Sodium Chloride (salt) has many uses - medical, health, manufacturing, food, roads etc.
One lesser known use, is in the manufacture of optical windows for spectroscopic applications including infrared. These are made from a high purity crystal.
Another application is for thermal storage linked to renewable energy capture - Solar Salt.
Transmission properties of NaCl window:
We have no known application reference (shared by our channel partners or from direct sales) at this time (June 2024).
Considering existing internet research articles, some assumptions may be made:
1. Much like "glass" , emissivity at higher (incandescent) temperatures should increase due to an increase in opacity.
2. With a melting point of 810°, these windows are practically useable up to 400° - we can assume that this is the temperature when transmission drops and opacity increases sharply.
3. Internet research of test papers show that molten salts display emissivity variance at various temperatures and wavelengths.
1.3um
https://www.osti.gov/servlets/purl/1984700
550° E=0.9
850°C E= 0.77
Solar Salt
https://www.sciencedirect.com/science/article/abs/pii/S1359431118351123
Extract from Solar Salt research paper:
https://www.sciencedirect.com/science/article/abs/pii/S1359431118351123
Molten salt is widely used in many fields as an important working fluid at high temperature as the increase in the use of new energy sources. In addition to studies on the thermal properties, research on the radiative properties of molten salts is mainly in the wavelength range of 0.3–2.5 μm, but most of emission energy falls in range of 2.5–25 μm at 600 °C.
"...at the operating temperature of some molten salts (usually above 600 °C), nearly 90% of the emission energy falls within the wavelength range of 2–13 μm."
There is a lack of data for temperatures higher that 850°C.
There is a lack of data for wavelengths between 1.3 and 5um.
There is also indication that emissivity varies.
If we take the example of emissivity reduction with temperature increase from the Pacific North West study, we may infer that emissivity will continue to reduce at high temperatures:
Based on the available data, our recommendation would look towards an E3MH (where we need to view through a short wave window).
Based on the emissivity variation at shorter wavelengths and higher temperatures - E1RL. Perhaps this shorter wave device offers a sub-surface measurement.
Recommended sensor - E3MH
Higher temperatures are worth evaluating with E1RL or E2RL.