Fokusvariation

Fokusvariation, known in English as focus variation, is a non-contact optical method for measuring 3D surface topography in industrial metrology. It works by capturing images of a part at several focal positions and, for each image pixel, identifying the focal plane where the image is sharpest to infer height.

Principle: An optical system moves through focus to produce an image stack. A focus metric is calculated for each image; for every pixel, the peak of the focus metric indicates the height at that point. Calibration converts the focal position to physical height, yielding a height map of the surface.

Equipment and workflow: A microscope or macro-imaging setup with a high numerical-aperture objective and a motorized focusing stage collects the image stack. Software analyzes the stack and reconstructs the 3D surface, often with tilt and scale calibration included to ensure accurate measurements.

Advantages: It handles rough or textured surfaces and does not require contact with the part. The method provides high vertical resolution and can measure features with a substantial depth range, making it suitable for components such as molds, gears, and micro-structured parts.

Limitations: Transparent or highly reflective surfaces can be challenging. Accuracy depends on calibration, optics, and surface texture. The process can be slower than some interferometric techniques for very smooth, flat parts and is sensitive to vibration or motion during acquisition.

Applications and history: Fokusvariation emerged in optical metrology during the 1990s and is widely used in industry for gear inspection, injection-mold inspection, plastics metrology, and semiconductor components. Related techniques include optical profilometry, confocal microscopy, and interferometry.