PROBLEM TO BE SOLVED: To calculate the amount of a low-absorbance substance in a body, using the principle of photometry.SOLUTION: The extinction of a first light beam is acquired based on a first intensity signal corresponding to the intensity of the first light beam, the first light beam containing a first wavelength λtransmitted through or reflected from a tissue of a living body. The extinction of a second light beam is acquired based on a second intensity signal corresponding to the intensity of the second light beam, the second light beam containing a second wavelength λtransmitted through or reflected from the tissue. The amount of water in the tissue is calculated based on the ratio between the first extinction and the second extinction. The ratio between the absorbance of water in the first wavelength λand the absorbance of water in the second wavelength λis deviated from 1. The absorbance of oxygenated hemoglobin in the first wavelength λis higher than the absorbance of water in the first wavelength λ. The absorbance of oxygenated hemoglobin in the second wavelength λis higher than the absorbance of water in the second wavelength λ. The ratio between the absorbance of oxygenated hemoglobin in the first wavelength λand the absorbance of oxygenated hemoglobin in the second wavelength λcan be approximated by 1.SELECTED DRAWING: Figure 3【課題】フォトメトリの原理を用いつつ、体内低吸光物質の量を算出する。【解決手段】生体の組織を透過あるいは反射した第一波長λ１を含む第一の光の強度に対応する第一強度信号に基づいて、当該第一の光の減光度が取得される。当該組織を透過あるいは反射した第二波長λ２を含む第二の光の強度に対応する第二強度信号に基づいて、当該第二の光の減光度が取得される。当該第一減光度と当該第二減光度の比に基づいて、当該組織における水の量が算出される。第一波長λ１における水の吸光度と第二波長λ２における水の吸光度の比率は１から乖離している。第一波長λ１における酸化ヘモグロビンの吸光度は、第一波長λ１における水の吸光度よりも高い。第二波長λ２における酸化ヘモグロビンの吸光度は、第二波長λ２における水の吸光度よりも高い。第一波長λ１における酸化ヘモグロビンの吸光度と第二波長λ２における酸化ヘモグロビンの吸光度の比率は１で近似できる。【選択図】図３
