Machine vision light geometry selection by surface and feature

Light geometry decides whether a defect is visible before any software runs. Most failed inspections come from a light with the right brightness but the wrong shape: a dome on matte black rubber, a ring on a curved mirror, a front light where a backlight belonged. This checklist walks the selection in the order you meet it — surface, angle, size, motion, colour — with worked arithmetic for a real part and the checks to run before ordering. Ring lights, bar lights and dome lights each solve one slice of the problem; the table below picks the slice.

Which light geometry fits a matte, shiny or curved surface?

Matte surfaces take a directional ring or bar at 30-45°, flat shiny surfaces take a dome or coaxial light, curved shiny surfaces take a dome, and any part whose outline matters more than its surface takes a backlight.

The law of reflection does the sorting. A matte surface scatters light in every direction, so geometry is forgiving and you optimise for evenness. A flat shiny surface mirrors light, so only a beam aligned with the camera axis returns to the sensor. A curved shiny surface sweeps that mirror through every angle at once, and only a fully diffuse field lights it. The earlier guide to machine vision illumination types covers the families in depth; the question of surface or silhouette settles front light against backlight.

When one part matches two rows — a printed matte face plus a polished boss — run two lights on separate controller channels and take two exposures. The sections below size each choice.

What light angle makes scratches, edges and engraving glow?

Strike the surface at 20-45° for dark field: a flat surface reflects the light away and only scratches, edges and dust scatter light into the lens, so defects glow on a dark background.

Measure angles between beam and surface. Below about 15° — the built-in beam of a low-angle ring — fine texture, dust and edges light up against a near-black background, and standoff barely matters. Between 20° and 45°, a bar light gives the same dark-field behaviour with more working room and stronger relief on coarse finishes. Above 45° you are in directional bright field: flat matte surfaces go bright and relief gains shadow, which suits print and texture. Straight down the axis is the one angle a mirror sends back, which is what coaxial light exploits.

At α = 45° the ring radius equals the working distance; grazing dark field from a plain ring needs the part closer than one ring radius, which is why low-angle rings with 0-15° optics exist as their own type. Low-angle lighting is also the standard answer for dents and pits in sheet metal, where the defect is deformation rather than colour — see detecting dents and pits in metal surfaces. For the path of each reflected ray, bright field versus dark field lighting is the companion piece.

A low-angle ring builds the grazing beam into its optics. The LS-RNG-0426 is a 120 mm diffused ring light at 0°, aimed at exactly this job.

How do you size a ring, bar or dome to the part?

Match the light to the field of view and the working distance: a ring's incidence angle at the part centre is atan(ring radius / WD), a bar's lit length should reach at least 1.3 × the field of view, and a dome's aperture at least 1.5 × the part width.

• Ring: the bore clears the lens barrel plus focus travel, and the outer diameter sets the angle — for 45° incidence at the centre, ring diameter ≈ 2 × WD. Ring lights are grouped by size class from under 50 mm to over 400 mm.
• Bar: lit length at 1.3 × field of view or more, because output falls away at both ends; tilting the bar shortens its footprint, so keep the margin when mounting shallow. Bar lights span the same size range.
• Dome: aperture at 1.5 × the widest face you light, with the part just inside the opening and the lens passing cleanly through the camera hole. Dome lights are listed the same way.

Worked example: anodized plate with a polished boss

Inputs: a 120 × 80 mm matte anodized aluminium plate, worst defect a 0.25 mm scratch; a polished boss of 40 mm diameter on the same plate; line speed 250 mm/s; a 2448 × 2048 pixel camera with 3.45 µm pixels (an 8.4 × 7.1 mm sensor) at a 300 mm working distance.

Field of view: 140 × 117 mm covers the plate with margin. Ground sample distance = 140 / 2448 = 0.057 mm per pixel, and the scratch spans 0.25 / 0.057 = 4.4 pixels — past the 3-pixel minimum for a line defect. Magnification m = 8.4 / 140 = 0.060, so f = WD × m / (1 + m) = 300 × 0.060 / 1.060 = 17 mm; a 16 mm stock lens delivers the same field at about 283 mm. The focal-length calculator runs this in one step:

Dark field bar for the scratch: at a 30° beam angle and a 120 mm offset from the defect line, the bar centre sits 120 × tan 30° = 120 × 0.577 = 69 mm above the surface. Lit length must reach 1.3 × 140 = 182 mm. The LS-BAR-1047 is a 290 x 30 mm bar light, past that with margin for end falloff.

Dome for the boss: the region to light is about 60 mm across, so the aperture needs 1.5 × 60 = 90 mm. The LS-DOM-0122 is a 134 mm OD dome light and clears that with room for the part to sit inside the opening. Fire it in a second exposure with the bar off.

Why not a ring for the plate: the 45° rule at a 283 mm working distance asks for a ring near 570 mm across (2 × 283). At that size a bar is the practical buy — the same conclusion the large field of view lighting guide reaches.

How do you set the exposure to freeze a moving part?

Keep motion blur under half a pixel: exposure time = (0.5 × ground sample distance) / part speed, so a part moving 250 mm/s under a 0.057 mm/px scale needs the light strobed for less than about 115 µs.

For the example plate: t_max = (0.5 × 0.057) / 250 = 0.000114 s, about 110 µs. A continuous light can only be gated that short if it is very bright, so strobe instead: a 50-100 µs controller pulse, overdriven because the duty cycle stays low, freezes the part and swamps ambient light at the same time. Strobing also gates out the 50/60 Hz ripple of overhead lighting that otherwise prints as banding — camera flicker under artificial lighting covers that diagnosis.

Aperture: plate height varies ±0.5 mm, and depth of field for one pixel of blur is DoF ≈ 2 × f-number × GSD × (1 + m). At f/8: 2 × 8 × 0.057 × 1.06 = 0.97 mm, which covers the ±0.5 mm band; at f/4 it is 0.48 mm and the corners soften. Stay at f/8 and let the strobe pay the light bill — the depth of field calculator checks the lens side.

When does the usual geometry rule fail?

Domes fail on matte or large parts, dark field fails on curved shiny surfaces, backlights cannot show surface print, and a single ring lights a field of view only about as wide as its own diameter.

• Matte black parts: a dome needs a specular path to redirect and matte black rubber has none — it returns flat black. Low-angle bars on the edges are the fix, as shown for black rubber inspection lighting.
• Curved shiny parts under dark field: grazing light skates along the curve and leaves bright streaks. Go diffuse with a dome, the trade-off laid out for shiny, curved and wrinkled parts.
• Mirror-flat surfaces with flat contamination: a stain or fingerprint has no relief, so dark field misses it. Use coaxial or dome light, adding cross polarization when the residue itself is the target.
• Relief defects on polished parts: a dome flattens a scratch into the background; dark field brings it back. The two-way trade is shown for lighting shiny metal pins.
• Transparent parts: dark field glows the edges and loses the faces. A backlight wins for outline, as covered in detecting small transparent parts.

Which colour and diffuser complete the selection?

Choose geometry first and colour second: red light gives the strongest monochrome response, blue resolves finer surface detail, complementary colours darken a coloured feature, and a diffuser turns a hard directional beam into even coverage.

Red delivers the highest output per watt and a monochrome sensor's strongest response. Blue scatters harder at fine surface structure, so fine scratches and texture read sharper. A coloured feature goes darkest under the complementary LED — the machine vision lighting color wheel picks it, and which LED colour gives the best contrast summarises the evidence. Keep white for a colour camera that must see true colour; the monochrome trade-offs sit in light colour and monochrome camera contrast. Near-IR at 850 or 940 nm behind a matching pass filter erases visible ambient; UV at 365-405 nm turns adhesives and oils fluorescent.

A diffuser in front of a directional light widens the beam, evens the falloff and softens the specular hotspot, at the cost of reach — pay it back with strobe current. The LS-RNG-0143 is a 100 mm ring light with diffuser at 30°, the default shape for matte print work.

What should you check before ordering?

Run seven checks: lens clearance through the ring or dome hole, lit length against field of view, working-distance window against part height variation, controller channels and strobe current, trigger wiring, connector family, and mounting angle.

Fit and optics

1. Ring bore or dome camera hole clears the lens barrel plus focus travel — a barrel that fouls the opening shifts the whole geometry.
2. Lit length at least 1.3 × the field of view — output falls away at the ends of every bar.
3. Depth of field covers the part height variation — recompute it whenever the standoff moved during layout.

Drive and wiring

4. One controller channel per light when geometries alternate, and strobe pulse width under your t_max.
5. Trigger source wired — encoder or photoelectric sensor — so the pulse lands with the part centred.
6. Connector family matches the harness; 3-pin flat and XS10-3T are the two you meet most often.

Environment

7. The bracket holds the angle you computed: a bar mounted flat by accident is a bright field you never chose. Add a shroud when the station sits under overhead lighting.

Next step: the light step of the configurator locks geometry, colour and drive together with a camera and lens, and the lights finder filters every shape by size, angle and wavelength. If the part is on the bench, the contact page reaches the applications team for a lit trial before you commit hardware.