Engraved part numbers and date codes on machined metal are routine OCR work until the text sits inside a recess. A ring light mounted around the lens then lights the raised rim of the part, the rim saturates the sensor, and the characters disappear. The fix is to move the light onto the camera axis. This guide explains why on-axis light turns a recessed face into a readable image, how a splitter coaxial light differs from a flat light with a center window, how to size the light and the optics with worked numbers, and where the method fails, so you can pick the winning geometry before ordering hardware.
What coaxial lighting is
A coaxial light puts an LED panel at 90 degrees to the optical axis, with a 50/50 beam splitter at 45 degrees between panel and part. Half the LED output reflects down onto the part along the lens axis; the half the part returns along that same axis passes through the splitter to the sensor. Rays that leave or return at any other angle are discarded. The part is lit from exactly the direction the lens looks.
A flat light with a camera window reaches the same geometry without the splitter: the emitting face has a clear hole at its center, the lens looks through the hole, and the face floods the field from every side of the axis. Nothing is split, so the part receives roughly four times more of the LED output at the same drive current.
Both builds are on-axis lighting. Machine vision illumination types places them among bright field, dark field and diffuse geometries.
Why a ring light washes out a recessed face
The law of reflection decides everything here: light leaves a surface at the angle it arrived. Bright field versus dark field lighting builds its whole selection method on that rule, and a recessed part is its clearest failure case.
A valve face, a bearing cap or an injector body usually carries a raised rim around a recessed floor. A ring light above the part fires inward at its emission angle, commonly 30 to 90 degrees from vertical. The rim, tilted toward the camera, mirrors those rays straight into the lens and saturates white. The recessed floor sees almost none of it: only light scattered off the walls reaches down, the floor images grey, and the engraved strokes drown in wall scatter. The classic bad image is a blown white rim around featureless grey.
Lighting geometry for image contrast works the same camera-light-part triangle for other part shapes.
How on-axis light makes the strokes readable
Turn the light onto the axis and the geometry flips in your favor. The machined floor of the recess is flat and square to the axis, so it mirrors the light straight back and reads bright. Each engraved character is a cut groove: its walls are tilted, they reflect the return downward or sideways instead of into the lens, and they read dark. Where the cutting or marking tool threw up a burr, the raised edge scatters light and rims each stroke bright. The result is dark strokes on a bright floor with crisp bright rims, which is as much contrast as OCR on metal ever gets.
Set the exposure so the floor sits at 70 to 90 percent of sensor saturation. The strokes then fall near 20 to 30 percent, and a fixed threshold holds across tool wear and alloy batches. One readable image is not the goal; margin against part-to-part variation is, which is the argument robust machine vision lighting makes in detail.
The same axis reads laser-etched marks on polished surfaces of any kind: DPM Data Matrix codes on metal and laser-marked OCR on reflective wafers are this problem with different marks.
Splitter coaxial or center-window flat light
Pick the splitter build for small fields and steep recess walls. The splitter emits a compact cone aligned with the lens, and narrow-angle models concentrate it further for deep pockets. Pay for it in photons: two passes through a 50/50 splitter leave about a quarter of the LED output on the imaging path, so the same shot needs roughly four times the exposure of a direct light.
Pick the center-window flat build when you want the full output and even coverage over a wide face. No splitter means no two-pass loss, and the emitting face spreads flat, even light across fields of 100 mm and more. The trade is clearance: the emitting face must exceed the field plus the window the lens sees through, and the lens-plus-light stack sits deeper over the part.
| Splitter coaxial | Center-window flat | |
|---|---|---|
| Light reaching the part | about 25 percent of LED output | nearly all |
| Coverage | compact cone, reaches deep pockets | wide, very even |
| Stack depth over the part | moderate | deeper |
| Best for | small recessed fields, steep walls | wide faces, bead grooves |
A spot light with a loose beam splitter is the build-it-yourself version for tiny fields; an integrated coaxial light saves the alignment and the stray-light leaks.
Size the light and the optics: a worked example
Take a laser-engraved date code on an intake valve head. Numbers first, hardware second.
- Print: character height 2 mm, engraved stroke width 0.2 mm
- Camera rule: at least 4 pixels across the narrowest stroke
- Field: the valve face is 32 mm across; allow 40 mm square for fixture tolerance
Any camera from 0.5 MP upward clears the 800 px bar. Take a common 5 MP monochrome sensor, 2448 x 2048 pixels at 3.45 µm, and ask what focal length delivers the 40 mm field at a workable distance. The area scan cameras finder filters by exactly these two numbers.
17.4 mm is not a stock lens. Rearranged as WD = f (1 + m) / m, a 16 mm lens lands at 92 mm working distance and a 25 mm lens at 143 mm. Take the 25 mm: 143 mm leaves room for the light stack above the part and keeps the emission cone tight on the floor.
Light sizing follows the same field number. The emitting window must cover the 40 mm field with margin on every side, because emitter brightness falls off toward the window edge. Margin per side is (ratio − 1) / 2 × field: a 1.5x window gives 0.25 × 40 mm = 10 mm per side, and a 2x window gives 20 mm. Treat 1.5x as the floor for a fixed fixture, 2x when parts sway.
The LS-CXL-0027 is a 60 x 60 mm coaxial light with 4.4 W of red and 7.2 W of white output at 24 V. At a 60 mm emitting face over a 40 mm field, it is exactly the 1.5x choice.
- Size
- 50-100 mm
- Voltage
- 24 V
Bigger faces need bigger windows: the 70 x 70 mm LS-CXL-0092, the 90 x 70 mm white LS-CXL-0241 and the 140 x 140 mm LS-CXL-0192 at 525 nm step the window up as the field grows, all on 24 V.
- Size
- 50-100 mm
- Voltage
- 24 V
- Size
- 50-100 mm
- Voltage
- 24 V
- Size
- 100-200 mm
- Voltage
- 24 V
Exposure closes the calculation. A direct light that needs 2 ms needs about 8 to 10 ms through the splitter at the same current: a factor of four for the two 50 percent passes, plus a few percent of surface reflection. Strobe instead of stretching the exposure: fire the light only during integration and run the drive four to five times above its continuous rating inside the duty-cycle limit. LED overdrive and current control sets that out, and a lighting controller supplies the strobe current and locks the pulse to the camera exposure.
Wavelength on machined steel
The geometry does the heavy lifting; wavelength decides which residues and micro-texture join the image. Red gives the cleanest read on bright machined steel and keeps thin oil films from stealing attention. White doubles as the general-purpose channel when one station inspects mixed features. Green often returns the brightest raw image per ampere, because monochrome sensors respond most strongly near its wavelength. Blue exaggerates fine scratches and films, which helps when the defect is itself a scratch and hurts when you only want the text. Infrared near 850 to 940 nm makes thin oils nearly vanish and removes surface color as a variable on mixed-alloy lines.
For the valve field above, the infrared option in the same size class is the 75 x 73 mm 940 nm coaxial light LS-CXL-0168.
Which LED colour gives the best contrast and light color and monochrome camera contrast work through the color-versus-sensor trade.
Where on-axis lighting fails
Four part families defeat it, and each has a direct alternative.
Curved floors. A domed or crowned face mirrors the on-axis return off-axis, brightness falls from center to edge, and no single threshold holds. Switch to diffuse: a dome light surrounds the part so every slope sees light along its own normal, and lighting shiny metal pins without glare compares dome, diffuse and polarizer answers on small curved steel.
Cast, blasted or textured surfaces. Rough floors scatter in every direction, floor and strokes return similar flux, and the image goes flat grey. Go dark field: grazing light makes pores, scratches and stroke edges scatter bright while the untouched face stays dark; detecting dents and pits in metal surfaces with low-angle lighting shows the layout.
Raised marks. Stamped and embossed characters displace metal instead of removing it, so the floor stays square to the axis and the shallow bulge barely shifts the return. Formed marks read under low-angle light, and stacked ring lights switch angles to catch them.
Polarizing by reflex. Crossed polarizers kill the specular return, and the specular return is the bright floor this method lives on. Keep cross polarization for diffuse set-ups where glare is the enemy and the feature itself scatters.
Deep, narrow pockets stay a coaxial job, but order a narrow-angle emitter: the LS-CXL-0019 is an 80 x 80 mm narrow angle coaxial light at 24 V, built to concentrate the cone for steep walls.
- Size
- 50-100 mm
- Voltage
- 24 V
Second case: a formed-in-place gasket bead
The same axis solves a second automotive job: inspecting the formed-in-place (FIPG) silicone bead laid in a groove around a metal flange. On-axis light makes the flange face bright, the groove walls dark, and the matte silicone bead a mid grey between them, because silicone scatters instead of mirroring. Bead width and continuity become a single threshold pair, with no edge hunting against a saturated rim, which is what an outer ring delivers on this part.
Fields run wider than valve faces: a transmission flange can span 100 mm or more. The LS-CXL-0067 covers 102 x 100 mm in one shot, and larger fields call for stepping the camera or scanning the part under the light.
- Size
- 100-200 mm
- Voltage
- 24 V
Checks before you order
- Window versus field. Emitting window at 1.5x field minimum, 2x if parts move in fixture; the field corners must sit inside the emitter's uniform center, not on its falloff.
- Stack clearance. Add lens length, splitter depth and working distance: about 143 mm plus the light's own height for the 25 mm lens above.
- Exposure budget. Multiply a direct-light exposure by four for a splitter build, or strobe at four to five times continuous current inside the duty cycle.
- Channels and wiring. Order two wavelengths at least, red plus white, and match the controller channel output to the 24 V input most coaxial lights take.
- Margin on real parts. Run the threshold across ten production parts, including a freshly tooled face and a worn-tool face; machined brightness moves with tool wear.
- Eye safety. No direct viewing of the window, and infrared variants get the same respect as a visible high-power LED.
The decision, case by case
- Engraved or laser-etched characters in a flat recess: on-axis light; splitter for small fields and steep walls, center-window flat for wide faces; red or white; floor at 70 to 90 percent saturation.
- Curved or crowned faces: a dome, not coaxial.
- Cast or rough surfaces: low-angle dark field.
- Stamped or embossed marks: low-angle rings, angle-switched if the marks vary.
- Bead in a groove: center-window flat sized at 1.5x the field.
Machine vision light geometry selection by surface and feature walks the same table in more depth, and machine vision lighting selection by inspection task starts from the task instead of the part. To spec hardware now, open the light finder or jump into the lighting step of the configurator; the lighting topic page collects the rest of the lighting guides.
Questions
How large should the coaxial emitting window be compared with the field of view?
Set the window at 1.5x the field of view as a working minimum, and prefer 2x when parts sway in the fixture. For the 40 mm valve-face field in the worked example, a 60 x 60 mm emitting face leaves 10 mm of margin per side. Also mind the image corners: emitter brightness falls off near its edge, so keep the field inside the uniformly lit center, not on the falloff.
How much light does the beam splitter cost me?
Two passes through a 50/50 splitter leave about a quarter of the emitted light on the imaging path, and surface reflections trim a few percent more. A shot that needs 2 ms from a direct light therefore needs 8 to 10 ms through a coaxial splitter at the same current. Strobing recovers most of it: fire only during exposure and run the LEDs four to five times above continuous rating, within the duty-cycle limit.
Can I fix the glare with a polarizer instead of changing the light?
Usually not. Crossed polarizers suppress the specular return, and the specular return from the flat floor is exactly what makes on-axis contrast work, so you would darken the background you need bright. Cross polarization belongs with diffuse or ring lighting, where glare is unwanted and the feature itself scatters. If a coaxial image speckles from micro-texture, first lower the exposure or move to a longer wavelength.
Does on-axis lighting work on cast or sand-blasted faces?
Not with usable margin. A rough surface scatters incoming light in every direction, so floor and strokes return similar amounts and the image turns flat grey. Use low-angle dark field instead: grazing light makes pores, scratches and stroke edges scatter bright while the untouched face stays dark. A coaxial trial may look technically readable, but the margin will not survive alloy and blast variation between parts.
Will the same set-up read stamped or embossed characters?
Weakly. Formed marks displace metal instead of removing it, so the floor stays continuous and square to the axis and returns on-axis light almost everywhere; the shallow bulge barely changes the return. Formed characters read best under low-angle light, where each sloped wall catches rays the flat floor misses. If one station must handle both mark types, plan two lighting channels and switch rather than compromise one geometry.
Which LED colour should I order for bright machined steel?
Order red and white at minimum, on two controller channels, and re-set the threshold after any channel change: each wavelength shifts the floor's grey level, so a threshold tuned on white misreads under red. Green often gives the brightest raw image because monochrome sensors respond most strongly near its wavelength. Blue helps only when the defect is itself a fine scratch or film; infrared suits mixed-alloy lines.