The proposed mid-infrared passive spectroscopic imager could measure blood glucose levels of internal bodies from a distance. In this report, we distinguished blood vessel areas and interstitial fluid areas in accordance with time-response differences. Freestyle Libre measures blood glucose levels in interstitial fluid. Thus, the measured value has time delay around 10 min compared with the conventional blood collection type. In accordance with correlation coefficients of radiation value, whose wavelength was 9.65μm, we identified these two areas at every pixel.
We developed a one-shot passive Fourier spectrometer to integrate with wearable devices for non-invasive blood glucose measurements. The apparatus was designed with a N.A. of 0.77 for passive spectroscopy of the living body. The field curvature due to the increased N.A. was improved by combining an imaging lens with a phase shifter. Because the apparatus was configured with two lenses, the transmission was high and the optical axis length was 14 mm. In addition, the apparatus was equipped with a multi-slit to prevent loss of interference sharpness. The multi-slit was designed with 5 lines with aperture patterns that enabled the detection of glucose. We measured a blackbody to evaluate the apparatus.
We developed a mid-infrared passive spectroscopic imaging apparatus that uses an uncooled micro-bolometer array sensor as a light-receiving device. This apparatus makes it possible to acquire component information such as that of glucose directly emitted from the skin without a light source. However, it is difficult to obtain the background for spectroscopic measurements inside the human body. In this paper, we propose a background correction method for calculating the spectral characteristics from the acquired spectral emission intensity. The proposed method estimates the emitted light through a fitting calculation using Planck’s law as a basis function.
We aim to measure the glucose concentration in the body through passive mid-infrared spectroscopy using a palm-sized imaging two-dimensional Fourier spectrometer. Radiation in the mid-infrared region (at a wavelength of approximately 10 µm) is emitted from the object surface, with the intensity of the radiated light corresponding to the object temperature. Passive spectroscopy acquires component information from the spectral intensity of the radiated light emitted from the object without a light source. Intrinsic vibrations of molecules in the object itself are detected, and the spectral characteristics are thus the emission spectrum of intrinsic vibration peaks. In contrast, conventional active spectroscopy irradiates the measurement target with light and acquires spectral characteristics from the reflected light. Molecular vibrations excited by the light source are measured, and the spectral characteristics are thus absorption spectra of the energy absorbed at the eigenfrequency of the molecule. The wavelengths that are confirmed as absorption wavelengths in active spectroscopy are confirmed as emission wavelengths in passive spectroscopy. Active spectroscopy and passive spectroscopy thus have a negative–positive relationship. The imaging-type two-dimensional Fourier spectrometer (7 to 14 µm) used in past measurement has transmission optics. Using three Ge lenses for the front lens, objective lens, and imaging lens, we constructed reflective optics using reflective mirrors for the objective and imaging lenses. The reflective mirror guarantees flatness and high spectral reflectance over a wide bandwidth (3 to 20 µm), and the measurement bandwidth is thus extended.
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