The refractive index and absorption coefficients of the samples were obtained using the differences in amplitude and phase signals between the frequency domain signals with and without a sample, as shown in Fig. 2.3 These constants decreased in the order of water, plasma, blood, and RBCs. The absorption coefficients of water, plasma, blood, and RBCs at 1 THz were 237.4, 231.1, 210.8, and , respectively. In other words, the absorbance of plasma, blood, and RBCs was 2.8%, 11.2%, and 21.6% lower than that of water, respectively. The differences could be explained by the different proportions of the water and biomaterials except water. The plasma, with an absorption coefficient close to that of water, was mainly water with only 8% of its composition being made up of various proteins, fats, sugars, inorganic salt, urea, amino acids, and uric acid.13 The RBCs had the smallest absorption coefficients, indicating that the water content of the RBCs was much lower than that of the plasma.