Scratch-Dig vs ISO 10110: Understanding Surface Quality Specifications
How MIL-PRF-13830B scratch-dig and ISO 10110-7 describe the same surface defects, where conversion breaks down, and how to pick a grade by application.
Surface quality is the most frequently mistranslated requirement in precision optics. A drawing that says 20/10 is unambiguous in one system and meaningless in another, and the confusion is expensive in both directions: a component specified too loosely fails at the detector, and one specified too tightly is rejected at goods-in for defects that no optical measurement would ever notice. This note explains how MIL-PRF-13830B scratch-dig and ISO 10110-7 describe the same physical problem, and how to choose between them.
What scratch-dig actually measures
Scratch-dig is a two-number code. The first number grades a scratch by visual comparison against a set of standard scratches, and the resulting figure corresponds approximately to the scratch width in micrometres. The second number grades a dig — a pit, bubble or coating void — by its diameter in hundredths of a millimetre, so a dig of 10 means a pit no larger than 0.10 mm across. Our three production grades are 40/20 (Commercial), 20/10 (Precision) and 10/5 (High Precision), with the same code applied across lenses, prisms and flats in the tolerance chart.
Two characteristics of the system deserve attention. First, it is comparative rather than dimensional: the inspector judges a scratch against a reference set under defined illumination, which introduces operator judgement, though the judgement is well bounded in practice. Second, the acceptance applies across the surface without a count limit — 20/10 means no scratch wider than the 20 reference and no dig larger than 0.10 mm, but the standard does not say how many smaller defects are permitted. In practice we apply our own internal count discipline on top of the code, but the drawing does not express it.
What ISO 10110-7 measures instead
ISO 10110-7 is quantitative. The tolerance is written into the drawing as an imperfection code in the form 5/N × A, where N is the permitted number of imperfections and A is the maximum permitted size in millimetres. Long scratches and localized digs are handled in separate notations, and the standard also covers edge chips and coating imperfections. Because size is expressed in millimetres and count is explicit, the requirement can be inspected with a measuring reticle rather than a comparison plate, and the result is closer to a dimensional measurement than to a visual judgement.
The practical consequence is that ISO notation can describe a zone-dependent requirement — for example a tighter imperfection count inside the central 80% of the clear aperture and a looser one outside it. MIL scratch-dig has no native mechanism for that, so zone-dependent requirements are usually written as a note on a MIL drawing, which invites interpretation.
| Attribute | MIL-PRF-13830B scratch-dig | ISO 10110-7 |
|---|---|---|
| Basis of acceptance | Visual comparison with standard scratches | Measured size in millimetres plus permitted count |
| Scratch definition | Reference width grade (10, 20, 40, 60, 80) | Maximum length and width, count-limited |
| Dig definition | Diameter in hundredths of a millimetre | Maximum size in millimetres, count-limited |
| Count limit | Not explicit in the code | Explicit (N in 5/N × A) |
| Zone-dependent limits | Written as a drawing note | Native to the notation |
| Coating defects | Covered by general practice | Separate coating imperfection notation |
Converting between the two systems
Dig conversion is straightforward because both systems are dimensional: MIL dig 10 corresponds to 0.10 mm, dig 5 to 0.05 mm, dig 20 to 0.20 mm. Scratch conversion is not exact. The MIL scratch number is a comparative grade derived from a reference scratch width, while ISO requires a measured width and an accompanying count, so a "20 scratch" and "0.02 mm scratch" are close but not formally equivalent. Anyone publishing a conversion table should treat scratch equivalence as indicative and confirm it with the supplier before writing it into a purchase specification.
Our approach is to manufacture and inspect to the system named on the drawing, and to supply the corresponding inspection record. Where a customer's end user requires ISO notation but the supply chain works in MIL, we will quote against the MIL grade that satisfies the equivalent ISO imperfection count, and state the basis on the certificate of conformity.
Choosing the right grade for the application
Surface quality should follow the optical function. A window protecting a sensor array can live with 40/20 if the defect sits outside the imaging conjugate; a lens imaging onto a small detector should sit at 20/10; a part in a laser cavity, a lithography relay or an interferometer reference arm usually needs 10/5. The flat optics range shows where this matters most — windows and beam splitters are inspected across a large aperture, so a single dig can sit exactly where the beam passes.
Two rules reduce cost without reducing performance. First, define the clear aperture explicitly: 80%, 90% or 95% of diameter. Most surface defects cluster at the edge, so a realistic clear aperture removes the majority of rejections. Second, do not tighten surface quality to compensate for a coating problem. Coating defects have their own acceptance criteria, and a scratch-dig call-out will not control pinholes, haze or delamination.
Lighting and inspection conditions also belong on the drawing when the requirement is tight. A 10/5 part inspected under conditions different from the supplier's will produce a disagreement that neither party can resolve from the certificate alone.
Where defects come from, and what to do about them
Most scratches originate in handling and cleaning rather than in polishing: a wipe with a dry lens tissue, a slide across a work surface, a part stored face-down. Dips are more often polishing or coating artefacts, and their distribution — clustered near the edge or scattered across the aperture — usually identifies the process step. When a lot fails repeatedly, the useful question is not "how do we inspect better" but "which step changed", and that is why our in-house manufacturing flow keeps polishing, coating and final inspection under one quality system with one corrective-action path.
Coatings change the picture. A coated surface shows defects more clearly than a bare one, and some coating processes can bridge a small pit, only for it to open during cleaning. For coated parts, specify both the substrate surface quality and the coating imperfection acceptance, and state the cleaning regime the optic will see in service. Repeated solvent cleaning is a legitimate durability requirement and should be declared at quotation stage.
Need a surface quality grade confirmed against your drawing? Send your drawing, specification or beam profile and our engineering team will review it and reply with a quotation.Talk to our quality engineers
Frequently Asked Questions
Is 20/10 scratch-dig better than 40/20 for imaging lenses?
Yes in the sense that it is tighter: a 20 scratch and a 0.10 mm dig are smaller defects than a 40 scratch and a 0.20 mm dig. Whether you need it depends on the imaging conjugate and the illumination. Most imaging and laser applications are satisfied by 20/10; 40/20 is adequate for illumination, protective windows outside the beam and non-critical condenser optics.
Can you supply parts inspected to ISO 10110-7 if the drawing uses scratch-dig?
Yes. We can inspect and report against ISO notation for the equivalent imperfection count, and we will state on the report which system was used. For programmes that must be documented in ISO form, it is cleaner to issue the drawing in ISO notation from the start so that the acceptance criteria and the inspection record use the same language.
Does scratch-dig apply to the coating as well as the substrate?
It is usually applied to the finished surface, which includes the coating. Some customers specify substrate quality and coating imperfection separately, which is the more precise approach for coated optics. We recommend the separate call-out whenever the coating is a functional layer rather than a simple anti-reflection layer.
How is the 10/5 grade achieved in production?
Through controlled polishing, dedicated handling from polish to coating, protected storage between steps, and final inspection under defined illumination before packing. The grade is maintained by process discipline rather than by sorting, which is why we do not offer it as an inspection-only upgrade on parts manufactured to a looser process.
What measurement do you report for surface quality?
We report the grade achieved, the inspection method and any defects outside the acceptance criteria, together with the measured dig sizes where relevant. Surface form and roughness are reported separately, since scratch-dig says nothing about figure accuracy — a part can be 10/5 and still be a full wave out of figure.
