Eliminate edge shadows in machine vision when parts sit off-center

The report is consistent: with the part under the middle of the lens the edge reads clean, and the same part pushed toward one side of the field grows a dark band along its edge. Raise the exposure and the band fades, but the true edge fades with it. Swap in a bar light, then a ring light, and the band survives both.

That pattern is not a lighting fault. A conventional lens views the part at an angle once it leaves the optical axis, so it images the side wall as a shadow-like band welded to the edge, and no repainting of the light removes it. Below you get the five-minute test that separates geometry from illumination, the arithmetic that predicts the band width from part height and position, and the fixes that hold: a telecentric lens with coaxial or collimated light, a larger coaxial light where the tolerance allows, or a backlight for outline gauging.

First, five minutes that separate geometry from light

Before changing any hardware, pin the cause. Three candidates cover nearly every edge band of this kind, most likely first:

1. Perspective error of a conventional lens. Chief rays tilt outward from the axis away from the field center, so past mid-field the lens looks onto the part's side wall and prints it as a dark band along the edge. Test: slide the part from center to a known offset with the light untouched. A geometric band grows smoothly with distance from the axis.
2. Directional illumination shadow. A bar light or a one-sided ring throws light from one direction, and a tall part casts a real shadow onto the background. Test: rotate the light or move it to the opposite side. A light shadow flips with the light; the geometric band does not.
3. A real feature on the part. A chamfer, radius or burr prints as its own dark or bright line at every position. Test: bring the part back to center. The feature keeps its width and stays; the perspective band collapses to nothing at the center.

If the band width also matches the arithmetic in the next section, the case is closed.

The arithmetic: the band follows the part, not the light

A lens of working distance WD imaging a part offset d from the optical axis views it at an angle θ, and a side wall of height h prints with this width:

With 3.45 µm camera pixels and 0.5x magnification, one object-side pixel measures 3.45 µm ÷ 0.5 = 6.9 µm, so the 10 mm-offset band spans 270 µm ÷ 6.9 µm ≈ 39 pixels. Whatever threshold you set lands inside the band, and it shifts with every part position; against a ±0.02 mm gauging tolerance the band is more than ten times too large. This is also why the bar light and the ring light failed: neither changes where the chief rays point, so both only repaint the wall's brightness.

Fix 1: a telecentric lens with coaxial or collimated light

A telecentric lens places the entrance pupil at infinity, so it accepts only chief rays parallel to the optical axis: a vertical side wall is seen edge-on at every part position. This is the textbook case for when a telecentric lens pays for itself, with the broader entocentric versus telecentric decision covering when the extra length is not warranted. The residual band comes from the lens's own telecentricity, the chief-ray angle tolerance that measurement-grade designs hold near 0.1° — the 3 mm wall then projects 3 mm × tan 0.1° = 5 µm, under one pixel at the 6.9 µm scale, constant across the whole field. Telecentricity is the number to compare between data sheets.

Telecentric imaging needs illumination traveling down the axis as well. The shortest path is a lens with the light built in; the port route leaves the light choice to you:

The LS-TCL-2194 integrates the coaxial light behind an internal beam splitter. Its printed specifications: 0.5x magnification, 110 mm working distance, 1" sensor format, C-mount, with the lens front group collimating the light by design.

The LS-TCL-0811 carries a coaxial light port for when you want to choose the light. Its printed specifications: 1.36x magnification, 73 mm WD, 2/3" sensor format, C-mount.

Telecentric-type collimated lights sized for fields of this class:

Size the light to the field, not to the part. The emitting area must flood the lens aperture across the part's full travel, with roughly 20-30% margin beyond the field edge in the worst position; beam diameters in the 90-100 mm class cover a 26 × 19 mm field with that margin.

Fix 2: a larger coaxial light flattens brightness, not geometry

The illumination side can still be wrong once. A coaxial light that is small relative to the part's off-center position strikes the wall at a slant and lays a gray gradient across the bright fixture. Be clear about the split: a bigger emitting area flattens that gradient, but the side-wall band is lens geometry and survives every light swap. The two sizes worth knowing:

The LS-CXL-0092 provides a 70 × 70 mm emitting area on 24 V. For wide travel, the LS-CXL-0192 spans 140 × 140 mm at 525 nm, also on 24 V.

As a stand-alone fix, the larger light suits low parts, roughly 1 mm tall or less, against tolerances of 0.5 mm or looser; any real metrology pairs it with Fix 1.

Fix 3: collimated backlight for outline-only gauging

When the inspection reads the silhouette — width, diameter, position against the fixture — put a collimated backlight under the part and keep a telecentric lens above. The edge then forms where the part interrupts a parallel beam: no side wall, no gray band, position-independent, the sharpest edge a vision station can produce. Read the collimated versus diffuse backlight trade-off for the alignment details, and surface versus silhouette lighting when some features still need top light. The costs are real: the silhouette frame carries no surface information, and the part needs free space below, so the fixture becomes glass or a windowed plate. Run both lights sequentially in one cycle when the top face carries features too.

Telecentric depth: keep the part inside it

Parallel chief rays hold magnification constant only inside the lens's depth range. A practical estimate:

with blur the permissible image-side blur, commonly two pixels. At 0.5x magnification, f/4 and 3.45 µm pixels: depth = 2 × 4 × 6.9 µm ÷ 0.25 = 220 µm, about ±110 µm around the working distance. Stopping down two stops, f/4 to f/8, doubles it to ±220 µm and costs four times the light or exposure. The fixture's height tolerance must fit inside the depth, or the measurement drifts with part height — a drift pattern covered in micron-level measurement accuracy.

Checks before ordering, and the decision per case

Run this list against the station before ordering anything:

• Field covers travel. Sensor width ÷ magnification ≥ part travel + longest part dimension + 10% margin. On the 1 inch example above: 12.8 mm ÷ 0.5 = 25.6 mm ≈ 26 mm across.
• Band budget. h × tan(telecentricity) ≤ tolerance ÷ 3 as a working rule; aim at the stricter 10:1 accuracy rule for metrology work.
• Depth covers height. The depth estimate must exceed the part's height tolerance, or stop down and re-check the exposure budget.
• Light area covers field and travel, with the 20-30% margin; telecentric-type lights behind port lenses, built-in light on the integrated route.
• Edge gradient survives exposure. After the fix, the edge should fall several tens of gray levels across one to two pixels at working exposure; if it does not, the illumination geometry is the remaining problem — see lighting geometry for contrast.

The decision per case: tolerance at 0.1 mm or tighter with free part position — a telecentric lens with built-in coaxial light, or a port lens with a telecentric collimated light. Tolerance of 0.5 mm or looser on a part under about 1 mm tall — the larger coaxial light first, as the smaller change. Outline-only gauging — a collimated backlight under the telecentric lens. Moving the camera four times farther away shrinks the band to roughly a quarter; it stays a stopgap, never a fix under a 0.1 mm budget. Size the set in the lens finder and light finder, start the configurator at the lens step or the light step, and if the numbers above do not close the case, bring the fixture details to us through the contact page.