The objective of this study is to visualize tooth hardness by mid-infrared passive spectroscopic imaging with an imaging type 2D Fourier spectrometer. Radiated light in the mid-infrared region (wavelength of approximately 10 μm) is radiated from the surface of an object with a radiation intensity that depends on the temperature of the object. Our proposed device has a multi-slit grating that prevents the cancellation of bright spots. The multi-slit grating enables an inexpensive uncooled microbolometer array sensor to acquire two-dimensional spectral information from the radiated light of the measured target without external irradiation. Because the intrinsic vibration of the molecule itself is detected in this case, the spectral characteristics are the radiation spectra due to the intrinsic vibration peak. Wavelengths that can be confirmed as absorption in active spectroscopy can be confirmed as radiation in passive spectroscopy, resulting in a negative-positive relationship. First, we measured bovine teeth with our spectrometer. A radiation peak assigned to hydroxyapatite, the main component of teeth, was detected. Next, to obtain the relationship between the tooth hardness and spectrum, the micro-Vickers hardness of the tooth surface was measured. By peak deconvolution, we found a negative correlation between the hardness and peak area ratio of the crystalline and amorphous hydroxyapatite phases. The results of this study will be used in the future for non-invasive and simple risk prediction of caries formation.
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.
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.
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