General-purpose optical glass is selected first on refractive index (Nd) and Abbe number, then on TCE, density, and transmission band, with the manufacturing route (block, strip, pressed blank) chosen to match the lot size and the anneal rate the fabricator can actually deliver [S3][S4].
Most general fabrication shops settle on a small catalog: BK7-style borosilicate crown for visible work, an SK16-class higher-index crown when f-number pushes down, fused silica for UV or low-TCE assemblies, and either optical-grade plastic or sapphire for window duties where impact or weight dominate the design [S4][S6].
Core Selection Criteria: Nd, Abbe, TCE, Density
Edmund Optics defines the crown vs. flint split at an Abbe number of 50: glasses with Abbe greater than 55 are considered low-dispersion crowns, while glasses with Abbe below 50 are treated as higher-dispersion flints, with the 50-55 band treated as transitional [S5]. Choosing inside that band, rather than across it, is the cheapest way to keep achromat doublets within a reasonable radius pair.
The Optics Online material map codes every entry by Nd and Abbe. SK16, a general-purpose crown with a higher index than BK7, carries Nd 1.62041, Abbe 60.32, TCE 7.3×10⁻⁶/K, density 3.58 g/cm³, MIL code 620603, and a 0.334-2.5 µm transmission window, with Chinese catalog equivalent C-ZK9 [S4]. The MIL code is a 6-digit convention: the first three digits are the first three digits of Nd, the next three are the first three digits of the Abbe number with the decimal removed [S4].
For comparison, BK7 (Nd 1.5168, Abbe 64.17) is the default visible-light crown, while SF6-like dense flints trade Abbe into the low 20s for higher Nd. The takeaway: if a design spec already pins a focal length and a wavefront error budget, the glass catalog is searched by Nd first, then by Abbe, before cost is even discussed [S4][S5].
Forms of Supply: Block, Strip, Rod, Pressed Blank
Raw optical glass ships from the melt house in block glass, large strips, and rods, with some vendors also supplying plates, cut prisms, or pressed blanks ready for fine anneal [S3]. A fine anneal runs on the order of 2 K per hour and is what locks in the published refractive index tolerance, so a block that has only been coarse-annealed will drift several units in the 5th decimal of Nd after re-heating during generating.
After glass production, blanks are cut or trepanned (cored) from block with about 1 mm of oversize per face, then handed off to the generating cell. Volume runs use embossing or molding for blanks; prototype or small runs go through diamond sawing and core-drilling, which is where most of the per-piece lead time lives [S3].
Comparing the Main Material Families
For a general fabrication shop, the four material families to keep on the shelf are BK7-class borosilicate crown, higher-index crowns like SK16, fused silica, and either polycarbonate or optical-grade plastic. BK7 wins on cost, polishability, and a 0.38-2.0 µm band; SK16 wins on a smaller radius for the same focal power at the cost of a slightly higher dispersion and a TCE of 7.3 versus BK7's roughly 7.1; fused silica wins on UV transmission and a near-zero TCE, but costs more per kg and generates slower [S4].
Specialty materials fill narrower slots. Optical quartz is birefringent and anisotropic, so it is restricted to optical low-pass filters and specialty fiber-optic components where that property is exploited, not fought. Sapphire is a hard, durable window material used to protect internal optics in harsh environments. Fused silica's UV transmission and thermal stability make it the default for UV lenses and prisms [S4].
Plastic optics cap out at a refractive index near 1.6 and are limited to low-precision lenses such as eyeglass stock, magnifiers, and educational optics, but the injection-molding route cuts unit cost dramatically at volume and can be hybrid-assembled with glass elements [S4].
Fabrication Workflow from Blank to Finished Surface
The general fabrication sequence is cutting or trepanning the blank, generating the curvature with bonded or pellet abrasives, fine grinding (lapping) with a graded slurry, then polishing with a pitch or polyurethane lap and cerium-oxide or zirconium-oxide slurry. Each step is sized to leave the next step the minimum stock it actually needs, because over-generating drives stress into the surface and adds to subsurface damage that polishing cannot fully remove [S3].
For custom and prototype work, single-point diamond turning is added when the surface is rotationally symmetric and the substrate is a soft glass or a plastic, and magnetorheological finishing (MRF) is the standard final-figure corrector when surface form needs to come down to lambda/20 or better on a plano or weakly curved part [S3]. For plastics, the route is injection or compression molding directly to near-net shape, with the as-molded surface often good enough that only a thin coat of hard anti-scratch lacquer is needed before coating [S6].
A short shop rule: if the surface roughness spec is sub-5 Å rms on a 25 mm aperture, plan on MRF or ion-beam figuring after polish; if the spec is 10-20 Å rms, pitch polish alone is usually enough, and the lap material (pine pitch vs. polyurethane vs. cloth) is what sets the achievable roughness on a given glass family.
Use Cases, Constraints, and Common Failure Modes
Visible-light imaging in the 400-700 nm band, with focal lengths in the 10-200 mm range and NVE coatings, runs almost entirely on BK7 and SK16 in a general fab. The known failure modes are striae and cord from inadequate mixing at the melt house, index inhomogeneity from a too-fast anneal, and subsurface damage carried over from a heavy generating pass that the polish step cannot fully remove [S1][S3].
For UV or laser-line work, fused silica is the safe default because it avoids the solarization and fluorescence seen in some borosilicates, and the very low TCE keeps wavefront stable under modest thermal load; the trade is a roughly 3-5× higher raw stock cost and longer cycle time on the polish line [S4].
For windows, sight glasses, and protective covers, sapphire is the top-end pick on hardness and chemical resistance; borosilicate soda-lime or BK7 is the budget pick when impact resistance is not on the spec, and engineered plastics such as polycarbonate or PMMA fill the weight- and cost-sensitive slots. The link between optical glass families and sight glass process-window hardware is mostly a shared raw-material supply chain, not a shared optical surface spec, and the two are not interchangeable without re-validating pressure and temperature ratings.
Standards, Traceability, and Sourcing Notes
There is no single international standard that defines "optical glass"; instead, the catalogs of Schott, Ohara, Hoya, CDGM, and a handful of US Defense Logistics Agency entries (encoded by MIL code) are treated as de facto references, and the Nd/Abbe/TCE data on the supplier certificate is what an incoming-quality lab measures against [S3][S4]. The optical comparator community uses these same catalogs as a common vocabulary: when a print calls out "SK16 or equivalent," the engineer reads that as Nd 1.620, Abbe 60.3, TCE 7.3, and a fine-anneal certificate, and the optical comparator stage on the shop floor is set up to verify radius and centration against that data set, not against a brand name.
Anneal grade is the easiest variable to get wrong in a general fab. A fine-annealed block carries a published Nd tolerance in the 5th decimal; a coarse-annealed block may drift a full unit in the 5th decimal after generating re-heats the bulk, which is enough to push a tight-tolerance achromat out of spec. A practical spec line is "fine-annealed, per supplier catalog, certificate of analysis with each lot," and that should be on every glass-fiber-fed glass fiber or glass quartz subassembly purchase order as well, since the same lot-traceability rule applies to those adjacent material families.
Trackable signals worth watching: published updates to the Schott, Ohara, and CDGM datasheet libraries, MIL code revisions issued by DLA, and new ISO 10110 drawings that pin surface-form and scratch-dig to tighter numbers than the legacy MIL-PRF-13830 calls for. For adjacent process work in the same plant, a quartz material selection guide covers the tooling-side constraints when the same fab cell is asked to switch from BK7 block to fused silica or quartz.