Partial bright field lighting: why matte parts need directed light

A light aimed straight down at a polished part throws its energy right back into the lens. Aimed at a sand-blasted casting, the same trick hides the detail you want. Partial bright field lighting solves that: directed light, an intentional angle, and a rough surface that scatters the beam into an even glow. Here you get the physics, the beam-angle arithmetic and a worked sizing example, so you can place the light with numbers instead of trial and error. The focus is castings, moulded plastics, textured labels and other matte surfaces.

What partial bright field lighting is

Partial bright field lighting is directed incident light aimed at the part from an angle, typically 40-60° off the camera axis, so the mirror-like reflection misses the lens while scattered light fills it. The part reads evenly lit, and surface texture shows up as gentle shading rather than glare. It is the most widely used incident-light geometry in production vision, and the closest cousin to how a desk lamp lights a workbench: the lamp sits beside you, not behind your head.

Full bright field, by contrast, puts the light on the camera axis, coaxially or in a ring around the lens, so a flat glossy surface returns the light straight into the aperture. That is exactly what you want when reading a laser mark on polished steel, and exactly what wastes power on a matte part: the scattered return is weak, so most of the light never reaches the sensor. The machine vision illumination types overview places partial bright field among the four incident geometries; this page works that one choice in full.

The physics: one law, two surfaces

Every ray obeys the law of reflection: angle of incidence equals angle of reflection (bright field vs dark field lighting: the law of reflection decides works that case in detail). What differs between surfaces is the spread of surface normals. On a polished part the normals sit near-parallel, so parallel rays leave near-parallel and the energy stays in a lobe only a few degrees wide. On a matte part, micro-roughness points the normals in every direction, so the same ray leaves somewhere within a hemisphere.

Each patch then follows Lambert's cosine law: brightness falls with the cosine of the incidence angle θ.

At 60° incidence a matte patch returns about half the light it returns face-on, because cos 60° = 0.50. Two consequences drive the design. A matte part cannot blow out the sensor with a specular flash, so exposure is forgiving. And because the scatter cone is wide, the camera accepts light from almost any lamp position, so you pick the angle for coverage and shadow control, not for reflectivity. The surface becomes its own diffuser. Finish feeds in too: a coarser blast widens the scatter, which is why finish specifications such as Type III hardcoat anodize for matte black parts belong in the lighting conversation.

Four incident geometries at a glance

Partial bright field sits between the on-axis and grazing extremes of the incident-light family. The table compresses the decision; machine vision light geometry selection by surface and feature turns it into a step-by-step chooser, and the lighting topic page collects the whole series.

The diagram shows the two extremes. Partial bright field borrows the directed cone of the dark-field set-up but aims it at a surface too rough to need grazing angles.

Worked example: sizing the cone for a 90 mm matte housing

The whole decision fits in one calculation. The case: a sand-blasted aluminium housing, 90 mm across, checked for stamp depth and surface damage on a static station. The inputs:

• Field of view target: 110 mm, from the 90 mm part plus 10 mm margin on each side (about 10%).
• Incidence angle: 45° from the surface normal, the compact-bracket starting point.
• Margin factor: 1.1 on the light footprint.
• Candidate cones: 32° and 21° full angle, the printed lens options in the catalog spots.

The rule in words: a cone of full angle θ aimed from distance d paints an ellipse on the surface. Across its narrow axis the width is 2 · d · tan(θ/2); the 45° tilt stretches the ellipse along one axis by 1 / cos 45°, so the narrow axis is the binding one.

• 32° cone: half-angle 16°, tan 16° = 0.287. At d = 220 mm, w = 2 × 220 × 0.287 = 126 mm. That clears the 121 mm requirement (1.1 × 110) and covers the field with about 15% to spare.
• 21° cone: half-angle 10.5°, tan 10.5° = 0.185. At d = 220 mm, w = 2 × 220 × 0.185 = 81 mm, roughly 40 mm short. Solving backwards, d = 121 / (2 × 0.185) = 326 mm, so this cone needs a stand-off of about 330 mm.

Result: the 32° cone fits at a 220 mm stand-off; the 21° cone needs about 330 mm. Decision: take the wide cone if the fixture allows 220 mm. Irradiance falls roughly with the square of distance for small sources, and (330 / 220)² = 2.25, so the 21° set-up needs about 2.25 times the light output to match image brightness. Keep the threshold: narrow-axis footprint ≥ 1.1 × field of view. Below it the field corners dim, the histogram shifts with part position, and fixed thresholds drift.

The wide-cone pick:

The narrow-cone alternative, for taller stands:

LS-SPT-0024 prints a 455 nm blue, 32° lens; LS-SPT-0037 prints a 505 nm cyan, 21° lens. Either colour images well on grey aluminium. The geometry above fixes the mounting, not the colour.

Where partial bright field wins, and where it loses

It earns its keep on castings, blast-finished and anodized metal, textured mouldings, unglazed ceramics, paper labels and matte food surfaces: anything that scatters. Set two lights at mirrored 45° angles and their shadows cancel, so embossed characters and raised beads read without a black band down one side.

It loses in four cases. Polished or curved shiny parts keep throwing a specular lobe toward the lens no matter where you stand; hand them to a dome or coaxial light, as in lighting shiny metal pins without glare: dome, diffuse and polarizer. Deep bores and recessed features hide from any external cone; grazing angles reach them, as in detecting dents and pits in metal surfaces with low-angle lighting. Semi-gloss parts sit in between: start here and shave the residual gloss with a polarizer, per cross polarization machine vision: reduce surface glare. And a silhouette dimension check needs no incident light at all; machine vision lighting angles: surface or silhouette? settles that call.

Picking the hardware: spots, bars and rings

Three families cover the job, and field size picks between them.

Spot lights suit fields up to roughly 150 mm: one lamp off-axis on an adjustable bracket. The cone-width trade is the one the worked example made: a wider lens shortens the stand-off but dilutes intensity. Diffused windows widen the cone further and smooth LED hot spots at a cost in peak output; the optics side is explained in LED lens beam shaping: focused versus diffused illumination. The spot lights finder filters by lit area and lens angle.

Ring lights suit small and medium fields where you want light all around the optics. An angled ring light at 30-60° gives concentric partial bright field with a clear hole for the lens. Keep the inner diameter clear of tall features, or the part shadows its own far side.

Bar lights take the long cases. Opposed bar lights at 45° wash a panel or an extruded profile evenly, and diffused types trade intensity for homogeneity along the length. Balancing coverage against brightness on wide fields is covered in large field of view lighting: balance coverage and brightness.

Two diffused units that fit the matte-part job:

LS-BAR-1276 is a diffused white bar in the 200-400 mm size band; LS-RNG-0143 is a diffused 100 mm ring with a 30° emission angle. Both run on 24 V from a standard lighting controller. Strobe rather than run continuous: the pulse freezes motion, drives the LEDs harder for the instants that matter, and holds brightness steady frame to frame. The case for the controller is in why use a machine vision lighting controller: five reasons, and the current side in LED overdrive and current control: stable machine vision lighting.

Checks before you lock the design

• Spin the part under a hand torch: if brightness barely changes as the angle changes, the surface is matte enough for this geometry.
• Recompute the footprint at the real stand-off, including bracket offsets, and keep narrow-axis coverage at 1.1 × field of view or better.
• Trace the specular lobe for every part variant and fixture position; no variant may clip its lobe into the lens.
• Pair lights at mirrored angles whenever the part has raised features, so each side's shadow is filled from the other.
• Compare corner and centre histograms and hold them within about 10% before fixing thresholds.
• Log light output over the first weeks of production: a slow drift points to LED ageing or dirt on the window, and both are cheaper to catch early.

When the usual answer is wrong

Three wrinkles. Mixed-gloss parts, say a glossy label zone on a matte bottle, need lighting split by zone rather than one compromised geometry for both. Translucent plastics blur the category: an angled lamp outside the field acts as a back-lighter through the material, which can beat any surface geometry on thin parts. And when the part is matte but the measurement is dimensional, light geometry was never the deciding choice; silhouette and telecentric techniques take over.

Next step: filter the lights finder by field size and cone angle, or step through the lighting configurator with your part dimensions. If a stubborn part resists, contact the applications team with a photo of the surface and a sketch of the fixture.