Bright field vs dark field lighting: the law of reflection decides

Flat, polished parts are where machine-vision lighting goes wrong most often. Light them the obvious way — from above, close to the lens — and a mirror surface returns every photon straight back: white glare hides the mark you need. Swing the same light to a grazing angle and the field turns black while a single scratch glows. Both results follow from one rule, the law of reflection, and the rule turns lighting selection into geometry you can calculate. This guide gives you the rule, the angles and mounting offsets that follow from it, a worked camera sizing for a scratch check, and the decision for each surface type.

The law of reflection decides

A ray hitting a flat surface leaves at the same angle on the other side of the surface normal. That one line, α = β, decides bright field versus dark field, because a lens is a tiny window: it only sees rays arriving inside its acceptance cone, and the cone is narrow.

A 25 mm lens at f/4 has an aperture of D = 25 / 4 = 6.25 mm. At a 200 mm working distance the acceptance half-angle is arctan(6.25 / 400) = 0.9°. A mirror-flat field therefore lights up only where light arrives within about a degree of the lens axis. Arrive vertically and the field is white — bright field. Arrive obliquely and the specular reflection sails past the lens and the field goes dark — dark field. The part, camera and LED did not change; a 20° angle change flipped the image.

A ring light is the everyday example of the middle ground: at a 70 mm radius and 100 mm working distance its rays arrive at arctan(70 / 100) = 35° from vertical — a directed, partly bright field, which is why rings handle matte parts and moderately flat shiny ones but never match a coaxial light on a mirror.

The same logic scales to every geometry in the illumination types guide; bright field and dark field are simply the two ends of the angle range.

Bright field: flat surfaces glare, marks go dark

Bright field arranges light so the mirror reflection of the flat part lands inside the lens. The flat field reads white. Anything that breaks flatness — an embossed character, a dot-peened code, a laser-etched mark — tilts the local normal and sends its light elsewhere, so marks read dark on white.

The cleanest source is a coaxial light: a diffuse LED panel above a 50/50 beam splitter sitting in the lens axis. Half the light is reflected down onto the part; the return loses half again through the same splitter on the way to the sensor. About a quarter of the emitted light reaches the sensor, which is why coaxial lights run high-power LEDs and strobe well. In return you get shadow-free, even illumination straight down the optical axis — the standard choice for mirror-like flat surfaces and direct part marks.

For marks on metal this is the geometry behind most direct part mark (DPM) reading; the DPM Data Matrix lighting guide carries it into code verification, and the metal surface defect guide shows where bright field stops and low angle starts.

Coaxial lights come in emitting faces from under 50 mm to over 400 mm. The three below are 24 V lights with emitting faces from 60 x 60 mm to 80 x 80 mm, including a narrow-angle variant:

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

Dark field: grazing light, black field, glowing defects

Dark field uses the same law the other way round. Aim the light at a shallow grazing angle — 15° to 30° from the surface is the practical band — and the mirror reflection of the flat field passes under or over the lens without entering it. The field reads near black. Every raised edge, burr, particle, pit and scratch interrupts the grazing ray, scatters light back toward the camera and glows. On polished metal this is the highest defect contrast available: bright lines on a black field.

The band has a geometric reason. The horizontal offset of a light head mounted at lens height and aimed at the field centre is:

At a 100 mm working distance: γ = 20° puts the head 100 / tan(20°) = 275 mm off-axis; γ = 30° puts it 173 mm away; γ = 10° needs 567 mm. Below roughly 15° you are buying bench length, not contrast. Mount the heads low instead — near surface height, firing across the field — and the offsets shrink to a few tens of millimetres, at the cost of needing a slim light that clears the field.

Narrow-beam spot lights keep the grazing ray where the geometry puts it:

The LS-SPT-0024 throws a 49.9 x 66.6 mm lit area through a 32° lens at 455 nm; the LS-SPT-0037 covers 99.9 x 141.6 mm at 505 nm with a 21° lens.

This is the geometry worked through in the low-angle dent and pit guide and the copper surface guide, both built on the contrast between a scattering defect and a mirror field.

Curved parts break the flat-field logic: go diffuse

On a curved surface no single angle describes the reflection. Every point has its own normal, so one grazing ray always lines up with some of them and burns a bright band into a dark field image, while a coaxial light only returns from the tangent zone. Curved, shiny parts — bottles, caps, handles, rings — need light arriving along every normal at once. That is a dome light: the part sits inside a diffuse emitting hemisphere, every surface point finds an LED along its own normal, and the surface images evenly with no specular bands.

Size the emitting area at about twice the part's largest dimension, and confirm the silhouette clears the camera hole from every angle the surface can present.

The LS-DOM-0085 pairs a Ø166 mm outer shell with a Ø116 mm emitting area. Half of 116 mm leaves room for parts around 55 mm across.

The dome lights guide works through dome sizing in detail, and the shiny metal pins guide puts dome, diffuse and polarizer head to head on fasteners. For the widest range start at dome lights.

Worked example: sizing the camera for a 20 µm scratch

Surface: polished stainless plate, mirror-flat, so dark field spots at 20° grazing. Target: detect a 20 µm wide scratch anywhere on a 22 mm face; an alignment margin of 3 mm per side sets the field of view (FOV) at 22 + 2 × 3 = 28 mm. Working distance 200 mm.

The sizing rule: a feature needs at least 3 pixels across to detect reliably and 5 pixels across to measure its width. Below that, threshold noise rather than optics sets the false-call rate.

Camera A — 5 MP class, 2448 px wide. Pixel scale: 28,000 µm / 2448 px = 11.4 µm/px. Scratch width in pixels: 20 / 11.4 = 1.7 px. Under the 3 px floor — detection fails.

Camera B — 12 MP class, 4096 px wide. Pixel scale: 28,000 / 4096 = 6.8 µm/px. Scratch: 20 / 6.8 = 2.9 px. Meets the 3 px floor — detection works across the full field.

To measure the width, 5 px needs a scale of 20 / 5 = 4.0 µm/px, so the field shrinks to 4096 × 4.0 µm = 16.4 mm. Two stations at 16 mm: 16,000 / 4096 = 3.9 µm/px, giving 20 / 3.9 = 5.1 px across — measurement passes. With a 3.45 µm pixel pitch the sensor is 4096 × 3.45 µm = 14.1 mm wide, magnification m = 14.1 / 28 = 0.504, and the focal length for the 28 mm view at 200 mm working distance is f = 200 × 0.504 / 1.504 = 67 mm. Run your own numbers through the lens focal length calculator.

Decision: detect with the 12 MP camera over one 28 mm dark field view; measure width only in a tighter 16 mm station. The camera resolution calculator does the same division for your feature size.

The decision table

Inside bores and pipes the geometry flips — light and camera fight for the same axis — so read the cylindrical bore layout guide and the pipe internal surface guide before borrowing flat-part rules. When several geometries could work, the inkjet OCV setup guide compares dome, dark field and backlight on one real print case, and the selection by task guide sorts tasks into geometries the same way.

When the textbook answer is wrong

Rough or matte surfaces. Dark field needs a field that can go dark. Roughness scatters light back at every angle, so a grazing set-up lifts the background to grey and the contrast you built the rig for disappears. On textured metal and plastic, use directed bright field, or move to photometric stereo, which reconstructs surface shape from several shaded images — the photometric stereo ring guide shows the set-up.

Ambient light. A dark field station under open shop lighting is not dark: 500 lx of bench light leaks into every scatter angle and lifts the black level. Enclose the station, or strobe the light and gate the exposure to the pulse — the timing trade-offs are in the LED strobe rise time guide. For why contrast margin beats a perfect threshold, see robust machine vision lighting.

Coaxial on stepped parts. Vertical walls send the vertical rays sideways, so walls read black. That is a feature when you inspect top-face marks and want the walls gone; it is a problem when the wall is the target. A partial bright field at 30-60° from vertical fills walls and steps instead.

Wavelength is not geometry. Blue scatters more strongly than red from micron-scale texture, but if the mirror reflection lands inside the lens, no wavelength rescues the image. Fix the angle with the law of reflection first, then pick colour with the LED colour contrast guide.

Checks before you order

• Shine before you buy. Aim any spot at the real part at the planned angle with the camera running. A field that stays bright from that angle means roughness is scattering — switch geometry before the order, not after.
• Cover the field. Emitting area at least the FOV plus 20% margin per side, so alignment drift and vignetting never clip the corners.
• Budget the offsets. d = WD / tan(γ): at 100 mm working distance, 20° grazing means a head 275 mm off-axis, or a low-mounted slim spot instead.
• Pay the splitter tax. Coaxial delivers about a quarter of the LED light to the sensor, so size the driver and strobe duty for 4× the on-part brightness a direct light would need.
• Set wavelength last. Blue brightens fine texture, infrared erases batch colour variation; geometry first, colour second.
• Plan for motion. Dark field contrast is strong enough to strobe, and strobing freezes motion — put the light pulse inside the exposure window.

The decision

Bright field for flat and mirror-like: marks dark on white. Dark field for flat and polished: scratches bright on black, spots at 15-30° grazing. Dome for curved and shiny: every normal gets its ray. Match your case in the table, size the camera with the worked example, then filter coaxial, dome and spot lights by emitting size and wavelength, or step through the configurator's lighting stage from camera to light. If the part resists both rules, send its photo and defect list through the contact page and we will sketch the geometry with you.