White Light Interferometry Testing for Inner Wall Surface Roughness

White light interferometry testing of inner wall surface roughness provides non-contact, nanometer-resolution three-dimensional topography measurement inside bores, tubes, and cylinders that conventional stylus profilometers cannot physically reach. This article explains the measurement principle, applicable sample types, evaluated roughness parameters, test methods in accordance with ISO 25178 and ISO 21920, and typical industrial applications of this advanced optical inspection technique.

Technical Overview

White light interferometry (WLI), also known as low-coherence interferometry or coherence scanning interferometry (CSI), is an optical metrology technique that uses a broadband white light source instead of monochromatic laser light. Because white light has a very short coherence length (typically a few micrometers), interference fringes appear only when the optical path difference between the measurement beam and the reference beam is close to zero. By vertically scanning the objective and locating the height at which fringe contrast is maximized, the instrument reconstructs the surface height at every pixel with sub-nanometer vertical resolution.

Inner wall surface roughness measurement is one of the most demanding applications in surface metrology. The surfaces of small-diameter bores, deep holes, tubing, and cylinder liners are inaccessible to conventional stylus instruments and standard microscope objectives. Purpose-built white light interferometers overcome this limitation through right-angle mirror adapters, long working distance objectives, side-viewing probes, swing-arm optics, and rotary staging, which redirect the optical axis onto the interior cylindrical surface. The result is a full three-dimensional areal topography map of the inner wall, from which both profile (Ra, Rz, Rq) and areal (Sa, Sq, Sz) roughness parameters are calculated.

Typical performance capabilities include vertical resolution below 1 nm, vertical scanning ranges of several millimeters, lateral resolution from approximately 0.3 um to a few micrometers depending on the objective numerical aperture, and measurable bore diameters from roughly 1 mm up to several tens of millimeters, with access depths reaching hundreds of millimeters in favorable geometries. The method complies with the framework of ISO 25178-604 for coherence scanning interferometers, and results can be reported according to ISO 25178 for areal parameters and ISO 21920 (the successor to ISO 4287 and ISO 4288) for profile parameters. Accredited third-party laboratories widely use this technique as an arbitration method for functional interior surfaces.

Testing Samples

Metallic Tubing and Machined Bores

Stainless steel and carbon steel hydraulic tubing, honed cylinder bores, gun-drilled deep holes, reamed and bored surfaces, and inner walls of valves and pump bodies are the most common samples. The technique distinguishes machining marks, honing cross-hatch angles, chatter, tearing, and smearing, which directly affect sealing performance, flow resistance, and fatigue life. Both bare machined surfaces and inner-wall coatings such as chromium plating, nickel alloys, thermal spray layers, and PVD films can be measured, provided the film is opaque or thick enough to avoid interference from underlying interfaces.

Fuel Injection and Precision Fluid Components

Diesel common-rail injector spray holes, nozzle inner seats, fuel injector needle bores, and precision orifices in aerospace and automotive fluid systems demand surface finishes in the sub-micrometer range. White light interferometry is one of the few methods able to quantify the internal roughness of holes with diameters below 1 mm to a few millimeters, where roughness strongly influences flow coefficient, atomization quality, cavitation, and deposit formation.

Medical Device Interior Surfaces

Hypodermic needle bores, stent tubing lumens, catheter delivery channels, syringe barrels, and drug-contacting flow paths are tested to verify cleanliness, polish quality, and biocompatibility-related texture. Non-contact measurement avoids scratching or contaminating sterile surfaces and captures defects such as draw marks, pits, and residual polishing compound traces that affect drug delivery performance.

Ceramic, Glass, and Polymer Bores

Technical ceramic liners, glass capillaries, extruded polymer tubing, and lined pipe sections can be evaluated when their reflectivity and transparency are within the measurable range. Anti-reflective, highly scattering, or fully transparent walls may require adjusted illumination, reference mirror settings, or replica techniques, and such feasibility limits should be confirmed before testing.

Testing Items

Testing typically covers the following measurable characteristics of inner wall surfaces:

• Profile roughness parameters: Ra (arithmetic mean deviation), Rq (root mean square deviation), Rz (maximum height of profile), Rt (total height), Rp (maximum peak height), Rv (maximum valley depth), Rsk (skewness), Rku (kurtosis), and Rsm (mean spacing) evaluated according to ISO 21920 / ISO 4287.
• Areal (3D) roughness parameters: Sa, Sq, Sz, Sp, Sv, Ssk, and Sku evaluated according to ISO 25178, providing statistically robust characterization of the whole measured area rather than a single line.
• Functional and bearing parameters: core roughness depth Sk, reduced peak height Spk, reduced valley depth Svk, areal material ratio Smr, and the Abbott-Firestone bearing area curve, which correlate strongly with sealing, lubricant retention, and wear behavior.
• Texture characteristics: texture aspect ratio Str, texture direction Std, honing cross-hatch angle, and lay orientation analysis.
• Step height, wear depth, erosion depth, corrosion pit depth and volume, scratch depth, and local form deviation of the inner wall.
• Defect mapping: pits, porosity, inclusions, scoring, and coating defects located and quantified in three dimensions.

Testing Methods

Vertical Scanning Interferometry (VSI)

In VSI mode, the interferometric objective is scanned vertically through focus while a camera records fringe contrast at each pixel. This mode handles rough or stepped inner wall surfaces with heights up to several millimeters and achieves vertical repeatability in the nanometer range. VSI is the standard approach for machined bores, honed cylinders, and technical tubing with medium to high roughness.

Phase Shifting Interferometry (PSI)

PSI mode uses phase extraction from several interferograms with sub-fringe vertical shifts to reach sub-nanometer resolution on smooth, continuous surfaces. It is suitable for highly polished inner walls such as injector needle bores, gauge bores, and lapped seats, where roughness values are below approximately Sa 0.03 um. Many systems combine both modes in a single acquisition to cover a wide dynamic range.

Inner Wall Access Configurations

Access to interior cylindrical surfaces is achieved with dedicated hardware: right-angle mirror tips that turn the optical axis 90 degrees into the bore; long working distance objectives that image the far wall of a transparent-adjacent opening; small-diameter probe optics for capillaries and spray holes; and rotary or linear stages that reposition the measurement spot for multiple axial and circumferential locations. Stitching algorithms merge overlapping fields of view into extended axial strips or full circumferential maps for pipeline-length roughness evaluation.

Data Evaluation Procedure

Raw topography data are processed in accordance with the ISO 25178 specification operators: S-filter and L-filter remove short-wavelength noise and long-wavelength form, and the F-operator removes nominal cylinder form to isolate roughness. After leveling and thresholding of non-measured points, profile and areal parameters are computed with calibrated scale factors traceable to length standards. Measurement uncertainty is verified with certified roughness or step height standards, and reports typically include the topography map, filter settings, evaluation area, cut-off wavelengths, and full parameter tables in accordance with ISO 25178-2, ISO 25178-3, ISO 21920, ASME B46.1, or GB/T 33523 as required by the client specification.

Sample Preparation and Environmental Control

Interior surfaces must be cleaned of oil, coolant residue, and loose debris because transparent films distort the optical signal, and contaminant particles generate false topographic features. Samples are fixtured so that the bore axis is aligned with the measurement direction, and vibration isolation is applied because interferometers are sensitive to mechanical disturbance. Tests are best performed in an accredited third-party laboratory environment with controlled temperature and humidity to ensure repeatability in the nanometer range.

Applications

White light interferometry testing of inner wall roughness supports quality control, process development, and failure analysis across many industries:

• Hydraulic and pneumatic systems: verifying honed cylinder bore finish to ensure seal life, leakage control, and friction performance.
• Automotive powertrain: qualifying fuel injector spray holes, valve guide bores, cylinder liners, crankshaft oil drillings, and turbocharger bearing surfaces.
• Aerospace: inspecting landing gear bores, actuator cylinders, gun-drilled spars and fittings, and turbine component cooling passages.
• Oil and gas: evaluating tubing internal finish for flow assurance, corrosion monitoring, and erosion assessment in downhole and process piping.
• Medical devices: confirming needle, catheter, and stent tubing interior polish for delivery performance, cleanliness validation, and regulatory submissions.
• Precision manufacturing: incoming inspection of deep-drilled components, research on honing, electrochemical machining, EDM, and laser drilling processes, and tribological studies of coated inner walls.
• Semiconductor and fluidics: characterizing micro-bore channels in manifolds, dispensing needles, and microfluidic devices.

FAQ

What is the smallest bore diameter that can be measured?

With dedicated micro objectives and mirror-tipped probes, bores down to approximately 1 mm in diameter can be accessed, and certain specialized configurations reach below that. Accessibility also depends on bore depth, straightness, and the required field of view, so a feasibility review of the specific geometry is recommended.

How accurate is white light interferometry compared with stylus profilometry?

Vertical resolution and repeatability of white light interferometry are typically at the nanometer level, equal to or better than stylus instruments, and the areal measurement provides far richer statistics than a single profile trace. Because it is non-contact, it also avoids stylus tip radius filtering and cannot deform or scratch soft surfaces. Where required, results can be cross-verified against contact methods in accordance with ISO 21920 and ISO 25178.

Which standards govern the test reports?

Areal parameters are evaluated per ISO 25178 series, profile parameters per ISO 21920 (or the legacy ISO 4287 and ISO 4288 where specifications still reference them), and instrument characteristics per ISO 25178-604 for coherence scanning interferometers. ASME B46.1 and GB/T 33523 are commonly invoked as regional equivalents.

Can transparent or highly reflective inner walls be measured?

Highly reflective surfaces are generally ideal for interferometry. Fully transparent or strongly scattering materials, however, may produce ambiguous signals; in such cases the laboratory adjusts illumination and evaluation settings, or applies replication materials to transfer the surface texture to an opaque replica that can be measured reliably.

What information should be provided when requesting a test?

Clients should supply the bore diameter, depth, and overall component dimensions, the material and expected finish range, the parameters and cut-off lengths required by the drawing, the locations to be measured, and any applicable acceptance limits. This allows the laboratory to select the correct optics, define the measurement plan, and report traceable results suitable for design verification, supplier audits, or failure analysis.