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Focus Variation in Optical Metrology

AZoM talks to Stefan Scherer, CEO of Alicona about focus variation, edge geometry and an overview of the InfiniteFocus 3D measurement device.

Please provide an overview of how InfiniteFocus measures form, roughness and contour.

InfiniteFocus is a high resolution optical 3D measurement device. Based on the technology of Focus-Variation, the system combines all functionalities of a 3D surface measurement device and a micro coordinate measurement system.

It provides measurements in a high vertical resolution with a high measurement point density also across large measurement volumes and highly structured (rough) surfaces. For a user this means that both form and surface roughness are measured with only one instrument.

In addition, InfiniteFocus can be equipped with a 3D rotation unit, enabling the full form measurement of components. This is based on an advanced 3D merging technology that merges single measurements into a full 3D data set.

This can be achieved as InfiniteFocus in combination with the rotation unit is not limited to the view from one perspective when performing measurements. A component can also be measured in 3D from different perspectives. Single measurements from various positions are automatically merged into a 3D data set. The “Real3D” technology allows the component´s visualization from different angles plus a measurement of contour, difference and form.

What is the definition of Focus-Variation?

Focus-Variation (ISO 4287/88 and ISO 25178) exploits the small depth of focus of an optical system with vertical scanning to provide topographical and color information from the variation of focus. In contrast to other optical techniques, two issues should be especially addressed.

First, the method is not limited to coaxial illumination or other special illumination techniques, which allows to overcome some limitations with respect to the maximum measurable slope angle. Second, the technology delivers registered true color information for each measurement point.

What is the size range for cutting tools, such as drills, mills and taps, the system is able to measure? How large can the measurement areas and volume be?

The range of measurable components includes micro tools to large and heavy components. As of example, also turbine blades are measured with the technology of Focus-Variation. Users measure micro tools in the µm and sub- µm range. For instance, radii down to 1µm are repeatably and traceably measured. The maximum slope angle is up to (approx.) 90°.

Is the measurement density more than 100 million measurement points? How does that compare with other measurement technologies?

Based on Focus-Variation, InfiniteFocus works with more than 100 million measure points in one measurement also on larger areas. To our knowledge there is no other measurement device that processes measure points to this extend.

In addition, Alicona also automatically provides an estimation of repeatability of every single measurement value analyzing the variance of each measurement point in Z-direction. The repeatability measure helps to evaluate the quality of individual measurement points. This again allows measurement results that are easy to interpret.

Is it only suitable for round tools and parts or can it measure cutting tool inserts? If so, does that alter how the system is operated?

InfiniteFocus is applied for all kind of surfaces and shapes, not only round components. Actually, the measurement of inserts and other cutting tools is a standard application. As soon as there is minimal roughness with a Ra above 10-15nm the system achieves high resolution measurements.

Applications range from highly structured and very rough surfaces with steep flanks to shiny, smooth and polished components. The handling of the system does not change with respect to the surface measured. But, also automated measurement of repeating measurement tasks is enabled. The technology of Focus-Variation is used in the lab and in production.

Why are tactile probes unable to accomplish the same tasks?

There are various reasons why tactile measurement techniques don´t provide satisfactory results for the measurement of very fine and functional surfaces. Whereas the optical measurement technique Focus-Variation is non-destructive, tactile systems are “touching”. This can lead to “destructive” measurements. Also, when using contact stylus instruments it can happen that the form of the contact stylus tip has a smoothing effect on the surface profile result.

Also, due to a quite high radius of the stylus tip sometimes very fine surface structures cannot be captured, which can lead to inadequate measurement results. Compared to coordinate measurement systems, Focus-Variation is faster, has a higher measurement point density and a higher vertical resolution and enables the measurement of complex forms. Also, when measuring complex forms such as cutting edges the stylus tip often cannot measure the edge as the stylus may slips off.

What are the challenges with measuring ground and polished components and how does the device measure those on a repeatable basis?

Ground, polished and coated surfaces show various reflection properties and reflection patterns. InfiniteFocus operates with the special illumination technology “SmartFlash”. SmartFlash is based on modulated light providing co-axial illumination throughout the whole measurement area.

As soon as the surface has a minimal roughness with a Ra above 10-15nm users achieve high resolution measurements.

How is data collected when objects slant beyond 20 or 30 degrees from the horizontal?

Light coming from different directions is used to positively influence a measurement. The measurement of the maximum flank angle is not restricted to the numerical aperture. Depending of the surface users measure flanks to of up to approximately 90°.

This is achieved via add-ons such as an optional ring light or polarization to support the measurement of complex geometries.

Is the system also being used to precisely inspect tool and part wear in addition to newly manufactured products? What are the benefits for this type of application?

Wear analysis is a standard application with InfiniteFocus. Also, the automatic comparison to CAD data or reference geometry is a typical field of use. Amongst other applications, this enables the optimal time for a tool change in a manufacturing process.

How do tool measurements from the Alicona system compare to those from a toolmaker’s optical microscope?

Microscopes are mainly viewing devices with hardly any measurement capabilities. Alicona offers optical 3D measurement solutions which provide both, high resolution traceable 3D measurements and registered 3D color information for easy, fast and traceable measurements of form and finish on micro structured surfaces.

What are the training requirements for the measurement system?

InfiniteFocus is very easy and intuitive to use. In less than an hour a user can operate with the system to perform standard measurements. A “tab-windows” system guides a user through the application and the 3D measurement of various surface parameters. User friendliness is increased throughout a series of export functions.

How do you provide a detailed reproduction of an edge geometry?

Usually, the common approach for measuring edges is to simply fit a radius into a relatively large section of the edge itself. However, our application work has shown the true edge is elliptic instead of being an ideal radius.

Measuring an elliptic shape requires a significantly higher lateral and vertical resolution than commonly thought. With a vertical resolution down to 10nm, InfiniteFocus enables elliptic shapes to be fit into the edge region, providing a more detailed geometric reproduction. The fitting element consists of an elliptic segment with two adjacent tangent lines.

About Stefan Scherer

Stefan Scherer Stefan studied technical physics at Graz University of technology (Austria) and received his Ph.D. degree in the field of informatics in 2000. During his four years as assistant professor leading the industrial machine vision group at the Institute for Computer Graphics and Vision he founded an initiative for digital microscopy. In this position Stefan also authored and co-authored a series of papers and supervised more than ten master theses.

2001 he started the company Alicona which is highly specialized and now a leading manufacturer in the field of high resolution optical 3D measurement. Today, the company is an international player with headquarters in Austria and subsidiaries in Germany, France, the UK, the US and South Korea.

Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.


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