Editing Sodium chloride (section)

===Infrared optics===
Sodium chloride crystals have a [[transmittance]] of at least 90% (through 1&nbsp;mm) for [[infrared]] light having wavelengths in the range 0.2– 18&nbsp;[[μm]].<ref>{{cite book |last1=Waynant |first1=Ronald W. |last2=Ediger |first2=Marwood N. |title=Electro-optics Handbook |chapter=Chapter 11: Optical Materials: Visible and Infrared |publisher=McGraw-Hill Professional Publishing |publication-place=New York |date=2000 |isbn=0-07-068716-1 |url=https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=1d1e91eee8d9531a5744405441940c50d7a69dde |page=11.20 |access-date=1 May 2024 |archive-date=1 May 2024 |archive-url=https://web.archive.org/web/20240501190105/https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=1d1e91eee8d9531a5744405441940c50d7a69dde |url-status=live }}</ref> They were used in optical components such as windows and lenses, where few non-absorbing alternatives existed in that spectral range. While inexpensive, NaCl crystals are soft and [[hygroscopy|hygroscopic]] – when exposed to the water in ambient air, they gradually cover with "frost". This limits application of NaCl to dry environments, vacuum-sealed areas, or short-term uses such as prototyping. Materials that are mechanically stronger and less sensitive to moisture, such as [[zinc selenide]] and [[chalcogenide glass]]es, more widely used than NaCl.