Published July 7, 2026 at 09:03
Verifying flatness and parallelism on the measuring surfaces of precision measuring instruments is a classic metrology challenge. A micrometer’s spindle and anvil should be flat within 0.1-0.3 µm for the instrument to hold its specified accuracy. An angled or wavy measuring surface produces small but systematic errors on every measurement. The problem: how do you measure flatness with sub-micrometer accuracy, when most measuring instruments themselves are only accurate to ±5-10 µm? The solution has been the same since the 1900s — optical interferometry with a plane-parallel optical flat.
What the Schut plane-parallel optical interference plate is
Plane-parallel optical interference plate, single piece thickness 12.00 mm (art. 826.630) from Schut ECONOMY LINE is a precision metrology tool for flatness and parallelism verification. The instrument:
- Is a Ø30 mm cylindrical plane-parallel glass plate.
- Has a thickness of 12.00 mm.
- Has flatness on both measuring surfaces ≤0.15 µm (less than 1/3 of the wavelength of light).
- Has parallelism between measuring surfaces ≤0.3 µm.
- Is designed for micrometers with 0.5 mm spindle pitch.
- Is supplied in aluminum protective cases.
- Is compatible with ISO 17025 calibration certificates.
Basic specifications:
- Diameter: 30 mm
- Thickness: 12.00 mm
- Flatness of measuring surfaces: ≤0.15 µm
- Parallelism of measuring surfaces: ≤0.3 µm
- Material: Optical glass (fused silica or optical BK7)
- Application: Micrometer spindle pitch 0.5 mm
- Supplied in: Aluminum case
- Item number: 64-826630
Newton’s rings and basic optical physics
When a plane-parallel glass plate is placed against another surface (e.g. a micrometer’s anvil), a thin wedge of air forms between the two surfaces. Light reflected from the underside of the glass and from the top of the anvil interferes with itself. Result:
- Constructive interference where the air wedge is an even multiple of ½ the wavelength of light → bright bands.
- Destructive interference where the air wedge is an odd multiple of ¼ the wavelength of light → dark bands.
In practice:
- Natural white light (monochromatic at ~550 nm optimal): each colored ring corresponds to ~275 nm (0.275 µm) of height variation.
- Sodium light (589 nm monochromatic): each dark band corresponds to ~295 nm (0.295 µm).
Practical consequence: if the operator sees 2 colored rings across the anvil surface, the maximum height variation is ~0.55 µm. If the surface were perfectly flat, only a single uniform color would be visible.
How to verify micrometer flatness
Verifying micrometer spindle flatness with an optical flat:
- Clean the measuring surface thoroughly with a lint-free optical cloth and pure acetone or isopropyl alcohol.
- Clean the optical flat the same way (never with a coarse cloth — it can scratch).
- Place the flat on the measuring surface under a microscope or in diffuse light (an office lamp works).
- Press the flat lightly against the measuring surface.
- Observe the interference color rings (Newton’s rings).
- Count the number of complete ring sequences across the measuring surface.
- Multiply by 0.275 µm per ring to get the height deviation.
Standard tolerances for micrometer measuring faces:
- New micrometer: 0-1 ring (max ~0.3 µm deviation).
- Used but OK: 1-2 rings.
- Near limit: 2-3 rings (requires adjustment or replacement).
- Not approved: >3 rings. The instrument produces sub-micrometer errors that compromise its accuracy specification.
Parallelism verification — two flats in a set
Parallelism verification is harder than flatness verification. The standard method is to use a set of 4 flats of different thickness (typically 12.00, 12.12, 12.25, and 12.37 mm). Each flat is placed between the micrometer’s spindle and anvil at full closure. If the spindle and anvil surfaces are parallel, even interference color rings appear on both measuring surfaces.
If they are not parallel (angled), line-shaped interference appears (parallel lines instead of concentric rings). Each line corresponds to ~0.3 µm of angular deviation per mm of width.
Four flats of different thickness are used because the micrometer’s spindle rotates 90° for every 0.125 mm of travel — so different thicknesses show the spindle at different rotational positions.
Why this method is so unique
Optical interference is one of the FEW measurement methods that doesn’t require a reference gauge with better accuracy than what’s being measured. The interference phenomenon provides absolute accuracy based on the wavelength of light — a fundamental physical constant. This provides:
- Sub-micrometer accuracy without needing external references: The wavelength of light is calibrated by nature.
- Direct visual measurement: The operator sees the interference pattern with the naked eye (or a microscope for higher precision).
- No electronics: No battery, no adjustment, no calibration drift.
- Long service life: An optical flat can last for decades without degradation, provided it isn’t scratched.
Drawback: the method only measures flatness and parallelism — not absolute dimension. Dimensional verification requires gauge blocks or other references.
Where an optical flat makes a real difference
Calibration labs: Verification of the measuring faces of micrometers, dial indicators, and comparators. Sub-micrometer truth regardless of the production calibration chain.
Micrometer manufacturers: Final QC of manufactured instruments before shipment.
Workshop QC: Annual flatness check of the company’s micrometer set.
Gauge block set verification: Individual gauge blocks are verified for flatness with an optical flat before use in a ”wrung stack”.
Measuring flat manufacturing: Reference plates for CMM calibration are themselves verified with an optical flat.
Optical instrument manufacturing: Precision optical surfaces in telescopes, microscopes, and laser instruments must be flat within ¼ wavelength of light (~140 nm).
Semiconductor industry (wafer testing): Calibration plates for wafer metrology.
Metrology training: Technical education demonstrating optical-physical measurement principles.
Restoration and calibration of old precision instruments: Antique metrology instruments from the 1900s can be verified with the same optical flat the factory used in 1950.
Reference metrology for ”new” surface quality: Anti-wear plates on the workshop table are verified for flatness.
Handling and care
Optical flats are extremely sensitive to mechanical and chemical damage:
- Never rub against another hard surface: Dust particles between the flat and the surface can scratch both surfaces instantly.
- Clean only with a lint-free optical cloth: Ordinary paper or cotton cloth can leave scratches.
- Chemicals: Pure acetone or isopropyl alcohol for removing grease. Never ammonia or other alkalis (can etch the glass over time).
- Storage: In the aluminum case or a padded box. Never loose in a workshop drawer.
- Thermal stability: Temperature-stable environment. Avoid direct sunlight or heat sources.
- Regular calibration verification: Even optical flats should be verified every 3-5 years against a reference flat at a DAkkS or SP lab.
What you get for your money
Schut ECONOMY LINE plane-parallel optical interference plate 826.630, Ø30 mm × 12.00 mm thick, flatness ≤0.15 µm on both measuring surfaces, parallelism ≤0.3 µm between measuring surfaces, optical glass (BK7 or fused silica), compatible with micrometer spindle pitch 0.5 mm, aluminum case for protected storage, compatible with ISO 17025 calibration certificates.
1,941 kronor is the instrument investment. Compared to CMM verification of micrometer flatness at an external calibration lab (500-1,500 kr per instrument per verification), an optical flat pays for itself after just 2-3 in-house verifications. For a workshop with 10-20 micrometers that should be verified annually, the investment is a no-brainer.
Read more: Schut optical interference plate 826.630 in the shop →
