Summary Description

BaF₂ is a low refractive index (1.40 @ 10 microns) material with low solubility in water. Since it has a good refractive index match to water, in liquid cell applications sampling aqueous solutions it shows minimal interference fringing. As result it has become the preferred window material for infrared biological samples. These applications are in the medical area, both research and clinical and the food and beverage area, both QC and process control.

BaF₂ has a similar refractive index to KBr. Therefore, the broad band beamsplitter coatings developed for KBr can be slightly modified to use on BaF₂ substrates. The longer wavelength range of KBr can be sacrificed to optimize near-IR performance.

Advantages

1. Very low water solubility
2. Strong and can be used in high pressure applications
3. Good spectral range
4. Very good index match to water

Disadvantages

1. Very subject to thermal shock
2. Spectral range not as good as the soluble materials
3. More costly than KBr or CaF₂

Physical Data

Melting Point: 1280 °C
Density: 4.89 g/cm³
Solubility in H₂O: 0.17 g/100 g at 20 °C
Hardness: 82 kg/mm²
Appearance: Crystal clear

Mid-IR Transmission – 6 mm thick BaF₂ window

Refractive Index¹

Specific index listed; Generic: 1.45 @ 5 microns

Spectral Range

Short Wavelength Limit: 66,600 cm^-1 (1 mm)
Long Wavelength Limit: 691 cm^-1 (2 mm), 782 cm^-1 (4 mm)

Coatings

Although BaF₂ has low H₂O solubility and is a hard material, both resistance to aqueous solutions and abrasive samples can be improved by applying a hard coat. Sapphire is often used for this purpose. For laser applications narrow band anti-reflective (AR) coatings can also be used.

Typical Uses

• Room temperature IR detector windows
• Liquid cell windows (particularly aqueous solutions)
• Protection windows for hygroscopic optics
• FT-IR beamsplitter substrate

Notes

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

1. I.H. Malitson; Journal of the Optical Society of America, Vol. 52, p.1377, 1962