Stylus Profilometer: How It Works, Where It Fails, and When to Use Optical Instead
A stylus profilometer drags a diamond contact stylus across a surface to trace one height line at a time, reporting Ra and Rz for that profile. On thin, soft or transparent films, optical systems measure the same step height and roughness without contact — 0.1 µm across 300 × 150 × 70 mm (SE310).
What a stylus profilometer does well
Contact stylus scanning is a mature technique, and it earns its place in three situations:
- Hard, machined or polished surfaces where the tip can track the real profile without deflecting the material — metals, ceramics, glass, hard-baked coatings.
- Deep, tall features — mould cavities, etched grooves, machined steps — where optical contrast is lost on steep walls.
- Line-profile conventions. If your process is documented and spec'd against a single Ra or Rz line, a stylus instrument returns exactly that number, in the same filtering convention your existing records use.
The limit is not primarily resolution — it is what the measurement physically is: one line, one touch, on a surface the stylus must survive.
Six places contact profilometry breaks on thin films
- Soft or tacky films. Photoresist, slot-die wet films, perovskite, polymers, sealants. The tip ploughs, and you measure the deformation you caused, not the film.
- Very thin or delicate coatings. A contact tip can scratch or penetrate what you came to characterise — the measurement destroys the sample you were reporting on.
- Fine-pitch and high-aspect structures. Tip radius and tip shape set the floor: narrow grooves and sharp steps get rounded, and the reported depth is the tip's shape, not your feature's.
- Smooth, sub-nanometre surfaces. Mechanical noise and tip wear swallow the signal where interferometry resolves it. TDM's interferometric bench publishes RMS repeatability ≤0.1 nm in phase-shift mode (SE320).
- Buried interfaces. The stylus reads a topography line; it cannot see through a transparent layer to a buried film boundary. Thickness without an edge is reflectometry or ellipsometry.
- Area statistics. One scanned line is not a surface. Roughness of a coated or printed area is a 3D question — the same reason optical systems scan a whole field at once.
Stylus profilometer vs optical profilometer
| Stylus (contact profilometer) | Optical (3D, non-contact) | |
|---|---|---|
| How it reads the surface | Diamond tip dragged along one line | Light-section triangulation, or white-light / phase-shift interferometry |
| What it returns | Ra, Rz and other line-profile values for one trace | Full 3D topography plus areal roughness, step height, warp |
| Contact | Yes — tip force on the sample | None |
| Risk to the sample | Can scratch, plough or deflect soft films | None; works on soft and delicate coatings |
| Typical speed | Point-by-point scanning of a line | TDM SE310 publishes 1 scan/second full-area |
| Area covered (TDM published) | Line scans; area requires repeated traces | 300 × 150 × 70 mm (SE310); 200 × 200 mm stage with 100 mm Z travel (SE320) |
| Deepest vertical capability (TDM published) | Limited by tip radius, wear and contact force | 0.1 µm light-section (SE310); ≤0.1 nm RMS repeatability in PSI mode (SE320) |
| Steep walls, dark or rough surfaces | Tracks them physically | Light-section keeps contrast; interferometry can lose it |
| Buried or transparent layers | Cannot see them | Cannot either — that is reflectometry / ellipsometry |
| Consumables and upkeep | Tip wear and tip condition directly move the numbers | No tip to wear; optics and vibration isolation instead |
| Best for | Hard machined parts, tall etched features, legacy Ra/Rz specs | Thin, soft, coated, printed or polished films; 3D area statistics; mapping |
Tip condition matters more than people admit: a worn or chipped stylus silently changes both roughness and step-height readings. Before comparing a stylus result with an optical one, confirm the tip was qualified that day — most "the two instruments disagree" cases start there. Which areal 3D parameters our software reports, and the exact line-profile filtering conventions applied, are confirmed per configuration — we will state them for your specific standard in writing.
Ra, Rz and the 3D numbers: which do you actually need
- Ra — the average deviation from the mean line, on one profile. The most quoted roughness number and the most misused: two surfaces with identical Ra can look and perform very differently.
- Rz — peak-to-valley on a profile, more sensitive to the extremes, less sensitive to the field.
- Sa and 3D areal statistics — computed over a measured area instead of a single line. For coated, printed, spun or polished films, this is the honest comparison, and it is what an optical 3D profilometer returns by default.
- Step height — the film-edge or terrace reading that confirms deposition and etch. Both techniques can produce it; only optical does it without touching the film.
Can a stylus profilometer measure film thickness?
Only where there is a step to measure — a scratched edge, a patterned terrace, an etched pit. A film lying flat under a transparent top layer has no edge, and a stylus has nothing to fall off. In that case thickness comes from spectral reflectometry (TDM SE200 publishes 4 nm–450 µm film thickness, repeatability ≤0.02 nm) or from fitting optical constants (SE100 publishes 5–1200 nm thickness with n/k, ≤±1% repeatability). Wet films measured at the coating head are a separate problem: see Wet Film Thickness Control Loop.
Which instrument fits your measurement
- Hard parts, tall etched steps, legacy Ra/Rz records → a stylus profilometer stays in the lab.
- Thin, soft, transparent or freshly coated films; 3D roughness and step height without contact → SE310 3D Optical Profiler (light-section, 0.1 µm)
- Sub-nanometre steps on smooth surfaces, large-area mapping → SE320 3D Optical Profiler with Scan Platform (WLI + PSI)
- 3D thickness of transparent and multi-layer films with uniformity maps → SE8000 Imaging 3D Thin-Film Thickness Measuring Instrument
- Micro-area thickness with a 1 µm spot → SE200 Microscopic Thin Film Thickness Measurement Instrument
How the two optical methods differ is covered here: Optical Profilometer: What It Measures, How It Works · How to Measure Thin Film Thickness · Micro-Spot Thin Film Measurement
Try it on your own sample
Send one part. We run it on the light-section bench and the WLI/PSI bench, and report the 3D map, the roughness and step-height values, and whether contact scanning was in fact the better tool for your surface — that answer comes back either way, and it costs nothing on our bench.
Written by TDM Metrology Team · published 2026-11-11
Frequently Asked Questions
What is a stylus profilometer?
A contact instrument that draws a diamond-tipped stylus across a surface and records height along that single line, reporting roughness values such as Ra and Rz for the trace.
How accurate is a stylus profilometer?
For hard, machined or polished surfaces it is a well-established method, and its accuracy is governed by tip condition, contact force and the filtering convention applied — a worn tip changes both roughness and step-height readings. On thin, soft or tacky films the practical limit is different: the tip deforms or scratches what you are measuring, so the number no longer describes the film.
What is the difference between Ra and Sa?
Ra averages height deviation along one profile line. Sa does the same over a measured 3D area. Two surfaces can share an Ra value and look completely different under an area scan, which is why coated, printed and polished films are usually specified with 3D areal statistics.
Can a stylus profilometer measure thin-film thickness?
Only across a step — a film edge, a patterned terrace or an etched pit. A film under a transparent top layer has no edge for the tip to reach; thickness then comes from spectral reflectometry (SE200, 4 nm–450 µm) or from fitting optical constants (SE100, 5–1200 nm).
Should I buy a stylus or an optical profilometer?
Buy contact for hard parts, tall etched features and matching an existing Ra/Rz spec. Buy optical when the sample is thin, soft, transparent, freshly coated, or when you need 3D area statistics or large-area mapping. When you are not sure, send a sample: the same part measured both ways settles it in one report, before you commit budget.
Related guides and products
- Optical Profilometer Guide
- How to Measure Thin Film Thickness
- SE310 3D Optical Profiler
- SE320 3D Optical Profiler with Scan Platform
- SE8000 Imaging 3D Thickness
- SE200 Microscopic Thin Film Thickness
