Dome Lighting for Curved and Reflective Surfaces: When It Works and When It Hides the Defect

A ring light on a curved, polished surface produces a bright arc that moves when the part moves and hides whatever is underneath it. A dome light removes the arc by removing the direction: its spherical inner surface diffuses the LEDs into light arriving from every angle, so a curved part is lit evenly and the reflection that was blinding the camera stops being a distinct feature.

That is the whole advantage, and it comes with a cost worth stating up front: a dome light also removes the directional shadows that make surface relief visible. It is the right choice for reading characters, codes and colour on a curved glossy part, and the wrong choice for finding a scratch in it.

All values below are published POMEAS figures for the GR dome series. This is a selection method, not a compatibility claim, and no third-party specification is quoted.

What the dome geometry does

The dome is a hemisphere with a diffuse inner coating and an aperture at the top for the camera. Light emitted at the inner surface reflects many times before reaching the part, so the part is illuminated from a very large solid angle. The result is that no single direction dominates, and the specular highlight that a ring light produces is spread out until it stops being visible.

The consequence for curved parts is significant: a part with curvature no longer produces a moving bright band, because there is no single source direction for the surface to reflect.

The consequence for defects is the opposite: a scratch, a dent or a step is visible largely because it casts or catches directional light. Under a dome, that mechanism is largely removed.

Published specifications for the POMEAS GR series

Choosing the size

The dome has to be large enough to enclose the part's field of view at the working distance, and POMEAS gives a direct mapping for the GR series:

- GR70 or GR116 — small parts, or where installation space is tight. - GR175 or GR198 — medium-sized products and conventional fields of view. - GR260 or GR325 — larger products or larger inspection areas.

Being too small reintroduces the problem the dome was bought to solve: the part sees the edge of the dome rather than a full hemisphere, and directionality returns. Being unnecessarily large costs working distance and light, since the same emitter output is spread over more area.

Colour: the same rule as the rest of the range

Red (≈ 620–630 nm), blue (≈ 470 nm) and white (≈ 6500 K) are the standard options. The choice follows the part, and POMEAS recommends confirming the effect by sample testing rather than assuming — different colours and materials respond differently to each band.

Where the output is a measurement rather than an appearance check, monochromatic light is generally preferable because it removes chromatic aberration from the edge position.

Three cases where a dome is the wrong answer

1. Finding scratches, dents or steps in the surface. These are directional features. Use coaxial illumination for flat specular surfaces, or low-angle / dark-field illumination for relief. A dome will hide them.

2. Measuring a dimension on a part with height variation. Uniform illumination is not the same as collimated illumination. A dimensional station needs a telecentric lens with a telecentric parallel backlight (VP-GLTD, 30–265 mm) — the dome gives even light, not orthographic imaging.

3. Transparent or multi-layer parts. Ghost edges from internal reflections need a different approach; POMEAS publishes separate guidance for transparent-part inspection.

Two practical build notes

The dome sits between the camera and the part, so its aperture has to clear the lens barrel and its depth has to fit the working distance — a deeper dome needs more space than a ring light at the same field. And because the dome cuts the light reaching the part substantially compared with a direct source, the exposure budget has to be checked after installing it, not before.

POMEAS also recommends pairing the dome with a constant-current controller (VP-DPI2404-4-RS232T is the published recommendation) for stable output and adjustable brightness, with the usual caveat: check the light's own forward voltage and current against the controller's channel limit, and confirm cable length at the design stage since longer extensions affect delivered current.

What this article does not cover

It does not cover exposure, gain or threshold settings, which interact with every lighting choice. It does not cover safety or enclosure requirements. And it does not claim a dome light will remove every reflection: POMEAS states plainly that a dome reduces highlights, shadows and over-exposure, and cannot eliminate all reflection — a mirror-like surface at the right angle still returns a strong signal.

FAQ

What is a dome light for?

Even illumination of curved, glossy or uneven surfaces, where a ring light would produce a moving bright band. Typical uses include metal curves, glass dials, PCB and IC marking, and printed codes.

How is it different from a ring light?

A ring emits from one direction around the lens; a dome emits from a full hemisphere after internal diffusion. Directionality — and therefore the specular highlight — is largely removed.

Which size do I need?

From the POMEAS mapping: GR70 / GR116 for small parts or tight spaces, GR175 / GR198 for medium parts, GR260 / GR325 for larger parts and larger fields.

Can a dome light find scratches?

Usually not. Scratches are directional features and a dome removes the directional light that makes them visible. Use coaxial or low-angle illumination instead.

Is dome lighting suitable for measurement?

Not for dimensional work on parts with height variation — that needs a telecentric lens with a telecentric parallel backlight. A dome gives even light, not orthographic imaging.

Does a dome eliminate all reflection?

No. It reduces highlights, shadows and over-exposure; a strongly specular surface can still return a strong reflection.

Next Step

Send the part material, surface condition, size and inspection target, and POMEAS will return the dome size from the GR range together with sample imaging results.