A device for measuring brain oxygen level of a subject, including a probe (210) and a detecting means (220), which are respectively coupled to a processor (230). According to the example, the probe (210) includes three light sources (215a, 215b, 215c) that simultaneously emit the first, second, and third NIR wavelengths across the brain of the subject. The first NIR wavelength is the isosbestic wavelength for oxy-hemoglobin (HbO2) and deoxy-hemoglobin (Hb), the second NIR wavelength is shorter than the first NIR wavelength, and the third NIR wavelength is longer than the first NIR wavelength. The detecting means (220) includes a first, second and third detectors (221, 222, 223) for respectively detecting the NIR intensities of the first, second and third NIR wavelengths traveled across the brain. The processor (230) is configured to determine blood oxygen level based on the measured NIR intensities of the first, second and third NIR wavelengths by use of build-in algorithm derived from Beer-Lambert Law. And method of detecting brain hematoma of a subject by use of the present device.
