A method for calibration of an optical system used for measuring optical properties of a portion of a body of a subject, the method comprising: during a calibration stage, deriving information regarding characteristics of the optical system , with reference to which the measurements that are performed upon the portion of the subject's body may be calibrated, by: illuminating a front surface of a calibration object having known reflectance characteristics with light from a light source of the optical system , the front surface being at least partially curved; detecting light reflected from a plurality of points on the calibration object , using at least one detector of the optical system ; measuring intensities of the light reflected from the plurality of points on the calibration object ; and determining a calibration value at each location upon the calibration object by calculating an integral over all angles with the known reflectance of the calibration object to determine the absolute reflection coefficient; and during a measurement stage: illuminating the portion of the subject's body with light from the light source ; detecting light reflected from the portion of the subject's body, using the at least one detector; and calibrating measurements performed upon the light reflected from the portion of the subject's body, using the measured intensities of the light reflected from the plurality of points on the calibration object . during a calibration stage, deriving information regarding characteristics of the optical system , with reference to which the measurements that are performed upon the portion of the subject's body may be calibrated, by: illuminating a front surface of a calibration object having known reflectance characteristics with light from a light source of the optical system , the front surface being at least partially curved; detecting light reflected from a plurality of points on the calibration object , using at least one detector of the optical system ; measuring intensities of the light reflected from the plurality of points on the calibration object ; and illuminating a front surface of a calibration object having known reflectance characteristics with light from a light source of the optical system , the front surface being at least partially curved; detecting light reflected from a plurality of points on the calibration object , using at least one detector of the optical system ; measuring intensities of the light reflected from the plurality of points on the calibration object ; and determining a calibration value at each location upon the calibration object by calculating an integral over all angles with the known reflectance of the calibration object to determine the absolute reflection coefficient; and during a measurement stage: illuminating the portion of the subject's body with light from the light source ; detecting light reflected from the portion of the subject's body, using the at least one detector; and calibrating measurements performed upon the light reflected from the portion of the subject's body, using the measured intensities of the light reflected from the plurality of points on the calibration object . illuminating the portion of the subject's body with light from the light source ; detecting light reflected from the portion of the subject's body, using the at least one detector; and calibrating measurements performed upon the light reflected from the portion of the subject's body, using the measured intensities of the light reflected from the plurality of points on the calibration object . <EOS>