A CaF2 grade qualified at 365 nm does not automatically qualify for 193 nm use — internal transmittance and laser durability are specified and validated at a particular wavelength. Comparing specifications across suppliers requires matching specimen thickness, measurement wavelength and surface state.

Key Takeaways

• A CaF2 grade qualified at 365 nm (i-line) cannot be assumed qualified at 193 nm or 248 nm; internal transmittance and laser durability ratings are wavelength-specific.
• Our CAF-A grade specifies >99.8% internal transmittance at 193 nm (10 mm), CAF-B at 248 nm, and CAF-C-I at 365 nm — each validated at its designated wavelength.
• Internal transmittance strips out surface reflection losses and reports only bulk-material absorption; it is not the same as the raw transmission value your spectrometer measures.
• Every transmittance comparison between suppliers must control for specimen thickness (typically 10 mm), measurement wavelength and surface condition.
• At 193 nm, nitrogen purge or vacuum optical paths are essential to prevent air absorption and ozone artifacts from corrupting measurements.
• The CaF2 industry has no unified grade-naming standard; names like "UV grade," "ArF grade" and "excimer grade" from different suppliers may or may not describe equivalent performance — always compare the underlying specifications.

Does a CaF2 Grade Qualified at 365 nm Also Qualify for Use at 193 nm?

No. A grade certified at 365 nm has its internal transmittance and laser durability validated only at that wavelength; these specifications do not transfer to shorter wavelengths.

The reason is physical: as operating wavelength decreases, bulk absorption mechanisms change. Trace impurities and lattice defects that are transparent at 365 nm can become significant absorbers at 193 nm. The laser durability rating reflects this wavelength-dependent damage behavior — CAF-C-I carries an LD-3 rating (entry tier for laser applications), while CAF-A carries LD-1 (the highest durability tier).

Consider our grade structure as an illustration. CAF-C-I specifies >99.8% internal transmittance at 365 nm, while CAF-A achieves the same >99.8% threshold but at 193 nm. The identical numerical specification does not mean interchangeable performance, because each value was measured and validated at a different wavelength.

The reverse direction — using a 193 nm–qualified grade at 365 nm — is physically feasible in terms of transmittance but may represent unnecessary cost and specification overhead. The correct approach is to match grade to operating wavelength.

No US or Chinese mandatory standard prescribes minimum internal transmittance or grade classification for CaF2 at these wavelengths, according to NIST. Supplier-specific grade definitions are the operative specification.

What Is Internal Transmittance and How Does It Differ from Measured External Transmission?

Internal transmittance is the fraction of light transmitted through the bulk material volume only, after mathematically removing the reflection losses at both surfaces. It isolates the material's own absorption behavior.

Measured (external) transmission is the raw value a spectrometer records. It includes surface reflection losses, any coating contributions and bulk absorption combined. This number is always lower than internal transmittance because it includes Fresnel reflection losses at each air-material interface.

The refractive index of CaF2 drives these reflection losses and varies with wavelength. According to Hellma Materials data, CaF2 has n = 1.50143 at 193 nm, n = 1.46791 at 248 nm and n = 1.44488 at 365 nm (all at 22 °C, nitrogen atmosphere, 1013 hPa). Higher refractive index at shorter wavelengths means proportionally larger reflection losses, widening the gap between internal and external values.

This distinction matters for specifications. A datasheet stating ">99.8% internal transmittance per 10 mm" describes a fundamentally different quantity than one stating "high measured transmission" without clarifying whether surface losses are included. Our own test data on a D25.4 × 4 mm sample showed measured T = 91.87% at 193 nm, which is consistent with high internal transmittance after accounting for uncoated surface reflections.

Converting between internal and external transmittance requires knowing the surface reflectance at the measurement wavelength. Contact our engineering team to request surface reflectance values at your operating wavelength.

Which Test Conditions Must Accompany Every CaF2 Transmittance Comparison?

A CaF2 transmittance value is meaningful only when the measurement conditions are fully documented. Below is the checklist every quality gatekeeper should apply.

Before comparing datasheets from any supplier, request or confirm measurement temperature, atmospheric conditions, spectral bandwidth and sample surface preparation method. These conditions are not standardized across the industry. For the conditions we report, refer to our specification documents.

How Do Our CAF-A, CAF-B and CAF-C-I Grades Map to 193, 248 and 365 nm?

Each of our CaF2 grades is engineered and validated for a specific operating wavelength. The table below provides the definitive mapping.

The laser durability tier system works as follows: LD-1 (CAF-A) represents the highest durability tier, LD-2 (CAF-B) is intermediate, and LD-3 (CAF-C-I) is the entry tier for laser applications. The specific quantitative difference between tiers is not published in the current datasheet; contact our engineering team to discuss laser durability requirements for your application.

Our CaF2 crystals cover a broad transparency range from 0.13 μm to 8 μm, but the grade designation determines at which wavelength the material has been optimized and validated for laser-grade performance. Available crystal orientations include (111), (001) and (100) as standard, with custom orientations available on request — orientation can affect birefringence and is part of a complete specification.

For visible-to-IR applications, our CAF-D-I (single crystal) and CAF-D-II (polycrystalline) grades are available but fall outside the scope of this DUV/i-line selection guide.

What Additional Parameters Separate DUV-Grade CaF2 from i-Line and UV Grades?

DUV grades require tighter specifications beyond transmittance alone. Several parameters become increasingly critical as wavelength decreases.

Laser damage threshold (LIDT). Our CaF2 has demonstrated LIDT of 6.22 J/cm² at 193 nm (6.7 ns pulse) and 7.5 J/cm² at 193 nm (10 ns pulse). For context, general literature reports CaF2 LIDT of approximately 7 J/cm² at 193 nm, according to Hellma Materials. LIDT is pulse-width-dependent, so values from different test conditions cannot be directly compared.

Long-term laser durability. We subjected a D25.4 × 4 mm sample to 2 billion pulses at 193 nm (600 Hz, 15 kV, 6.2 mJ, spot size 5.5 mm × 3.5 mm). The sample showed no surface or internal damage, with measured T = 91.87% at 193 nm. This demonstrates sustained performance under high-dose exposure.

Subsurface damage and surface quality. Our DUV-grade optical finishing can achieve subsurface damage ≤70 nm and surface roughness Rq ≤0.35 nm. These parameters matter more at shorter wavelengths, where scatter losses scale inversely with wavelength.

Stress birefringence. Achievable levels reach as low as ≤1 nm/cm. CAF-C-I and CAF-C-II offer customizable birefringence in the 0–10 nm/cm range. Stress birefringence specifications for CAF-A and CAF-B are not published in the current datasheet; contact our engineering team to discuss your requirements.

Abbe number and thermal design. Our CaF2 has Vd = 95.23, making it a key material for apochromatic lens designs. Combined with its negative dn/dT, CaF2 paired with fused silica enables athermal optical system design. NIST identified CaF2 as essential for 193 nm lithography optics because fused silica alone cannot provide the required dispersion characteristics.

How Should You Compare CaF2 Datasheets Across Suppliers?

There is no industry-wide grade-naming standard for CaF2. Different suppliers use names like "UV grade," "ArF grade," "excimer grade," "laser durable" and proprietary designations (e.g., Hellma's LD-A/B/C/D system) — all potentially describing 193 nm–suitable material but with different underlying test protocols. The following steps provide a normalization method.

Step 1: Are You Comparing Internal Transmittance or External Transmission?

Confirm whether the stated value is internal transmittance (surface losses removed) or measured external transmission (surface losses included). If the datasheet does not specify, the value cannot be reliably compared.

Step 2: Is the Specimen Thickness Consistent?

Normalize all values to the same reference thickness — 10 mm is the most common basis. If a supplier reports at a different thickness, request the absorption coefficient or a 10 mm–normalized value. Do not assume linear scaling without confirming the absorption regime.

Step 3: Does the Measurement Wavelength Match Your Operating Wavelength?

A transmittance value at 248 nm does not predict performance at 193 nm. Ensure the datasheet reports at your exact operating wavelength.

Step 4: What Atmospheric and Environmental Controls Were Used?

For 193 nm measurements, confirm nitrogen purge or vacuum path was used. Measurements in ambient air at this wavelength are unreliable due to oxygen absorption and ozone generation.

Step 5: What Laser Durability Data Is Provided?

Compare laser durability ratings, but recognize that rating systems differ between suppliers (e.g., our LD-1/2/3 versus Hellma's LD-A/B/C/D). Request the underlying test protocol — pulse count, repetition rate, fluence and wavelength — to make a meaningful comparison.

No mandatory US or Chinese standard governs CaF2 grade classification or minimum transmittance, so the burden of specification falls on the buyer-supplier agreement.

What Does a Practical Grade-Selection Workflow Look Like?

The following decision sequence synthesizes the preceding technical content into an actionable process.

1. Identify your operating wavelength — 193 nm, 248 nm, 365 nm or another wavelength.
2. Select the grade designated for that wavelength — CAF-A for 193 nm, CAF-B for 248 nm, CAF-C-I for 365 nm. Do not substitute a longer-wavelength grade for a shorter-wavelength application.
3. Confirm the internal transmittance specification meets your optical budget at the correct specimen thickness (10 mm reference).
4. Specify additional parameters — crystal orientation, stress birefringence requirement, surface quality and subsurface damage limits.
5. Request the supplier's test conditions and laser durability data at your operating wavelength to enable valid comparison.
6. Confirm dimensional availability — our blanks and components are available in sizes up to 600 mm.

Request-Data Checklist

This article covers grade selection for DUV and i-line bulk material. For coating-material applications, visit our CaF2 coating material page.

FAQ

What is the difference between internal transmittance and external transmission for CaF2 optics?

Internal transmittance measures only the light lost to absorption within the bulk CaF2 material, with surface reflection losses mathematically removed. External (measured) transmission is the raw spectrometer reading that includes both bulk absorption and Fresnel reflection losses at each surface. Because CaF2's refractive index is higher at 193 nm (n = 1.50143) than at 365 nm (n = 1.44488), the gap between internal and external values is larger at shorter wavelengths. Always confirm whether a datasheet value is internal or external before comparing suppliers.

Which CaF2 grade is recommended for 193 nm ArF excimer lithography systems?

For 193 nm ArF excimer applications, our CAF-A grade is the recommended choice, with an internal transmittance specification of >99.8% per 10 mm and the highest laser durability rating of LD-1. A grade qualified only at longer wavelengths (248 nm or 365 nm) must not be substituted, as its transmittance and durability have not been validated at 193 nm.

Can a CaF2 grade qualified at 365 nm (i-line) be used at 248 nm or 193 nm?

No. A CaF2 grade qualified at 365 nm cannot be assumed to meet 248 nm or 193 nm requirements, because internal transmittance and laser durability specifications are validated only at the designated wavelength. Our CAF-C-I grade specifies >99.8% internal transmittance at 365 nm with LD-3 laser durability, while CAF-A achieves the same transmittance threshold at 193 nm with the more demanding LD-1 rating. Always select the grade matched to your operating wavelength.

What test conditions must be reported alongside a CaF2 internal transmittance value?

A valid CaF2 internal transmittance specification must report at minimum the specimen thickness, measurement wavelength and surface state. For 193 nm measurements, the atmospheric environment (nitrogen purge or vacuum) must also be documented to prevent air-absorption artifacts. Confirm measurement temperature, spectral bandwidth and surface preparation with the supplier, as these conditions are not standardized across the industry.

How does CaF2 internal transmittance vary with specimen thickness, and how should I normalize data across vendors?

Internal transmittance decreases with increasing specimen thickness because the optical path through absorbing material is longer; CaF2 specifications are typically stated per 10 mm of material thickness. To compare values from suppliers who report at different thicknesses, request the absorption coefficient at the measurement wavelength or ask for a 10 mm–normalized value. Do not assume a simple proportional relationship without confirming the absorption regime at your specific wavelength.

How do DESIOPTOE CAF-A, CAF-B, and CAF-C-I grades map to 193 nm, 248 nm, and 365 nm operating wavelengths?

CAF-A is designated for 193 nm (ArF excimer) with >99.8% internal transmittance per 10 mm and LD-1 laser durability. CAF-B is designated for 248 nm (KrF excimer) with the same transmittance threshold and LD-2 durability. CAF-C-I is designated for 365 nm (i-line) with >99.8% per 10 mm, LD-3 durability and customizable stress birefringence of 0–10 nm/cm. Each grade is validated exclusively at its designated wavelength.

Why do different CaF2 suppliers use different grade names for the same wavelength, and how can I identify the correct grade?

There is no industry-wide standard for CaF2 grade nomenclature; suppliers independently define names such as "UV grade," "ArF grade," "excimer grade" and "laser durable" to describe material for similar wavelengths, and no US or Chinese mandatory standard prescribes minimum transmittance or grade classification. To identify the correct grade, ignore the trade name and compare the underlying specifications: internal transmittance at your operating wavelength, specimen thickness, laser durability test protocol and stress birefringence — all measured under documented conditions.

Request Grade-Specific Data or Discuss Your Wavelength Requirements

Ready to specify the right CaF2 grade for your DUV or i-line system? Contact our engineering team at desioptoe.com to request detailed grade data, test-condition documentation or a custom specification review.

If you need finished optical components after grade selection, explore our CaF2 lens and window/plate product pages. Our CaF2 blanks and components are available in sizes up to 600 mm, and custom crystal orientations beyond the standard (111), (001) and (100) can be discussed with our team.