Engraved text lighting on metal: low-angle grazing light vs dome

Engraved serial numbers, date codes and part markings on machined metal are easy inspections to justify and easy ones to ruin. Light the part from above and the flat finish returns a bright, even sheet of glare, with the recessed strokes only a few grey levels inside it. The fix is geometric, not a better camera: send light in nearly parallel to the surface, so the flat finish mirrors away from the lens while the groove walls scatter light back. This guide gives the working angle range, the mounting-height arithmetic, the finishes where a dome light still wins, and a complete worked camera-and-light set-up for reading engraved text on metal.

Why flat metal hides its own engraving

A machined or polished face behaves like a horizontal mirror. A ray arriving at angle θ from the normal leaves at θ on the far side, so the camera only sees the lamp that stands in its mirror direction. Under a dome or a coaxial light that lamp is everywhere above the part, so the field images white. Swing the lamp to the side and the field images black. That is the law of reflection that separates bright field from dark field, and it decides this inspection before the sensor is chosen.

An engraved stroke is a groove with tilted, usually rough walls. Walls sit at every angle except the surface plane, so each one catches light from some directions and misses it from others. Under full-angle diffuse lighting, wall and field receive nearly the same illumination: on an 8-bit camera the stroke sits near grey 200 against a field near 230. That 30-level gap is an example reading, but the collapse is what you see on the bench, and it moves with every part swap. Swing the same lamp to grazing and the numbers flip: the field drops towards 30 and the walls return 180.

The flip, not resolution or exposure, is what makes characters readable. The camera, light and part geometry sets this contrast before the image exists, and the same reasoning drives light selection by surface and feature.

The grazing set-up: a low-angle ring as dark field

A low-angle ring light mounts around the lens and fires inward almost horizontally. The machined field mirrors the rays off to the sides, so it images black. A groove wall tilted steeper than the incoming ray intercepts that ray, scatters part of it upward, and the stroke images bright. You are not lighting the characters; you are lighting their walls and starving everything around them.

Two numbers set the geometry: the ring's inner radius r, half its inner bore, and the mounting height h above the surface. Rays aimed at the part centre arrive at θ above the surface:

Work between 10° and 30°. Below 10°, the tolerances of height, flatness and part seating eat the margin; above 30°, the field starts catching enough stray light to grey out. The wall tilt sets the limit: a wall catches direct light only when it leans off the surface by more than θ, so steep cut marks read at 25-30° while near-flat etched textures need the shallow end of the band.

A ring with a 48 mm inner bore has r = 24 mm, so the 20° height is 24 mm × tan(20°) = 8.7 mm and the 30° height is 24 mm × tan(30°) = 13.9 mm. Mount inside that 9-14 mm band and sweep for the peak.

The set angle holds at the part centre. Toward the field rim the rays shallow out and the strokes dim, so keep the text inside the middle half of the field or expect a gain gradient across it.

The LS-RNG-0730 is a 103 mm ring light with a 48 mm inner bore.

Larger fields want a larger ring so the grazing rays still reach the centre from the sides; the same formula sets the height from that ring's bore. The LS-RNG-0426 is a 120 mm diffused ring that emits at 0°.

Dome lights: even light that evens out the relief

A dome light surrounds the part with diffuse light from nearly every upward direction; the camera looks in through a small port. Every point receives light from every angle, so the mirror directions are always fed. A polished field images bright and uniform, and a groove wall, lit from all sides at once, images nearly the same. Uniformity is the dome's purpose, and it is exactly what shallow relief cannot survive on.

Three cases still belong to the dome:

• Satin, bead-blasted, cast or sand-blasted finishes scatter on their own, so grazing has nothing to mirror away. The mark reads by its small reflectance difference, and the dome holds that difference steady across the field and across part-to-part variation.
• Curved parts: a ring sets one angle and a curved surface breaks it everywhere at once. A dome covers the profile evenly; the dome light guide walks through sizing and placement.
• Mixed-part lines: one dome gives shiny, matte and curved variants the same image statistics, which beats peak contrast when several part numbers share a station.

Size the emitting area to at least 1.5-2 times the field's long dimension; below that, edge falloff re-introduces the gradient you bought the dome to remove. For satin work the right tool is a large, even diffuse source: the LS-DOM-0085 is a 166 mm dome with a 116 mm emitting area.

Coaxial light: steep walls against a mirror field

A coaxial light folds the illumination onto the camera axis with a beamsplitter, so rays leave along the optical axis and strike the part at 90°. A flat polished field returns them straight back into the lens, so the field images bright. Near-vertical walls — stamped characters, dot-peen marks, deep engraving — catch that axial light and scatter it diffusely, so the mark reads against the bright mirror. Grazing starves the field and lights shallow relief; coaxial floods the field and lights steep relief.

For direct part marks this is the default first trial, and the DPM Data Matrix lighting guide applies the same geometry to codes. On satin or brushed surfaces, a diffuse coaxial panel softens the field texture while keeping some relief shading, which is why it is the usual answer for engraved text on housings and hoods.

The LS-CXL-0067 is a 102 × 100 mm coaxial light source.

Choosing by finish and mark

Read the surface first, the mark second:

The full matrix sits in lighting selection by inspection task. The shop splits the hardware the same way: ring lights, dome lights and coaxial lights, with the light finder filtering by emission angle, size and wavelength.

Worked example: a serial on a machined aluminium block

The part: a milled aluminium block with the serial engraved in a 30 × 10 mm field, stroke width about 0.3 mm. The station reads it in one shot, no rotation.

Resolution first. Text needs about 3 pixels across a stroke, so the pixel must be no larger than 0.3 mm / 3 = 0.1 mm on the part. Take a 2448 × 2048 sensor with 3.45 µm pixels as the example camera: the sensor is 2448 × 3.45 µm = 8.45 mm wide. Sizing the field at 40 mm for placement margin gives 40 mm / 2448 px = 16.3 µm per pixel, so a stroke spans 0.3 mm / 0.0163 mm ≈ 18 pixels. Comfortable.

Focal length next, at a 100 mm working distance:

17.4 mm is not a stock lens. With a 16 mm lens the numbers re-check as m = 16 / 84 = 0.190, field = 8.45 mm / 0.190 = 44.4 mm, pixel scale 44.4 mm / 2448 = 18.1 µm and stroke width 16.5 pixels — the 30 mm text still fits with margin. The focal length calculator runs the same arithmetic.

Now the light. With a 48 mm bore (r = 24 mm), set the ring at h = 12 mm: θ = arctan(12 / 24) = 26.6°, mid-band grazing. Check clearance: the field edge sits 22.2 mm off-axis and a small lens front at roughly 15 mm radius, so the sightline passes 22.2 − 7.2 × (12 / 100) = 21.3 mm from the axis at the ring's height, inside the 24 mm bore radius. Nothing vignettes. Strobe the ring for 1-2 ms to freeze line motion. Expect the machined field to image near black, each stroke to return a bright line, and a single mid-grey threshold to separate characters from background with contrast far wider than a top-light view gives.

Checks before you commit

• Sweep the height in 2 mm steps through the computed band and keep the height with the strongest stroke contrast; the peak normally sits within ±2 mm of the calculation.
• Fold in the part-height tolerance. At h = 12 mm, a ±1 mm seating swing moves the angle between arctan(11 / 24) = 24.6° and arctan(13 / 24) = 28.4°, about ±2°, which is harmless. A ±5 mm stack-up swing is not, so fixture the height or measure it in the station.
• Rotate the ring 90° on brushed parts. Rays running along the brush lines keep the field dark; rays crossing the lines light up the texture and bury the strokes.
• Test white against red and infrared on coloured or anodized finishes before locking the wavelength.
• Skip the polarizer at grazing angles: the specular return already leaves the lens path. Crossed polarizers pay off in coaxial and dome bright field, as the cross-polarization guide shows.
• If the line runs fast, strobe through a lighting controller and mind the overdrive and current-control limits instead of simply turning the drive up.

When the grazing answer is wrong

• Curved or cylindrical parts: the ring sets one angle and the surface breaks it everywhere except the tangent band. Use a dome for uniformity, or index the part so each character zone crosses the band.
• Annealed, printed or ink-filled marks: there is no wall to scatter light, so grazing returns a featureless grey. Read these in bright field with wavelength contrast; the LED colour guide covers the trial sequence.
• Coloured reflective metals such as copper: wavelength and finish interact, and the copper surface lighting guide works the finish-specific version of this decision.
• Recessed features, bores and threads: grazing throws long shadows across them. Use axial light or the recessed-contact techniques instead.

Where this leaves the design

Commit by surface: polished or machined and flat means a low-angle ring at the computed height; satin or curved means a dome or diffuse coaxial panel; stamped and steep means coaxial. Clamp a sample, set the height from h = r × tan(θ), sweep ±3 mm and photograph the sweep before ordering anything. Once you know the field size, angle and wavelength, the lighting configurator narrows the candidates, and the team answers set-up questions through the contact page. The lighting topic page collects the neighbouring techniques — domes, dark field, polarization — around this one.