Summary Description

KRS-5 is a medium refractive index material (2.37 @ 10 microns) with excellent long wavelength transmission in the infrared spectral region. It is non-hygroscopic with very good chemical resistance, but KRS-5 will occasionally exhibit strange and sometimes even bizarre cold flow behavior. Therefore it is not possible to polish flat. Pressure windows will “balloon” very slowly over time. The primary use is as an acid resistant window or ATR element or as a protective window for hygroscopic optical elements. It is extremely toxic and is absorbed through the skin and should only be handled with glove protection.

Advantages

• Very low water solubility
• Good chemical resistance except against very strong acids, for example, HNO₃
• Excellent spectral range
• Fairly good refractive index match with diamond

Disadvantages

• High toxicity (through skin); Personnel handling KRS-5 (even using protective gloves) should be routinely tested for Thallium
• Unstable
• Higher cost

Physical Data

Melting point: 414.5 °C
Density: 5.65 g/cm³
Solubility in H₂O: 0.05 g/100 g at 20 °C
Hardness: 40 kg/mm²
Appearance: Red clear

Far-IR Transmission – 1 mm thick KRS-5 window

Far-IR Transmission – 4 mm thick KRS-5 window

Refractive Index¹

Specific index listed; Generic: 2.37 @ 10 microns

Typical Uses

• IR detector windows
• Liquid cell windows (particularly aqueous solutions)
• Protection windows for hygroscopic optics
• ATR elements

In the last few years several successful sampling devices have been manufactured by diamond turning KRS-5 and contacting it with diamond to construct a device with extreme durability and wavelength coverage.

Spectral Range

Short Wavelength Range: 17,900 cm^-1 (1 mm)
Long Wavelength Range: 204 cm^-1 (1 mm), 232 cm^-1 (4 mm)

Coatings

KRS-5 can be coated with a BBAR (Broad Band Anti- reflection) coating. However, since KRS-5 will cold flow, this coating is much thinner than those on more stable substrate materials and therefore not as effective. Also, it will not improve transmission at wavelengths longer than 16 microns. KRS-5 may also be hard coated when used for wavelengths less than about 18 microns.

Notes

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

1. Tilton, Pyler & Stephens; Journal of Research of the National Bureau of Standards, Vol. 43, p.81–86, 1949