Measurement of emission spectra from the forearm by long-wave infrared passive spectroscopic imaging for non-invasive blood glucose sensors

We intend to realize a non-invasive blood glucose sensor system based on long-wave infrared (LWIR) passive spectral imaging. We have developed an imaging-type two-dimensional Fourier transform spectrometer equipped with a microbolometer. Fourier transform spectroscopy is highly sensitive because of its high light-use efficiency. Our spectrometer is currently the only instrument capable of performing passive spectroscopic measurements at body temperature. The LWIR band is called the fingerprint region and is known to be highly suitable for component identification applications. In addition, LWIR radiation is emitted by the human body at room temperature, as can be observed in thermography. In active spectroscopy, light is used to illuminate the sample under test, and the amount of light absorbed as a result of molecular vibrations is measured. Therefore, in water-containing samples such as human skin, only the spectral characteristics near the skin surface can be observed because the absorption of the light by water is very strong. However, passive spectroscopy does not require illumination and thus can measure the amount of emitted light that arises from molecular vibrations. We therefore propose that it is possible to determine a subject’s blood glucose levels by measuring the radiation from the deep skin layer in which the blood vessels are located. In this report, we present passive spectroscopic measurements of a human forearm and report successful detection of the characteristic emission peak of glucose.