Wrist camera flash reflection on a shiny part: light it off-axis

Your wrist camera carries its own LED flash, and on a shiny part that flash is the problem: mounted a few centimetres from the lens, it lights the surface along the camera's own axis, so the mirror reflection comes straight back as a white hotspot that hides whatever sits under it. This page gives the diagnosis order, the three off-axis fixes that work on a robot arm, and the numbers that set them up: dome emitting area against part width, grazing landing offset against stand-off height, and flash pulse against exposure time. The approach applies to any arm-mounted vision camera with an integrated light.

Why the flash makes a hotspot

A polished surface is a mirror. The law of reflection returns each ray at the angle it arrived, measured from the surface normal, and a wrist camera's flash sits so close to the lens that the two directions nearly coincide. Light leaves along the camera axis, bounces off a flat face close to perpendicular, and comes back down the same axis into the sensor: a saturated white patch exactly where the part should be. Which contrast any light position gives you — bright field or dark field — follows that same law, worked through in the bright field vs dark field guide.

Curved parts smear the highlight and move it as the camera moves, so print or surface finish under it becomes unreadable. The lighting geometry guide shows how camera, light and part positions set contrast before any software sees the image.

Diagnose before you change anything

Work down the list; each check costs one snapshot.

1. Cover the flash. Put opaque tape over the flash window and shoot again. If the hotspot vanishes and the frame goes dark, the flash is your only light and the part is specular — you need a replacement light, not a flash switch.
2. Move the camera, not the part. Jog the arm 50-100 mm and shoot. A hotspot that tracks the camera is on-axis glare; one that stays fixed on the part comes from a plant light or a window, and the cure is re-aiming the cell or cross polarization on lens and light.
3. Open the camera's settings. Look for flash intensity or duration controls among its capture and lighting options. Dimming the pulse alone can drop the hotspot below saturation on semi-matte parts.
4. Read the histogram. A well-exposed frame with one blown patch is a geometry problem. A dark, noisy frame means the flash is too weak for the stand-off, and a brighter external light fixes that too.

Checks 1, 2 and 4 usually land in the same place: the light belongs somewhere other than the camera axis.

Move the light off the camera axis

Off-axis lighting places the LEDs where a flat shiny face reflects their light away from the lens. What returns instead is scattered light from edges, embossing, texture and defects. That split is dark field, and the lateral illumination angle guide shows how beam angle alone sets it.

Three geometries cover wrist-camera work:

Dome light for curved shiny parts

A dome wraps the part in diffuse light from every direction, so no single mirror angle collects the energy. It is the default fix for caps, knobs, handles, housings and lids — anything where the highlight moves when the camera moves. The dome lights range runs from under-50 mm bodies to 400 mm+, and sizing starts from the emitting area, not the housing:

The 1.5× keeps rays arriving at useful angles right out to the edge of the field, so a curved flank still sees steep light instead of grazing. The dome lights for shiny, curved and wrinkled parts guide covers selection in depth.

The LS-DOM-0085 has a Ø166 mm housing with a Ø116 mm emitting area and runs on 24 V.

That emitting area covers a Ø55 mm cap with 30 mm of wrap per side — (116 − 55) / 2 — enough for a strongly domed lid. For small shiny pins and connector shells the same logic scales down; the shiny metal pins guide walks a small-part example.

One dome-specific artefact: the camera looks through a hole in the dome centre, and a flat mirror face reflects that hole as a dark patch or ring at image centre. The camera-hole dark spot fix lists the offsets and diffuser tricks that remove it.

Low-angle ring for flat and gently curved parts

A ring mounted around the lens with its LEDs firing at a shallow angle turns flat faces dark and leaves raised features bright: burrs, embossed date codes, chips and dents all catch the grazing light. The number to check is the landing offset — a beam angled θ above horizontal, launched h above the surface, touches down h / tan θ away. At a 25 mm stand-off that is 25 / tan 15° ≈ 93 mm for a 15° beam and 25 / tan 30° ≈ 43 mm at 30°. Small parts therefore want a low mount — at 10 mm the 15° beam lands 37 mm out — or a steeper angle.

Browse ring lights by beam angle, and check the inner opening too: leave 10-20 mm between lens barrel and inner edge so the barrel does not shadow the beam. The LS-RNG-0730 is a 103 mm dia. ring with a 48 mm inner dia., listed at 24 V with beam angles from Low angle (0-15°) to 85°, so one body serves grazing and steeper layouts alike.

Where the part only needs softer, more even light than the bare flash gives — semi-matte surfaces, printed lids — a diffused ring is the middle ground between flash and dome:

Grazing spot for small fields

When the field is small — a datamatrix code, an engraved logo, a pin header — a single spot swung low from one side is the cheapest dark field you can build. Aim it across the feature and height changes pop out as shadows; the engraved text on metal guide compares exactly this choice against a dome. The spot lights range includes heads in the under-50 mm class, small enough to ride on a bracket beside a wrist camera.

The LS-SPT-0133 is a third generation spot light rated at 4 W on 24 V in the under-50 mm size class.

Fire the light with the exposure

An external light replaces the flash only if it fires at the right moment. Two routes, neither needing the camera's software library: a light with an integrated strobe driver takes a trigger line straight into its pulse driver, as covered in what integrated strobe drivers change, or a digital output from the robot controller fires shortly before the snapshot command, the route set out in adding an external strobe with no camera API.

Give the pulse margin at both ends, because trigger paths add jitter of tens of microseconds; the strobe rise time guide shows where those microseconds go.

Worked example: a 55 mm stainless cap

Example inputs: polished domed cap, Ø55 mm across; the arm pauses 400 ms at the snapshot pose; bright overhead shop lighting.

1. Dome size. Minimum emitting diameter = 1.5 × 55 mm = 82.5 mm. The Ø116 mm emitting dome clears it by 116 − 82.5 = 33.5 mm, leaving (116 − 55) / 2 = 30 mm of wrap per side.
2. Exposure. The arm has stopped but still rings slightly, so freeze it: exposure = 2 ms.
3. Pulse. Pulse width = exposure + margin + margin = 2 + 0.25 + 0.25 = 2.5 ms, started 0.25 ms before the exposure opens. With ±50 µs of trigger jitter, 250 − 50 = 200 µs of margin survives at each end.
4. Ambient. Ambient energy scales with exposure time, so the 2 ms strobe collects 2 / 400 = 1/200 of the overhead light a continuous full-pause exposure would.
5. Accept. Flash alone: hotspot present. Dome plus strobe: no saturated pixels on the crown and the embossed logo reads cleanly — the cell passes.

When the usual answer is wrong

• Matte or rough dark parts. Little mirror energy returns, so the flash rarely saturates anything; fix exposure or brightness instead and skip the new hardware.
• You need the highlight. Finish and dent checks read the specular image itself. Keep the light near the axis and diffuse it — the direct incident light guide covers when that wins, and the dents and pits guide shows the low-angle route.
• The part outgrows the dome. Past half the emitting area, wrap is gone; a 250 mm casting needs a large dome or grazing bars aimed across the face from two sides.
• Copper, brass and bright anodised finishes act as mirror plus absorber; the copper surface guide tunes the same geometry for high-reflection metal.

Set-up checklist

• Diagnose. Tape-over-flash test, camera-move test, histogram check — each one isolates glare, ambient or brightness before money is spent.
• Size. Dome emitting diameter ≥ 1.5 × part width; ring inner opening 10-20 mm clear of the lens barrel; spot throw covers the field from its mount.
• Aim. Beam within about 30° of horizontal for dark field on flat faces; steeper on concave parts.
• Trigger. Pulse = exposure + 0.25 ms at each end, or twice your measured jitter, wired from a rated output.
• Verify and keep the evidence. Shoot with the new light off and on and store both images with the golden parts; a bracket knocked out of line shows up within days, and the pose drift guide explains why lighting is the first thing to recheck.

Take the part width and the exposure time to the lighting step of the configurator, or filter by size, angle and colour in the light finder. The lighting topic page collects the other lighting guides. If the hotspot survives all of this, send a photo of it through the contact page — one image usually tells us the geometry.