Optical Surface Profiler: How WLI Maps Real 3D Surface Topography

A surface is never just a line on a drawing. After machining, molding, lapping or coating, the real topography of a part decides whether it seals, reflects, bonds, wears or fails. An optical surface profiler is the instrument that captures that topography in three dimensions without touching the part. This guide explains what a surface profiler actually measures, how the three mainstream optical principles differ, which jobs each one should own, and how to pick measurement modes on a real instrument: the POMEAS WLI100 3D white light interferometer.

What an Optical Surface Profiler Actually Measures

Unlike a 2D vision system, which sees edges and silhouettes, a surface profiler reconstructs height information across an area. From a single 3D height map you can derive several families of results at once:

- 3D topography: the height field itself, exported as a point cloud or color map, useful for warpage, flatness and shape deviation.

- Surface roughness: area-based roughness parameters computed over the measured patch, such as Sa and Sq, instead of the single-line Ra of a stylus trace.

- Step height: the height difference between two regions, for film stacks, etch depth, coating thickness steps and machined ledges.

- Texture and defects: scratches, pits, orange peel and polish marks that are visible in the height map but invisible in a brightness image.

Because all of these come from the same non-contact height map, one profiling session can replace several separate gages, and fragile or sticky surfaces can be measured without damage.

Three Optical Principles, Three Different Jobs

The market often lumps three very different optical techniques under the same "3D measurement" label. They are not interchangeable. The division of labor below is the single most useful thing to understand before buying or specifying an optical profiler.

White Light Interferometry (WLI): the Area-Scan Profiler

In a white light interferometer, the objective itself is an interferometric optical unit. Light from a broadband source is split, reflected from the reference and from the sample, and recombined. Interference contrast peaks when the optical path difference is zero, so the system can find the height of every pixel in the field of view with nanometer-level sensitivity. A vertical scan of the objective through focus captures the full topography in one pass.

The defining strength of WLI is that it measures an area at once with very fine vertical resolution. That combination makes it the natural tool for ultra-smooth and optical surfaces, precision machined and lapped parts, and any application where roughness must be quantified, not just eyeballed. It is also fast: a full-area measurement in a few seconds, as shown in the WLI100 specification table below.

Its boundaries are equally clear. WLI works best on surfaces that reflect light back to the objective. Steep local slopes, deep narrow holes and strongly scattering transparent materials can drop signal, and lateral resolution is bounded by the objective magnification, not by the vertical resolution.

Point Spectral Confocal: the Displacement Sensor Family

A chromatic confocal sensor is a different animal. It focuses polychromatic light through a single point, and the wavelength that comes back in focus encodes the distance to that point. Raster or scan the spot and you build a profile; pair it with a controller and you get a displacement gage with nanometer-class repeatability on almost any surface, including glass, film and liquids.

Confocal sensing is therefore the right choice for point or line displacement, thickness of transparent layers, and inline single-value checks at production speed. It is not an area profiler: building a 3D map with a point sensor means mechanical scanning, and each pixel is acquired one at a time. For a detailed comparison of point confocal versus triangulation sensors, see our dedicated article: Point Spectral Confocal vs Laser Triangulation Sensors.

Focus Variation: the Robust All-Rounder

Focus variation sweeps a telecentric optics stack through focus and uses the focus response of each pixel to compute height. Because it tolerates steep slopes, rough textures and larger working distances, it handles fixtures, castings and complex machined parts that challenge an interferometer. The trade-off is vertical resolution: focus variation reaches the micrometer class, which is plenty for shape and waviness but is not the tool for sub-nanometer smoothness evaluation.

Quick Comparison: Which Principle Owns Which Job

One sentence to remember: the profiler answers "what does this whole surface look like", the confocal sensor answers "how far is this point", and focus variation answers "what does this large part look like even when it is difficult".

POMEAS WLI100: Core Specifications

The POMEAS WLI100 3D white light interferometer is the POMEAS optics answer to the area-scan profiling job. Its current specification set, as published on the product page, is reproduced below.

Three rows deserve attention when you specify an instrument. The 0-10 mm Z-axis range defines how tall a part you can measure without re-fixture. The motorized 50 x 50 mm XY stage lets you stitch fields into a larger map unattended. And the 0.1 to 100 percent reflectivity window means the same head works on polished steel as well as on matte or coated surfaces.

PSI or VSI: Picking the Mode for Your Surface

WLI100 measurement modes divide by how they scan. Phase Shifting Interferometry (PSI) steps the objective over a very short range and uses the interference phase itself, which delivers the finest vertical resolution and the sub-5-second measurement time quoted above. Use PSI when the height variation within one field of view is small: polished optics, lapped seals, thin-film steps.

Vertical Scanning Interferometry (VSI) sweeps the objective through the full Z range and locates the envelope of interference contrast at every pixel. It trades some vertical sensitivity for a much larger measurable height span, so use VSI when the part has real form: machined pockets, molded texture, multi-millimeter steps. Extended Depth of Field composites a sharp image for documentation, and Bright Field mode gives a conventional view for locating features before profiling.

In practice a session starts in Bright Field to frame the region, then measures once in VSI for topography and once in PSI for fine roughness on the same patch. Both data sets come from the same non-contact scan, so they stay registered.

Where WLI Profiling Pays for Itself

Across the industries we serve, WLI profiling consistently earns its place in four situations:

- Roughness as a specification: when a drawing states a roughness limit, an area-based measurement over the whole patch is far more defensible than a single stylus line that can miss a defect.

- Step and film-stack metrology: etch depths, coating steps and polished ledges are measured in one shot with nm-class vertical resolution, without cross-sectioning.

- Flatness and warpage on precision parts: sealing faces, optical mounts and wafer-like parts are mapped as a full field, feeding directly into flatness evaluation workflows such as those described in Flatness Measurement Methods Compared.

- Advanced packaging and semiconductor surfaces: wafer-level metrology increasingly combines WLI area scans with line-scan sensors, a pairing we discuss in the panel-level packaging article: Panel-Level Packaging Metrology.

Choosing Between Profiler, Confocal and Focus Variation: Five Practical Rules

1. If the specification is about roughness or fine steps, start with a WLI profiler; nothing else gives nanometer-class vertical resolution over a full field in seconds.

2. If the question is a single distance or a transparent layer thickness on a running line, use a chromatic confocal displacement sensor and keep the profiler in the lab.

3. If the part is large, steep and rough, test focus variation first; it will hold form accuracy where an interferometer loses signal.

4. If you need flatness verdicts rather than full topography, check whether a multi-point flatness system is the leaner answer: Multi-Point vs Single-Point Flatness Measurement.

5. If the deliverable is a go/no-go inspection workflow, walk the five-step equipment selection path before choosing the instrument: How to Choose Flatness Inspection Equipment in Five Steps.

Frequently asked questions

What is an optical surface profiler used for?

An optical surface profiler captures the 3D height map of a surface without contact, then derives roughness, step height, flatness and full topography from that single data set. Typical uses include polished optics, lapped sealing faces, machined precision parts, coatings and semiconductor surfaces where touching the part with a stylus would damage it or miss area information.

How is white light interferometry different from a chromatic confocal sensor?

A WLI profiler measures an entire area in one vertical scan and reports area-based roughness and topography. A chromatic confocal sensor measures one point at a time and is built for displacement, thickness and transparent layers at production speed. The two are complementary: the profiler owns the surface, the sensor owns the point. See the point confocal versus triangulation comparison for the sensor-side detail.

When should I choose focus variation instead of WLI?

Choose focus variation when the part has steep slopes, hard reflections at odd angles or a large form to capture, because the focus-based principle tolerates geometry that starves an interferometer of signal. Its vertical resolution is micrometer-class, so keep WLI whenever the spec is roughness or sub-micrometer steps on reasonably reflective surfaces.

Which WLI100 measurement mode should I use, PSI or VSI?

Use PSI when the height variation inside one field of view is small and you need the finest vertical resolution, for example on polished or lapped surfaces. Use VSI when the part contains real form, such as multi-millimeter steps or molded texture, because it scans the full 0-10 mm Z range. Many sessions use both: VSI for topography, then PSI for fine roughness on the same patch.

Related Resources and Next Steps

Browse the full optical surface profiler product family for hardware options, or read the flatness method comparison and the five-step selection guide linked above for the surrounding decision framework. For a measurement trial on your own parts, send samples drawings and tolerance calls to POMEAS contact and our engineers will map them to the right profiling configuration.