White Light Interferometers for High-Precision Surface Measurement

A white light interferometer measures surface height by analyzing patterns created when light waves interact. The system splits a light beam into two paths—one reflects off a reference mirror while the other bounces off the sample surface. When these beams recombine, they create interference patterns that reveal height variations across the surface.

This optical approach captures topography without touching the sample. The technology works particularly well on smooth, reflective surfaces where interference patterns form clearly. Engineers use these instruments to measure film thickness, surface roughness, step heights, and other features requiring nanometer or sub-nanometer precision in semiconductor, optics, and materials research applications.

Light interference occurs when two light waves collide, causing each to strengthen or weaken. This section describes the interference of two lights reaching point P at a certain distance from the surface of the target. If the difference in distance between the two light paths S1P and S2P is an integer multiple of the light wavelength λ, the two light waves will strengthen and become brighter at point P due to the wave peaks overlapping. If the optical path difference is an integer multiple of the wavelength λ + 1/2 the wavelength λ, the peaks and valleys of the waves will overlap, causing the waves to weaken and become darker.

The interference light becomes lighter and darker at intervals equal to half the wavelength of the light source (λ/2). These patterns of light and dark are called interference stripes. The height of a target can be determined by counting the number of interference stripes.
The physicist Christiaan Huygens and others have proved that interference stripes of light form a graph (waveform) with a fixed period, as in the figure below. Optical interferometers use this physical phenomenon to ensure high-resolution measurement even at low magnifications.

Determining whether the shape of the target is on an upward or downward slope is not possible when using interference stripes generated from a single-wavelength light source. However, this problem can be solved using phase shift interferometry.

KEYENCE's 3D Optical Profiling Microscope with a built-in white light interferometer uses a white LED to determine the interference stripe intensity at each height interval by moving the objective lens. Height information from the focal point position is obtained by using a linear scale to measure the lens position at the point where the interference stripes become stronger. This method is called vertical scanning interferometry (VSI).

Unlike many optical measurement techniques, the height resolution of an interferometer is independent of the magnification of the objective lens. This is because the composite waveform representing interference fringe intensity can be accurately reconstructed through calculation, even when the objective lens has a large depth of field.

Interference fringes appear at regular, predictable intervals due to the constant wavelength of light. Since the wavelength is known in advance, the expected shape of the composite waveform derived from the fringe intensity can be calculated mathematically. By capturing interference intensity data at regular intervals and reconstructing this composite waveform, the system can then isolate and process individual components of the waveform and achieve high height resolution regardless of the magnification used.

White light interferometry operates through a process known as vertical scanning. The system moves the objective lens up and down while continuously monitoring interference patterns at each height. White light contains multiple wavelengths, creating a composite interference signal that peaks sharply when the surface is at the focal plane.

Software tracks the vertical position of this intensity peak for every pixel across the entire field of view. A linear encoder measures lens position precisely as it scans, linking each interference maximum to a specific height coordinate. This technique, known as Vertical Scanning Interferometry (VSI), offers a significant advantage over single-wavelength systems, particularly for measuring larger step heights. The distinct, unambiguous nature of the white light interference peak eliminates the ambiguity that can confuse single-wavelength measurements, ensuring reliable data across complex surface features.

One of the standout advantages of white light interference is its wide-field measurement capability. Rather than building images point by point like scanning probe methods, white light systems capture entire areas at once, dramatically reducing measurement time.

The technique is also non-contact, meaning delicate samples such as thin films, soft coatings, and cleanroom components can be fully characterized without any risk of contamination or damage. Combined with sub-nanometer vertical resolution, this makes white light interference well-suited for detecting subtle surface features that influence optical performance, friction, and sealing across a wide range of precision manufacturing applications.

Modern systems combine white light interference with laser confocal scanning to expand measurement capabilities. Our Optical Profilometer Resources page demonstrates how KEYENCE 3D Optical Profilers use this hybrid approach to adapt to diverse surface characteristics.

The system uses white light scanning for maximum resolution on reflective areas and can switch to confocal methods for steep angles, rough textures, or difficult materials. This allows users to measure complex samples in a single session without needing multiple instruments. Additionally, the high-resolution objective lenses can produce stunning, true-color images of a surface.

White light interferometers are used across industries where surface geometry must be measured at the nanometer to micrometer scale without contact.

Semiconductor manufacturing is one of the most demanding application areas. Wafer step height verification, film thickness measurement, and substrate flatness all require sub-nanometer precision, since defects at that scale directly impact device yield. As feature sizes continue to shrink, interferometry has become a standard process control tool at multiple stages of fabrication.

Optics manufacturers rely on these systems to verify lens curvature, mirror flatness, and the uniformity of optical coatings since any surface deviation introduces aberration that degrades optical performance. Similarly, precision machined components such as cutting tools, bearing surfaces, and mold cavities are measured to confirm surface finish and dimensional tolerances that contact-based profilometers cannot reach without risking part damage.

In materials research and failure analysis, interferometry is used to characterize wear patterns, corrosion, fracture surfaces, and thin film behavior. Because measurements are non-destructive, the same sample can be measured repeatedly across test intervals to track how a surface evolves over time.

Medical device manufacturing represents a growing application area, where surface texture of implants and the geometry of micro-machined components must meet tight regulatory and functional tolerances. Data storage component inspection, such as read/write head flatness and disk surface quality, has historically been another high-precision use case driven by the same need for nanoscale surface control.