Focus-Variation versus Optical Techniques

When surface qualities and micro geometric features need to be assessed, it is necessary to have accurate measurement solutions with high resolution. In contrast to other optical technologies, the gap between usual 3D coordinate measuring technology and classical surface metrology devices is closed with Focus-Variation.

Profile Projectors

Profile projectors have been preceded by current optical measurement systems. They are, however, still useful for understanding optical measurement technology. Profile projectors project an enlarged image of the components‘ surface characteristics onto a screen and compare this image to a reference using pattern matching.

Measurements can be taken within seconds but the automatic measurement of geometric features limits this technology to two-dimensional measurements. Profile projectors are also sensitive to alignment, meaning that measurements can differ depending on the orientation of the object.

Structured Light

With structured light, objects are illuminated by a projector with several bright and dark stripes. At least one camera is then used to capture this image. The striped pattern of the projector is distorted by the topography of the sample. A camera is used to record the distorted pattern and, finally, image processing is used to calculate the topography.

Measurements over large surfaces can be taken quickly with structured light and as such it is commonly used in the measurement of very large parts (for example, bodywork). However, the technology is rarely appropriate for high-resolution sub-µm depth measurements (for example, roughness measurements). Furthermore, the application range is substantially limited by low depth of field and high sensitivity to differing surface characteristics.

Confocal Systems

Confocal systems contain additional aperture right at the focal point inside the detector, which then blocks light from above and below the focal plane, meaning that only light within the focal plane can pass through the detector.

Depth is measured by detecting the strongest signal. As such, confocal measurements are characterized by high lateral resolution. They are especially useful for measuring smooth surfaces found on silicon structures or semiconductor geometries. One disadvantage, however, is their sensitivity towards vibrations.

Focus-Variation

Focus-Variation collects depth information and true color information of the surface. Real3D technology is used to measure complex geometries from different perspectives. These are then combined into a full 3D dataset. Focus-Variation measures form, dimension, position and roughness in one system, thus closing the gap between usual 3D coordinate measuring technology and classical surface metrology.

The components 3D surface is measured differently to when using profile projectors, which simply measures an outline. Intensity peaks or intensity modulation is measured only within a very narrow band around the focal point of the system with confocal systems and interferometers. Focus-Variation is much more tolerant against vibrations as it measures sharpness over a much larger region.

This information has been sourced, reviewed and adapted from materials provided by Bruker Alicona.

For more information on this source, please visit Bruker Alicona.

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