Under vehicle inspection with a line scan camera: system plan

An under-vehicle scanner has one job: hand the operator a single continuous image of the whole underbody while the vehicle creeps past. A line scan camera does this naturally — it photographs one narrow strip per line and builds an image as long as the lane, with no frames to stitch. This plan takes you from scan width to camera, line rate, lens geometry, lighting and mounting, and works the arithmetic for a typical case: a 2 m lane, a 2K camera, a 0.5 m/s crawl. By the end you can size every part of the system with the numbers written down.

Why line scan suits the underbody

The underbody is a continuous web moving at crawl speed, exactly what line scan was built for. An area camera would need overlapping frames and stitching software; a line camera emits one unbroken image. Exposure applies per line, so it can stay short enough to freeze motion even with almost no ambient light. The task here is screening — attached objects, damage, leaks, tampering — which needs about 1 mm ground resolution, not microns. The same pixels, speed and blur arithmetic that sizes a conveyor camera sizes this system; the sorting conveyor camera guide works that general case.

Choose the line length: 2K or 4K

Start from the lane, not the camera. Measure the widest vehicle you must cover and allow 2.0 m for vans. Pixels divide that width: a 2K sensor gives 2000 mm ÷ 2048 ≈ 1 mm per pixel; a 4K sensor halves it to about 0.5 mm. For security screening, 1 mm is plenty, because anything hidden under a car spans dozens of pixels. For rust, hairline cracks and defect work, take the 4K.

A mount note before you default to photographic glass: only 8 of the 170 line scan cameras in the catalog carry C-mount, against 71 on M42 and 66 on M72, so plan the optics around M42.

Need more than 2 m from one sensor? Two cameras side by side with matched line rates, overlapped so the seam falls on repeatable content — the line scan image overlap guide covers the boundary math.

For this lane, the 2K GigE camera below is the base choice:

For color, use the MV-CL042-91GC-V2: a 4K GigE color camera with 7 μm pixels, a 29 mm image circle and an M42 mount. Over the same 2 m strip that is about 0.5 mm per pixel.

Worked example: 2048 pixels over a 2 m lane at 0.5 m/s

Inputs, and where each comes from:

• Sensor line: 2048 pixels at 7 μm pitch, so the line is 2048 × 7 μm = 14.3 mm wide (catalog data for the 2K camera above).
• Scan width: 2.0 m, measured on the widest vehicle served.
• Speed: 0.5 m/s, a walking-pace crawl under attendant control.
• Working distance: 1.2 m from optic to underbody, set by the ramp or mirror geometry.

The rule in words: for square pixels, the camera must expose one line every time the vehicle advances one ground pixel.

Arithmetic:

• Ground pixel: 2000 mm ÷ 2048 px = 0.98 mm — call it 1 mm.
• Line rate: 500 mm/s ÷ 1 mm per line = 500 lines per second.
• Exposure: 0.5 mm ÷ 500 mm/s = 1 ms ceiling, to keep blur under half a pixel.
• Data rate: 2048 px × 500 lines/s ≈ 1 MB/s, about a hundred times under a GigE link.

Result and decisions. The lane asks 500 lines per second. The LS-LCM-0016 runs to 58 kHz, more than a hundred times that, so any crawl speed works and camera choice is not the hard part. The binding constraint is light: with exposure capped at 1 ms, the line light must deliver contrast inside that window, which is the arithmetic in the line scan light power guide. Focal length follows from the geometry, next section.

Match the line rate to the actual speed

500 lines per second is only correct at exactly 0.5 m/s. A driver at 1 m/s on a fixed line rate stretches motion pixels to 2 mm — twice the cross-track pixel — and any overlay against a previous scan stops registering. Hold the match one of three ways:

1. Encoder. A roller in the wheel path pulses one line per pixel of travel, so the image stays square at any speed. Wiring is a quadrature input; see the encoder-triggered capture guide.
2. Runtime follow. Read vehicle speed and update the line rate continuously; the runtime line-rate guide shows the control loop.
3. Fixed rate plus lane discipline. Mark the crawl speed, brief the driver, accept some stretch. Simplest, weakest.

Whatever you choose, set the value deliberately — the line rate guide walks the setting itself.

Lens and geometry: direct view or a 45° mirror

The lens must squeeze a 2 m strip onto a 14.3 mm sensor, so focal length follows from the field of view; the field of view guide shows the full method, and the short form is:

• Direct view, optic 1.2 m under the vehicle: f = 14.3 mm × 1200 mm ÷ 2000 mm = 8.6 mm.
• 45° mirror beside the wheel path, folded path 2.2 m: f = 14.3 mm × 2200 mm ÷ 2000 mm = 15.7 mm — an ordinary 16 mm lens.

Below roughly 10 mm focal, optics go extreme wide-angle: perspective stretch grows toward the frame edges and the viewing angle varies hard across the strip. That is why the mirror wins in most lanes — the camera and lens sit at ground level out of the wheel paths, and only the mirror lives under the vehicle.

Buy from the line scan lenses range rather than photographic glass: resolution is graded for pixel pitch and line length, as the line scan lens selection guide explains, and the image circle must cover the sensor — the 4K color unit needs a 29 mm circle.

Lighting: graze the underbody from the sides

An underbody sees almost no daylight, so the line scan lights supply every photon, and low angle does the work: light entering at 30-45° from horizontal throws a shadow behind every attachment, bolt head and rust blister, which is what makes objects read in an otherwise flat gray image. Mount the light alongside the scan line, angled across, and make the lit length at least the scan width.

Two catalog units cover this lane. The LS-LSC-0017-L25 is a 1092 mm line scan light on 24 V, and the LS-LSC-0441 is a 493 x 60 mm oblique line light with natural cooling, also 24 V. Two of the long units end to end span 2.18 m over the 2 m strip; place one oblique unit at each lane edge to cross-fire the low-angle component.

With a mono camera, pick one color and match the light to it — white, red or blue all give strong contrast on road grime, so test on the dirtiest underbody you serve. With the color camera, use white. Exposure demand is modest at a 1 ms ceiling, but work the numbers in the light power guide above, and run dimming through a light controller; the line scan lighting controller guide covers what they fix.

Mounting, environment and the interlock

Three mechanical facts decide whether images stay usable. Rigid mounting: one pixel is 1 mm of underbody, so a housing that shakes with passing traffic smears lines visibly — bolt the camera to a mass, not a cantilever. Clean optics: the mirror or window lives in road spray, mud and salt, so specify a sealed, spray-rated housing with a window you can wipe down, and check it nightly. Shrouding: block daylight at the lane edges so the 1 ms exposure budget is spent on light you control.

Calibration is one ruler: lay a scale across the lane, scan it, confirm the mm-per-pixel value in both directions. The moving checkerboard calibration guide covers the full method.

Re-fitting an older gate whose camera has an analog output, rather than building fresh? The optics and mounting carry over; the signal side does not — see replacing a legacy line scan camera with a 4-20 mA output.

Checks before you order

• Scan width measured on the widest vehicle served, not assumed.
• Ground pixel target set: 1 mm for screening, 0.5 mm for defect work.
• Line rate computed as fastest realistic speed ÷ ground pixel, with encoder triggering if drivers vary.
• Exposure ceiling of half a ground pixel ÷ speed, and a light that delivers contrast inside it.
• Focal length from f = sensor × WD ÷ FOV, with image circle at least the sensor width.
• M42 optics planned, unless you specifically pick one of the 8 C-mount models.
• Housing sealed against spray, wheel-load rated, with a cleanable window.
• Trigger source chosen — encoder or runtime follow — and wired before commissioning.

Next step

Size the camera in the camera finder or start a configured set in the camera configurator. Another worked application plan is the line scan weld seam inspection guide, and the team answers layout questions — send the lane drawing and the widest vehicle you serve — through the contact page.