If you've ever tried to get a suntan through a window, you already know the answer intuitively: ordinary glass blocks UV light. But swap that window for quartz glass, and UV passes straight through. Both materials are transparent to the eye, both are based on silicon dioxide, and both look nearly identical on a lab bench. So why does one block ultraviolet radiation almost entirely while the other transmits it down to wavelengths as short as 160 nm?
The answer comes down to composition—specifically, what's in the glass beyond SiO₂, and how those impurities interact with short-wavelength photons.

Why Ordinary Glass Blocks Ultraviolet Light

Standard soda-lime glass—the material used in windows, bottles, and most everyday glass products—is not pure silicon dioxide. It is a mixture of oxides formulated to reduce the melting temperature of silica and make the glass easier and cheaper to manufacture at scale.

A typical soda-lime glass composition by weight is approximately 73% SiO₂, 14% Na₂O (sodium oxide), 9% CaO (calcium oxide), and smaller amounts of MgO, Al₂O₃, and other oxides. These network-modifying oxides lower the glass transition temperature from around 1,200°C for pure silica to approximately 550–600°C, which makes industrial production practical.
The problem is that these metallic oxides absorb UV radiation. Iron oxide (Fe₂O₃) is particularly effective at this—even at concentrations of just 0.01–0.1% by weight, iron impurities create strong absorption bands in the UV and blue-visible range. Titanium dioxide (TiO₂) contributes similarly. Sodium and calcium oxides modify the electronic structure of the glass network in ways that shift its fundamental absorption edge toward longer wavelengths, cutting into what would otherwise be the UV transmission window.
The result is a UV cutoff at approximately 300–320 nm for standard soda-lime glass. Radiation below this wavelength is absorbed within the first fraction of a millimeter of glass thickness and never reaches the other side. For visible light, where wavelengths range from 380 to 700 nm, this absorption is negligible—hence ordinary glass appears perfectly transparent. But for UV applications, the glass is effectively opaque.

Why Quartz Glass Allows UV Transmission

Quartz glass—whether natural fused quartz or synthetic fused silica—is composed almost entirely of silicon dioxide (SiO₂), typically exceeding 99.9% purity and reaching above 99.999% in high-purity synthetic grades. Unlike conventional glasses, it contains virtually no sodium oxide, calcium oxide, or other network-modifying oxides. Instead, the structure consists almost entirely of Si–O–Si bonds forming a continuous amorphous network.

This structural composition is critical for ultraviolet transmission. The intrinsic electronic absorption edge of the Si–O bond occurs in the vacuum-ultraviolet region, typically around 160–180 nm. Photons with wavelengths longer than this threshold do not carry enough energy to excite these electronic transitions, allowing the material to transmit light efficiently from roughly 180–200 nm through the visible spectrum and well into the near-infrared region, reaching approximately 3,500 nm.

Equally important is the extremely low concentration of metallic impurities. In high-quality quartz glass, iron content is often controlled below 1 ppm by weight—far lower than in conventional soda-lime glass, which may contain hundreds or thousands of ppm. Other transition metals such as titanium, copper, and chromium are also reduced to trace levels. Because these elements create strong absorption bands in the UV and visible regions, minimizing their presence prevents significant attenuation of UV photons in the 200–400 nm range.

The structural homogeneity of fused silica further enhances optical performance. The continuous amorphous Si–O–Si network contains no grain boundaries and exhibits extremely low defect density, which minimizes scattering at short wavelengths. As a result, polished optical-grade quartz glass can transmit ultraviolet light with losses on the order of about 1–3% per millimeter of path length at 200 nm, with transmission improving rapidly at longer wavelengths.

What Types of Quartz Glass Offer the Best UV Transmission

Not all quartz glass performs equally in the UV range. Within the quartz glass family, UV transmission varies significantly depending on manufacturing method, raw material source, and hydroxyl (OH) content.
Natural fused quartz is produced by melting high-purity natural quartz crystals. It achieves UV transmission down to approximately 240–250 nm in standard grades. Residual metallic impurities from the source crystal—primarily aluminum and iron at low ppm levels—create absorption features in the deep UV that limit performance below this threshold. Natural fused quartz is adequate for many UV lamp and industrial heating applications but is not suitable for deep-UV optical systems.
Synthetic fused silica is produced from chemical precursors via flame hydrolysis or chemical vapor deposition rather than from mined quartz. The elimination of the natural crystal source removes the metallic impurity limitations almost entirely. Depending on the specific grade, synthetic fused silica transmits down to approximately 160–180 nm, and high-OH grades extend this further into the vacuum UV. This is the material of choice for UV laser optics, deep-UV spectroscopy, and semiconductor photolithography equipment operating at 193 nm (ArF excimer laser) or 248 nm (KrF excimer laser).
UV-grade fused silica is a designation applied to synthetic fused silica that has been specifically optimized and screened for UV applications. Suppliers test these materials for internal transmission at key UV wavelengths (typically 185 nm, 193 nm, 248 nm, and 266 nm) and guarantee minimum transmission values. UV-grade materials also have controlled OH content, since high hydroxyl concentration creates an absorption band at approximately 2,730 nm in the infrared but can also affect deep-UV performance in very high-OH grades. Representative products include Heraeus Suprasil® 300 series, Corning HPFS® 7979, and Schott Lithosil® Q0.
The practical selection criterion is the shortest wavelength at which the application requires transmission. For UV-C sterilization at 254 nm, natural fused quartz is generally sufficient. For ArF excimer laser optics at 193 nm, only UV-grade synthetic fused silica meets specification. For vacuum UV applications below 160 nm, even quartz glass is insufficient, and calcium fluoride (CaF₂) or magnesium fluoride (MgF₂) windows are required instead.

Conclusion

The difference in UV transmission between quartz glass and ordinary glass is not a matter of degree—it is a fundamental consequence of composition. Soda-lime glass contains metal oxide impurities and network modifiers that shift its absorption edge into the near-UV, making it opaque to the wavelengths that matter most in UV applications. Quartz glass, with its near-pure SiO₂ structure and sub-ppm impurity levels, has an absorption edge defined by the Si–O bond itself at approximately 160 nm, leaving the entire UV spectrum open for transmission.

For engineers specifying optical windows, lamp envelopes, or process equipment where UV performance is critical, the choice of quartz glass grade should be matched to the specific wavelength requirement. Canal glass shop supplies and machines quartz glass components to precise dimensional specifications for UV optical, semiconductor, and industrial applications. Request a quote to discuss your requirements.