Dark spot from the camera hole in dome lights: how to fix it

You mount a dome light over a shiny part, grab an image, and a dark disc sits in the middle of the frame, right over the feature you need to read. That disc is not a fault in the light: it is the camera hole in the dome's top plate, which the part mirrors straight back into the lens. On matte parts you never see it; on mirror-like and curved parts it can swallow a third of the field of view. This guide predicts its size with one formula, then removes it three ways: an aperture plate that shrinks the hole to your lens, a small dome fitted over the port, or a mirror-bright backlight field that turns marks dark instead. The arithmetic, the trade-offs and the decision for each case are set out below.

What the dark spot actually is

A dome light wraps the part in diffuse light from every direction except one: the port at the apex where the camera looks through. That port is an unlit patch in an otherwise glowing hemisphere. On a matte surface each point mixes light from a wide cone of directions, so the missing cone is diluted into nothing. A specular surface does not blend: the law of reflection sends one direction straight back at the lens. Wherever that reflected line of sight lands on the port rather than the lit dome wall, the camera receives an image of the hole.

That image is the dark spot. On a flat mirror-like part it is a disc at the centre of the field; on a curved part it becomes a blob sitting wherever the local surface normal aims the port at the lens, and it drifts whenever the part shifts.

For a flat, mirror-like surface with the lens on axis, the disc diameter in the plane of the part is:

D is the port's clear diameter, H_c the lens-to-part distance and H_d the port height above the part. The disc is always smaller than the hole and grows as the camera retreats: with the lens right at the port (H_c = H_d) you see half the hole diameter, and with the camera far above the dome the disc approaches the full hole diameter. Real set-ups sit between the bounds, so expect a disc of 0.5 to 0.9 times the port diameter on flat shiny surfaces, and a smeared blob of the same order on curved ones.

If you are still choosing the dome itself, dome lights for shiny, curved and wrinkled parts covers size, height and colour; this guide takes over once the dome is on the bench and the disc appears.

Fix 1: an aperture plate that shrinks the hole

Dome ports ship oversized so any camera and lens fits. Your set-up only needs enough clear diameter for the lens barrel and the cone of rays that reaches the edge of the field of view; the rest can be covered by a plate. Some dome series include aperture adaptors in several sizes; if yours did not, a machined plate with a clean edge does the same job, and it is worth checking the mounting parts in the lighting accessories range before cutting your own. The plate must pass the ray from the edge of the lens to the edge of the field, which sets the minimum clear radius:

Here a is the lens front radius, R_FOV half the field of view, g the lens-to-port gap and WD the lens-to-part working distance.

Worked example: inkjet code on a can end

An inkjet date code on the shoulder of an aluminium can is the classic victim: a curved, near-mirror surface with a code that lands close to the centre. Run the numbers before touching hardware.

1. Resolution first. A 1/2 inch sensor, 6.4 mm wide with 1600 pixels across, over an 84 mm field of view gives 84 / 1600 = 0.053 mm per pixel. A 2.5 mm character then covers 47 pixels, comfortable for OCR.
2. Lens. Magnification m = 6.4 / 84 = 0.076. With the lens 170 mm above the part, f = 170 × 0.076 / (1 + 0.076) = 12 mm, using the working distance and field of view formula; the focal-length calculator below runs it for you.
3. Disc before the fix. The port measures 60 mm clear (example value, measure yours) and sits 150 mm above the part, leaving the lens 20 mm above the port. The disc is 60 × 170 / (170 + 150) = 32 mm, which is 32 / 84 = 38 percent of the field, a dead circle about 600 pixels wide.
4. Plate hole. With a 25 mm lens front (a = 12.5 mm), R_FOV = 42 mm, g = 20 mm and WD = 170 mm: r_hole = 12.5 + (42 − 12.5) × 20 / 170 = 16 mm. The geometric floor is a 32 mm hole; cut 34 mm to keep 1 mm of clearance per side.
5. Disc after the fix. 34 × 170 / 320 = 18 mm, or 21 percent of the field. The code zone, offset from the can's centre, now clears the disc with margin.

Decision: when the feature sits at least a fifth of the field off centre, the plate alone usually wins. It is a five-minute change that costs nothing in brightness or height. A feature dead centre on a mirror surface needs the next two fixes.

Domes in the 130 to 170 mm class suit parts of this size. The LS-DOM-0085 is a Ø166 mm dome light with a Ø116 mm emitting area.

Fix 2: a small dome over the port

Why shrink the hole when you can light it? A small dome bolted over the port turns the one dark direction into diffuse output; the residual disc is then set by the small dome's own top port, a much smaller hole. Some flat domes carry threaded mounting holes around the port for exactly this.

The price is height. The lens moves back by the height of the added dome, and the small dome's own port clips the outer field. Extend the example: a 40 mm tall small dome (example value) puts the lens at 210 mm, the field of view grows to 6.4 × (210 − 12) / 12 = 106 mm, and pixel scale drops from 0.053 to 0.066 mm per pixel. The residual disc, set by a 20 mm top port now 190 mm above the part, is 20 × 210 / (210 + 190) = 10 mm, under 10 percent of the field and centred where the part is flattest.

Choose this when the feature sits at the centre of a flat shiny part and the station can absorb the height. It also beats a plate alone on strongly curved mirrors, where any residual disc lands on the highest point of the part.

Fix 3: make the surface a mirror and read dark on bright

Both fixes above shrink a dark disc; this one removes the concept. Mount a bright, homogeneous backlight above the part facing down, and let the camera look at the part the way it looks into a mirror. A perfect mirror returns the backlight as a uniformly bright field, and every mark that breaks the surface, a dot-peen character, a scratch, an inkjet ring, scatters light away from the camera and reads dark on bright. That is the standard answer for dot-peen and engraved codes on polished metal; DPM Data Matrix lighting for readable marks on metal shows it in that role.

The reflected path is what catches people out: the backlight must cover the field of view projected through the mirror, which widens with the light's height.

H_b is the backlight height above the part. The 84 mm field with the backlight 100 mm up and the lens at 170 mm needs 84 × (1 + 100 / 170) = 133 mm of homogeneous face. Mount the light as close to the part as the mechanics allow and the factor falls toward 1. Brightness matters for the same reason, because you fill a face about 1.6 times wider with the same output, so plan to strobe the light and match it to the driver as LED strobe duty cycle matching explains.

Depth of field earns one check here, because the marks sit in a surface that is often curved. A working estimate:

At f/4 with a circle of confusion of 8 µm (two 4 µm pixels) and m = 0.076, that is 2 × 4 × 0.008 × 1.076 / 0.076² ≈ 12 mm. Open to f/2 for brightness and it halves to 6 mm, at which point a curved shoulder 8 mm tall softens at its edges. Run your own values through the depth of field calculator before you lock the aperture.

On flat parts a coaxial light is this same geometry folded into one unit: the beam splitter illuminates the axial direction and the camera views through it, so no hole ever appears in the field, which is why coaxial units became the traditional cure for dark spots. The trade is shape: coaxial needs a flat, squarely presented part, and it spends working distance on the splitter box. Two options from the coaxial lights range:

Choosing the fix

Size the dome to the part, not the camera: the emitting area should overhang the whole inspection zone with margin. Three mid-size domes to compare for parts in this range:

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

Checks before you cut or order

• Measure the port's clear inner diameter with calipers, and note any threaded holes around it; they decide how a plate or small dome fixes.
• Lock the lens and working distance first, because both plate and dome sizes derive from them.
• Compute the plate hole from the ray formula and add about 1 mm of clearance per side; a hole cut small vignettes the field corners, which reads like a lens fault.
• Recompute the disc after the change and compare it with where the feature actually sits.
• For the mirror set-up, budget the backlight width from the projection formula and confirm the controller can strobe it inside its rating.
• Re-run field of view and pixel scale whenever you add height, as a small dome does.

When the usual answer is wrong

Wide fields break the plate. If the lens front is large, 40 mm and up, typical with bigger sensors, the lens term dominates the ray formula and the hole barely shrinks. Go straight to a small dome or the mirror set-up.

Loose part presentation breaks every fix but the last. A smaller disc is still a fixed disc in the image, so if the part stops anywhere under the dome, choose a geometry with no disc at all.

Crossed polarizers look tempting on flat glossy parts: they remove the disc by removing all specular reflection, including the glare signal some inspections feed on. Lighting shiny metal pins without glare weighs that trade.

And if the real task is reading a code, pick the geometry before the part number: inkjet OCV lighting setup compares dome, dark field and backlight for exactly that decision.

Next steps

Dark spots are geometry, not bad luck: one formula predicts the disc and three moves remove it. Browse the dome lights range by part size, or run the full chain, light, controller, strobe timing, in the configurator:

For a surface that resists all three fixes, send the part details through the contact page and we will run the numbers with you.