1. Read the Drawing as an Acceptance Agreement

An optical drawing defines more than the shape of a part. It connects material, dimensions, optical performance, surface condition and inspection into one manufacturing agreement. A component can meet its diameter tolerance and still fail an optical requirement; an excellent polished surface can still be unusable if its mounting features are incorrect.

Read the drawing number, revision, units, material specification and referenced standards first. Then identify the functional surfaces, datums, clear aperture and inspection notes. Resolve contradictory requirements before manufacture rather than assuming that the strictest-looking number explains the designer’s intent.

2. Map Each Requirement to Its Meaning

3. Interpret Dimensional Tolerances Correctly

A bilateral tolerance permits variation on both sides of the nominal dimension. For example, 25.00 ±0.05 mm gives an allowed interval from 24.95 to 25.05 mm. A unilateral callout of 25.00 +0.00/−0.10 mm gives 24.90 to 25.00 mm. These are arithmetic examples, not suggested tolerances for every optical part.

Confirm whether dimensions apply before or after coating, strengthening or assembly. For thin windows and curved parts, specify where thickness is measured: center thickness, edge thickness and dome wall thickness are not interchangeable.

A fit designation such as h6 requires the nominal size and the applicable fit standard and edition. It is not one fixed micrometre tolerance for all diameters. General title-block tolerances apply only within their defined scope; explicitly toleranced features require their own interpretation.

4. Use Datums, Wedge and Centration Deliberately

Datums establish references for measurement and assembly. A diameter, plane or axis used as a datum must be identified clearly. Optical and mechanical axes may not coincide unless the drawing requires that relationship.

For a nominally parallel window, wedge describes the angular difference between its faces. For a lens, centration or tilt requirements connect optical surfaces to an intended axis or datum structure. Do not replace these requirements with an outside-diameter tolerance.

Use a consistent geometric-tolerancing system and its stated edition. Avoid mixing notation from different systems without explaining how acceptance will be determined. Inspection fixtures should reproduce the intended datum references without introducing excessive clamping stress.

5. Define Clear Aperture and Edge Zones

The clear aperture is the region over which designated optical requirements apply. State its size, shape and location, and identify which requirements apply outside it. For domes or off-axis surfaces, clarify whether aperture dimensions are projected or measured on the surface.

A “90% clear aperture” statement needs a definition. For a centered circular part, 90% of diameter gives 81% of the full circular area. It is therefore not equivalent to 90% of area. An explicit diameter or bounded region avoids this ambiguity.

Specify chamfers, edge radii, permitted chips, coating exclusions and mounting lands separately. Defects outside the optical aperture may still matter for structural integrity, sealing or appearance. “Outside CA” does not automatically mean unrestricted.

6. Specify Curved Surfaces and Dome Geometry

A curved component needs enough independent dimensions to define its shape without conflict. For lenses, identify surface radii, signs or orientation conventions, center thickness and any aspheric prescription. For domes, identify inner and outer geometry, opening size, overall height, flange details and wall-thickness definition.

Do not calculate a general dome wall thickness as half the difference between two diameters unless the drawing actually defines concentric spherical surfaces and the relevant diameter convention. Offset centers or different surface shapes can produce nonuniform walls.

Radius tolerance and surface-form tolerance address different departures. A surface may be very smooth yet have an unacceptable radius. Define whether power or best-fit radius removal is allowed when evaluating irregularity.

7. Understand Scratch-Dig Surface Quality

Scratch-dig specifies discrete imperfections using a defined inspection system. Under the commonly used MIL-PRF-13830B approach, the scratch number is a visual brightness comparison with reference standards; it is not a direct scratch-width measurement in micrometres.

The dig number relates to dig size in hundredths of a millimetre. However, a complete acceptance decision also requires the applicable rules for defect count, distribution and cumulative limits. A 60-40 callout is not fully interpreted by checking only the largest visible mark.

Name the standard and edition, inspected region, reference set and viewing conditions. ISO surface-imperfection notation must be interpreted under the specified ISO method; do not silently convert a scratch-dig pair into an ISO indication. Surface quality is not a general ranking of image sharpness or laser performance.

8. Separate Surface Form from Wavefront Error

Surface form describes departure from the intended geometric surface. Transmitted wavefront error describes optical-path variation through a component and can include contributions from both faces, material homogeneity and mounting stress. Reflected wavefront error has its own geometry and interpretation.

If a requirement is written as λ/4, state the reference wavelength and whether the limit applies to surface height or wavefront, together with the evaluation aperture and metric. At 632.8 nm, one quarter of the wavelength is 158.2 nm, but that arithmetic alone does not identify the measured physical quantity.

For a simple single reflection at normal incidence, surface-height deviation produces approximately twice that deviation in reflected optical path. Do not apply this factor indiscriminately to transmission, oblique incidence, curved tests or a report already converted to surface units.

ISO 10110 treats surface form and wavefront specifications separately. The drawing must identify the intended requirement rather than relying on the word “flatness” to cover every optical error.

9. Distinguish P-V, RMS and Surface Roughness

Peak-to-valley (P-V) is the difference between the largest and smallest deviations within an evaluated data set. RMS summarizes the distribution of deviations relative to the stated reference. One cannot generally be converted to the other using a fixed factor.

Specify permitted data removal, filtering, sampling and invalid-pixel handling. Removing tilt, power or other fitted terms can substantially change a reported result. Reports from different settings should not be compared as if they were equivalent.

Surface roughness needs a parameter, units, measurement area and spatial-band or filter definition. Ra and Rq are different parameters; a roughness value does not replace scratch-dig or surface-form requirements. A polished surface can satisfy one category while failing another.

10. Write Complete Coating and Material Notes

Identify the exact substrate grade and any permitted substitutions. Include crystal orientation, homogeneity, inclusions or stress-birefringence requirements when they affect the application. A generic material family is not always sufficient for procurement.

For coatings, label S1 and S2 and specify wavelength range, incidence angle, polarization and contacting medium. State whether a transmission requirement is minimum, average or point-specific, and whether it describes the complete component or an individual surface.

Define masks, bonding zones, handling faces and relevant durability criteria. A note such as “AR both sides” does not establish a measurable spectral acceptance limit. Likewise, “optical polish” alone does not define roughness, surface form or defect acceptance.

11. Match Inspection Methods to the Requirement

These are method families, not universal equipment mandates. Choose a method with suitable capability and agree on how its result will be expressed.

12. Agree on Decision Rules and Inspection Records

Measurement uncertainty and instrument resolution are different concepts. Near a tolerance boundary, the conformity decision depends on the agreed decision rule. State whether and how uncertainty is considered; do not invent a guard band after parts have been produced.

Define sampling by characteristic: some features may need individual inspection, while others can use an agreed lot plan. “100% inspected” still needs a list of the characteristics tested on every part.

Request records that identify drawing revision, lot or part identification, method, test conditions, result and acceptance decision. Distinguish witness-coupon data from finished-component data. For curved or mounted optics, establish how representative the test is of the delivered configuration.

13. Resolve Ambiguities Before Quotation and Release

Create a written clarification list and approve changes against the drawing revision. Do not assume the 3D model overrides the drawing, or the reverse, unless the order establishes that hierarchy. Tolerance changes should preserve the functional requirement and receive approval before manufacture.

14. Frequently Asked Questions

15. Submit Your Drawing for Technical Review

Felix Glass can review custom optical component requirements with your drawing and application details. Send the controlled PDF, a 3D model where useful, operating wavelengths, quantities and required inspection documentation. Highlight critical mating features and optical zones.

Any proposed material substitution, tolerance adjustment or inspection alternative should be documented and approved before manufacture. The agreed drawing and order define the acceptance basis.

Dongguan Felix Glass Co., Ltd.
Email: james@felixglass.com