Window Materials

Both the outer vacuum housing (warm) and inner radiation shield (cold) windows may be easily replaced by the user within minutes. A variety of optical materials are available for different wavelengths and applications. The standard option is a VIS-NIR (400-1000nm) with AR coating.

The size of the windows is determined by the outer housing configuration. See Sample Chamber Housing & Lid Options for available window sizes. For a summary of window quantities and sizes for each housing, see Dimensions---Windows.pdf.

Check out our How to Guide on Removing and Replacing Windows and our Window Buyers Guide for more information on changing out your windows.

Window Substrate Wavelength Application Notes
400 - 1000nm
Standard windows
200 - 2000nm
350nm - 2um
300nm - 4um
200nm - 8um
250nm - 26um
Germanium (Ge)
3 - 12um
1.2 - 7µm
Sodium Chloride (NaCl)
300nm - 10um
Water soluble!
Zinc Selenide (ZnSe) uncoated
0.7 - 20um
Only 70% transmission
3 - 12um
Only 95% transmission
80um - 2mm
80% transmission
100um - 1mm
55% transmission
100um - 1mm
minimal birefringence
Spectrosil 2000 190 - 2000nm deep UV, fluorescence
BaF2 3 - 5um ultraviolet transmission
  for X-Ray work
Other   user specified

Not all windows are available in all sizes. Typical sizes required are 50mm, 30mm, and 20mm. The low working distance option has a very thin window and can be coated fused silica, BK7, or sapphire. 

Fused Silica

Made from UV-grade synthetic fused silica. Applications include laser set-ups (i.e. at Brewster’s angle), emitter/detector protection devices (such as in spectrophotometers), and imaging systems involving ultraviolet wavelengths. These windows are typically used for both the inner and outer windows of the sample chamber, unless otherwise specified. Figure 4 below is a transmission curve for the uncoated substrate. This standard substrate is available with various anti-reflection coatings. These coatings reduce the surface losses at these wavelengths. The curves in Figure 5 indicate the surface losses. You can estimate transmission by doubling these curves and subtracting from 100%. Outside these bands, the transmission of a coated surface is not predictable.

Transmission curve for uncoated substrate

Surface Loss Curves
Standard VIS-NIR Coating Optical UV Coating Optical UV-VIS Coating


These windows are well suited for low power laser applications. They have low distortion and low dispersion. These are available at 0.2mm thick and can be used for low working distance applications.


These windows are made from single crystal sapphire, with extreme surface hardness, and high thermal conductivity. Sapphire is the second hardest crystal next to diamonds and, because of their structural strength, sapphire windows can be made much thinner than other common dielectric windows with improved transmittance in a transmission range from 0.15 - 5.5u.

Calcium Fluoride

Calcium fluoride is often used in spectroscopic windows and lenses due to its high transmission from 250nm to 7μm. Its low absorption and high damage threshold makes it a popular choice for excimer laser optics. Calcium fluoride’s low index of refraction allows it to be used without an anti reflective coating.

Potassium Bromide (KBr)

Potassium Bromide (KBr) is commonly used in FTIR spectroscopy. KBr has a slightly larger spectral range than Sodium Chloride (NaCl) and has excellent transmission from 250nm – 26um. KBr is water soluble and its surfaces should be protected from exposure to moisture.

Silicon (Si)

Silicon windows are popular for the 1.2-7m spectral region.

Zinc Selenide (ZnSe)

Zinc Selenide windows are ideal for a wide variety of infrared applications including thermal imaging, FLIR, and medical systems. This chemically vapor deposited material has wide usage in high power CO2 laser systems because of its low absorption coefficient and high resistance to thermal shock. Zinc selenide (ZnSe) is a relatively soft material that scratches easily. When handling, wear Latex gloves to prevent contamination. Special care should be taken when handling Zinc Selenide as it is a toxic material.


Hi Resistivity Float Zone Silicon for Terahertz applications.


TPX is optically transparent in UV, visible, and THz ranges, allowing using a HeNe laser beam for alignment. TPX windows are used in cryostats as “cold” windows as the THz transparency does not change in dependence on temperature. Note: TPX windows are not completely vacuum tight, resulting in a temperature increase of approximately 1K. 

Z-Cut Crystal Quartz

Z-cut crystal quartz is transparent in the visible range allowing easy adjustment with HeNe laser and does not change the state of light polarization.

Aluminized Kapton

These windows are suitable for x-ray work. They consist of a 0.005” thick Kapton layer, aluminized on each side with 300 Angstroms of aluminum. This window is made custom by Montana Instruments in 30mm and 50mm sizes.