Inline Diameter Measurement: Optical Routes, Real Limits and How to Choose

Direct answer: for continuous outer-diameter measurement on a running line there are two fundamentally different optical families, and they are often confused. A projection (shadow) diameter gauge illuminates the part with collimated light and reads the shadow edges on a line-scan sensor — it measures one projected diameter per reading, at very high sampling rates, and is the right choice for wire, tube, rod, shaft and other continuously moving products. A vision (image) measurement system forms a real image of a field of view and extracts many 2D features at once — it is the right choice when you need several dimensions, positions or contours of a discrete part, not just one diameter. A third family, laser triangulation profiling, produces a height/contour cross-section; it is a different technology from both and is not what the products described on this page use.

Route 1: Projection Diameter Measurement (LM-1030 class)

Working principle, as implemented on the POMEAS LM-1030: a GaN green LED produces the measurement light, which passes through a dedicated diffuser and a collimating lens to form a single, stable parallel beam. When the workpiece enters the measurement area between the emitter and the receiver it blocks part of the light and forms a projected silhouette at the receiving end. A telecentric lens in the receiver transfers that silhouette onto an HL-CCD, and the system locates the edges between the bright and dark regions and calculates the outer diameter, wire diameter or related contour dimension from the distance between the two edges. Results are displayed in real time for process judgement.

Stated specification (LM-1030):

How this differs from a laser scan gauge. A conventional laser-scanning gauge sweeps a laser line across the part with a rotating polygon mirror; dimensional changes occurring between two scan passes may not be captured. The LM-1030 design uses continuous HL-CCD exposure instead: the silhouette is integrated over the whole exposure time, which reduces the blind window on fast-moving product. Removing the high-speed motor and polygon mirror also removes the moving-part wear, vibration and maintenance that come with them.

Typical applications stated for this class: wire and filament diameter (metal wire, enamelled wire, steel wire, copper wire, optical fibre), tube outer diameter (plastic tube, rubber tube, small hose, medical tubing, precision extruded tube), bar stock, shafts and pins (precision shafts, pin shafts, needle shafts, needle rollers), extrusion process monitoring, automatic sorting and quality control, and incoming inspection of connector pins, electronic components and miniature cylindrical parts.

Route 2: Online Projection Image Measurement (HM-1120 class)

When the task is not "one diameter on a moving strand" but "several dimensions of a discrete part inside one field of view", an online image measurement head is the appropriate route. The POMEAS HM-1120 is the large-field model of the HM series:

Documented application: online measurement of medical bone screws — small parts around 3 mm in diameter carrying threads, steps, conical surfaces and a shaped head. The difficulty there is not the diameter alone but the combination of features and the elimination of repeated point-taking, manual recording and switching between gauges; stable edge imaging is the basis for repeatable measurement and tolerance judgement, and stored measurement programs can be re-called for the same part number.

Choosing Between the Routes

1. Is the product continuous or discrete? Continuous strand, wire, tube, extrusion → projection diameter gauge. Discrete parts with multiple features → image measurement.
2. How many dimensions per part? One or two diameters → projection. Five, twenty, fifty dimensions including positions and angles → image measurement.
3. Diameter range. 0.3–30 mm fits the LM-1030 class; parts up to Ø120 mm fit the HM-1120 field, with the tighter accuracy inside the Ø60 mm high-precision zone.
4. Line speed. High-speed continuous product favours the 2400 samples/s projection gauge; discrete handling at up to ≈200 fps favours the image head in 1/4 mode.
5. Available depth of field. Moving product that changes height within the field needs the stated depth of field (40 mm / 20 mm on HM-1120) — verify on the line, not on the drawing.
6. Inspection coverage. For 100 % inspection the measurement rate and the handling rate must both be inside the cycle time; see the station-count calculation linked below.

What These Routes Do Not Do

• They are not laser triangulation profilers. They do not output a height cross-section by laser triangulation; height/Z measurement on POMEAS systems is handled by the spectral confocal sensor on IMAGE 3 PRO-H / MAX (optional on MAX 2).
• They measure what the optics can see. Internal walls, deep holes and hidden features are out of scope for both routes.
• Stated accuracy comes from stated test conditions. Vibration, temperature, surface contrast, fixturing and calibration all move the result — the HM data above is accompanied by the note that actual results depend on sample contrast, fixturing, vibration, calibration method, algorithm and part position, and must be verified on samples before final selection.

FAQ

Is inline diameter measurement the same as laser profile measurement?

No. The systems described here use collimated LED light with telecentric imaging and a line-scan detector (projection/shadow method). Laser triangulation profiling is a separate technology that reconstructs a height cross-section; it is not used by LM-1030 or HM-1120.

Can a projection diameter gauge measure a part that is not round?

It measures the projected silhouette in one direction. Width, step and contour dimensions can be derived, but full 2D feature extraction is the domain of image measurement.

What is the smallest object that can be measured?

For the LM-1030 class: 0.3 mm minimum detectable object, with a 0.3–30 mm measuring range.

How fast can it sample?

LM-1030: 2400 samples per second. HM-1120: approx. 34 ms per frame in FULL mode (≈30 fps), approx. 5 ms in 1/4 mode (≈200 fps), with 35/70/150 µs exposure options.

Can transparent tube be measured?

Transparent and highly reflective surfaces need evaluation on the actual sample and lighting setup. [Test required]

Is 100 % inspection achievable on the line?

Only if both the measurement rate and the part-handling rate fit the cycle time. Use the station-count calculation and verify with a trial.

Where should the gauge be mounted?

Emitter-to-receiver distance is fixed by model (160 ± 40 mm for LM-1030, 436.7 mm for HM-1120); both are IP64 for industrial environments. [Engineer confirmation required for your line layout]

Related Tools and Next Step

• Inline vs offline optical measurement: how to decide
• Inline measurement cycle time and station count calculation
• LM-1030 high-precision non-contact inline outer diameter gauge (product page)
• Medical bone screw case: online image measurement and contour inspection

Evaluate Your Line Before Selecting

Send the part or strand specification — diameter range, tolerance, line speed, material and surface, and whether inspection must be 100 % — and we will state which route applies, which stated accuracy figure is relevant, and what must still be verified on a trial before any commitment.