Window Buyers' Guide

The Montana Instruments products typically ship with 50mm outer vacuum windows and 30mm inner radiation shield windows, unless specified by the customer or the customer has a custom housing configuration, such as a castle. The window material is fused silica, and typically anti-reflection (AR) coated for VIS-NIR wavelengths. Please see Window Materials for more details.

It is simple to replace the windows in most cases (see our How to Guide on Removing & Replacing Windows). It is important that the vacuum window is thick enough to support the pressure differential from atmospheric pressure to the vacuum inside the cryostat. Our Window Calculator can help you determine the thickness. 

Most standard windows are easily replaced once the system is purchased with the exception of the low working distance windows, in which the windows are epoxied in place. However, if the user knows prior to ordering what their needs are then it is best to specify from the beginning.

Window Sizes

The standard vacuum windows are 50mm and held in place with a delrin retaining ring against an o-ring. The window size can be slightly bigger than 50mm since this is not a tight fitting window. The thickness should be between 3-5mm; the standard window is 4mm thick. The radiation windows are 30mm and 3mm thick, however this thickness can also vary. Since the radiation shield is in uniform vacuum, the thickness is not as important as the vacuum windows which have atmospheric pressure on one side and about 10^-7 Torr on the other. It is best to be within +/- 1mm of the thickness of the original window. See Dimensions---Windows.pdf for more specific window dimensions. 

Window Manufacturers

Edmund Optics :: Edmund Optics has the widest range of options for optical windows. They have a variety of window materials and AR coatings, and also have thin windows available for low working distance windows. They also have a several window materials suitable for IR optics*.

Thorlabs :: Thorlabs has standard optics as well and a good NIR coating which they call their C range coating (1050-1700nm). Note the thickness of these windows and make sure it’s compatible. Some users integrate Thorlabs aspheric lenses into their systems.

Crystran :: Crystran has a variety of UV, visible, and IR optics in a variety of sizes and thicknesses.

Esco Optics :: Esco Optics has fused silica and quartz windows, also in a variety of sizes. Typically they are uncoated, and can be ordered very thin for low working distance options. They keep a lot of inventory on hand and are budget-friendly.

Tydex Optics :: Tydex OPtics offers a variety of Terahertz windows made of HRFZ-Si, TPX, Quartz, and HDPE**.

Kapton windows** can be used for x-ray experiments, as well as windows from Moxtek.

Buyer Beware

Sapphire can be made incredibly thin, so if you’re trying to achieve the lowest working distance possible this may seem like an enticing option. However, note that sapphire is birefringent, so if your experiment involves imaging, then this is not a great option. Instead, go with fused silica or BK7 to achieve a fairly low working distance setup and image clearly.

*Some IR windows do not perform well in the presence of water; if the Cryostation leaks there could be water and this could damage the windows. Also, some IR window material is a health hazard, please check the MSDS prior to purchasing and be sure to take the proper handling precautions.

**Kapton windows, as well as the Terahertz windows, with the exception of Quartz, can slowly leak. This could lead to a slight amount of condensation on the windows when warming up.

Always be careful of getting windows that are too thin in the vacuum housing. If windows that are too thin are put in place then they could break into the vacuum and cause damage to the system. Please check the Window Thickness Calculator and/or consult with Montana Instruments.