At 121.6 nm (hydrogen Lyman-α), the vacuum ultraviolet absorption edge of most optical materials renders them completely opaque. Only MgF₂ (cutoff ~115 nm) and LiF (cutoff below 110 nm) offer practical transmission at this wavelength among common window materials. Always request measured transmittance data at 121.6 nm with sample thickness and measurement conditions stated.

Key Takeaways

• 121.6 nm is the hydrogen Lyman-α line, deep in the vacuum ultraviolet; air absorbs strongly below ~200 nm, so the entire optical path must operate under vacuum or inert-gas purge.
• Only MgF₂ (cutoff ~115 nm) and LiF (cutoff below 110 nm) offer practical transmission at 121.6 nm among common window materials.
• UV-grade CaF₂ cuts off at roughly 120–130nm—the "UV grade" label does not extend its intrinsic absorption edge to Lyman-α wavelengths.
• Supplier transmittance data must include sample thickness, measurement wavelength, vacuum/purge conditions, and uncertainty to be specification-grade.
• Thinner windows reduce bulk absorption loss at 121.6 nm, but mechanical strength and vacuum-seal requirements set a practical lower limit on thickness.
• LiF is hygroscopic and requires strict moisture-controlled storage and handling; otherwise surface quality and transmittance degrade significantly.

Why Does 121.6 nm Impose a Hard Cutoff on Window Materials?

Most materials marketed as "ultraviolet" are completely opaque at the Lyman-α wavelength. Understanding why narrows the candidate list before you ever contact a supplier.

The 121.6 nm line is the hydrogen Lyman-α transition, situated in the vacuum ultraviolet (VUV) band. The VUV region spans roughly 100–200 nm; below about 200 nm, atmospheric oxygen absorbs so strongly that the optical path must run under vacuum or a controlled inert-gas environment.

Common "UV" materials fall far short of 121.6 nm. UV fused silica has an effective transmission floor around 180–193 nm. Sapphire typically transmits down to 150–170 nm. Neither comes close to the Lyman-α line. The short-wave cutoff of any material is set by its electronic bandgap—a fundamental physical limit that no purity grade or manufacturing process can override. Only a few fluoride crystals have intrinsic absorption edges low enough to pass 121.6 nm photons.

One additional factor deserves attention early: hydrocarbon contamination or water films on window surfaces create absorption layers that further reduce real-world transmittance at 121.6 nm. System cleanliness management is critical in any VUV optical path.

What Window Materials Should You Consider for a 121.6 nm VUV Optical Path?

The practical answer is two: MgF₂ and LiF. Everything else commonly found in optics catalogs is ruled out by physics.

MgF₂ is the most widely used VUV window material. According to NIST, the short-wave cutoff of MgF₂ windows used in VUV instruments is approximately 115 nm. MgF₂ is also employed in VUV lenses, prisms, and optical coatings, and is a standard choice in excimer-laser systems.

LiF has a broad transmission range extending from below 110 nm in the deep VUV to approximately 7 µm in the infrared, covering a wider spectral span than other common optical materials. It is used in spectroscopy, synchrotron-radiation beamlines, and dedicated VUV and X-ray optical systems.

CaF₂ transmits to about 120–130 nm—useful for F₂ (157 nm) and ArF (193 nm) laser optics, but not for 121.6 nm.

We at DESIOPTOE manufacture fluoride crystal materials including MgF₂, LiF, CaF₂, and BaF₂, and we provide VUV transmittance curves for our MgF₂ and LiF crystals to support engineering decisions at this wavelength.

How Do MgF₂ and LiF Compare as VUV Window Materials?

Both transmit at 121.6 nm, but they differ in spectral range, environmental robustness, and handling requirements. The right choice depends on your system's wavelength coverage, operating environment, and maintenance constraints.

Transmission range. LiF extends below 110 nm, giving it a wider short-wave window than MgF₂ (cutoff ~115 nm). If your system must also cover wavelengths between 110 nm and 115 nm, LiF is the only option among these two.

Proven use at 121.6 nm. NIST radiometric-standards literature documents measurements made through a 2 mm thick MgF₂ arc-lamp window at 121.6 nm, according to NIST SP 250-3. Separately, NIST has published optical-constant data for MgF₂ at 1216 Å (121.6 nm), according to NIST SP 697. Both confirm MgF₂'s technical suitability at this wavelength.

Environmental stability. LiF is hygroscopic and will degrade if exposed to ambient moisture—surface quality and transmittance drop significantly without strict moisture control during storage and installation. MgF₂ is considerably more stable under normal laboratory and cleanroom conditions.

Refractive index (reference values for coating materials). Our MgF₂ coating material has a refractive index of 1.389 (@19 °C); our LiF coating material is 1.39126 (@19 °C). These values are close in the visible range, but VUV dispersion behavior differs and should be evaluated using each material's full optical-constant curve.

Product grades. Our LiF crystals are available in three grades: LIF-A (single-crystal, optical grade), LIF-B (single-crystal, window grade), and LIF-C (polycrystalline). We supply crystal blanks and finished optical elements up to 300 mm, with custom crystal-orientation options.

A note on direct comparison data. No source currently provides a side-by-side transmittance comparison of MgF₂ and LiF at 121.6 nm under identical thickness conditions. Engineers should request measured curves for the specific thickness they intend to use from their supplier.

Can a UV-Grade CaF₂ Window Be Used Below 150 nm?

No. CaF₂ transmits in the deep UV down to approximately 120–130 nm. At 121.6 nm the material is well below its intrinsic absorption edge and effectively opaque.

The label "UV grade" indicates that the crystal has been manufactured and selected for optimized UV performance—higher purity, fewer inclusions, tighter homogeneity. It does not shift the material's electronic bandgap. The short-wave cutoff is a fundamental property of the CaF₂ lattice, and no processing step can push it to 121.6 nm.

Where CaF₂ excels: our CaF₂-A grade crystal achieves an internal transmittance greater than 99.8 % at 193 nm (10 mm sample), and our CaF₂ products serve wavelengths including F₂ 157 nm, ArF 193 nm, KrF 248 nm, and i-line 365 nm. The reference refractive index for our CaF₂ coating material is 1.43288 (@22 °C).

Do not extrapolate CaF₂ transmittance data from 157 nm or 193 nm down to 121.6 nm. Material absorption does not follow a simple linear trend across wavelengths, and no published source provides measured CaF₂ transmittance at 121.6 nm—which itself signals that the material has no practical value at that wavelength.

What Transmittance Data Should You Request for an MgF₂ VUV Window?

According to NIST, when deriving source radiance through a window, the measured transmittance of that specific window must be used for correction. In practice, this means every window needs its own measured data—catalog "typical" curves are not sufficient for rigorous work.

Request the following from your supplier:

1. Measured transmittance at 121.6 nm — a full wavelength-vs-transmittance curve, not just a stated cutoff wavelength.
2. Exact sample thickness — for reference, our MgF₂ VUV transmittance curve is measured on a 3 mm sample over 120–220 nm.
3. Surface finish and polish specification.
4. Coating status — whether the window is uncoated or has an antireflection or protective coating, and the coating type.
5. Crystal-axis orientation — critical for birefringent materials like MgF₂.
6. Measurement environment — vacuum or inert-gas purge.
7. Measurement geometry and uncertainty.
8. Environmental exposure history of the sample.

A stated "cutoff wavelength" or "transmission range" alone is not adequate. At 121.6 nm, you may be on the steep roll-off portion of the transmittance curve, where a few nanometers of wavelength shift correspond to a large change in throughput.

How Do You Distinguish Bulk Transmittance from Finished-Window Transmittance?

This distinction is a persistent source of specification errors and procurement mismatches.

Bulk (internal) transmittance reflects only the material's volume absorption. It excludes surface-reflection losses and represents the fraction of light that survives passage through the material body alone.

Finished-window (external) transmittance includes both bulk absorption and Fresnel reflection losses at the two air–crystal interfaces. This is the transmittance your optical path actually sees.

Supplier datasheets may report either value without always making the distinction explicit. Engineers must confirm which type of transmittance is quoted. NIST's reporting requirements—sample thickness, measurement geometry, and conditions—exist precisely to eliminate this ambiguity, as described in NIST SP 250-3.

Because 121.6 nm sits near the absorption edge of both MgF₂ and LiF, bulk absorption rises steeply with thickness. Use the thinnest window that still meets mechanical-strength and vacuum-seal requirements. And remember that hydrocarbon contamination or water films on the surfaces add an absorption layer that pushes real-world transmittance below what clean-condition measurements show.

What Should a VUV Window Procurement Checklist Include?

The table below consolidates the technical requirements discussed above into a single procurement tool.

If your system demands the highest possible throughput at 121.6 nm and window-material absorption or surface contamination cannot meet your specification, evaluate a reflective optical path or coated-filter approach as an alternative.

We provide VUV transmittance curves for both our MgF₂ and LiF crystals. Specific finished-window availability, size ranges, and lead times should be confirmed through a project consultation—reach out to us for details.

FAQ

Which Window Materials Have Practical Transmittance at 121.6 nm (Lyman-α)?

Only MgF₂ and LiF among common optical materials transmit at 121.6 nm. MgF₂ has a short-wave cutoff near 115 nm, and LiF extends below 110 nm. UV fused silica (cutoff ~180–193 nm), sapphire (~150–170 nm), and CaF₂ (~120–130 nm) are all opaque at this wavelength and cannot be used in a Lyman-α optical path.

How Do MgF₂ and LiF Differ in Transmission Performance at 121.6 nm?

LiF offers a wider transmission window, extending below 110 nm, compared with MgF₂'s cutoff near 115 nm—both transmit at 121.6 nm. The key practical difference is environmental stability: LiF is hygroscopic and degrades without strict moisture control, while MgF₂ is far more robust. No published source provides a direct same-thickness transmittance comparison at 121.6 nm; request measured curves for your specific thickness from the supplier.

Why Can't UV-Grade CaF₂ Serve as a VUV Window Below 150 nm?

CaF₂ transmits down to approximately 120–130 nm. At 121.6 nm it is well past its intrinsic absorption edge. "UV grade" is a purity and process designation—it does not change the material's electronic bandgap or shift its cutoff wavelength. For 121.6 nm transmission, choose MgF₂ or LiF instead.

Why Is UV Fused Silica Unsuitable for a 121.6 nm VUV Window?

UV fused silica has an effective transmission floor around 180–193 nm and is completely opaque at 121.6 nm. Engineers familiar with silica's reliable performance in the near-UV should not assume it extends into the VUV. A fluoride crystal—MgF₂ or LiF—is required for Lyman-α work.

What Transmittance Data Should You Request When Purchasing MgF₂ or LiF VUV Windows?

Request a measured transmittance curve at 121.6 nm with the sample thickness, measurement wavelength range, measurement environment (vacuum or inert-gas purge), and uncertainty all explicitly stated. Also confirm whether the data represents bulk or finished-window transmittance, and obtain surface-finish, coating-status, and crystal-orientation information.

How Does Window Thickness Affect Transmittance at 121.6 nm?

Thicker windows suffer greater bulk absorption at 121.6 nm because this wavelength sits near the material's absorption edge. Choose the thinnest window that satisfies mechanical-strength and vacuum-seal requirements. Always confirm the exact sample thickness associated with any transmittance data so you can accurately model optical-path performance.

When Should a 121.6 nm System Consider a Reflective Optical Path Instead of a Transmission Window?

When bulk absorption and surface-contamination losses through MgF₂ or LiF windows cannot meet your throughput specification, evaluate reflective optics or coated-filter designs as alternatives. This decision involves trade-offs in optical-path geometry, alignment complexity, and overall system cost.

Discuss Your 121.6 nm Window Requirements with DESIOPTOE

If you are designing or specifying a Lyman-α optical system, we can help you evaluate MgF₂ and LiF crystal materials for your application. We supply MgF₂ and LiF crystals with VUV transmittance data, LiF in multiple grades (optical, window, and polycrystalline) with custom crystal-orientation options, and blanks up to 300 mm. For engineers building a complete VUV optical path, we also offer optical windows, plates, and lenses in fluoride crystal materials.

Finished-window specifications, measured transmittance data for your required thickness, and delivery timelines are confirmed on a project basis. Visit desioptoe.com to submit your requirements or start a technical discussion with our team.