Summary Description

CaF₂ is a low index (1.40 @ 4 microns) material which is used in a wide variety of commercial applications. In spectroscopy it is a substitute for SiO₂ materials, fused quartz, fused silica, BK-7, Suprasil®, Infrasil®, etc. In near-IR applications it is superior to these materials because it has no OH absorption to limit the accuracy of quantitative results. Since it is easy to polish to tight specifications it is the material of choice for near-IR beamsplitters and windows.

Advantages

1. No infrared impurity absorption bands
2. Near zero solubility in water
3. No OH absorption
4. Can be polished to extremely tight specifications
5. Low index of refraction
6. Relatively low cost as compared to SiO₂

Disadvantages

1. Limited spectral coverage in the mid-IR
2. Slightly sensitive to thermal shock

Physical Data

Melting Point: 1360 °C
Density: 3.18 g/cm³
Solubility in H₂O: 0.05 g/100 g at 20 °C
Hardness: 158.3 kg/mm²
Appearance: Crystalline clear

Mid-IR Transmission – 3 mm thick CaF₂ window

Refractive Index¹

Specific index listed; Generic: 1.40 @ 5 microns

Spectral Range

Short Wavelength Limit: 79,500 cm_-1 (1 mm)
Long Wavelength Limit: 896 cm_-1 (2 mm), 1025 cm_-1 (4 mm)

Coatings

Broad band (visible to 8 microns) beamsplitters for FT near-IR spectrometers are in routine production. Narrow band anti-reflective (AR) coatings for laser applications can also be done. CaF₂ is often used as a substrate for interference filters.

Typical Uses

• High resolution large size UV Optics
• High resolution visible optics, particularly video cameras
• Near-IR optics and beamsplitter substrates for spectroscopy
• Mid-IR sampling optics usually aqueous samples

Notes

Short and Long Wavelength Ranges defined for which transmissivity is greater than 50% of stated crystal thickness.

1. W. Kaiser, W.G. Spitzer, R.H. Kaiser & L.E. Howarth; Physical Review. Vol. 127, p. 1950, 1962