Most optical component disputes do not come from a supplier doing the job badly. They come from a drawing that left a decision open, and from two people reading the same line differently. This guide sets out the six groups of information that a workable optical glass specification contains, in the order in which they should be decided.
Buyer's Guides in practice
This article collects the practical guidance we give most often when customers ask us to specify this range, together with the numbers behind each recommendation.
Start with the function, not the material
The first line of a specification should say what the part does: a cover window, a filter substrate, a measurement path element, a mechanical spacer that happens to be transparent. The function determines which attributes can be relaxed, and almost every part has at least one that can. A cover glass needs hardness and clarity; a filter substrate needs homogeneity and coating compatibility; an interferometer window needs flatness and homogeneity above everything else. Naming the function also gives the supplier room to propose a cheaper material when the specified one is unnecessary, which is the single most common source of cost reduction on a first quotation.
Pin down the substrate by standard designation
Write the material as a standard designation rather than as a trade name, and add the attributes that matter for the application: index tolerance, homogeneity class, bubble and inclusion class, and striae grade. Where the part is supplied as sheet or wafer stock, add thickness tolerance and total thickness variation. If a substitution would be acceptable, say so in writing, because a substitution with no optical consequence can otherwise trigger a pointless change-control cycle - and an unapproved substitution with an optical consequence is exactly what a certificate requirement is meant to prevent.
Assign tolerances per feature, and price them
Give every dimension its own tolerance and know which ones are tight. Dimensional tolerances of plus or minus 0.05 mm are standard; plus or minus 0.01 mm adds cost. Optical tolerances are quoted in their own units: flatness in fringes, surface quality in scratch-dig, parallelism in arcseconds, roughness in nanometers. Chamfer, edge chip size and cosmetic criteria belong on the drawing too, because an unspecified edge will be whatever the operator considers normal. When the first quotation arrives, the fastest way to reduce it is usually to relax one optical tolerance rather than to shop for a cheaper supplier.
Decide coating and edge finish before the material is cut
Coating requirements change the substrate specification and the edge preparation, so they have to be settled early. State the band, the target reflectance or transmission, the angle of incidence at which the numbers are valid, and the environmental tests the coating must pass. State where the coating is allowed to extend and where it must stop, leaving a clear margin for mounting. On the edge, state whether a ground, chamfered or polished finish is required - a laser-cut edge and a polished edge are not interchangeable, and the difference usually reveals itself in thermal cycling rather than at incoming inspection.
Define inspection and the documentation package
Say what will be measured, on how many parts, and what record travels with the shipment. For most parts, 100 percent dimensional gauging plus sampling for cosmetics is the sensible split, with a spectral measurement added when the part is coated. The documentation package is worth specifying explicitly: material certificate, dimensional report, coating curve, environmental test results and lot traceability. Buyers who ask for these at quotation get them at no additional cost; buyers who ask after delivery usually pay for a re-inspection.
Write down the assumptions
Finish the specification with the conditions the part will experience: maximum temperature, cleaning chemicals, mounting method, whether it will be bonded, and the service life expected. These lines cost nothing and they resolve most of the ambiguity that otherwise surfaces at first article. They also give the supplier the information needed to choose a process rather than merely to meet a tolerance, which is where most of the difference between a good part and an adequate one is decided.
Selection data at a glance
| Material | Index (nd) | Typical use | Notes |
|---|---|---|---|
| B270 / soda-lime | 1.523 | Cover glass, filters, lighting | Lowest cost, quick polishing |
| BK7 (N-BK7) | 1.517 | General optics, windows | Excellent homogeneity, low bubble |
| Borosilicate 3.3 | 1.471 | Heat-resistant windows, thin glass | CTE 3.3 ppm/K |
| Eagle XG | 1.510 | Display, thin-sheet optics | Alkali-free, very low warpage |
| Fused silica / quartz | 1.458 | UV, laser, high-temperature | Deep UV transmission, high damage threshold |
| Sapphire | 1.768 | Scratch-proof windows, IR | Mohs 9, anisotropic machining |
| Blue glass (BG-type) | 1.52 - 1.55 | IR-cut for cameras | Absorptive NIR, color correction |
| Glass wafer | varies | Sensors, packaging | 0.1-1 mm, low TTV |
Frequently asked questions
Can you substitute B270 for BK7?
Only after checking the optical design. Their indices differ enough to move a coating's effective center wavelength and to change the focus of a lens. A substitution is fine for a cover glass or a mechanical window, and a bad idea for a filter edge or an imaging element, unless the design is re-optimized.
Which material should I use for a part that will be soldered or autoclaved?
A low-expansion substrate such as borosilicate 3.3, Eagle XG or fused silica. The failure you are avoiding is thermal shock and post-process warp, and both are driven by the coefficient of thermal expansion more than by strength.
How much does material choice affect lead time?
Soda-lime, B270 and standard borosilicate ship from stock in days. Fused silica, quartz and sapphire are frequently made to order, and their slower grinding and polishing rates add days to weeks on top of the material lead time.
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