 They were therefore used in optical components (windows and prisms) operating in that spectral range, where few non-absorbing alternatives exist and where requirements for absence of microscopic inhomogeneities are less strict than in the visible range.While inexpensive, NaCl crystals are soft and hygroscopic – when exposed to the ambient air, they gradually cover with "frost".This limits application of NaCl to dry environments, vacuum sealed assembly areas or for short-term uses such as prototyping.