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BaF2 Barium Fluoride Technical Data

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

baf2

Refractive Index1

WAVELENGTH (Microns) INDEX
0.2652 1.51217
0.28035 1.50668
0.28936 1.5039
0.296728 1.50186
0.30215 1.50044
0.313 1.49782
0.32546 1.49521
0.334448 1.49363
0.340365 1.49257
0.34662 1.49158
0.361051 1.48939
0.366328 1.48869
0.404656 1.48438
0.589262 1.47443
0.643847 1.47302
WAVELENGTH (Microns) INDEX
0.89435 1.46942
1.12866 1.46779
1.36728 1.46673
1.52952 1.46613
1.681 1.46561
1.97009 1.46472
2.1526 1.46440
2.5766 1.46262
3.422 1.45940
5.138 1.45012
5.549 1.44732
6.8559 1.43694
7.268 1.43314
9.724 1.40514
10.346 1.39636

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

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