Focus-Variation in comparison

Profile projectors

and other image processing systems are the predecessors of present optical measurement systems and are still relevant for understanding optical measurement technology. Profile projectors enlarge the components‘ surface characteristics and project the image onto a screen. Through pattern matching the image is compared to an appropriate reference.

Advantages are measurements that can be executed within seconds, although the automatic measurement of geometric features is limited to two-dimensional applications only. One main disadvantage is its sensitivity to object alignment. Depending on its orientation, differing measurement results can be obtained.

?Structured light

is based on a projector that illuminates the measurement object with several bright and dark stripes and captures it with at least one camera. The topography of the sample distorts the stripe pattern of the projector. The distorted pattern is recorded with a camera and, finally, the topography is calculated via image processing. One advantage of structured light is the high measurement speed when measuring large surfaces. Therefore, the technology is primarily used for the measurement of very large parts (e.g.: bodywork). The technology is of limited suitability for high-resolution sub-μm depth measurements as, for example, with roughness measurements. In addition, the low depth of field and high sensitivity to varying surface characteristics substantially limit the application range.

?Confocal measurement

is characterized by its high lateral resolution. Right at the focal point inside the detector an additional aperture is used to block light from above and below the focal plane. Thereby, only light within the focal plane passes through the detector. By detecting the strongest signal depth is measured. Confocal systems are particularly suitable for measurements of very smooth surfaces that can be found on silicon structures or semiconductor geometries. The advantage of high resolution in z is accompanied with increased sensitivity towards vibrations.

?White light interferometer (WLI)

A coaxial illumination beam is split onto a reference surface and onto the measurement surface. The reflected light coming from the reference and the surface interfere on the sensor. The distance to the measurement surface is precisely calculated based on this Moiré effects.??One advantage is the high vertical resolution and the independence from lateral field of view. Therefore, this technique is widely used for flatness measurements of larger areas and additionally shows its strength in roughness measurements on smooth surfaces as for example highly polished parts, optical lenses and semiconductor materials.

?Focus-Variation

gathers not only depth information but also registered true color information of the surface. Roughness of micro and nano-structures is measured both profile and area based. Complex geometries are measured via the Real3D technology from different perspectives which are then merged into a full 3D dataset. By measuring form, dimension, position and roughness in one system, Focus-Variation closes the gap between typical 3D coordinate measuring technology and classical surface metrology.

In contrast to profile projectors not an outline is measured, but the components‘ 3D surface. While confocal systems and interferometers measure intensity peaks or intensity modulation only in a very narrow band around the focal point of the system, Focus-Variation measures sharpness over a considerably larger region. Therefore, the technology is much more tolerant against vibrations.