Coaxial Illumination for Reflective Metal: 3 Types, 7 Steps

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Polished, brushed and coated metal fail differently under a ring light: the mirror throws light away, the brushed surface fans it out, the coated surface does both. Coaxial illumination — light injected on the camera's own axis — is the primary choice for flat mirror-like and coated parts; curved or strongly brushed geometry needs a diffuse or dome source.

1. Three surfaces, three reflection paths

2. Why coaxial illumination works, and where it stops

A coaxial unit folds light onto the camera axis with a half-mirror, so the part is lit perpendicular to its surface and the specular return retraces the same path into the lens. The flat background turns bright and even — the precondition for seeing anything by contrast — while a scratch, burr, laser mark or 2D code that tilts or diffuses the return breaks that field.

The limit is geometric: the on-axis condition holds only where the surface normal is parallel to the axis, so a cylinder returns a narrow top band and a sphere a small spot. Curved metal belongs to a dome source. The axis itself comes from an external coaxial light or from the lens: POMEAS builds coaxial telecentric models such as PMS-LTC1-110C (1.0X, WD 110 mm, 1/2″) and PMS-LTCM1-110C (1.0X, WD 110 mm, 2/3″), matched at the factory.

3. Selection matrix: coaxial vs dome vs bar

4. Hotspot suppression and polarizer pairing

A hotspot is energy concentrated in the middle of the field. The causes are the fold-mirror geometry and the collimation of the emitted beam: a centre-biased beam is brightest at the centre, auto-exposure darkens the frame, and corner contrast is lost. Three remedies, in order:

1. Diffuser plate. Widens the angular spread, keeping the on-axis return while redistributing energy; it is mechanical, so nothing else changes.
2. Working distance. A window far larger than the field, or a working distance shorter than design, skews the distribution; run the light at the lens working distance.
3. Crossed polarizers. A polarizer on the illumination side plus an analyser on the camera side, set to crossed: the specular return from a smooth surface largely retains its polarization and is blocked, while light depolarised by a scratch, edge or printed feature passes. A blinding mirror becomes a dark background with a readable defect, at the cost of exposure headroom — so verify the gain on the real part rather than trusting a single setting.

5. Sizing the illumination window: formula and example

Formula (calculated): β = sensor size ÷ FOV, so FOV = sensor size ÷ β and FOV_diag = √(FOV_W² + FOV_H²). The window must cover that diagonal plus an alignment margin.

Worked example. PMS-LTCM12-110C is a 1.2X coaxial telecentric lens, WD 110 mm, 2/3″ format, publishing a 7.3 × 5.5 mm field. The check runs backwards: 7.3 × 1.2 = 8.76 mm, matching the standard 2/3″ width of 8.8 mm. Its diagonal is √(7.3² + 5.5²) = 9.14 mm (calculated), so a window near 9 mm is the minimum and 11–12 mm is comfortable. Low magnification is harder — PMS-LTCM03-110-C (0.3X, WD 110 mm) gives roughly 29 × 22 mm, a diagonal near 36 mm.

6. Camera–lens–light coaxiality: assembly points

Most coaxial-light failures on metal are assembly problems:

1. Axis. Verify one shared axis against a flat mirror-like reference; the return must be a single bright, centred spot.
2. Interfaces. Check light-to-lens and lens-to-camera threads for burrs or partial engagement.
3. Working distance. Keep light and lens on the lens specification — 110 mm on the 110-series — and re-check focus.
4. Clear aperture. The window or port bore must not vignette the field; the LZL-12101 12.5X coaxial zoom lens specifies a Ø25 mm port.
5. Stray light. Lamps, windows and bright fixtures enter a folded path as background offset; shield the cell.
6. Re-verify after every mechanical change, including a lens swap.

7. Step-by-step selection

1. Classify each surface region as mirror-like, brushed or coated.
2. List the defects: the defect type, not the material, decides the wavelength.
3. Fix magnification and field with FOV = sensor size ÷ β, then size the window from the diagonal (section 5).
4. Choose the axis: for flat mirror-like or coated parts, an external coaxial light or a lens with a built-in coaxial channel such as PMS-LTC1-200C (1.0X, built-in coaxial); for curved surfaces, plan a dome channel.
5. Start from a standard colour — red 625 nm, white 6500K or blue 470 nm — and add green, infrared or ultraviolet only if contrast is short.
6. Add the accessory that fixes the known weakness: diffuser for a hotspot, crossed polarizers for a clipping return.
7. Verify on the real part and record the recipe.

8. Specifications, troubleshooting and next step

If the finish and defect set are defined, send the part drawing, the required field of view and the defect specification and we will confirm the coaxial configuration and control option. Lens side: the telecentric lens selection guide and what a telecentric lens is and when to use one. For glass, wafers or fibre end faces: fibre end face inspection with a 10X telecentric lens. Share your workpiece details through the contact page.

Frequently Asked Questions

Can coaxial illumination inspect a curved or cylindrical metal surface?

Partly, and the limit is geometric. A coaxial system illuminates effectively only where the surface normal is parallel to the coaxial axis, so a cylinder shows a narrow bright band along its top line and a sphere a small spot. That band is enough to inspect a mark near the top of the part, and it suits a shaft rotated under the camera. For full-surface work, use a diffuse dome source and keep the coaxial channel for the top band.

Do I need a polarizer with coaxial light?

Not always, but it is the standard remedy when the specular return saturates the sensor and flattens contrast. A linear polarizer on the illumination side plus an analyser on the camera side, set to crossed, blocks the return that kept its polarization at the smooth surface and passes the depolarised light scattered by a scratch, an edge or a printed feature. The cost is exposure headroom, since the analyser also removes useful signal. Start without polarizers, add them when the histogram shows clipping.

Should I choose a lens with a built-in coaxial port or a separate coaxial light?

It depends on whether the lens choice is still open. A built-in coaxial design matches the illumination path, front aperture and working distance at the factory, which removes the front-window alignment error that most often produces a dark ring at the field edge. POMEAS supplies telecentric lenses with a built-in coaxial channel across several magnifications. A separate coaxial light gives more freedom on window size but adds one interface to align; for a new station the built-in route is lower risk.

Can coaxial light read laser-marked or dot-peened 2D codes on mirror-finished metal?

Often better than an off-axis light, because the on-axis return makes the flat background bright and the code elements appear as a local modulation of it. Laser marks on polished metal generally work well under coaxial illumination, and a shorter wavelength such as blue 470 nm suits a mark that changes the surface rather than the colour. Dot-peened codes create small pits that scatter off axis, so they may decode better under a diffuse or low-angle bar source. Verify with the reader software, since a code that looks acceptable can still fail decode.

My image became too dark after adding a crossed polarizer. What can I do?

Raise the illumination power first, then lengthen the exposure if the part is not moving; the coaxial light family covers a wide power range depending on model and is specified for use with a brightness controller. If power is already at the limit, move from fully crossed to a partially crossed position, which returns some of the specular component while keeping part of the contrast gain. A third option: if the hotspot rather than the specular level is the problem, a diffuser plate redistributes energy without costing exposure.

Does every POMEAS telecentric lens support coaxial illumination?

No, the coaxial versions must be selected by model. Built-in coaxial and coaxial-illumination models are specific catalogue items listed with their magnification and working distance, and non-coaxial models in the same magnification range do not have the coaxial channel. In the low-magnification range, for example, the 0.3X coaxial model is a distinct part number while the 0.2X model in the same family is not a coaxial design. Check the catalogue rather than assume, and state the coaxial requirement when requesting a quotation.