An irradiated turbid medium scatters the light in accordance to its optical properties. Near-infrared (NIR) clinical methods, which are based on spectral-dependent absorption, suffer from an inherent error due to spectral-dependent scattering. We present here a unique spatial point, that is, iso-pathlength (IPL) point, on the surface of a tissue at which the intensity of re-emitted light remains constant. This scattering-indifferent point depends solely on the medium geometry. On the basis of this natural phenomenon, we suggest a novel optical method for self-calibrated clinical measurements. We found that the IPL point exists in both cylindrical and semi-infinite tissue geometries (Supporting Information, Video file). Finally, in vivo human finger and mice measurements are used to validate the crossing point between the intensity profiles of two wavelengths. Hence, measurements at the IPL point yield an accurate absorption assessment while eliminating the scattering dependence. This finding can be useful for oxygen saturation determination, NIR spectroscopy, photoplethysmography measurements, and a wide range of optical sensing methods for physiological aims.

中文翻译：

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近红外临床测量的自校准现象：理论，模拟和实验

辐射的混浊介质根据其光学特性将光散射。基于光谱依赖性吸收的近红外（NIR）临床方法由于光谱依赖性散射而遭受固有误差。在这里，我们在组织的表面上呈现一个唯一的空间点，即等光程（IPL）点，在该点处重新发射的光的强度保持恒定。该无散射点仅取决于介质的几何形状。基于这种自然现象，我们建议一种用于自校准临床测量的新型光学方法。我们发现IPL点同时存在于圆柱和半无限组织几何形状中（支持信息，视频文件）。最后，体内人手指和小鼠的测量值用于验证两个波长的强度曲线之间的交叉点。因此，在IPL点进行测量可得到准确的吸收评估，同时消除了散射依赖性。这一发现对于确定血氧饱和度，近红外光谱，光电容积描记术以及各种用于生理目的的光学传感方法都是有用的。