Optical glass is the raw material behind every precision lens, prism, and imaging system - from camera phones to semiconductors. But the step opticians often find most difficult to understand is how to choose the optimal grade. This simple field guide to transparent optical materials deconstructs measurable optical properties, generic glass families, real-world manufacturing impacts, and design considerations that separate a successful optical project from an expensive re-engineering nightmare. At its core, optical glass is no different than simple float glass as a type of glass. It is a formulation of silica, alumina, alkalis, and other oxides “engineered” to a very high degree of purity & homogeneity and, for the most part, arranged into a predetermined formula to yield specific refractive and dispersive properties. By comparison, the average sheet of house-glass contains thousands of tiny bubbles and inclusions and has no variation in properties beyond 110.
“Modern optical glass materials began with Otto Schott in the 1880s when he systematically separated various glass compositions and produced transparent forms with specific refractive index & dispersion values - a move that propelled glass from an art form to a science. Four factors separate optical glass from window glass. First, purity: optical glass raw materials contain impurity levels below 10 ppm for contaminants like iron oxide (Fe₂O₃), which causes visible light absorption - normal glass tolerates hundreds of ppm. Second, homogeneity: optical blanks undergo 72+ hours of controlled furnace annealing until the refractive index variation across the piece falls below ±2×10⁻⁶ (grade H5 per ISO 12123). Third, thermal history: the entire annealing profile is engineered to lock in a specific refractive index to the fifth decimal place - a level of control that simply does not exist for architectural glass. Optical glass classification is based on a system implemented using the Abbe diagram, which compares nd against Vd.
Crown glass grades have an Vd值greater than 55, resulting in little dispersion across the visible spectrum. BK7(borosilicate crown) has been adopted by industry as the primary glass for all standard applications, with Vd=64.17. It is a durable glass, with high transmission in the visible range, predictable polishing character, and good chemical durability. Flint grades tend to be below 50 in Vd, with higher indices of refraction, conferring the ability to bend the light path in more aggressive ways in smaller packages. Dense flint varieties such as SF11 - historically formulated with lead oxide and barium for high refractive index - can be combined with crown elements in achromatic doublet designs -the classic two element lens designation that corrects colour fringing at two separate wavelengths. Specialty types maintain transmission over specific wavelength ranges or working environments where crown/flint image correction combinations are insufficient.
Fused oxide components such as fused silica can have transmission well into the deep ultraviolet down to 185 nm, essential for semiconductor lithography at 193 nm (ArF laser).Chalcogenide types extend TIR transmission well into the thermal infrared, 1-12um, for FLIR and missile seekers. Specific types of optical filter substrate glass, including color optical filters and neutral density filter glass, are fabricated to selectively absorb or transmit specific wavebands with short tolerances. Laser gain media, such as phosphate glass, can incorporate a broad range of rare-earth dopants including phosphorus and boron without thermal lensing effect present in traditional silicate host material types.
One detail overlooked by many competitors when discussing glass types: N-BK7 is not the same as BK7. The ‘N’ is for SCHOTT’s lead-free reformulation, developed for soda-lime and arsenic-free glass manufacturing in order to be compliant with RoHS. While the optical indices and Abbe number are similar, there are differences to do with the transmission in the near-UV (350-400 nm), and there are small shifts with regard to the thermal properties; which may mean the mounting calculations will require correction if replacing BK7 with N-BK7. Data in the tables sourced from the available catalog data from Ohara Corporation, TIE catalog from SCHOTT, and entries from the RP-Photonics encyclopedia.
Refractive index; nd of the glass is the ratio of the velocity of light within a vacuum to the velocity of light passing through the glass. This index is measured for the helium d-line ( 587.6 nm). Higher the refractive index of the glass, greater is the deviation of light ray for unit thickness of the glass allowing the optical designer to develop shorter and more compact lens assemblies. Optical glasses have a refractive index varying from about 1.44 (fused silica) to over 1.95 (dense lanthanum flint) of which the majority of general purpose grades are found to be between 1.50 and 1.75.
Abbe number (Vd) quantifies chromatic dispersion - how much the refractive index changes across different wavelengths. It is calculated as Vd = (nd − 1) / (nF − nC), where nF and nC are the refractive indices at the hydrogen F-line and C-line respectively. A high Abbe number (e.g., BK7 at 64.17) means low dispersion: the glass bends blue and red light almost equally. A low Abbe number (e.g., SF11 at 25.76) means high dispersion, which is actually desirable in achromatic doublet designs where a flint element cancels the chromatic error of a crown element. As a working tool, the Abbe diagram - a scatter plot of nd versus Vd for all available glass families - is the primary tool optical designers use to select glass pairs for color correction. Transmittance - the percentage of light that passes through a given thickness - determines the usable wavelength range. In general, most of the common silicate optical glasses transmit well from around 350 nm up to 2,000 nm.
Fused silica is good down to 185 nm, thus it is commonly used in all UV and deep-UV work, such as the ArF excimer (193 nm) lithography. Longward of 2,000 nm, the transmissive range of silicates terminates against the strong fundamental SiO vibrational band at 4.45 μm, leaving a hard 5 μm cutoff. UV transmission in the near-ultraviolet range (200-380 nm) is a critical specification for spectral analysis and photolithography applications. Homogeneity: scores the amount of variation in the index of refraction over the width of the glass blank. A score of H1, n 110 on the homogeneity scale made by ISO 12123:2018 is usable for simple condenser optics; for interferometric and lithographic optics, a glass blank must be a H5, n 210. Among material properties, the coefficient of thermal expansion might seem minor, but it is absolutely not at the system level. When optical glass element is cemented or clamped into a metal or ceramic holder, thermal expansion mismatch produces stresses at the interface when there is a cycle of temperature change.
For reference, thermal expansion of BK7 of 7.110/K is well-borne by aluminum mounts, but poor for titanium 8.610/K while the extreme simplicity of fused silica’s low 0.5510/K expansion requires for dimensional stability the use of Invar or carbon-fiber construction. Four factors compound to make optical glass expensive. First, raw materials need to have the impurities removed down to parts per million, cost 10 to 100 times more than commodity glass batch; the melting must use platinum-lined crucibles so as not to contaminate the raw materials (also prohibitively expensive) and be done at temperatures of 1400-2000C or more for fused silica; energy input into the blank per kilogram using such large crucibles is also huge. Lastly each flat blank must be tested on an individual basis for interferometric homogeneity, spectrophotometric transmission, and stress birefringence before being certified for sale.
Market data shows the optical glass industry was valued at an estimated $2 billion in 2024 and predicted to scale at a CAGR of 5.5% through 2033, driven by growth in semiconductor lithography, medical imaging, and consumer AR/VR devices. For example, autonomous vehicles now carry more than 15 optical sensors each - LiDAR, cameras, rain sensors - all requiring precision glass components. Scenario - AR/VR Wafer Lens Selection: A consumer electronics manufacturer working on a future AR headset candidate seeks to minimize lens wafer thinner than 2 mm to hit weight goals of less than 85 grams. Using standard BK7 at nd = 1.5168, the minimum thickness necessary for the lens to meet these goals is 3.2 mm, due to the measured power of the lens. By shifting to a lanthanum crown glass with nd = 1.80, the minimum focal length again is 3.2 mm, but the glass only must be 1.8 mm thick to support the power. Here is the tradeoff: lanthanum crown is generally 4+ per kg more than BK7 and the mold tooling will be more difficult without a thermal runaway in the molding run.
Common Mistake: Engineers often choose BK7 for UV work below 350 nm without realizing internal transmission drops significantly at this wavelength. This can result in loss of signal, errors in measurements taking 2-4 weeks to troubleshoot, because while the blank “looks clear” to the eye it will in fact be absorbing in the region of interest. Glass manufacturing for optical applications is a six-stage process that is every bit as demanding as optical design itself. Raw material batching- High purity silica (SiO, B2O3, BaO, La2O3, etc.) silicate oxides are precisely weighed and blended to within a few grams. The crucible prevents contamination from the raw materials themselves. Fine annealing- The most time-consuming step, blank is cooled through the temperature range at rates of 2-10 K per hour or less. This is the temperature range where the glass’ index of refraction is “frozen in” to the material. Inspection and testing- The blanks are checked for index of refraction (to 0.00001), homogeneity (by transmission interferometry), striae by shadowgraphs, and bubbles/inclusions by visual count in a 100 cm volume. Any blanks that don’t pass are scrapped or downgraded to lower performance grades. Cutting and grinding- Qualified blanks are diamond sawn to the near net shape and then CNC ground close to net shape before the final finish. Final Polish- Optical surfaces are wet polished to specification using standard abrasive-driven polishing plates.
Sample Scenario-Semiconductor Supplier Qualification: A semiconductor fabrication facility needs fused silica blanks at H4 homogeneity (n 510) for a new 193 nm lithography stepper lens assembly. The qualification process takes about 6 months: 3 months for the glass manufacturer to produce test blanks from a tightly controlled melt, then another 3 months of incoming inspection, environment stress screening, and pilot-run integration at the fab. Any single failed qualification-say, one out-of-spec blank in a sample lot of 20-resets the date clock to 0. Knoop hardness over 600 (BK7 measures about 610) indicates a glass tough enough to take a polishing load without chipping out. One reality reports never high light: a single good prototype blank does not promise production repeatability. Glass choice begins with a well defined specification of what the optical system needs to do. Selection follows a sequential puzzle-operating wavelength span, required dispersion control, thermal stability, mechanical durability, cost. Doubling back to an earlier decision step causes re-design.
Sample Customer-Medical Endoscope Sourcing: A medical device startup develops a next-generation endoscope. Design intent: small, high resolution, rugged tip optics. Design specification: N-LAK lanthanum crown, nd 1.6727 for optimized compactness. Regulatory requirement: the entire lot of every material supplier in the device must hold ISO 13485 certification for medical device quality management. During qualification, the major device design engineers found three optical glass suppliers with precision grinding experience, but only one could supply the N-LAK batch and provide every step of the ISO 13485 documentation chain-from raw glass certificate to incoming inspection report. Pro Tip-Get a measurement-based glass datasheet when comparing optical glass shop quotes! Measured values of range, index, amp; other specs on a specific batch cut down on lot-to-lot surprises. The most common and widely used optical glass in the world is BK7, and the lead-free N-BK7 alternative. K9 is typical of the Chinese BK7: plate manufactured to the CDGM (Chengdu Optoelectronic Materials) glass catalog. The K9 optical constants are very similar to BK7: nd = 1.5163, Vd = 64.06. It can depend on the grade. Standard BK7 will transmit down to around 350 nm but will absorb strongly below this and so wouldn’t be suitable for deep-UV work. For the 193 nm Ar F excimer laser line for semiconductor lithography only high purity excimer grade fused silica will transmit and meet you damage threshold requirements.
The Abbe number (Vd) measures how much a glass disperses light - that is, how much its refractive index changes across different wavelengths. Higher Abbe numbers (55+) mean lower dispersion and less chromatic aberration, which is desirable for imaging. Lower Abbe numbers indicate higher dispersion, useful for achromatic doublet correction. Borosilicate (Pyrex) and optical (something like BK7) glass share the same basic chemistry - both are SiO and BO. The defining factor is accuracy/quality of manufacture. The lab grade borosilicate used for beakers and test tubes isn’t given to the same optical certification, has additional contaminants and exhibits a variation in refractive index not permitted in a lens or a prism. Same family, very different quality levels. ISO 9001:2015 would be the absolute minimum for general quality management. For aerospace and defense optics most prime contractors require AS9100D certification. This guide was developed to provide optical engineers and purchasers with a single reference to compare optical glass types, find information on refractive index and dispersion specifications, and assist in selecting the appropriate glass.
All technical data has been taken from NIST publication, ISO standards and manufacturer catalogs, but not from proprietary testing. Používáme soubory cookies, které nám pomáhají rozvíjet a zlepšovat náš web. Poskytnuté informace jsou u nás v bezpečí. Více informací najdete na stránce "Podmínky ochrany".
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Dr. II. Na našem e-shopu si vyberte dioptrickou obrubu, která vám padne. 1. Čím vyšší index lomu, tím budou brýlové čočky tenčí. Existuje obecná poučka, že do 2 dioptrií je vhodné používat brýlové čočky s nižším indexem lomu - tedy index 1.5, 1.53 a 1.6. Při 2 až 4 dioptriích doporučujeme index lomu minimálně 1.6. Nejvýhodnější v tomto případě je používat brýlová skla s indexem lomu 1.67. Pro vyšší dioptrie je vhodné používat brýlová skla s indexem lomu 1.67 nebo ideálně 1.74. 2. Zvolte, jestli si přejete brýlová skla čirá nebo barevná. Samozabarvovací - intenzita zvolené barvy automaticky roste v závislosti na míře slunečního svitu. Zabarvení se odvíjí i od výše teplot. Sluneční barvené dioptrické - zabarvená sluneční dioptrická skla s UV filtrem. 3. Dioptrické sluneční brýle Proměníme vámi vybrané rámy ve sluneční brýle se všemi náležitostmi. Nabízíme několik úrovní zatmavení. 85% zabarvení odpovídá klasickým slunečním brýlím. Navíc si můžete k čočkám přidat polarizační filtr nebo zrcadlový efekt v několika odstínech! Přečtěte si více o slunečních brýlích s dioptrickými skly
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