Lateral illumination angle sets image contrast in machine vision

Move the same LED bar from above a part to beside it and the camera image can invert: a bright, flat metal face goes black while a scratch, a laser-etched Data Matrix or a machined chamfer lights up. The flip is not luck. The angle between the light and the surface normal decides whether you are imaging bright field or dark field, and the hand-over sits near 45°. This guide gives you the angle bands, the shadow and glare arithmetic that fixes where the bars go, the light families that work from the side, and a bench sweep that finds your working angle before you order hardware. It is written for anyone building an inspection around flat metal, laser marks or machined edges.

What is lateral illumination in machine vision?

Lateral illumination means the LEDs sit off to the side of the camera axis, so light strikes the part at an angle anywhere from 0° (straight down the lens axis) to nearly 90° from the surface normal, and that angle, not the light's brightness, decides what the image shows.

Measure the angle from the surface normal: 0° is light running parallel to the camera axis, 90° is light skimming the surface. Every ray leaves the LED, bounces once off the part and either reaches the lens or misses it; the finish decides how much is mirrored and how much is scattered. Two consequences follow. First, the same part can look completely different at 10° and at 70°, so the angle is a design parameter, not a setup detail.

Second, geometry is what counts: tilt the camera toward the part while the light stays put and the image flips exactly as if you had moved the light. The geometry of camera, light and part positions page works through that triangle.

At what angle does bright field turn into dark field?

The hand-over sits near 45° from the surface normal: from 0° to about 30° a flat surface still sends light back toward the camera and reads bright, and from about 45° upward the specular reflection leaves the lens, so flats go dark and only edges facing the light glow.

The rule underneath is the law of reflection: the reflected ray leaves at the same angle from the normal at which the incoming ray arrived, on the far side. A camera looking almost straight down, within 0-10° of the normal, therefore catches a flat face's mirror reflection only when the light also sits near the axis. The bands look like this:

Where a part sits inside the 10-30° band is a finish question: a bead-blasted face scatters enough light to stay readable at 40°, while a polished or rolled face is already black by 45°. The two endpoints are worked through in the bright field vs dark field comparison; matte surfaces sit closer to the partial bright field case.

Why does a laser-marked Data Matrix light up under side light?

A laser mark scatters light in every direction while the polished metal around it reflects like a mirror, so at 60-75° from the normal the code returns light to the camera and the smooth conductor reflects it away: a bright code on a dark background.

The laser re-melts and microscopically roughens the surface it writes on. To your eye the mark is slightly duller metal; optically it is a scatterer, sending a share of the incoming light into every direction, while the surrounding rolled face behaves like a mirror.

Park the light at 60-75° from the normal and read the pair together: the mirror reflection of the flat lands far from the lens, the scattered light from the mark comes back at any viewing angle, and the code prints bright on black. At bright-field angles the same part gives the inverse image, dark print on grey, which is usually the weaker contrast on a reflective conductor. The dot peen Data Matrix dark field case and laser-marked OCR on reflective wafers both run on exactly this difference.

How do you stop side light shadows and a brightness gradient?

Illuminate from at least two opposite sides at the same angle: one side light casts a shadow of length equal to the feature height times the tangent of the angle from the normal, and it fades the image from bright to dark across the field.

The shadow behind a raised feature of height h, lit at angle θ from the normal, stretches

A 2 mm boss lit at 60° hides 2 mm × tan(60°) = 3.5 mm of surface behind it; the same boss lit at 30° hides 2 mm × tan(30°) = 1.2 mm. Steeper lateral light means longer shadows, which is why the two sides must match.

Two bars at equal angle and equal intensity light every point from both directions at once: the shadows cancel and the left-to-right brightness slope disappears. Fire them together from one trigger so both pulses land inside the same exposure.

For rectangular fields, bar lights are the natural shape, with lit lengths from under 50 mm to more than 1.2 m; the ring light vs bar light trade-off comes down to part shape and glare, and matched low-angle bars on a moving line show the two-sided pattern in production.

How far from the lens axis do the bars sit so the flat top stays dark?

Treat the flat surface as a mirror and keep the glare streak outside the field of view: with the lens 150 mm above the part and the bar face 40 mm up, the glare lands at 0.79 of the bar's horizontal offset, so a 40 mm offset puts it about 32 mm out.

Worked example. A station reads a laser-marked Data Matrix on an aluminium plate. The camera pairs a 7.2 mm wide sensor with a 25 mm lens at a 150 mm working distance, so the field of view is 7.2 mm × 150 mm ÷ 25 mm = 43.2 mm, half-width 21.6 mm. Reflect the bar face through the surface and draw a straight line from the lens to that virtual image: where the line crosses the surface is the glare point.

d is the horizontal offset of a point on the bar face, H the lens height above the surface (150 mm) and h the bar face height (40 mm). The factor is 150 ÷ 190 = 0.79, so to push the glare past the half-field, d must exceed 21.6 mm ÷ 0.79 = 27 mm.

Choose d = 40 mm: the near edge reflects to 40 mm × 0.79 = 32 mm and the far edge of a 40 mm wide face to 80 mm × 0.79 = 63 mm, so the whole streak misses the 21.6 mm half-field with margin. Mount each bar's near edge about 40 mm out from the lens axis at the face height, tilt it to the 60-70° the code wants, and the flat stays black.

Which light families work from the side?

Bar lights are the default on flat, rectangular parts because you can run two of them at any angle you choose; low-angle rings take over on round parts, and small spot lights cover tiny fields or corners inside a machine.

A bar pair brackets the field, and every angle from 30° to 85° is a bracket adjustment. Narrow 12-16 mm profiles sit low beside the part, which grazing work wants; wider 30-40 mm faces throw more light when the exposure budget is tight. Spots suit fields under about 50 mm and mounts where nothing bigger fits.

The three pieces below cover a two-sided flat-part bench and a small-field spot job:

On round parts, two bars cannot follow the curve. The LS-RNG-0256 is a 96 mm outer diameter ring that lights at low angle (0-15°) from every side, so a cylinder goes dark all around with no aiming. That elevation band equals 75-90° from the surface normal, the deep end of dark field.

For the angle sweep itself, the LS-RNG-0730 lists every candidate on one 103 mm diameter body: low angle (0-15°), 50°, 65°, 70°, 75°, 80° and 85°. Sweep that list on the bench and the bright-to-dark transition shows up live in the image. Rings live in the ring lights category, spots in spot lights.

How do you find the working angle before ordering?

Find the angle with one light before you commit: clamp it on a swivel arm, sweep the tilt from 0° to 85° in 10° steps and watch where the flat drops dark and where your feature peaks — for laser marks the peak usually sits between 60° and 75°.

1. Square the camera to the surface first, because every angle afterwards is measured from the normal.
2. Power one light, sweep 0° to 85° in 10° steps and save an image at each position.
3. Record the grey value of the flat background and of the feature at every step; your working angle is where the background collapses and the feature holds.
4. Clone the bar on the opposite side at the matched angle and re-shoot; the gradient and the single-sided shadows should disappear together.
5. Nudge the part ±0.5 mm in every direction and re-shoot; if contrast holds, the station survives placement error.

Fire the two bars in sequence instead of together and the same two images yield surface normals nearly for free — photometric stereo from hardware already on the bench. The same sweep settles engraved text on metal, and for dent and pit work it usually lands at 70° and above, as in the low-angle dent detection guide.

When is lateral illumination the wrong answer?

Skip side lighting when the defect hides on an uneven or strongly curved face, when presence checks need the whole surface uniformly bright, or when transparent material mixes transmitted and reflected light — coaxial, dome and backlight each beat lateral light in one of those cases.

Match the exception to the alternative: print and labels on matte tops prefer soft, directed top light, as in the food packaging lighting chooser; deep bores and recesses want axial light delivered through the lens, the coaxial approach for recessed parts; fields too wide for one frame belong on a line scan camera with its own illumination options. Everything flat, opaque and reflective is side-light country.

Take three numbers to the bench — part size, camera height, finish — then filter by size, angle and wavelength in the lighting finder or start the configurator at the light step. The lighting topic page collects the geometry guides, and a sample photo through the contact page gets two angles to try first.