Line scan camera illumination techniques: bright field to backlight

A line scan camera never photographs an area, so it never needs a lit area. It photographs one line at a time, and the whole illumination job is to put the right light on exactly the strip that line sees — from a few tenths of a millimetre to a few millimetres wide — for every metre of the pass. Which geometry wins follows one rule: the law of reflection. This guide explains the three line scan camera illumination techniques — bright field with the camera in the reflection, dark field with the light near grazing, backlight with the light opposite the lens — and gives the numbers that size each one: stripe width, line rate, exposure, light offset and the brightness factor dark field pays. It ends with the stripe-alignment procedure and a worked example you can copy.

One line of light changes the lighting job

A line scan sensor puts 1,024 to 16,384 pixels in one or a few rows, and the lens images that single row onto the moving web. The only surface that matters at any instant is the strip the row sees; every other photon the light emits is stray or wasted. That is why line scan lights are built as narrow, intense strips rather than broad panels. The stripe has to land exactly on the sensor line, and the alignment procedure below finds that strip in about fifteen minutes.

Two demands follow. Brightness: a 30 kHz line rate leaves per-line exposure of tens of microseconds, so the light has to be intense and driven hard. Stability: drive the LEDs at high frequency from a proper controller, because slow ripple repeats as bright and dark bands down the image — the guide to lighting controllers for line scan cameras lists what such a controller fixes. Before paying for a bigger light, read how much power a fast line really needs; the answer is often drive current and cooling, not the watts on the label.

The category runs to 761 models, from 29 units under 50 mm to 471 at 400 mm and longer, split by voltage, colour, wavelength and connector. Match the light to the web width first, then to the technique.

The law of reflection picks the technique

A smooth surface reflects light like a mirror: the angle of incidence equals the angle of reflection, both measured from the surface normal. A rough surface scatters the same light in every direction. Each illumination technique is a choice of where the camera sits relative to that reflected ray. The companion piece on bright field vs dark field lighting derives the rule in detail; here is the line scan version.

• Bright field — the camera sits in the reflection path, so the surface sets the image: matte areas mid-grey, print and stains dark, and any facet tilted enough to steer the reflection away goes black.
• Dark field — the light runs almost flat across the surface, the mirror reflection passes over the camera, and only scattered light reaches the sensor; roughness, scratches and raised edges glow on a near-black background.
• Backlight — light and camera face each other with the part between, so the image is a silhouette and surface finish stops mattering.

Bright field: the default for matte, opaque surfaces

Place the line light and the camera roughly mirrored about the surface normal, each 30-45° off it to start. On matte material — paper, label stock, fabric, wood, unglazed ceramic — the grey levels carry print, colour and stains, which is why this is the standard set-up for print inspection on webs. Steep scratches and embossing also read, because they tilt the local surface and steer the reflection away from the lens; on matte stock that darkening is the defect signature.

The same physics carries a warning: on anything glossy, bright field shows the lamp. A shiny film or polished strip mirrors the stripe straight into the lens and the contrast washes out; switch techniques instead of fighting it.

For a web a few hundred millimetres wide, pick the light by line length: the LS-LSC-0527 emits a 300 × 17 mm line at 87 W in red or 108 W in white, and the LS-LSC-0074 is a 484 mm high-uniformity, fan-cooled line scan light.

With a mono sensor, red usually buys more contrast per watt than white; the explainer on light colour and monochrome camera contrast works through the spectral reason.

Dark field: grazing light that makes defects glow

Lay the light almost flat against the surface — 60° to 85° from the normal is the working range, and 75° is a good first setting. The mirror reflection now passes over the camera and a smooth surface reads near-black. What remains is the light that scratches, particles, embossed lettering and raised edges scatter, and those features show as bright marks on a dark field. That is the standard recipe for surface defects on smooth, glossy or metallic webs, and it barely cares about print or colour underneath, because surface tone contributes far less than the scatter you are amplifying.

Scatter is weak, so budget roughly ten times the bright-field brightness as a starting rule. Exposure cannot absorb it: the line period caps exposure, and at 30 kHz that ceiling is 33 μs. The practical routes are overdrive strobing — the article on strobe overdrive, duty cycle and timing sets the limits — a physically larger line light, or pixel binning on a line scan camera, which trades resolution for roughly double the signal per pixel.

Light makers build the geometry in: oblique line lights aim their LEDs at grazing angles out of the box. The LS-LSC-0441 is a 493 × 60 mm oblique line light with natural cooling on 24 V, and the LS-LSC-0046 is a 500 × 53 × 121.5 mm double-angle oblique light, also 24 V — its two opposing angles light the strip from both sides and average out surface lay.

Backlight: contours, edges and dimensions

Put a wide, diffused line light directly opposite the camera with the web running between them. Opaque parts become silhouettes, so width, hole presence, edge chipping, shape and position all read with no sensitivity to surface finish. In transparent film, transmitted light reveals bubbles, cracks, gels and inclusions that incident light barely shows — clear laminates, foil and glass are the classic cases.

The trade is intensity and background care. A diffuser spreads the same watts over a wider stripe, so size the light for the fastest web you must run, or strobe it. The area behind the part must stay even, because any brightness step reads as an edge: width measurement in particular needs a stable dark background, and this width measurement and stable dark background guide walks through the acceptance-test consequences.

Alignment is the easy part here — no reflection angle to chase, just a stripe over the camera's line.

Worked example: dark field for a 40 mm film strip at 300 mm/s

Inputs, each with its source:

• Web: printed glossy film strip, 40 mm wide, 300 mm/s — a chosen typical case; every result scales linearly with speed.
• Camera: MV-CL042-91GC-V2 — 4K GigE colour line scan, 7 μm pixels, 29 mm image circle, M42 mount (catalog data).
• Sensor line: 4,096 px × 7 μm = 28.7 mm, which sits inside the printed 29 mm image circle — the geometry closes.
• Lens: chosen to map the line to 40.96 mm of web, an object pixel of 10.0 μm (illustrative choice; fix it from the working distance with the lens focal length calculator).
• Feature: scratches and print voids on a glossy surface — dark field indicated.

The rules, in words: the line rate follows from one pixel of travel per line; the exposure follows from a half-pixel blur limit; the light offset follows from the grazing angle; the brightness follows from the ten-times scatter rule.

Line rate: 300 mm/s ÷ 0.010 mm = 30,000 lines/s = 30 kHz — one line per 10 μm of travel. Fire it from the encoder; the divider arithmetic is in the guide to encoder ticks per pixel, and the camera-side settings in how to set the line rate of a line scan camera. The 40 mm strip sits in the 40.96 mm field with 0.48 mm of margin each side.

Exposure: the half-pixel limit is (0.010 mm ÷ 2) ÷ 300 mm/s = 16.7 μs. Set 15 μs — blur is then 300 μm/ms × 0.015 ms = 4.5 μm, under half a pixel, and 15 μs fits inside the 33.3 μs line period.

Light geometry: mount two grazing line lights 50 mm above the web, aimed 75° from the normal. Offset = 50 mm × tan 75° = 50 mm × 3.73 ≈ 187 mm on each side of the scanned line. Stripe across the web: 40.96 mm + 5 mm margin ≈ 46 mm, so the 50-100 mm length class fits (the size filter lists 17 models there).

Brightness: dark field wants about 10× the bright-field signal and exposure is pinned at 15 μs, so intensity must deliver it. Overdrive the strobes from a controller with headroom — LS-CTL-0264 is a 2-channel, 400 W high-power digital controller with a network option for 24 V lights — and keep the duty cycle inside the strobe limits.

Decision: two grazing lights at ±187 mm offset and 50 mm height, 75° aim, stripe ≥ 46 mm across, encoder-triggered at 30 kHz, 15 μs exposure, overdriven strobes within duty cycle. If the strip turns out matte after all, keep the camera and the 30 kHz rate, switch to bright field with the light mirrored about the normal, and drop the ten-times brightness demand.

Aligning the light stripe to the sensor line

The camera reads a strip from well under a millimetre to a few millimetres wide; a stripe aimed 5 mm to the side lights nothing the camera can see. Find the exact strip first, then aim the light at it.

1. Make a calibration target: a flat plate with a matte black stripe slightly wider than the strip you expect to image, laid square to the conveyor.
2. Set the camera to free run — no encoder, no hardware trigger — so it stacks lines into a continuous live image.
3. Pass the target under the lens and shift it until the black stripe covers the imaged strip over its full length; with many lines stacked, the band must read completely black.
4. Draw a pen line across the stripe and read where the black band sits in the image; mark the sensed strip on the target, so its position on the part is known.
5. Only then bring in the light: adjust angle and working distance until the bright stripe lands on the marked strip, and focus or re-aim it for maximum brightness there.

Combined and sequential illumination

Two techniques can share one pass. Run two lights at once and every line carries both signals: the classic case is incident light plus blue transmitted light on a colour line scan camera, so surface print and internal defects arrive in one image. Pairing colours is its own decision — the guide to sequencing light wavelengths shows what one camera gains from a colour plan.

Sequentially, strobe bright field and dark field on alternating lines and split even and odd lines into two images afterwards. Each technique then samples every second line, so along-travel resolution halves unless the line rate doubles — and the controller must settle the LED between lines, which the article on LED strobe optical rise time shows how to measure. Four-angle line lights for photometric stereo push the same idea further on moving webs.

For overdriven or sequenced stripes, size the controller headroom first: the LS-CTL-0264 is a 2-channel, 400 W high-power digital controller with a network option for 24 V lights.

Which technique for which job

The decisions, committed: print on matte label stock — bright field, light beside the lens. Scratches on polished strip or glossy film — dark field at 75° with overdrive headroom. Width, holes or edge chipping — backlight, no contest. Curved shiny caps and bottles defeat every stripe technique because the reflection angle changes across the part, so go diffuse: the guide on machine vision light geometry selection maps surfaces to geometries, and the inkjet OCV lighting article walks the dome, dark field and backlight decision on a real part.

When the usual answer is wrong

• Very fast lines: past roughly 100 kHz, the exposure window drops under 10 μs and brightness becomes a power problem rather than a geometry problem. The LS-LCM-0016 is a 2K GigE line scan camera with 7 μm TDI pixels, 58 kHz, M42 mount. Binning helps near that ceiling; beyond it, TDI does.
• Hall light and daylight: ambient adds to your stripe and shifts through the day. Shroud the station or strobe against it, and write the rejection requirement into the acceptance test — the ambient light rejection specification guide supplies the test language.
• Waving or wrinkled webs: the strip moves in height, so focus and incidence angle wander and dark field contrast breathes. Keep the stripe wider along travel, accept some diffusion, and align at the worst case, not the average.

Checks before you order

• Light line length ≥ web width + 10%, so the web edges never pass unlit.
• Sum of camera field widths across the web ≥ web width; compute each field of view before the mounts are drilled.
• Stripe across the web ≥ field width + about 5 mm; along travel, 2-10 mm focused for bright and dark field, wide and diffused for backlight.
• Line rate = speed ÷ object pixel size; exposure below the line period and blur within half a pixel — the worked example above shows both.
• Dark field: strobe duty cycle and controller watts for the ten-times demand, confirmed before the light is ordered.
• One alignment pass with the black-stripe target before the first production run, and again after any mechanical change.

Send the web width, speed and surface to the line scan light finder or start at the light configurator; the line scan cameras and light controllers pages cover the other two sides of the system. More lighting-geometry guides sit on the lighting topic page.