An ultrasonic standing wave produced near the skin surface by an ultrasonic vibrator can form refractive index boundaries between areas of high and low density. Additionally, we can use a low ultrasonic frequency (1 MHz) that does not suffer from heavy damping by parametric standing waves, which appear in non-rigid samples and produce reflection planes at the same depth when using a high frequency (10 MHz). At the refractive index boundary nearest the skin surface, the light from inside biological tissues is reflected and can be used to obtain information of biological components. The optical path length can be set by changing the measurement depth by altering the ultrasonication frequency. We measured internal reflected light from the ears of mice (blood glucose level = 120 mg/dL) non-invasively using our unique mid-infrared spectroscopic imager and obtained absorption peaks for blood glucose (λ = 9.3 μm and 9.7 μm). Mid-infrared spectroscopy can be applied to measurements in samples with high water contents.
We propose the point one-shot mid-infrared Fourier spectroscopic imager, which is composed of only a single Ge lens (diameter: 6 mm; thickness: 5 mm) and a two-dimensional array device. The lens is a nonspherical lens on the front side and a dual-axis inclined wedge prism at the rear side. The objective beams, which have different optical path lengths because of the effects of the prism, are imaged using the array device and we obtain a two-dimensional spatial fringe pattern. We can improve the wavelength resolution analytically by connecting the same optical path difference (OPD) pixels of the horizontal lines at different rows, even though we use low-resolution cameras.
Smart toilets could be used to monitor different components of urine in daily life for early detection of lifestyle-related diseases and prompt provision of treatment. For analysis of biological samples such as urine by midinfrared spectroscopy, thin-film samples like liquid cells are needed because of the strong absorption of midinfrared light by water. Conventional liquid cells or fixed cells are prepared based on the liquid membrane method and solution technique, but these are not quantitative and are difficult to set up and clean. We generated an ultrasonic standing wave reflection plane in a sample and produced an ultrasonic liquid cell. In this cell, the thickness of the optical path length was adjustable, as in the conventional method. The reflection plane could be generated at an arbitrary depth and internal reflected light could be detected by changing the frequency of the ultrasonic wave. We could generate refractive index boundaries using the density difference created by the ultrasonic standing wave. Creation of the reflection plane in the sample was confirmed by optical coherence tomography. Using the proposed method and midinfrared spectroscopy, we discriminated between normal urine samples spiked with glucose at different concentrations and obtained a high correlation coefficient.
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