QMT Features: January 2008
Focus on a 3D future
Focus-Variation, a new optical surface metrology technique to ISO 25178 standards, provides a robust D surface measurement technique both for QC lab and in-line applications.

Optical surface measurement techniques are well established in research and industry but do not still offer robust 3D measurement of surfaces. The Focus-Variation technique, pioneered by Alicona and forming the principle of operation of the InfiniteFocus system, allows advanced optical 3D surface metrology of most surfaces including those with steep flanks and varying reflective properties down to a vertical resolution of 10nm.

This new optical technology provides dense measurements over large areas with a density of 2 Mio – 16 Mio measurement points.  Irrespective of whether it is used in the lab or during production, this technique works faster and more accurately than conventional methods with the potential to measure over a high vertical scan range (up to 20mm)  and x-y range  (up to 100mm x 100mm) enabling the robust measurement of relatively large components.

Robust measurement
The operating principle of Focus-Variation combines the small depth of field of an optical system with vertical scanning to provide topographical and colour information from the variation of focus. Depending on the topography of a surface the information from the variation of focus is analyzed in relation to the distance to the optics.
Using conventional optical measurement techniques a high vertical resolution can only be reached with a small vertical scanning range, whereas, the use of the focus variation technique yields a high vertical resolution over the entire scanning range, allowing a dynamic of 1:200000.

Focus-Variation simultaneously captures the entire surface topographic information in combination with its true colour information. Both, the topographic and colour information are perfectly registered to each other. This can be of major importance to understand if a particular surface feature is an artefact such as contamination or a real surface feature. Additionally, a quality measure is determined for each measurement point.

This advanced focus variation technology is the core of the InfiniteFocus. The main component of this optical metrology instrument is a precision optic system made consisting of various lens systems that can be equipped with different objectives allowing data collection with different resolutions. Using a beam splitting mirror, light emerging from a white light source is inserted into the optical path of the system and focused onto the specimen via the objective.

Depending on the topography of the specimen, the light is reflected into several directions as soon as it hits the specimen. If the topography includes diffuse reflective properties, the light is reflected equally strong into each direction. In case of specular reflections, the light is reflected mainly into one direction. All rays emerging from the specimen and hitting the objective are bundled in the optics and gathered by a light sensitive sensor behind the beam splitting mirror.

Due to the small depth of field of the optics only small regions of the object are sharply imaged. To allow a complete detection of the surface with full depth of field, the precision optic is moved vertically along the optical axis. This means that each region of the object is sharply focused. A sensor captures a series of 2D datasets during this scanning process. Thereby, all sensor parameters are optimized at each vertical position according to the reflective properties of the surface.
After the scanning process, the 2D datasets are evaluated to generate 3D information as well as an image with full depth of field. This is achieved by analyzing the variation of focus along the vertical axis.  Due to the large amount of data mechanical restrictions can be eliminated allowing measurement results with high resolution. Once all height measurements are determined, an image with full depth of field is computed.

The technique of Focus-Variation has been accepted as a unique technique in ISO 25178, which has been recently developed as a standard for the classification of topographical measurement techniques.

The Focus variation technique has uses in many different areas of micro metrology for detailed 3D surface characterisation and form measurement. It  can be used for:

  • Evaluation of tolerances and wear analysis in 3D;
    The technique allows verification of data based on CAD models and the display and measurement of worn materials. This can be used effectively in tool making, wear on bearings and failure analysis.
  • Precision machining, milling and drilling, mould and tool making
    Working in the micro and nano means the focus variation technique can be used in tool and mould making. Particularly this is the case with surfaces with a large lateral and vertical range 
  • Accurate 3D measurement of micro gear wheels
    This is a typical industrial application that can be easily achieved due to the measurement capabilities reached with Focus-Variation. Geometries with steep flanks of more than 80° can be measured accurately in seconds. Additionally, parameters such as the inner and outer diameter, flank angles and angels of angular geared components can be computed.

The technology is not only used in the lab, it is also capable of inline applications.  Depending on the measurement task and application, several sensors provide inline quality assurance. The full spectrum of measurement and analysis performance that can be achieved in the lab is also obtainable with the sensors used in the inline-system. l


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