Mid-Spatial Frequency Errors: An Optical Insert Review Plan

An optical insert can have an acceptable overall shape and still show a repeating surface pattern that matters to the optical system. A useful purchasing specification should explain how such patterns will be measured, reported and evaluated, rather than relying on one surface-quality number.

Mid-spatial frequency, often abbreviated MSF, describes surface variations between broad form errors and fine roughness. The practical boundary depends on the measurement and optical application. This article proposes a review workflow for buyers of diamond-machined optical inserts; it does not supply a universal acceptance limit.

Start with the optical consequence

Ansys’s tooling-error guidance explains that mid-spatial surface structure can affect imaging resolution, stray light and illumination uniformity. It also discusses periodic errors associated with diamond turning. A surface that looks bright under ordinary inspection therefore still needs a measurement appropriate to its intended use.

Ask the optical designer to identify the performance metric at risk. For an imaging part, that may be contrast at specified field positions. For an illumination component, it may be an agreed intensity distribution. Write the intended decision in the inspection plan so the measurement team knows what information the designer needs.

Define the measurement band and reporting method

A request for “low MSF” cannot be priced or inspected consistently. Agree the spatial-frequency or spatial-period range, units, usable aperture and analysis method. If a power spectral density report is requested, identify the convention and units used in the software and whether the result comes from a line profile or an area map.

Ask for the measurement resolution and coverage to be reviewed against the specified band. Record filtering, detrending, form removal and excluded regions. The supplier and buyer should exchange a representative data file before the production inspection, then confirm that both obtain a comparable result using the agreed processing settings.

Keep the unprocessed measurement file as well as the released report. A screenshot can explain a result, but it may not retain enough information to investigate a disagreement. The existing guide to optical insert measurement correlation provides a related handoff framework.

Look at direction and location

Define the orientation of the insert and the coordinates of each reported region. A radial trace, a tangential trace and an area map answer different inspection questions. Request the views needed for the actual geometry instead of assuming one center trace represents the entire optical aperture.

For an array, identify which features are measured and how they are registered to the component datum. If only selected locations are inspected, state that coverage explicitly. Additional sampling should follow a documented reason, such as an observed pattern or a design-sensitive region, rather than a search for a better-looking result.

Separate evidence from cause

A 2024 optical-manufacturing research abstract lists several potential contributors to MSF errors, including machine dynamics, tool condition and process parameters. A measured repeating pattern alone is not sufficient to assign a particular root cause.

When a result fails the agreed requirement, ask the manufacturer to connect its investigation to the insert identifier, machining route and measurement evidence. Record the proposed correction and the required reinspection before approving further cutting or polishing. Keep responsibility for optical acceptance with the designated reviewer.

Connect insert review to molded-part approval

The release plan should retain separate records for the insert surface and the resulting molded optic. Identify which trial parts came from the reviewed insert and which molding conditions were used. If optical performance is unacceptable, compare those records before concluding that the insert alone caused the problem.

• Document the optical performance concern and responsible reviewer.
• Specify the measurement band, units, aperture and data processing.
• Agree spatial coverage, orientation and sample locations.
• Retain raw data, released reports and any exclusions.
• Record correction approval and post-correction inspection.
• Link insert identification to trial-part and system-test results.

This approach makes an MSF requirement actionable. It lets the buyer ask for evidence that supports a specific acceptance decision without imposing an arbitrary number copied from an unrelated optical design.