Color images from a monochrome line scan camera: RGB light sequencing

A PCB carries two kinds of evidence at once: resist stains and silkscreen print want color contrast, while engraved text and circuit patterning only show under light aimed straight down the optical axis. One monochrome line scan camera can capture both on a single pass of the stage — you switch the lights instead of changing sensors. This guide sizes that station: how many passes your line rate must cover, how wide each strobe can be, which camera, line lights and controller carry the channels, and how the interleaved stream becomes one aligned color image. The arithmetic runs at 25 μm per pixel throughout, so you can rescale every result to your own field width and speed.

Can a monochrome line scan camera deliver a color image?

Yes — cycle red, green and blue line lights across successive scan lines, sort the stream into three images and merge them as RGB channels; a 2K camera running a four-pass sequence at 48 kHz returns a full color image at 300 mm/s with every pixel measured in every channel.

Every scan line is exposed under exactly one light, so all 2,048 pixels across the field contribute to whichever channel is lit — no color filter takes pixels away and no interpolation invents them. A fourth pass under white light folded in by a coaxial beam splitter adds a flat, top-down texture image on the same stage. Choosing red, green and blue is a contrast decision made defect by defect, the same way as any monochrome light color choice, and the sequence extends past three bands exactly as the multi-wavelength light guide shows for four-band work.

Why switch lights instead of using a Bayer color camera?

A Bayer color sensor gives half its pixels to green and a quarter each to red and blue, then interpolates the missing colors from neighbors; switching lights under a monochrome sensor measures every color at every pixel, and it keeps 88 of the 170 line scan cameras in the range open to you.

Interpolation is where false color comes from: a pad edge or a hairline in the print picks up a hue that is not on the board, the same artifact family the color camera artifacts guide diagnoses. Trilinear color line scan cameras read each channel from its own row, which removes interpolation but offsets the channels and punishes any speed error. Switching lights has neither problem: color lives in the illumination, every pixel measures it, and each channel takes its own exposure and gain — the single-band argument of the monochrome camera with optical filter guide run three times, plus a coaxial pass for texture.

How many passes must your line rate cover?

Multiply the moving part's own line rate by the number of illumination passes: 300 mm/s at 25 μm per pixel needs 12,000 lines/s per pass, so red, green, blue and coaxial white run the camera at 48 kHz, and a 58 kHz ceiling sets a top speed of 362.5 mm/s.

One pass at 25 μm per pixel and 300 mm/s is 12,000 lines/s. Four passes — red, green, blue, coaxial white — put the camera at 48,000 lines/s, and the data rate comes with it: 2,048 px × 48,000 lines/s × 1 byte ≈ 98 MB/s, inside a GigE link's roughly 118 MB/s of payload in 8-bit and outside it in 10-bit (123 MB/s). With a 58 kHz ceiling the speed limit is 58,000 ÷ 4 × 0.025 mm = 362.5 mm/s, about 21.7 m/min; drop the coaxial pass and three passes stretch it to 483 mm/s. Set the rate as the line rate guide describes, and keep shutter and speed consistent per the fps and shutter speed guide.

How do you keep every strobe inside its own camera line?

Every strobe must finish inside one line period: at 48 kHz that period is 20.8 μs, so pulses up to about 15 μs are safe; the 30 μs coaxial pass used for engraved-text contrast instead needs a 40 μs period — 25 kHz on the camera and 125 mm/s of stage travel.

The period is the budget: 1 ÷ 48,000 = 20.8 μs, so pulses up to about 15 μs leave margin for LED rise time and trigger jitter. Blur is not the limiter — 300,000 μm/s × 0.000015 s = 4.5 μm of smear, under a fifth of the 25 μm pixel.

The five-pass variant exists because one coaxial pass cannot do both texture jobs: a 10 μs pulse carries less charge, holds bright metal reflections below saturation and keeps engraved text crisp, while a 30 μs pulse integrates deeper and pulls low-contrast waviness and stains out of the surface. Thirty microseconds needs a period of at least 40 μs, so the camera steps down to 25 kHz — 1 ÷ 25,000 = 40 μs — and five passes then allow 25,000 ÷ 5 × 0.025 mm = 125 mm/s of stage travel, 7.5 m/min. Set camera exposure between the widest pulse and the period; 35 μs sits between the 30 μs strobe and the 40 μs line.

Drive the line trigger from the stage encoder, wired as in the quadrature input guide and scaled by the encoder ticks per pixel method. The controller takes the same trigger and fires one channel per line — the job described in the line scan controller guide, with channel-by-channel sequencing already proven in the photometric stereo sequencing setup. Duty cycle follows from width × rate: 15 μs at 25 kHz is 37.5%, and the power guide for fast lines sizes the light behind it. If a channel still comes back dim, pixel binning buys sensitivity without touching speed.

What does the full five-pass PCB station add up to?

For a 51.2 mm field: 2,048 px × 25 μm sets the field width, 14.3 mm ÷ 51.2 mm gives 0.28 magnification, a 25 mm lens at 115 mm working distance covers it, and five passes at 125 mm/s run the camera at 25 kHz and 51.2 MB/s of data.

The lens must also cover the sensor: its image circle needs at least the 14.3 mm sensor width. Data lands at 2,048 px × 25,000 lines/s × 1 byte = 51.2 MB/s, half a GigE link in 8-bit. Five passes do not force five light channels — white fires twice per cycle, at 10 μs and 30 μs, from one channel when pulse width is set per trigger; otherwise reserve one channel per coaxial pass and count five. White comes from a beam-splitter coaxial unit aligned with the optical axis, and the oblique and double-angle bodies in the line scan lights range stand ready for any angled pass you add later.

In the line scan cameras range, the example camera is the LS-LCM-0016, a 2K GigE line scan camera with 7 μm pixels and a 58 kHz line rate.

For red and white in one body, the LS-LSC-0527 line light source pairs 87 W of red with 108 W of white behind a 16M-6J connector in 300 × 17 mm; green and blue line lights of matched length sit beside it.

Pulse widths and channel selection live in the light controller — the LS-CTL-0070 digital control box provides four channels, 200 W, a strobe function and RS-232 control for exactly this pattern.

How do interleaved passes become one aligned color image?

Sort captured lines by index modulo five into five buffers, shift each color buffer by its one-line stage offset — 5 μm per line at 125 mm/s — balance the channels with per-color gain, then merge red, green and blue and keep both coaxial images as separate grayscale outputs.

Line indices 0, 5, 10, … carry red; 1, 6, 11, … green; 2, 7, 12, … blue; 3, 8, 13, … the 10 μs coaxial pass; 4, 9, 14, … the 30 μs pass. The stage advances 125,000 μm/s ÷ 25,000 lines/s = 5 μm per camera line, so green sits 5 μm after red and blue 10 μm after it — 0.2 and 0.4 of a 25 μm pixel. Shift each buffer by its stage offset during compositing, or accept the error at this scale. Balance the three channels with per-color digital gain until histograms align on a plain white target, then merge and keep the two coaxial buffers separate — one for engraved text, one for surface texture. The same synchronize-capture pattern is worked through for four-angle lighting in the moving-part synchronization guide.

When is RGB sequencing the wrong choice?

Skip sequencing when the speed ceiling blocks you, when added bands divide the line-rate budget past your target speed, or when the deliverable is calibrated color rather than defect contrast — those cases want a faster camera, fewer passes or a color line scan camera under white light.

• Speed: at 25 μm per pixel a 58 kHz camera carries four passes only to 362.5 mm/s and five passes only to 290 mm/s. Beyond that, buy line rate or drop a pass.
• Pass creep: every band divides the budget again — six passes under the same ceiling allow 58,000 ÷ 6 × 0.025 mm ≈ 242 mm/s.
• Calibrated color: if a person or a print standard judges the color, a sequenced composite is a defect-contrast image, not a color measurement; use a color camera under broadband white.
• Single defect: if one defect type is the whole job, one wavelength under a mono camera wins on simplicity — the light color and contrast guide walks that choice, and angle-based topography belongs to four-angle line scan photometric stereo.

What do you check before ordering?

Before ordering, check five numbers: camera line rate against passes × speed ÷ pixel pitch, strobe width against 80% of the line period, controller channels against colors plus coaxial passes, light length against field width plus margin, and lens image circle against the 14.3 mm sensor width.

• Line rate budget: passes × speed ÷ pixel pitch against the camera ceiling — the example lands at 25 kHz against 58 kHz, a 2.3× margin.
• Strobe width: every pulse at or under 80% of the line period — the widest here is 30 μs against a 40 μs period.
• Controller channels: one per color plus coaxial coverage, with a strobe input and per-trigger pulse width control.
• Light length: longer than the 51.2 mm field so brightness holds edge to edge.
• Image circle: at least the 14.3 mm sensor width, with a 25 mm focal length fitting the example geometry.
• Data rate: 51.2 MB/s against the interface payload — GigE holds it in 8-bit.
• Encoder scale: enough ticks that every color samples the same strip of material, set with the ticks-per-pixel method above.

Send your speed, field and pass list through the contact page and we will check the budget with you, or walk the light-and-controller pairing step by step in the configurator. The other line scan lighting methods sit on the lighting topic hub.