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How to Use an Optical Flat
Guide · Updated October 27, 2026
An optical flat is a polished glass or quartz disk flat to a few millionths of an inch. Set it on a lapped surface under single-color light and you see dark and light interference bands. Their shape tells you how flat the surface is, without any other gauge.
Checking blocks before a stack?
Once the faces check flat, build the fewest-block stack in the free Gage Block Calculator. Free, no login.
How the bands work
The air wedge between flat and surface reflects light twice. Each dark band marks a step of half a wavelength in gap: about 11.6 µin (0.29 µm) under helium light. More bands just means more tilt. The shape of the bands shows flatness.
Reading the pattern
| What you see | What it means |
|---|---|
| Straight, parallel, even bands | Flat |
| Bands curve the same way | Convex or concave face; error = bow ÷ spacing × 11.6 µin |
| Closed rings | A high or low spot (press the center: rings move out if it's high) |
| Bands bend near one edge | Worn or rolled edge, or a burr |
Step by step
- Clean everything: Wipe the flat and the work with solvent and a lint-free cloth. One speck of dust lifts the flat and spreads the bands.
- Use monochromatic light: Work under a helium or other single-color lamp so the bands are sharp and each one has a known value.
- Seat the flat: Lay the flat on the surface and press lightly at one edge to tilt it until you see a few straight bands.
- Read the bands: Lay a straightedge along a band and compare the bow with the band spacing.
- Work out the error: Multiply bow ÷ spacing by 11.6 µin (0.29 µm) for the flatness error.
Worked example
Bands are spaced 4 mm apart and the middle band bows 1 mm from a straightedge: 1 ÷ 4 × 11.6 = about 2.9 µin out of flat. That's normal for a used gage block face.
Care of the flat
- Never slide a flat on a dirty surface, because grit scratches it for good.
- Store it in its case and handle it by the edges. Hand heat distorts the bands.
- See gage block accessories for stones, holders and cases, and gage block care for burr removal.
FAQ
How much is one optical flat band?
Half the wavelength of the light. Under a helium lamp (587.6 nm) one band is about 0.29 µm, or 11.6 microinches. Under ordinary white light the bands are colored and harder to judge.
How do you read flatness from the bands?
Lay a straightedge along the bands. Divide how far a band bows by the spacing between bands, then multiply by 11.6 µin. A bow of half a band spacing is about 5.8 µin out of flat.
What do straight, evenly spaced bands mean?
The surface is flat within a fraction of a band over the area you can see. The number of bands only shows the tilt of the flat, not error.
Can an optical flat check micrometer anvils?
Yes. Rest the flat on each anvil face and count the rings or bands. Many makers allow only a few bands on a micrometer anvil, so check your spec.
Can I check gage block faces with an optical flat?
Yes. Clean both, rest or lightly wring the flat on the block and look for curved bands near the edges, which show wear or a raised burr. Stone burrs off before you wring blocks again.
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