CaF2 vs BaF2 Optical Windows

Representative bulk-material ranges only — actual transmission depends on grade, thickness, wavelength, surface finish, temperature and coating.

1. Transmission Range

Both CaF₂ and BaF₂ cover broad portions of the ultraviolet, visible and infrared spectrum.

Representative bulk-material data commonly places CaF₂ at approximately 0.13–10 µm and BaF₂ at approximately 0.2–11 µm. These limits should not be interpreted as uniform or guaranteed finished-window transmission.

CaF₂ is often the preferred starting point for UV applications because specially selected high-purity grades are available for UV, VUV and excimer-laser systems.

BaF₂ provides an advantage toward longer infrared wavelengths. It may be selected when CaF₂ transmission decreases before the required upper wavelength.

Useful transmission can be narrower because of:

• Material purity
• UV, IR, Raman or scintillator grade
• Window thickness
• Surface reflection
• Internal absorption
• Operating temperature
• Coating range
• Required minimum transmission

The material should be evaluated at the actual wavelength and window thickness rather than only by its nominal endpoints.

2. Material Grade

Material grade is especially important for fluoride crystals.

CaF₂ produced for general infrared applications may contain impurities that restrict UV or VUV transmission. UV, excimer and Raman systems normally require specially selected material.

The purchase specification should identify whether the application requires:

• General IR grade
• UV grade
• VUV grade
• Excimer-laser grade
• Raman grade
• Low-fluorescence material
• Scintillator-grade material

BaF₂ intended for infrared use may also have restricted short-wavelength performance compared with high-purity UV-grade material.

Writing only “CaF₂” or “BaF₂” on the drawing may therefore be insufficient for a demanding optical system.

3. Hardness and Mechanical Durability

CaF₂ is relatively soft compared with common optical glass and sapphire, but it is approximately twice as hard as BaF₂ based on representative Knoop hardness values.

BaF₂ is more mechanically fragile and can cleave easily. Particular care is required during fabrication, cleaning, assembly and use.

Mechanical reliability depends on:

• Window diameter
• Thickness
• Clear aperture
• Edge finish
• Chamfer design
• Mounting pressure
• Pressure differential
• Shock and vibration
• Cleaning method

A sharp or damaged edge can become a starting point for fracture. The component drawing should therefore specify a suitable chamfer or edge-break requirement.

Neither material should be mounted with excessive clamping force. A compliant mounting design may be required to accommodate dimensional changes and reduce localized stress.

4. Water and Humidity Resistance

CaF₂ has very low water solubility and is generally more suitable than BaF₂ when occasional moisture exposure is possible.

BaF₂ is considerably more soluble in water. Direct water contact, condensation or unsuitable aqueous cleaning fluids can degrade its polished surface.

For a BaF₂ window, review:

• Expected humidity
• Condensation risk
• Outdoor exposure
• Cleaning solution
• Storage conditions
• Sealing or purging
• Protective coating
• Packaging requirements

BaF₂ should not automatically be selected for an exposed outdoor window simply because it provides longer infrared transmission.

5. Thermal Performance

CaF₂ and BaF₂ have similar representative coefficients of thermal expansion, but this does not mean they provide identical resistance to thermal shock.

BaF₂ is particularly susceptible to rapid temperature changes. Sudden heating, cooling or an uneven temperature gradient may cause cracking or cleavage.

For thermally demanding systems, specify:

• Minimum and maximum temperature
• Temperature-change rate
• Thermal gradients
• Optical power
• Window dimensions
• Mounting material
• Heating or cooling method
• Thermal cycling requirements

CaF₂ also requires careful thermal design, especially for large or thin windows. Neither fluoride material should be treated like ordinary durable optical glass.

6. Weight and Component Size

BaF₂ has a density of approximately 4.83 g/cm³, compared with approximately 3.18 g/cm³ for CaF₂.

A BaF₂ window will therefore be substantially heavier than a CaF₂ window with the same dimensions.

CaF₂ may be preferred when:

• Component weight is limited
• The optic is installed in a moving assembly
• The system contains several large optical elements
• Mounting load must be reduced
• A larger aperture is required

Material availability, crystal diameter, thickness and required grade must also be confirmed before completing the design.

7. Surface Reflection and Coating

CaF₂ and BaF₂ have relatively low refractive indices compared with germanium, silicon or ZnSe. Their uncoated surface reflection is therefore lower than that of many common IR materials.

An AR coating may still be required when:

• Maximum transmission is important
• The system contains multiple transmissive surfaces
• Ghost reflections must be controlled
• The window operates at a non-zero angle of incidence
• A specific laser wavelength is used
• Surface protection is required

The coating specification should include:

• Wavelength or wavelength band
• Angle of incidence
• Required reflectance or transmission
• Polarization, if applicable
• Number of coated surfaces
• Environmental durability
• Cleaning requirements
• Laser damage requirement

A coating designed for UV use may not provide the required IR performance, and a broadband IR coating may restrict short-wavelength transmission.

General selection guide only — final material choice depends on grade, wavelength, thickness, environment and coating.

Which Material Is Better for Different Applications?

UV and Excimer-Laser Windows

High-purity CaF₂ is normally the more relevant starting point. It is widely used for UV, VUV and excimer-laser applications.

The required grade should be stated because general IR-grade CaF₂ may not provide suitable UV transmission.

UV-to-IR Spectroscopy

Both materials may be used.

CaF₂ is generally preferred when greater hardness, lower weight and better moisture resistance are important. BaF₂ may be considered when the measurement must extend farther into the infrared.

Infrared Spectroscopy

BaF₂ provides longer-wavelength coverage than CaF₂ and may be useful for infrared spectroscopy.

However, the required transmission should be checked at the actual thickness, particularly near the material’s long-wavelength limit.

Thermography Viewports

BaF₂ can be used for selected passive infrared and thermography viewports. However, it does not necessarily provide complete high transmission across the entire 8–14 µm atmospheric band.

The detector band, window thickness, environment and required minimum transmission should be confirmed before selection.

Moist or Outdoor Environments

CaF₂ is normally the safer choice because it is much less soluble in water.

For direct exposure to rain, condensation or aggressive cleaning, another more durable window material may be more appropriate than either fluoride.

Thermally Demanding Systems

CaF₂ is generally preferred over BaF₂ when thermal shock is a concern. The temperature-change rate, mount and window geometry must still be evaluated carefully.

How to Choose Between CaF₂ and BaF₂

Use this practical selection sequence:

1. Define the complete operating wavelength band.
2. Set the required finished-window transmission.
3. Identify the required UV, IR or spectroscopy grade.
4. Confirm the window diameter and thickness.
5. Review humidity, water and cleaning exposure.
6. Define the operating temperature and change rate.
7. Evaluate pressure, shock, vibration and mounting stress.
8. Select the AR or protective coating.
9. Confirm material availability and manufacturing limits.
10. Approve the complete finished-window specification.

Important: These recommendations are starting points rather than universal design rules. Final performance must be evaluated using the exact material grade, component geometry, coating and actual operating conditions.

Common Selection Mistakes

Choosing Only by the Published Transmission Endpoints

A stated material range does not guarantee high transmission throughout that range.

Treating Every CaF₂ or BaF₂ Grade as Equal

UV, IR, Raman and scintillator grades can have different purity and transmission characteristics.

Assuming BaF₂ Covers the Entire LWIR Band

BaF₂ extends farther into the infrared than CaF₂, but it should not automatically be presented as providing complete 8–14 µm transmission.

Ignoring Water Sensitivity

BaF₂ is significantly more water-soluble than CaF₂. Humidity, condensation and cleaning methods must be considered.

Ignoring Thermal Shock

BaF₂ is particularly susceptible to rapid temperature changes and uneven heating.

Overlooking Edge and Mounting Stress

Both materials can cleave. Edge quality, chamfer design and mounting force can affect component survival.

Frequently Asked Questions

Is BaF₂ Better Than CaF₂ for Infrared Optics?

Not for every application. BaF₂ extends farther into the infrared, while CaF₂ is harder, lighter, less water-soluble and generally easier to use in demanding environments.

Which Material Is Better for UV Windows?

High-purity CaF₂ is commonly preferred for UV and excimer-laser windows. The exact UV or excimer grade should be specified.

Which Material Is More Durable?

CaF₂ is generally more mechanically durable. It has a higher representative Knoop hardness and is less susceptible to moisture than BaF₂.

Does BaF₂ Transmit Across 8–14 µm?

BaF₂ transmits into part of this region, but it does not necessarily provide suitable performance across the complete 8–14 µm band. Actual transmission depends on thickness, grade, temperature and coating.

Are CaF₂ and BaF₂ Water-Soluble?

Both have measurable water solubility, but BaF₂ is significantly more soluble. CaF₂ is generally more suitable where limited moisture exposure is possible.

Do CaF₂ and BaF₂ Require AR Coatings?

Not always. Their relatively low refractive indices produce lower reflection than high-index IR materials. An AR coating is still useful when maximum transmission or reflection control is required.

Are CaF₂ and BaF₂ Birefringent?

Both have cubic crystal structures and are normally treated as optically isotropic. They do not have the inherent birefringence associated with non-cubic crystals such as sapphire.

Custom CaF₂ and BaF₂ Optical Windows from Chenyu Optics

Chenyu Optics manufactures custom calcium fluoride and barium fluoride optical windows and substrates for UV, infrared, spectroscopy, laser, sensing and scientific optical systems.

Custom components can be evaluated according to:

• CaF₂ or BaF₂ material
• UV, IR, Raman or other material grade
• Operating wavelength
• Diameter or rectangular dimensions
• Thickness and dimensional tolerance
• Surface flatness
• Transmitted wavefront error
• Surface quality
• Parallelism or wedge
• Clear aperture
• Optical coating
• Temperature and environmental requirements
• Prototype or production quantity
• Inspection and packaging requirements

Final specifications should be confirmed according to the customer drawing, operating wavelength, selected material grade, coating design, environmental conditions and available inspection methods.