Optical constants n and k, i.e., the real and imaginary part of the refractive index, of pure amorphous ice and crystalline ice depending on the temperature. Ice (a) and ice (c) indicate amorphous ice (solid line) and crystalline ice (dashed line), respectively. Optical constants n of ice (c) and k of ice (c) in 0.1 to 62 μm show the same regardless of the temperature. However, k of ice (c) at ~62 to 1000 μm shows differences that sensitively depend on the temperature. A more detailed description can be found in Sect. 3.