A packaging or print line rarely carries one ink. A red date code, a blue logo and black legal text can share a single part, and each color answers light differently. White light splits the difference and leaves every print at half-strength contrast. The fix is to drive one multi-wavelength RGB light through its red, green and blue channels and shoot one monochrome frame per color, so every ink is inspected under the channel that maximizes its separation from the background. This guide gives you the like-brightens, opposite-darkens rule, the channel pairing for each ink color, the timing math against your line rate, the ambient-light options and the controller set-up that keeps all three frames comparable.
Like colors brighten, opposite colors darken
LED colors split into a warm family — red, orange, yellow — and a cool family — green, cyan, blue, violet. Two rules carry most color work: like colors brighten a feature, and opposite colors darken it. Paper reflects light across the whole visible band, while ink reflects its own band and absorbs the rest. Under red light, red print brightens toward the paper's gray level and the print disappears. Under green light the same print absorbs nearly everything while the paper stays bright, which is the widest separation you can build. Blue light also darkens red print, but the separation comes out smaller than under green.
On a monochrome camera all of this reads directly as gray-level separation. The color wheel method shows how to pick a side for a single feature, the best LED color for contrast applies the same rule per channel, and light color on monochrome cameras covers the brighten-versus-darken trade from the sensor's side.
Where white light is still the right answer
White light carries warm and cool bands at once, so it lights every ink partially: flexible, and mediocre on all of them. It stays the correct choice in two cases. When a color camera must judge the ink's actual hue — shade checks, brand-color verification — broad-spectrum light preserves the spectral balance a Bayer sensor needs, and the white LED color temperature guide explains how to specify that repeatably. And when no single channel dominates the feature list, one white frame may carry everything acceptably. The camera side matters too: a monochrome sensor turns more of each strobe into signal than a Bayer sensor, because no color filter array sits over the photodiodes.
The sequence: one light, three optimized frames
The mechanics are plain. Trigger the camera three times per part. The controller strobes the red channel on frame 1, green on frame 2 and blue on frame 3. Your code inspects each feature in its assigned frame — the red date code in the green frame, the blue logo in the red frame — or merges the set. Keep the channel order fixed so frame N always maps to the same channel, and tag frames by channel rather than by arrival.
This is the same sequencing pattern that drives photometric stereo through a multi-channel controller and RGB color images on a monochrome line scan camera. The software side of capturing several frames per trigger is the multi-image capture pattern, and the general wavelength-sequencing method is laid out in the multi-wavelength lighting guide — this article applies it to mixed inks on one part.
In the range, the bar lights include a multi-colour RGB type. The 1272 x 22 mm diffused LS-BAR-0295 and the 1002 x 12 mm LS-BAR-0567 carry RGBW in their model codes: three color channels plus white, so a fourth frame can use broadband light where a feature genuinely needs it.
- Size
- 400 mm +
- Type
- Diffused
- Size
- 400 mm +
- Type
- Standard
Pair each ink with its channel
| Ink on the part | Best channel | Why it wins | Second choice |
|---|---|---|---|
| Red or orange | Green | Opposite on the wheel: the ink reflects almost no green while the paper reflects it fully | Blue |
| Blue | Red | Blue ink absorbs red and the paper reflects it | Green |
| Green | Red | Green ink absorbs red; the wheel's opposite of green sits in the red-magenta family | Blue |
| Black on white | Violet or UV-A | The print stays black while whitened paper fluoresces and glows | Any bright channel |
| White or metallic ink on dark plastic | Match the ink color | Like colors brighten, and white reflects every channel | Any bright channel |
LED peaks sit near 630 nm for red, 520-530 nm for green and 450-470 nm for blue; violet is 405 nm and UV-A runs 365-395 nm. For small fields, the spot lights carry these bands in single heads. The LS-SPT-0024 emits 455 nm blue through a 32° lens over a 49.9 x 66.6 mm lit area. The LS-SPT-0037 moves up the wheel to 505 nm cyan through a 21° lens, with a 99.9 x 141.6 mm lit area — cyan sits beside green and darkens red ink nearly as hard. Against blue inks across a wide web, red does the darkening: the LS-BAR-1168 is a 423 x 40 mm red diffused bar light. For black-on-white, violet earns its slot: the LS-SPT-0068 emits 405 nm from a 73.7 x 9.4 mm head with a PNP trigger for strobing.
- Size
- Under 50 mm
- Voltage
- 24 V
- Size
- 50-100 mm
- Voltage
- 24 V
Black print absorbs every channel, so the move is on the paper side. Optical whitening agents in paper, labels and many plastics fluoresce under near-UV and violet light: a 365-405 nm frame makes the background glow while the print stays black, and no other channel produces that gap.
Timing the three-shot sequence on a 6,000 parts per hour line
Inputs (example values; substitute your line's and camera's own):
- 6,000 parts per hour — a representative cartoning rate; it sets the takt.
- 3 frames per part — fixed by the method, one per channel.
- 1 ms strobed exposure per frame — a strobed LED far exceeds its continuous rating during the pulse, so 1 ms still fills the gray scale.
- 9 ms overhead per frame — trigger-to-exposure latency plus readout and transfer: a 0.5-megapixel monochrome frame is about 0.5 MB, roughly 100 MB/s on GigE gives 5 ms, and trigger handling adds a few ms.
The rule in words: the sequence must finish inside the takt.
The arithmetic: takt = 3,600,000 / 6,000 = 600 ms per part. Each frame costs 1 + 9 = 10 ms, so three frames cost 3 × 10 = 30 ms. Headroom = 600 / 30 = 20×: imaging takes 5% of the cycle and 570 ms remains for part handling.
The decision: run the sequence while the takt stays above 30 ms, which is line rates up to 3,600,000 / 30 = 120,000 parts per hour, or 2,000 per minute. Faster lines drop to two channels — 2 × 10 = 20 ms lifts the ceiling to 3,600,000 / 20 = 180,000 parts per hour, or 3,000 per minute — or split the channels across two synchronized cameras and keep all three colors. The channel-to-channel switch itself is not what eats the budget: drivers reach full output within microseconds, and the strobe rise time guide shows why those microseconds still matter at the exposure edge.
Keeping ambient light out
The strobe is the first rejection stage: ambient reaches the sensor only during the exposure window, so a 1 ms pulse collects 1 ms of ambient and nothing else. Wavelength matching is the second. Narrowband LED emission spans tens of nanometres, so a bandpass filter just wider than the LED line passes your light and blocks most of the surrounding spectrum; the red LED plus bandpass combination shows the technique on one channel. With three channels, a multi-band filter passes all three LED bands at once, or you dedicate the filter to the channel carrying the critical feature and accept ambient in the others. Band choice per part is covered in the wavelength selection guide. Shrouding is the third: if sunlight, skylights or high-bay banks hit the station, the ambient variation guide covers the diagnosis and the fixes.
Controller and camera set-up that keeps the frames comparable
Turn the camera's auto exposure and auto gain off. A mode that re-adjusts per frame will equalize away the contrast you built with light, and the three frames will no longer share one calibration; the brightness drift guide explains what auto modes do over time.
Drive each channel independently from the controller. Strobe one channel at a time at the fixed camera exposure, and set each drive level so the brightest area of interest sits clearly below saturation, leaving headroom for part-to-part reflectance swing. Record the values and keep them fixed; recallable per-channel settings are what the LED lighting controller guide argues a digital box buys you.
A 4-channel digital box with a strobe feature covers red, green, blue and one spare channel:
- Output
- Constant voltage 24 V
- Channels
- 4 channels
- Control
- Digital
- Feature
- Strobe
Checks before ordering
- Shoot the real part under each channel first. Ink pigments vary, and a cyan-leaning red may prefer blue over green; a sample under a swap-out light settles the pairing in minutes.
- Confirm the camera is monochrome. A Bayer sensor filters your colored light a second time.
- Re-run the timing formula with your camera's measured overhead rather than the example's 9 ms.
- Match lit length to the web with margin past both edges of the print field.
- Count the channels: colors in play plus the spare, then size the light controllers to the total wattage.
- Walk the cell at production light levels; sunlight leaks and high-bay banks decide the filter and shroud choices early.
When the sequence is the wrong tool
Three cases. When the question is whether the ink is the right shade, only a color camera under broad-spectrum white light can judge hue; the sequence maximizes gray separation and cannot answer color-fidelity questions. When the part carries a single ink color, one monochrome head does the job with one frame and one wavelength-matched filter — no sequence, no channel bookkeeping. And when print sits under glossy varnish or film, specular glare dominates the image and no channel choice repairs geometry: fix the lighting angle first, using the bright field versus dark field guide.
The decision, in one place
Mixed inks and gray-level contrast: sequence an RGB light, one monochrome frame per channel, pairing each ink by the table above, and budget 30 ms for three frames. Faster than 2,000 parts per minute: two channels or two cameras. One ink only: a single-wavelength head. Hue matters: white light and a color camera. Black-on-white: add the violet frame.
The lights finder filters by wavelength, size and voltage, and the light controller finder by channel count and strobe feature. The configurator walks the chain from camera to light in order:
The configurator matches a camera, lens, light, light controller and cables to each other, step by step, and lists only parts that fit what you already picked.
If the part is unusual, send us a photo of it under its current lighting plus your takt rate through the contact page — those two facts usually settle the sequence decision.
Questions
Can I run the RGB sequence with a color camera?
Not usefully. A color sensor splits incoming light with its Bayer filter before a pixel ever sees it, so a green strobe mostly excites green-sensitive pixels while red and blue pixels stay dark. Sequencing pays off on monochrome sensors, where every pixel receives every strobe. Keep white light with the color camera, and reserve the sequence for monochrome heads.
Which channel works best for black ink on white paper?
Black print absorbs every channel, so brighten the paper instead. Whitening agents in paper, labels and plastics fluoresce under 365-405 nm light, so a violet or UV-A frame makes the background glow while the print stays black — usually the deepest separation available. Without a violet or white channel, green typically gives the brightest paper gray level.
Do I need a bandpass filter if the station sits inside an enclosed cell?
Usually not. The strobe already limits ambient to the exposure window, so a shrouded cell at 1 ms exposures rarely needs more. A wavelength-matched bandpass, or a multi-band filter covering all three LED bands, earns its place when the line faces sunlight, skylights, or welding and high-bay lighting nearby; the filter buys margin rather than fixing a broken image.
How do I balance the brightness of the three channels?
Drive them independently from the controller at one fixed camera exposure. Raise each channel until the brightest area of interest lands at 70-80% of full scale, leaving headroom for part-to-part reflectance swing. Green channels usually need less drive than red to reach the same gray level, so set each one from its own frame, record the values, and leave them fixed.
What if the part carries four or more ink colors?
Add frames rather than compromise. A 4-channel light or controller runs red, green, blue and violet or white as a fourth frame; at 10 ms per frame the four-shot sequence costs 40 ms, still inside a 600 ms takt. Past four colors, inspect the critical inks under their own channels and read the remainder under white, accepting partial contrast.