Some surfaces must be flat to fractions of a micron. Micrometer anvils, gauge block faces, lapped seals and valve seats all fall into this group. No dial or probe reads flatness at this level as quickly as light does. The optical flat uses light interference to show flatness errors you can count by eye.
What is an optical flat?
An optical flat is a precision disc of glass or fused quartz polished to a very high flatness. When placed on a reflective surface under monochromatic light, it produces interference bands called fringes. The shape and count of these fringes show how flat the surface is, to fractions of a micron.
Optical flats are reference standards, not measuring tools with a scale. They compare a test surface against their own near-perfect face. This makes them a core tool in toolrooms, calibration labs and precision lapping shops.
How does an optical flat work?
An optical flat works on light interference. A thin air wedge forms between the flat and the test surface. Light reflecting from both surfaces either reinforces or cancels, creating dark and light bands. Each dark band marks a change in air gap of half the light’s wavelength.
With a helium light source, the wavelength is about 0.59 µm. Each fringe therefore represents a height change of roughly 0.29 µm. Counting fringes and their curvature converts directly into a flatness value.
What do the fringe patterns mean?
| Fringe pattern | What it shows |
|---|---|
| Straight, parallel, evenly spaced bands | Surface is flat |
| Curved bands | Surface is convex or concave |
| Concentric circles | Spherical high or low spot |
| Irregular, wavy bands | Uneven or poorly lapped surface |
| Bands bend near the edge | Rolled-off or raised edges |
| Many closely spaced bands | Larger air wedge; press the flat more evenly |
Whether a curve is convex or concave is confirmed by a light press test on the flat, following the manufacturer’s reading guide.
How do you check flatness with an optical flat?
- Clean the test surface and the optical flat with lint-free tissue and a suitable cleaner.
- Let both reach the same temperature; handling warms the glass quickly.
- Place the flat gently on the surface. Do not slide it, as grit scratches both faces.
- Press lightly to reduce the air wedge until a few broad fringes appear.
- View the fringes square to the surface under a monochromatic light source.
- Lay a straight edge or imaginary line across the ends of one fringe.
- Estimate how far the fringe curves from that line, as a fraction of the band spacing.
- Multiply that fraction by about 0.29 µm (helium light) to get the flatness error.
Worked example
| Observation | Value |
|---|---|
| Fringe curvature | Half of one band spacing |
| Value of one band (helium light) | About 0.29 µm |
| Flatness error | 0.5 × 0.29 µm ≈ 0.15 µm |
Optical flat vs optical parallel: what is the difference?
An optical flat has one precision working face and checks flatness. An optical parallel has two flat faces that are also parallel to each other. Optical parallels check the parallelism of micrometer anvil and spindle faces. They usually come in sets of slightly different thicknesses, so the spindle can be tested at several rotation positions.
| Feature | Optical flat | Optical parallel |
|---|---|---|
| Precision faces | One (or two, used one at a time) | Two, flat and parallel |
| Main check | Flatness | Parallelism of two measuring faces |
| Typical use | Gauge blocks, seals, anvils, lapped parts | Micrometer anvil and spindle parallelism |
| Supplied as | Single disc | Set of graded thicknesses |
Where are optical flats used?
- Micrometer calibration: flatness of anvil and spindle faces
- Gauge block inspection: flatness of measuring faces
- Mechanical seals: lapped sealing faces in pumps and compressors
- Valve seats and fuel system parts: sealing surfaces in automotive and hydraulic components
- Precision lapping shops: checking parts during finishing
For the full micrometer calibration routine, see how to calibrate a micrometer. For gauge block care and setting, read how to use gauge blocks.
How do you care for an optical flat?
- Always lift the flat; never slide it across a surface.
- Store it in its case, working face protected.
- Clean with lint-free tissue and a residue-free cleaner only.
- Hold it by the edges to limit heat and fingerprints.
- Check the flat itself against a reference at defined intervals.
BTSA supports calibration of reference standards through NABL accredited calibration services at SV Precision Instruments. To check a certificate before filing it, read how to read a calibration certificate.
Where can you buy optical flats and optical parallels?
BTSA has supplied precision measuring instruments since 1942. View optical flats and optical parallels, or browse Mitutoyo measuring instruments.
Request a quote for optical flats and parallels
Frequently asked questions
What is an optical flat used for?
It checks the flatness of precision surfaces such as micrometer anvils, gauge blocks and lapped seals.
How does an optical flat measure flatness?
It creates interference fringes. Their shape and curvature show the flatness error.
What is the value of one fringe on an optical flat?
With helium light, one fringe equals about 0.29 µm, which is half the wavelength.
What do straight fringes mean?
Straight, parallel, evenly spaced fringes mean the surface is flat.
What is the difference between an optical flat and an optical parallel?
An optical flat checks flatness. An optical parallel checks the parallelism of two faces, such as micrometer anvils.
Why should you never slide an optical flat?
Sliding drags grit across both surfaces and scratches the precision face.
