Calcium fluoride is the optical material that uniquely covers the widest useful wavelength range. From 130 nm in the vacuum-UV (deep enough for ArF excimer laser work and synchrotron beamlines) through 9 µm in the mid-IR (covering the entire FTIR fingerprint region), CaF₂ transmits where neither fused quartz nor sapphire can. That single advantage makes it the de facto window material for FTIR sample cells, ATR crystals, photolithography optics, and any application that combines deep-UV and mid-IR requirements in one optical path.

The trade-off is mechanical and chemical fragility. CaF₂ is Mohs 4 (soft, scratches easily), partially soluble in water and acids, hygroscopic over months, and sensitive to thermal shock. Working with CaF₂ productively means treating it differently from sapphire or quartz: dry storage in a desiccator, lens-tissue cleaning only, controlled temperature ramping. This guide covers when CaF₂ is the right material, how to handle it correctly, and the FTIR sample-cell, ATR-crystal, and excimer-optic applications where it is essentially irreplaceable. The 61-SKU MachinedQuartz CaF₂ range covers stock dimensions from Ø 6 mm to Ø 50 mm; custom geometry is routine for OEM work.

1. Why CaF₂ — the spectral-range argument

Calcium fluoride is justified for one specific reason: spectral coverage that no other common optical material matches. The 130–9000 nm range covers vacuum-UV (synchrotron beamlines, ArF excimer lasers at 193 nm), the entire UV-Vis-NIR (where quartz and sapphire also work), and crucially the mid-IR fingerprint region (5–9 µm) where neither quartz nor sapphire transmits.

Where CaF₂ is the only practical choice

• FTIR sample cells in the fingerprint region (5–9 µm): sapphire cuts off at 5 µm, quartz at 2.5 µm. CaF₂ covers the entire FTIR analytical band. ZnSe and Ge extend beyond 9 µm but are darker and more expensive.
• 193 nm ArF excimer laser optics: CaF₂ transmits cleanly at 193 nm; fused quartz starts to absorb significantly there; sapphire is at its UV cutoff. CaF₂ is the standard for ArF photolithography optics, ophthalmic surgery, and 193 nm research.
• VUV / vacuum-UV applications below 190 nm: CaF₂ transmits to ~130 nm (the only common material with significant transmission below 150 nm). Used in synchrotron beamline windows, vacuum-UV spectroscopy, and astrophysics instruments.
• Combined deep-UV + mid-IR applications: when an optical path needs both UV and mid-IR (multi-wavelength sensors, cross-coupled spectroscopy systems), CaF₂ is the single-material solution.

Where CaF₂ is NOT the right choice

• UV-Vis routine work (200–700 nm): fused quartz at 1/3 to 1/10 the cost is the better choice; CaF₂ is overkill
• High-pressure or mechanical-stress applications: CaF₂ is soft (Mohs 4) and brittle — sapphire is the right material for pressure cells
• Hygroscopic environments (long-term humid air exposure): CaF₂ degrades over months; choose sapphire or quartz
• Mid-IR beyond 9 µm: ZnSe (transparent to 14 µm) or germanium (to 14 µm) take over
• Aggressive acid environments (mineral acid splash): CaF₂ is slowly soluble in HCl, HNO₃, H₂SO₄ — not for direct acid exposure

2. CaF₂ fundamentals — growth methods and grades

Calcium fluoride is a cubic single-crystal material (fluorite structure) grown synthetically. Three production methods cover the commercial market.

Growth methods

• Bridgman-Stockbarger: the dominant method. Molten CaF₂ in a graphite or Pt crucible is slowly translated through a temperature gradient, producing single-crystal boules up to 200–300 mm diameter. Used for almost all optical-grade CaF₂ windows.
• Czochralski: the same method used for sapphire and silicon. Less common for CaF₂ because Bridgman gives better optical homogeneity for fluorides. Used for very-high-purity laser-grade material.
• Naturally occurring fluorite: mined fluorite (the same mineral) is sometimes used in budget IR optics, but contains impurities that degrade transmission and is not used for analytical FTIR work. All MachinedQuartz CaF₂ is synthetic single crystal.

Grades

• UV grade (sometimes called “UV-grade” or “VUV-grade”): highest purity, lowest scatter, transmission optimised for 130–300 nm. Used for ArF excimer laser optics, VUV spectroscopy. ~ 50% premium over standard.
• IR grade / standard grade: the workhorse for FTIR sample cells, ATR crystals, mid-IR optics. Slightly higher impurity than UV-grade but sufficient for the 1–9 µm range where standard CaF₂ impurities don’t absorb. The MachinedQuartz default.
• Laser grade: tighter scatter and absorption specifications for high-power 193 nm ArF laser cavity optics; ~ 100% premium. Available on request for OEM laser-system orders.

Crystal orientation and birefringence

CaF₂ is cubic (no inherent birefringence). Standard windows are typically supplied with the (111) crystal plane parallel to the polished face because this is the natural cleavage plane and provides slightly better polish quality. (100) orientation is also available but more expensive to polish. For most window applications the orientation is not specified explicitly.

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3. Transmission spectrum — what makes CaF₂ unique

The 130–9000 nm transmission range is what justifies CaF₂’s premium over fused quartz for specific applications. The key features:

Deep-UV cutoff at 130 nm

CaF₂ transmits down to about 130 nm in the vacuum ultraviolet. This is below the practical cutoff of every other common optical material: fused quartz cuts off at 190 nm, sapphire at 185 nm, MgF₂ at 120 nm (better than CaF₂ in the deep-UV but with worse mid-IR), LiF at 105 nm (best UV but mechanically weak). For 193 nm ArF excimer laser work, CaF₂ gives 92% transmission per surface uncoated, vs 88% for fused quartz and 85% for sapphire.

Excellent UV-Vis-NIR transmission

Across the standard UV-Vis-NIR range (200–1500 nm), CaF₂ transmits at > 92% per surface. This is comparable to fused quartz and sapphire; CaF₂ offers no UV-Vis advantage that justifies its cost for routine visible-light work.

Mid-IR transmission to 9 µm

This is the killer application. The mid-IR fingerprint region (5–9 µm, equivalent to 2000–1100 cm⁻¹) is where most organic molecular vibrations show up: C-H bend, C-N stretch, C-O stretch, P=O stretch. CaF₂ transmits cleanly through this entire range; sapphire cuts off at 5 µm and quartz at 2.5 µm. For FTIR sample cells covering the analytical fingerprint region, CaF₂ is the standard.

Where CaF₂ cuts off

Above 9 µm the Ca-F lattice vibrations begin to absorb and transmission drops sharply: 80% at 9 µm, 50% at 10 µm, near zero by 12 µm. For FTIR work above 9 µm, ZnSe (8% absorption per surface, transparent to 14 µm) is the standard alternative; for the far-IR above 14 µm, KBr or polyethylene-windowed cells take over.

Refractive index and dispersion

CaF₂ refractive index at 588 nm: 1.434 (lower than quartz at 1.458 and sapphire at 1.768). The low refractive index gives CaF₂ the lowest Fresnel reflection of the three (~3% per surface uncoated, vs 4% for quartz and 7.4% for sapphire). For broadband uncoated windows, this is a small but real efficiency advantage.

4. The soft-and-fragile reality — mechanical properties

CaF₂ is mechanically the most fragile of the three common optical-window materials. This shapes how it must be handled and where it should not be specified.

What “Mohs 4” means in handling practice

• Sand particles scratch CaF₂: any silica or quartz dust above ~5 µm can leave a visible scratch under normal hand pressure. Always blow-clean before wiping.
• Paper towel (Kimwipe) leaves micro-scratches: use only lens tissue, chamois leather, or laser-cleaning wipes designed for soft optics.
• Tweezers contact damages the optical face: always handle by the edge using gloves; never let metal tweezers contact the polished face.
• Cleavage along (111) plane: CaF₂ cleaves naturally along the (111) crystallographic plane. Avoid impact loads — sharp impact can split a window cleanly along the cleavage direction.

Thermal shock sensitivity

CaF₂’s thermal expansion coefficient is 19 × 10⁻⁶ /K — about 35x that of fused quartz and 3x that of sapphire. This high CTE combined with the (111) cleavage makes CaF₂ highly sensitive to thermal shock:

• Maximum thermal ramp: ~50 °C/min during heating; ~30 °C/min during cooling
• Sudden contact between hot CaF₂ and cold solvent (e.g., dropping a 60 °C window into 20 °C cleaning bath) causes immediate cracking
• Hot windows must be allowed to cool to within 30 °C of ambient before any cleaning step
• Mounting in thermally cycling environments requires CTE-tolerant mounting (O-ring face seal or floating mount, not direct epoxy bond)

5. Hygroscopic handling & chemical resistance

CaF₂ degrades on prolonged exposure to atmospheric water vapour. The degradation is slow but cumulative: over months, an exposed surface develops a translucent fog (the result of slow hydrolysis to Ca(OH)₂ and HF). Surface roughness increases, transmission drops, and the polished face becomes irreparable.

Storage protocol

• Active use: store windows in a desiccator with fresh silica gel or anhydrous calcium sulfate, RH < 10%. Replace desiccant when it changes color (typically blue silica gel turns pink at saturation).
• Long-term storage: dry-N₂ purged cabinet or vacuum desiccator, RH < 5%. Premium reference standards (laser-grade laboratory work) sometimes use sealed evacuated containers.
• In-mounted use (FTIR cell): the assembled cell with O-ring seal protects the inner faces from air. The outer faces of the windows are still exposed and need protection during storage of the assembled cell.
• Travel / shipping: CaF₂ windows ship in foam-cushion boxes with a small desiccant packet. The desiccant should be replaced if the package has been in transit more than 30 days.

Cleaning protocol

1. Remove dust by air: compressed air or rubber bulb (no liquid) blows away particles that would scratch during wipe
2. Lens tissue with dry methanol: a single drop of HPLC-grade methanol on lens tissue (Kimtech laser-cleaning, Thorlabs lens tissue, or equivalent); wipe gently in one direction only
3. Inspect under lamp: hold under a strong oblique-angle light; any visible streaking or droplet marks means re-clean
4. Air-dry inverted: on lens tissue or in the desiccator
5. Never use water alone: CaF₂ is partially soluble in water; even pure water leaves a slight surface etch
6. Never use acetone repeatedly: some commercial acetone contains traces of HF or HCl that slowly etch CaF₂; methanol or anhydrous ethanol is preferred

Chemical resistance

6. FTIR sample cells — the dominant CaF₂ application

The FTIR sample cell is what most CaF₂ windows are used for. The standard construction is two CaF₂ windows separated by a calibrated PTFE (or lead) spacer that defines the optical path length, all clamped in a stainless steel demountable holder. Sample is loaded between the windows as a thin film, neat liquid, or solution. The IR beam passes through one window, through the sample film, and out the second window to the detector.

Standard FTIR cell dimensions

Path-length selection by sample type

• Neat liquids (organics, oils): 25–50 µm spacer (the chromophores are concentrated; thin path needed)
• Aqueous samples (proteins, carbohydrates): 50–100 µm (water absorbs strongly in the fingerprint region; thicker paths saturate)
• Dilute samples in non-aqueous solvent: 100–500 µm
• Polymer films cast on window: film thickness 5–25 µm typical, no spacer needed
• Thin solid samples (KBr pellets, mulls): 0.5–1 mm; uses single window backing

7. ATR crystals — surface analysis at 45°

Attenuated Total Reflectance (ATR) is the dominant FTIR technique for solid samples, polymers, biofilms, biological tissues, and any sample where preparing a thin film for transmission FTIR is impractical. The IR beam enters a bevelled crystal at 45°, undergoes multiple total internal reflections, and exits through the opposite bevel. At each TIR point on the top face, the evanescent field penetrates ~1 µm into whatever sample is in contact with the crystal — this is what produces the ATR spectrum.

CaF₂ vs other ATR crystal materials

• Diamond (single-bounce ATR): the modern standard for routine FTIR-ATR. Hard, chemically inert, broad spectral range. We do not supply diamond ATR crystals; outside our manufacturing scope.
• Zinc selenide (ZnSe): the workhorse for multi-reflection ATR. 600 nm to 15 µm range; refractive index 2.4 (shallower penetration than CaF₂); moderate chemical resistance. We do not supply ZnSe crystals; specialised crystal-growth chain.
• Germanium (Ge): for high-refractive-index work where shallow penetration matters. 2–15 µm; opaque to visible (no visual alignment); inert to most chemicals. Specialty supply.
• CaF₂: available, but limited to mild samples because of CaF₂’s solubility. Used for short-path single-bounce work where sample is non-aqueous and non-acidic. Most ATR work uses ZnSe or diamond instead of CaF₂.

When CaF₂ ATR makes sense

CaF₂ ATR is used when the application combines UV-Vis-IR multi-modal detection on the same sample (CaF₂ transmits visible/UV in the same crystal where ATR provides mid-IR analysis) or when the sample is genuinely non-aqueous and stable (e.g., dry polymer films, organic crystals, neat oils). For typical biological or aqueous samples, ZnSe or diamond is the right ATR material; CaF₂ ATR is a specialty.

8. 193 nm ArF excimer laser optics

Argon fluoride excimer lasers at 193 nm have become a major industrial technology: photolithography for advanced semiconductor manufacture (DUV process at 193 nm), refractive eye surgery (LASIK at 193 nm), and research-grade photochemistry. CaF₂ is the standard window material for these lasers because:

• 193 nm transmission: CaF₂ transmits ~92% per surface uncoated; fused quartz absorbs significantly even in JGS1 deep-UV grade; sapphire is at its UV cutoff
• Low absorption: CaF₂’s 193 nm absorption coefficient is < 0.001 cm⁻¹ for laser-grade material; this matters for high-power systems where absorbed power becomes thermal load
• High laser-induced damage threshold (LIDT): CaF₂ LIDT at 193 nm is ~ 5–15 J/cm² for short-pulse work, sufficient for almost all commercial ArF systems

9. Mid-IR spectroscopy applications

Beyond benchtop FTIR sample cells, CaF₂ serves several specialty mid-IR applications.

Process FTIR (in-line monitoring)

FTIR analysers integrated into chemical process lines (pharma, polymer, petrochemical) use CaF₂ windows for the sample interface. Process pressure typically modest (1–5 bar), temperature 25–80 °C, with continuous flow of process medium past the window. Window replacement cadence: 6 months to 2 years depending on process aggressiveness.

Mid-IR sensors (gas detection, leak detection)

Non-dispersive infrared (NDIR) sensors for CO₂, CO, CH₄, NO₂, and other small-molecule gas detection use CaF₂ windows in the optical path. Long-path absorption cells (50–100 cm folded path) for atmospheric or industrial monitoring also use CaF₂ or ZnSe end windows.

Microspectroscopy & imaging FTIR

FTIR microscopy (Bruker Hyperion, ThermoFisher Continuum, PerkinElmer Spotlight) uses CaF₂ or BaF₂ substrates and condensers. Sample-cell formats are smaller (~ 5–13 mm) and need tighter dimensional tolerance for the focused-beam geometry.

Synchrotron mid-IR beamlines

Synchrotron-source mid-IR beamlines (typically 4 µm to 100 µm via combined CaF₂ + diamond windows) use CaF₂ for the visible/UV-end window where wide-band transmission is needed. Specialty work; usually custom-quoted.

10. VUV / synchrotron applications below 200 nm

Below 200 nm in the vacuum-ultraviolet (VUV), most materials become opaque. CaF₂ is one of the few that transmits down to 130 nm:

• Synchrotron beamline windows: CaF₂ or LiF entrance/exit windows for VUV synchrotron beamlines. For wavelengths below 130 nm, MgF₂ (transmits to ~120 nm) or LiF (to ~105 nm) are the alternatives.
• VUV spectroscopy: astrophysics, atmospheric chemistry, plasma diagnostics in the 130–200 nm range use CaF₂ cell windows. Examples: Lyman-alpha (121.6 nm) detection requires LiF; molecular absorption at 150–180 nm typically uses CaF₂.
• Excimer lamp optics: 172 nm xenon excimer lamp systems use CaF₂ for the lamp envelope or external optical window when light extraction matters.
• Vacuum monochromators: McPherson, Acton VUV monochromator systems use CaF₂ or MgF₂ entrance/exit windows depending on the wavelength range required.

11. Mounting & sealing — the desiccated environment problem

CaF₂ mounting has all the standard considerations of optical-window mounting (vacuum compatibility, thermal cycling, mechanical compression) plus the unique constraint of moisture protection. Three approaches dominate.

O-ring face seal (the standard for FTIR cells)

Window compresses against a Viton O-ring in a stainless steel holder. O-ring seal is gas-tight to ~10⁻⁶ mbar; the inner faces of the windows are protected from atmospheric moisture by the seal. Outer faces remain exposed and need protection during storage of the assembled cell. Standard FTIR sample cell holders (Specac, PIKE, ThermoFisher) all use this approach.

Epoxy bonding (for permanent installations)

For one-piece sealed assemblies (industrial process probes, aerospace optics), CaF₂ can be epoxy-bonded to a metal flange. Use low-outgassing optical epoxy (Norland 61, EPO-TEK 301) and CTE-matched flange material (Invar at CTE 1.6, or aluminium at 23 with controlled bondline thickness). Direct bond to stainless steel (CTE 17) is the worst combination because of differential thermal expansion vs CaF₂ at 19 — small but cumulative stress over thermal cycles.

Brazing or metallised bond (for UHV / high-temperature)

CaF₂ can be metallised and brazed to a Kovar or molybdenum flange for ultra-high-vacuum applications. The metallisation chemistry is different from sapphire (different surface chemistry); typical process: vapour-deposited Cr/Au layer, then nickel-plated, then brazed with Au-Sn or Cu-Ag eutectic. Specialty operation; longer lead times. Used for synchrotron beamline windows and high-temperature process probes.

CTE mismatch tolerance

CaF₂’s thermal expansion (19 × 10⁻⁶ /K) is high relative to sapphire (5.6) but close to typical metal flanges. For thermal-cycling environments:

• Aluminium flange (CTE 23) is reasonable match for CaF₂ — CTE differential ~ 4 ppm/K; tolerates ~100 °C cycle without significant stress
• Stainless steel (CTE 17) is also reasonable — differential ~ 2 ppm/K
• Invar (CTE 1.6) is the wrong match for thermal cycling because the differential is 17 ppm/K; window will see significant stress during cycling
• For high-cycle-count or wide-temperature applications, use a soft-metal or O-ring seal to absorb the mismatch

12. Custom geometry & what MachinedQuartz makes

The 61-SKU stock catalog covers standard round windows from Ø 6 to 50 mm. For OEM partners and specialty applications, custom geometry is the norm.

Standard custom variations (no tooling fee, in-envelope)

• Custom round diameters: Ø 5 mm to Ø 100 mm routine; larger up to ~150 mm with longer lead time
• Custom thicknesses: 0.5 to 8 mm
• Square / rectangular plates: any L × W up to 75 × 75 mm
• Wedged windows: 1–5 arcmin wedge for laser cavity etalon suppression
• Bevelled or chamfered edges: for sealing applications and stress reduction
• Pair-matched windows: two windows matched within transmission tolerance for FTIR sample cells
• AR coatings: single-line V-coat at your design wavelength; broadband or dual-band; outsourced to qualified vacuum-coating partners (V-coat for 193 nm or 9 µm commonly requested)

Specialty custom (one-time engineering or external partner)

For specialty needs beyond the standard machining envelope, we work with partners on:

• Trapezoidal ATR crystals (45° bevels): available through partner relationship; longer lead time and 5-piece MOQ
• Hemispherical IR domes: small (Ø < 50 mm) on request; large diameters require specialty fabrication
• Brazed CaF₂ to Kovar UHV assemblies: outsourced to ceramic-metal bonding partner; 5-piece MOQ
• VUV-grade laser optics: sourced through laser-grade material chain; longer lead time
• Specific crystal orientation ((111) vs (100)): (111) is the natural cleavage plane and is the default; (100) available on request through orientation-controlled supply

13. Catalog & ordering

The MachinedQuartz CaF₂ catalog has 61 stock SKUs covering Ø 6 mm to Ø 50 mm round windows with thickness 0.6 to 3 mm. All stock SKUs are optical-grade single-crystal CaF₂ with 130–9000 nm transmission. Custom geometry is the default for OEM work.

Stock dimensional range

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14. Frequently asked questions

15. Disclaimer & notes