Height Measurement on a Flash Measuring Machine: The Sensor, the Range and the Four Limits

A one-shot system measures X and Y from a single exposure. Height is a different operation: a point sensor scans, and the number it returns depends on a working distance, a spot size and a slope. Knowing those four numbers is the difference between a height feature that works and one that becomes a project.

The Sensor, Stated Plainly

On the IMAGE 3 series height measurement is provided by the 8055 spectral confocal sensor: working distance 55 mm, measuring range ±3 mm, repeatability ±2 µm, spot size 17 µm, linearity 0.02 %F.S. It is fitted as standard on IMAGE 3 MAX and available as an option on IMAGE 3 MAX2. No other model in the series carries a height sensor.

Two of those numbers decide most applications before any measurement is made. The ±3 mm range sets the deepest step the sensor can follow in one setup. The 17 µm spot sets the narrowest feature it can isolate — a 10 µm-wide groove is below the spot and will be averaged with its surroundings.

What It Measures Well

Step height between two flat faces, groove depth, plane flatness over an accessible area, the thickness of a transparent layer where both interfaces return signal, and the height of a raised feature whose top face is wider than the spot. These are all cases where the surface is reasonably perpendicular to the optical axis and the feature is larger than the spot.

Transparent layers are the interesting case: a spectral confocal sensor resolves interfaces rather than surfaces, so a glass plate or a film can return two peaks and the separation is the thickness. That is a genuine capability a 2D system does not have, and it is worth stating precisely rather than implying general transparency measurement.

Model range: which IMAGE 3 fits this part

Model range, as stated in the IMAGE 3 series specification sheet. Repeatability here is the figure printed in the sheet; it is not a guarantee for your own part, your fixture or your operator. Treat it as the floor you start from when you run a trial, not as the number you can quote to a customer on day one.

The full specification sheet, including the camera and lighting options and the workpiece-to-model mapping, sits on the IMAGE 3 series product page. If you only want a first-pass sanity check on whether your part fits any model, the instant measurement selection tool takes the part envelope and returns the models that can cover it.

The Four Conditions That Break It

1. A step deeper than ±3 mm from the sensor’s working point. The sensor loses the surface and the reading is invalid rather than merely inaccurate.
2. A feature narrower than the 17 µm spot. The returned value is a weighted average over the illuminated area, which is not the feature height.
3. A steeply inclined surface. Confocal return falls off with slope; a near-vertical wall of a deep groove does not return usable signal.
4. A transparent layer whose interfaces are closer than the sensor resolves, or a strongly absorbing layer. No second peak, no thickness.

Any one of these is a specification issue, not a tuning issue. They belong on the feasibility sheet before a purchase decision, not in the acceptance report afterwards.

Cycle Time: Height Scales With Points, Not Parts

In 2D mode the capture term is one exposure regardless of feature count. In height mode each probed location is a separate acquisition, so the capture term scales with the number of points. A coplanarity check on twelve contacts is twelve acquisitions; a flatness map over a 10 × 10 grid is a hundred.

The practical consequence is that height measurement changes the economics of a batch cell. A cell that was comfortably inside takt in 2D mode may not be once a height scan is added, and the answer is usually to reduce the point count to the minimum the tolerance actually needs rather than to buy a faster sensor.

Combining 2D and Height in One Part

The usual requirement is both: contour in the image plane, and one or two heights. The sequence matters. Measure the 2D features first from a single exposure, then scan the height points, because the 2D result establishes the coordinate frame the height points are reported in. Doing it the other way round means the coordinate frame depends on a scan whose own positioning is less certain.

When Height Should Use Another Method

Large-area flatness is better served by a full-field height method than by any point scan, because the scan time grows with area and the sampling is always partial. Deep bores beyond ±3 mm range need a different sensor. And surfaces with steep walls or high roughness are better handled by a laser triangulation or an interferometric method, depending on the scale. A one-shot system with a confocal sensor is the right instrument for shallow steps and layer thicknesses, and it should be sold as that.

FAQ

Which IMAGE 3 models include height measurement?

Only IMAGE 3 MAX and IMAGE 3 MAX2. MAX carries the 8055 spectral confocal sensor as standard, MAX2 offers it as an option. The other four models in the series measure in the image plane only.

What is the measuring range of the height sensor on a flash measuring machine?

The 8055 spectral confocal sensor has a measuring range of ±3 mm at a working distance of 55 mm, with ±2 µm repeatability, a 17 µm spot size and 0.02 %F.S. linearity. A step deeper than ±3 mm from the working point cannot be followed in one setup.

Can a flash measuring machine measure glass or film thickness?

A spectral confocal sensor resolves interfaces rather than surfaces, so a transparent layer can return two peaks and the separation gives the thickness. It requires both interfaces to return usable signal and to be separable within the sensor resolution; a strongly absorbing layer or interfaces closer than that resolution will not work.

Why does height measurement slow down a batch inspection cell?

Because each probed location is a separate acquisition, so the capture term scales with point count rather than with parts per shot. A coplanarity check on twelve contacts is twelve acquisitions, and a flatness grid multiplies that further.

Can the height sensor measure a groove narrower than 17 µm?

No. The spot size is 17 µm, so a narrower feature returns a weighted average over the illuminated area rather than the feature height. That is a specification limit, not a setting that can be tuned.

Should 2D features or height points be measured first on the same part?

Measure the 2D features first, from a single exposure, then scan the height points. The 2D result establishes the coordinate frame that the height values are reported in, and building that frame from a point scan makes the frame less certain than it needs to be.

Related reading

• What is a flash measuring machine and how does one-click measurement work
• Instant measurement vs CMM: when a one-shot system is enough
• How to read an instant measurement accuracy spec
• Closing the Loop
• Batch Dimensional Inspection
• Flash Measurement for Connectors

Send Us One Part and We Will Tell You Which Route Fits

The fastest way to close this question is a real part, not a discussion. Send one sample with the drawing and the features you actually gate, and we will run it and report what the measurement returns — including the features that come back as a no. A short written feasibility note is more useful than a brochure.