portion of the experiment the results show that the method of the present embodiment correctly reflects the effect of the infusion by showing an increase in total tissue water fraction from approximately 75 to 79 while the infusion is continuing these data suggest that the disclosed embodiment has a clear value as a monitor of rehydration therapy in a critical care setting in another embodiment of the present invention the water fraction f w is estimated according to equation 2 below based on the measurement of reflectances r u03bb at three wavelengths and the empirically chosen calibration constants c 0 c 1 and c 2 f w c 2 log r u03bb 1 r u03bb 2 c 1 log r u03bb 2 r u03bb 3 c 0 2 better absolute accuracy can be attained using equation 2 which incorporates reflectance measurements at an additional wavelength the results of in vitro experiments on excised skin indicate that the wavelength triple u03bb 1 1190 nm u03bb 2 1170 nm u03bb 3 1274 nm yields accurate estimates of total tissue water content based on equation 2 in yet another embodiment of the present invention the water fraction f w is estimated according to equation 3 below based on the measurement of reflectances r u03bb at three wavelengths and the empirically chosen calibration constants c 0 and c 1 better absolute accuracy can be attained using equations 3 as is attained using equations 2 which also incorporates reflectance measurements at an additional wavelength numerical simulations as shown in fig2 indicate that total tissue water accuracy better than u2212 0 5 can be achieved using equation 3 with reflectances measured at three.