To read light bands, rest an optical flat on the clean, lapped seal face under a monochromatic lamp, find where the flat touches, and count how many band spacings one band bends away from straight: each band of bend is one helium light band, or 0.29 µm (11.6 µin) of flatness error.

Step 1: Get an Optical Flat and a Monochromatic Light

You need an optical flat at least as large as the seal face and a monochromatic lamp, normally helium.

A working-grade flat is good enough for seal faces. A sodium lamp’s band is well under 1 % larger than a helium band, so either lamp gives the same reading on a seal face.

Step 2: Prepare the Seal Face for Reading

Clean the face and the flat until nothing sits between them, then let both reach the same temperature before you read.

Wash every trace of lapping compound off the face with solvent and wipe it with lint-free tissue. Wipe the flat’s working surface the same way, and handle both by their edges. Leave the face and the flat side by side on the bench before reading.

Step 3: Place the Optical Flat on the Face

Lower the flat gently onto the face, working surface down, and let it rest under its own weight without sliding it across.

To keep dust from settling as you lower the flat, you can lay a sheet of clean lens paper on the face, set the flat on the paper and draw the paper out from between them.

Read the face in the condition it will run in. A face that reads flat on its own can read well out of flat once it is pressed or fitted into its holder, as a 2024 EPRI technical brief on main reactor pump seals found, so check a pressed-in carbon or clamped face before and after it goes into its holder.

If No Bands Appear

No bands usually means the flat lies parallel to the face, and a crowd of fine bands means something is holding one edge of the flat up.

  • An even field with no bands: press lightly on one side of the flat to tilt it, and bands appear.
  • No pattern at all on a dull face: a face that is not lapped to a reflective finish cannot show bands, and has to be lapped before it can be read.
  • Many fine, tightly packed bands: a particle or burr is almost always holding up one edge of the flat. Lift the flat off, clean both surfaces and set it down again; bands that stay crowded after cleaning point to a raised burr on the face.

Step 4: Find the Point of Contact

The flat touches the face where the gap is zero, and you find that point by pressing lightly on the flat near one edge and watching the bands move.

Pressing on the side away from the contact spreads the bands out and moves them away from the contact, so fewer of them cross the face. Pressing directly over the contact changes almost nothing. Ease the pressure until a few bands lie across the face, far enough apart to follow each one.

Look straight down through the flat, with your eye square above the face.

Step 5: Count the Bands

Pick one band that runs across the whole face, imagine a straight line joining its two ends, and count how many band spacings the middle of the band departs from that line: that number is the face’s flatness error in light bands.

If the middle of a band reaches the next band, the face is one band out of flat; if it reaches halfway, half a band. A thin straightedge laid on top of the flat along the band’s ends makes the line easier to hold.

The number of bands crossing the face says nothing about flatness: it is set by the tilt you gave the flat in Step 4.

On a seal face the bore interrupts the bands. Read a band as one line from outer edge to outer edge, joining its pieces either side of the bore.

Converting Bands to Microns

Multiply the band count by the value of one helium light band, 0.29 µm or 11.6 µin.

Flatness error (µm) = light bands × 0.29

Flatness error (µin) = light bands × 11.6

Whether that number passes depends on the face material, its size and the seal’s duty; the limits are in our guide to seal face flatness tolerance by material and standard.

Step 6: Tell Convex From Concave

With the flat touching along one edge, bands whose middles bow away from the contact line mean the face is convex, high in the centre, and bands whose middles bow toward the contact line mean it is concave, low in the centre.

When the flat rests on the centre of the face or on its rim, the bands close into rings and there is no contact line to compare against. Press on one edge of the flat until the rings open into curved bands with the contact along one side, then check which way the middles of those bands bow against the contact line.

Step 7: Read the Band Pattern

Straight bands mean a flat face; every other pattern shows where the face is high or low, and most of those shapes trace back to the lap.

  • Straight, parallel, evenly spaced bands from edge to edge: the face is flat.
  • Bands curved evenly in one direction: the whole face is convex or concave, and Step 6 tells you which. A convex face usually comes from rocking the part during hand lapping or from a lap plate that has worn dished; a concave face from a plate worn high in the middle.
  • Concentric rings: the face has a dome or a dish centred on the rings. With no tilt on the flat, each ring is one band of height, so the ring count from centre to edge is the error.
  • Bands straight across the face that bend sharply at the inner or outer edge: that edge is rounded off, usually by too much lapping pressure or by rocking. A deliberate chamfer drops the bands too, so leave the chamfer itself out of the reading.
  • S-shaped or saddle-shaped bands: the face is twisted or wavy, which on a seal face often comes from clamping or an uneven holder rather than from lapping.
  • Bands with local bumps or breaks: a local high or low spot, such as a scratch, a pit or a patch the lap has not reached.

A face whose error is inside its tolerance goes back into service; one outside it goes back to the lap, with the pattern showing what to correct first.