Scanning a pipe surface for cracks with line scan imaging

Cracks on the outside of a pipe hide in a curved, often reflective surface that no single area-scan photo covers at crack-level resolution. The arrangement that works is mechanical: mount the pipe on a rotary stage, park a line scan camera above the top of the tube, and let rotation sweep every line of the surface under the sensor. This plan gives you the numbers that size the system: how fine the pixels must be for a given crack width, how wide a strip one pass covers, what line rate the spin demands, and which light makes a crack visible on bare metal. A worked example sizes the complete set-up for cracks down to 0.1 mm on a 100 mm pipe spinning at one revolution per second.

Why spin the pipe under a line scan camera

The sensor line lies across the pipe axis, just above the tangent point, and rotation feeds new surface through that line at a constant rate. The image builds line by line: 2048 pixels along the axis, thousands of lines around the circumference. Because the camera never moves and the lens-to-surface distance never changes, focus and light hold for the whole sweep, and every pixel keeps the same scale — no perspective stretching of a curved part, which is why this job calls for line scan cameras rather than one area snapshot.

One pass covers a narrow strip (60 mm wide in the worked example); longer pipes are inspected by stepping the camera along the axis between rotations, with about 10% overlap so no seam slips between strips. Motion sync works exactly as it does on a moving web, and the line scan field of view calculation carries over unchanged.

Start from the crack width, not the camera

Decide the smallest crack you must catch, then set the pixel scale to half of it or finer. Two pixels across a crack is the practical detection floor; three pixels or more let you measure its width. So the inspection target asks for 0.05 mm per pixel, and a 0.05 mm target asks for 0.025 mm. Pixels set the ceiling — contrast comes from the light — but no lighting trick recovers a feature finer than one pixel. Fix the pixel scale first; every other number follows from it, and the same defect-size-to-pixel logic used in semiconductor wafer inspection applies here unchanged.

Worked example: cracks on a 100 mm pipe at 1 rev/s

Inputs, each from its source:

• Smallest crack: 0.1 mm wide — the inspection target.
• Camera: the LS-LCM-0016, a 2K GigE line scan camera with 7 μm pixels, TDI and a 58 kHz line rate (catalog data).
• Lens: the LS-LSL-0215, a 25 mm line scan lens working at 0.24x on an M42 mount (catalog data).
• Pipe: 100 mm outside diameter, bare metal.
• Spin: 1 rev/s (60 rpm), servo driven.

The rules in words: surface pixel equals pixel size divided by magnification; line rate equals surface speed divided by surface pixel; exposure per line is the reciprocal of line rate.

The arithmetic:

1. Surface pixel: 7 μm ÷ 0.24 = 29.2 μm ≈ 0.029 mm — finer than the 0.05 mm the target requires. Call this the pixel scale for the rest of the example.
2. Strip width: 2048 pixels × the pixel scale = 60 mm along the pipe axis (sensor length 2048 × 0.007 mm = 14.3 mm, well inside the lens image circle).
3. Surface speed: π × 100 mm × 1 rev/s = 314 mm/s.
4. Line rate: 314 mm/s ÷ the pixel scale = 10,770 Hz ≈ 10.8 kHz.
5. Exposure cap: 1 ÷ 10,770 Hz = 93 μs per line.
6. Data rate: 2048 bytes × 10,770 lines/s ≈ 22 MB/s at 8-bit output, comfortable on a GigE link.
7. Crack floor: 2 × 29.2 μm ≈ 0.058 mm detected; 3 × 29.2 μm ≈ 0.088 mm measured.

Result: 10.8 kHz demanded against a 58 kHz ceiling — 5.4x headroom. Decision: build it exactly as listed for any spin up to about 5 rev/s (1,690 mm/s of surface speed) at this pixel scale. The threshold that disqualifies the camera is surface speed ÷ pixel scale passing 58,000 Hz; below that line, this hardware clears the target at the 0.058 mm detection floor, with room left if the target tightens later.

Camera, lens and light

TDI earns its place here: with the example's 93 μs exposure cap, the sensor accumulates the moving image over successive rows before readout, lifting signal-to-noise without slowing the scan. TDI needs constant image speed in the sensor's integration direction, which a steady servo drive on the stage provides.

The LS-LSL-0215 sets the optics: 25 mm focal length, 0.24x, M42 mount. Object distance at 0.24x is f × (1 + m) ÷ m = 25 mm × 1.24 ÷ 0.24 ≈ 129 mm to the surface, before the barrel's offset — set the working distance by focusing on the tangent line, then measure the achieved strip on a target. The line scan lenses category sorts alternatives by magnification and image circle.

Mount the line light so the beam meets the pipe at a shallow angle, roughly 15-30° above the surface, with the bar running across the strip at right angles to the pipe axis, so the rake direction crosses axial cracks. Cracks then read as thin dark or bright lines against the flat surface — the dark-field behaviour behind most crack and scratch lighting on line scan systems. Rotate the light 90° and take a second pass for circumferential cracks, or fit a double-angle bar and cover both directions in one pass. The bright-field to backlight overview places these options on the map.

The example's light is the LS-LSC-0441, a 493 x 60 mm oblique line light, natural cooled, on 24 V. For both rake directions without a second pass, the LS-LSC-0046 is a 500 x 53 x 121.5 mm double-angle oblique light, also 24 V. The line scan lights category filters by length, wavelength and connector.

Short exposures put the load on the light. Drive it from a lighting controller that strobes or overdrives the LEDs in step with the line trigger, and size the power the way the fast-line power guide works it out. A pulse that short, repeated for a one-second pass, is an easy duty cycle; continuous scanning across a full shift is where the power budget tightens.

Exposure, sync and the motion-blur trap

If the line rate does not match the surface speed, the image stretches or compresses along the motion direction: a crack drawn 20% too long is a measurement error, not a defect clue. Square pixels mean millimetres per line equal to the pixel scale — the line rate walkthrough shows the camera-side settings, and encoder-driven capture removes speed drift entirely by clocking lines from the stage's rotary encoder. TDI adds one more requirement: the image must travel in the sensor's integration direction, and drive speed ripple shows up as banded lines, so the stage's motion quality matters as much as the camera's.

The decision, and when imaging is not the answer

• Cracks 0.1 mm and wider, pipe up to 200 mm OD, 1-2 rev/s: build the worked example as listed — it clears the target with 5.4x line-rate margin.
• Cracks down to 0.05 mm: tighten the pixel scale to 0.025 mm (0.28x magnification). Line rate rises to 12.6 kHz and the strip narrows to about 51 mm, so passes increase.
• Spin faster than about 5 rev/s at this pixel scale: the 58 kHz ceiling binds. Pick a faster-rated model from the camera finder or start at the camera step of the configurator.
• Subsurface cracks or the pipe bore: a visible-light camera sees only the surface. Interior work is its own geometry — the glare control plan for pipe interiors and the inspection of a pipe under pressure cover that case — and true subsurface crack detection belongs to ultrasonic or eddy-current NDT, with imaging as the surface record.

Parts list for the worked example

More build plans like this sit on the applications topic page. If your pipe's diameter, length or crack target differs from the example, the same four steps — crack width to pixel scale, pixel scale to lens, geometry to line rate, contrast to light — resize the whole system, and the contact page is where the numbers get checked against your part before ordering.