Choosing the right thickness-measurement method is ultimately about fit: alloy and product mix, line layout, accuracy and response requirements, environmental constraints, and total cost of ownership. Friedrich Vollmer is unusual in offering all three core technologies—laser, X-ray, and contact—so you can select (or combine) the method that matches each line rather than forcing one tool everywhere.
The three methods at a glance
Laser (VTLG)
How it works: Two opposing laser triangulation heads measure distance to each strip surface; the system computes true thickness. Designed for installation very close to the roll gap, enabling fast control and high accuracy on thin strip and foil. Modern VTLG units offer precision down to ±0.5 µm and scanning rates up to tens of kHz; throat depths up to 1,480 mm support transverse profiling on wide strip. Interfaces include PROFINET/PROFIBUS/TCP-IP plus analog/digital I/O.
Best for: Cold-rolling stands and lines where rapid, alloy-independent, non-contact measurement near the roll gap is valuable (e.g., foil 0.003–2.0 mm).
See product: VTLG – Laser Thickness Gauge.
X-ray (radiometric)
How it works: Measures attenuation of X-rays through the strip with digital detectors. Operates non-contact from a safe distance; very slim C-frames are possible (down to about 120 mm), which helps when space is tight or the strip is hot/hostile.
Best for: Lines where distance from the strip is required (heat, steam, mist), or where you need stable measurement across a wide range and materials are suitable for radiometric gauging.
See product: X-ray Tjockleksmätare.
Contact
How it works: Two diamond-tipped transducers touch the upper and lower strip surfaces simultaneously; the summed displacement equals absolute thickness. Delivers repeatability down to ~0.5 µm, independent of alloy, surface finish, or oil film, and can optionally measure width.
Best for: Applications where physical contact is acceptable and you need robust, alloy-independent absolute measurements with comparatively simple lifecycle costs.
See product: Contact Thickness Gauges.
Head-to-head comparison
| Criterion | Laser (VTLG) | X-ray | Contact |
| Measurement principle | Non-contact, dual-head laser triangulation near roll gap | Non-contact, radiometric at a safe distance | Dual contacting probes (diamond tips) |
| Alloy/material dependence | Independent of alloy (optical) | Material/attenuation dependent (application-specific) | Independent of alloy |
| Typical precision | Down to ±0.5 µm; high scan rates (kHz) | High precision; depends on setup/energy | Repeatability down to ~0.5 µm |
| Where it shines | Cold stands, foil/thin strip, fast AGC response, transverse profile | Hot/hostile environments, tight spaces (slim C-frames) | Robust absolute thickness, width + thickness in one |
| Environmental sensitivity | Needs clean optics and alignment; handles high speed | Needs radiometric compliance; good when distance is needed | Requires clean contact and good mechanics |
| Install constraints | Can be installed very close to the roll gap | Operates away from strip; compact frames available | Requires physical contact; not for delicate surfaces |
Selection criteria
- Line environment & access
- Hot, wet, or misty with limited access? X-ray excels at standoff measurement and compact frames.
- Cold stand / foil with aggressive control demands? Laser near the roll gap gives fast feedback.
- Alloy & surface
- If you need alloy-independent readings, laser and contact are straightforward choices.
- Target accuracy & dynamics
- For ±µm-level control and high sampling rates for AGC/CP-CPK, VTLG provides precision and speed; transverse profiles on wide strip are supported by large throat depth.
- Product sensitivity
- Sensitive surfaces (e.g., cosmetic aluminum foil): non-contact methods (laser/X-ray) avoid marking.
- Compliance, safety, lifecycle
- Radiometric systems entail radiation safety and procedures; contact systems need mechanical care; lasers need clean protective windows and alignment. (See product pages for specifics.)
- Integration & data
- Vollmer gauges offer industrial interfaces (e.g., PROFINET/PROFIBUS/TCP-IP). For analytics and coil reports, pair gauges with VGraph/VRecoS for logging, SPC (Cp/Cpk), and visualization.
When to combine methods
Many mills use laser near the stand for fast control and X-ray downstream for verification and profile/flatness workflows—or pair contact on specific lines for alloy-independent absolutes. Vollmer’s portfolio supports mixed deployments across your mill from a single vendor.
Conclusion
Start from environment and control speed: if you can place a sensor at or near the stand and want the fastest closed-loop response on thin products, laser (VTLG) is usually the best fit. If heat, steam, or layout demand standoff and compact frames, X-ray leads. Where low-maintenance absolute numbers (and optional width) are desired and contact is acceptable, contact gauges deliver. Vollmer’s breadth means you can tailor per line—and combine methods where that yields better quality, uptime, and reporting.