White Light Interferometry (WLI)

White light interferometry (WLI) is a non-destructive, non-contact, optical surface topography measurement that uses coherence scanning interferometry to generate 2D and 3D models of surface height.

Strengths

• Rapid results
• Superior lateral resolution (limit is ~160 nm) as compared to chromatic confocal, Laser Confocal and Wide Area Patterned Light Mapping
• Nondestructive measurement
• High vertical resolution: ~1 nm

Limitations

• Steep cut-offs and sharp topography angles impair WLI measurement accuracy around these features
• Interpretation of fringe changes can be difficult in certain types of samples

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Example Outputs Instruments Used How WLI Works

• Solid phase
• Material: opaque, transparent, coated, uncoated, specular, or rough
• Maximum lateral dimension: 147 mm
• Maximum vertical height: 100 mm
  1. Larger sample width and depth possible with partial coverage / scan area
• Sample reflectivity: 0.05 – 100 %

WLI instruments use a white light source to illuminate the surface of the sample.

A beam-splitter divides the white light beam into two optical paths: one that reflects or scatters from the sample and one that reflects from a flat, known reference mirror. These two signal beams are then mixed together and the resulting image is projected onto a CCD image sensor.

The two beams, when mixed, form an interference pattern whose intensity can be related to the sample surface height: any difference in optical path length between the reference and sample beams changes the measured interference intensity at each scanned point, providing an indirect measure of the height variance in the sample.

The interference fringes are analyzed at each point to build up a 3D map of the sample surface.