Segmented ring light photometric stereo on patterned parts

Printed patterns, woven textures and colour changes defeat the classic character check: threshold one grey image and the pattern swamps the mark. Glossy spots add halation, and a part family that spans several colours looks like one lighting tune per colour. Photometric stereo with a segmented ring light breaks that deadlock. The ring fires its sectors one after another while the camera and part hold still, and per-pixel ratio math turns surface slope into strong contrast while flat ink stays quiet. Here you get the principle, the combination math with a worked threshold calculation, the choice between four and eight zones, and the controller requirements to order against.

What photometric stereo separates

Photometric stereo treats every pixel as a small measurement. How bright a point appears depends on two things: the slope of the surface at that point, and the direction the light comes from. Fire the light from N directions while camera and part stay fixed, and each pixel returns a vector of N counts. A flat area printed with ink or dye returns nearly the same count in every frame, because reflectance sets its brightness and reflectance does not change with direction. A slope returns strongly direction-dependent counts: lit from one side it flashes, from the opposite side it goes dark. The count vector therefore separates shape from flat 2D texture, which is exactly the split a character check needs — the mold mark is shape, the printed pattern around it is texture.

The same measurement supports full surface-defect maps with continuous rings; photometric stereo ring lighting for surface inspection covers that broader use. The segmented ring is the compact way to build the same directions around a single lens.

How a segmented ring light makes the directions

A standard ring drives every sector at once, so it produces one image in which print and height are mixed beyond separation. A segmented ring splits the emitting annulus into zones and wires each zone to its own driver channel. Fire zone 1 and store a frame, fire zone 2 and store a frame, and so on: N zones give N images from N azimuths, all through the same lens at the same focus and gain. Low-angle zones — the 0-15° class used for edge and embossing work — graze the surface, so flank slopes flash bright while flats stay dark. Steeper zones flood more evenly and suit shallow relief.

Zone count, zone elevation and emission colour define the ring build. Segmented rings are configured to order: you specify zones, diameter, angle per zone and connector layout, then pair the ring with a controller offering one strobed channel per zone. The cards show the two ring geometry classes the ratio math leans on:

For the segmented build itself, take the geometry above into the configurator and specify the zone layout alongside it. A coarser version of the idea stacks two complete rings at different angles and switches between them; stacked ring lights: switching angles to catch more defect types covers that two-image variant, which needs no segmentation at all.

The combination math: ratios that cancel print

Both frames of a pair show the same surface, so the per-pixel division meets the same ink, dye and colour in numerator and denominator. A flat printed area divides to almost exactly 1. A slope pushes one frame up and its partner down, so the ratio moves far from 1. C keeps the larger excursion of the two pairs, so the flank orientation of a character stops mattering. In the classic formulation each frame measures a projection of the surface normal, I_i = ρ (n · s_i), and three or more directions solve the normal and reflectance ρ per pixel; the two-pair ratio form is the special case that is simple to implement and easy to threshold.

Worked example: molded date characters on a black tire sidewall

This is the classic case, and a hard one: molded, not printed, characters on black rubber, a patterned tread band around them, and a recessed hub that casts shadows under any single light. Four low-angle zones fire in sequence; a monochrome camera stores four 8-bit frames. The counts below are illustrative pixel readings typical of flank and pattern response at this geometry.

The arithmetic, plainly: the flank divides 180/40 = 4.50 north-south and 95/105 = 0.90 east-west, so C = max(4.50, 1/4.50, 0.90, 1/0.90) = max(4.50, 0.22, 0.90, 1.11) = 4.50. The pattern divides 120/110 = 1.09 and 115/118 = 0.97, so its C = 1.09. Mark-to-pattern separation is 4.50/1.09 ≈ 4.1×.

Place the threshold at the geometric mean of the two clusters: sqrt(4.50 × 1.09) = sqrt(4.91) ≈ 2.21. That point sits about 2× from each cluster in relative terms (2.2/1.09 ≈ 2.0 and 4.50/2.2 ≈ 2.0), the equal-margin choice. Decision: set the threshold at 2.2 on C; every character flank passes and every printed pixel rejects, with roughly a factor of 2 of margin on each side. If trials show pattern C creeping above 1.3 from ink sheen, tighten by adding a cross-polarizer or moving to eight zones. If flank C falls below about 3.0, raise the zone current or lower the elevation rather than lowering the threshold; LED overdrive and current control: stable machine vision lighting shows how strobed pulses recover the brightness short exposures give up.

Black rubber is the reason the ratio beats plain binarization: absolute counts sit low and close to the pattern, while division amplifies the direction-dependent flank response about fourfold here. The recessed hub behaves too. A hub shadow darkens a pixel in one frame of a pair and leaves it normally lit in the other, so shadow boundaries land in C as strong slopes — the real shape of the hub edge — and the date characters outside the shadow keep their full ratio contrast. If the tool needs a clean background there, mask the hub polygon.

Four zones or eight

Choose four zones when the marks face the camera and their flanks run in two preferred directions; the tire date code, the patterned tile and the heat seal all resolve with four. Choose eight when text wraps around a hub, when flank orientation varies from part to part, or when the same image set must also grade fine scratches, where azimuth resolution shows directly in the slope map. Hybrid builds put different zone counts at inner and outer heights for parts with both a flat face and a raised rim; the only new requirement is a channel count that matches the total.

Sequencing the captures: controller requirements

Each zone needs its own strobed channel, pulsed strictly inside its camera exposure, in order. Three requirements decide the controller: channels at or above the zone count, a strobe mode fast enough to pulse within short exposures, and per-channel dimming so the flat-reference calibration is possible. Constant-current output holds per-zone brightness steady as the LEDs warm, which keeps the ratios stable across a shift. The sequence itself is ordinary trigger logic — exposure gate, zone pulse, frame read, next zone — and multi channel light controller sequencing for photometric stereo walks through the wiring and timing. When the part cannot hold still for four frames, moving part photometric stereo: synchronize image capture is the companion design step.

All three controllers below drive four channels. The LS-CTL-0070 is the 200 W unit with a strobe feature and RS232; the LS-CTL-0365 runs its four channels at 16-24 V with constant-current output; the LS-CTL-0180 is a constant-current controller, four channels, 120 W, for 24 V lights.

Values as printed in each product's data sheet; open a part for its drawing and full specifications.

To size channels, power and triggering in one pass, start the controller configurator.

Where the method wins, and where the usual answer is wrong

It wins when:

• Molded or embossed marks sit under heavy print — tread-band date codes, patterned tiles, textured enclosure faces.
• Production mixes colours — ratio images are close to colour-blind, because both frames of a pair carry the same colour.
• Halation plagues the part, such as heat seals — a saturated highlight returns the same near-maximum count from every zone, so its ratio lands near 1 and the glare drops out of the slope image instead of blooming over the characters.
• Scratches hide in texture — texture is flat noise, a scratch is a V-groove with strong direction response.

The usual answer is wrong when:

• The part is a deep cavity or bore: grazing ring light cannot reach the walls. Use a dome light for curved specular parts or a 360° inspection lens for bore walls.
• The surface is mirror-finish metal: flanks saturate in every frame, so ratios saturate too. Cut exposure first, then move to cross polarization if sheen still dominates.
• The part is transparent or strongly translucent: the method assumes an opaque surface that scatters light back from its top layer.
• The material moves continuously: four frames need four static instants. For webs, the directions run as parallel line lights instead — line scan photometric stereo: four-angle line lights for moving webs.

Checks before you order

Geometry:

• Zone elevation matches the relief: low-angle zones (the 0-15° class) for character flanks and embossing, steeper zones for shallow relief. Lighting geometry for image contrast explains the angle-contrast relationship behind this choice.
• Ring inner diameter clears the lens cone at the working distance; the LS-RNG-0730, for instance, carries a 48 mm inner diameter in a 103 mm ring.
• Working distance short enough that grazing light rakes the surface; low-angle contrast falls off quickly with distance.

Electrical:

• One strobed channel per zone, plus a spare if the panel allows.
• Constant-current or calibrated per-channel dimming for repeatable ratios.
• Trigger chain arranged so no zone fires outside an exposure window.

Imaging:

• Monochrome camera, manual exposure and gain, the same LED colour in every zone.
• Brightest flat area below saturation with headroom for part wobble; a common working point keeps peaks at 70-85% of full scale.
• Frame count × (exposure + readout) checked against takt time; zones multiply the capture.

Calibration:

• Flat reference dividing to 1.00 per opposite pair before production.
• Threshold set from measured flank and pattern clusters, as in the worked example above, not from a guessed grey level.

If the parts on your line match the date-code-and-pattern class, pick the ring geometry in the ring lights category and specify zones and connectors through the configurator below; the light controllers category covers the channel side. The lighting topic page gathers every illumination guide on the site, or tell us the part, the print and your takt time and we will size the zones with you.