Optical non-destructive methods for 3D surface metrology have become increasingly important in contrast to traditional tactile measurement techniques. Alicona’s InfiniteFocus optical 3D measurement system utilises focus variation technology to perform quality assurance processes both in the lab and in production.

In contrast to traditional tactile methods, InfiniteFocus is able to perform 3D measurements without touching the surface. It measures whole areas instead of only surface profiles, and delivers true colour information in addition to the 3D data.

Focus variation combines the small depth of focus of an optical system with vertical scanning to provide topographical and colour information from the variation of focus. The main component of the system is a precision optic containing various lens systems that can be equipped with different objectives, allowing measurements with different resolution.

With 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 via the objective. If the topography shows diffuse reflective properties, the light is reflected equally strongly into each direction. In case of specular reflections, the light is scattered mainly into one direction. All rays emerging from the specimen and hitting the objective lens 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 perform a complete detection of the surface with full depth of field, the precision optic is moved vertically along the optical axis while continuously capturing data from the surface. This means that each region of the object is sharply focused. Algorithms convert the acquired sensor data into 3D information and a true colour image with full depth of field. This is achieved by analysing the variation of focus along the vertical axis.

The vertical resolution depends on the chosen objective and can be as low as 10nm. The vertical scan range depends on the working distance of the objective and ranges from 4.5mm to 23.5mm. In contrast to conventional techniques, the vertical resolution is achieved regardless of the scan height, leading to a vertical resolution dynamic of 1: 500000.

The XY range is determined by the used objective and typically ranges from 0.16mm x 0.16mm to 5.63mm x 5.63mm for a single measurement. By using special algorithms and a motorized XY stage the XY range can be exceeded up to 100mm x 100mm.

In contrast to other optical techniques that are limited to coaxial illumination, the maximum measurable slope angle is not only dependent on the numerical aperture of the objective. Focus variation can be used with a large range of different illumination sources (such as a ring light), which allows the measurement of slope angles exceeding 87 degrees. Basically, focus variation is applicable to surfaces with a large range of different optical reflectance values.

As the optical technique is very flexible in terms of using light, typical limitations such as measuring surfaces with strongly varying reflection properties, even within the same field of view, can be avoided. Specimens can vary from shiny to diffuse reflecting, from homogeneous to compound material, and from smooth to rough surface properties. Focus variation overcomes the aspect of limited measurement capabilities in terms of reflectance by a combination of modulated illumination, controlling the sensor parameters, and integrated polarisation. Modulated illumination means that the illumination intensity is not constant, but varying. The complex variation of the intensity can be generated by a signal generator. Through the constantly changing intensity, far more information is gathered from the specimen’s surface.

In addition to the scanned height data, Focus-Variation also delivers a colour image with full depth of field that is registered to the 3D points. This provides an optical colour image which eases measurements as far as the identification and localisation of measurement fields or distinctive surface features are concerned. The visual correlation between the optical colour image of the specimens’ surface and its depth information are often linked to each other and are therefore an essential aspect of meaningful 3D measurement.

Since the described technique relies on analysing the variation of focus, it is only applicable to surfaces where the focus varies sufficiently during the vertical scanning process. Surfaces not fulfilling this requirement such as transparent specimen or components with only a small local roughness are hardly measurable. Typically, focus variation delivers repeatable measurement results for surfaces with a local Ra of 0.009 microns at a lc of 2 microns.

Focus variation is used to perform high-resolution 3D surface measurement for quality assurance in production as well as research & development activities in the lab. Key applications are surface analysis and characterisation such as in tool & mould making, aerospace industry, precision manufacturing, automotive industry, all kinds of materials science, corrosion and tribology, electronics and medical device development. Due to its technical specifications the focus variation technique is used for form and roughness measurements.

Focus variation in modern manufacturing

Globally, industry deals with the need to manufacture complex high-tech products with tight tolerances and a high degree of automation. Consequently, there is a tremendous pressure on companies to invest in state-of-the-art technology that meets both efficient processes and efficient resource management. Here, inline metrology, achieved for example with a robot in combination with a high-resolution optical 3D measurement sensor, and closed-loop manufacturing are the key to success, especially for miniaturised products. Both concepts are based on production-integrated metrology, which contrasts with the traditional handling of manufacturing and quality assurance as two independently achieved processes.

Metrology-wise, several requirements have to be met to obtain real-time data about process and product. First, measurements have to be area-based instead of only being profile-based. Second, measurements have to be traceable, in a high resolution and with high repeatability. Third, there has to be a high measurement speed together with a large number of measure points that are processed.

Alicona’s focus variation technology is designed to fulfil exactly these requirements and to consequently enable the interaction between machine and measurement. In particular, this is enabled by the high measurement speed and the high vertical resolution across small and large measurement volumes. Also, the capability to measure complex components with steep flanks and varying reflection properties has to be taken into account when it comes to advanced manufacturing processes. Focus variation is also suitable for high resolution measurements in production. Alicona measurement systems are robust against vibrations, insensitive to ambient light, and can be easily integrated in existing systems.

Manufactured right first time

Closed-loop manufacturing is achieved through a production-ready optical measurement sensor that is directly integrated in a machining centre, enabling self-optimised production and an up to fourfold increase in machining accuracy. Components are measured directly in the machine, and the internal measurement evaluates possible deviations from the target geometry. Based upon internal evaluation, the machine automatically modifies the process parameters for further manufacturing. Important real-time information on the state of the process and product is received through the fast supply of reliable measuring data. Deficient workpieces are early recognised and expensive resources conserved.

Another option to achieve fully automated production integrated quality assurance can be a robot-based metrology solution. A robot enables flexible positioning of the measurement system, allowing its use in an assembly line. In one measurement cycle all necessary parameters at various positions can be measured. Also, the measurement of one selected parameter on several workpieces is possible by one simple command. Again, an immediate evaluation shows if all values are within the geometrical tolerance. Similar to the closed-loop concept, high-resolution measurements of complex components depend on production-ready optical metrology.

Real3D – How to measure undercuts

In combination with Real3D, users measure surfaces from numerous perspectives. Components are measured in 3D from various perspectives and then automatically merged into a full 3D dataset. High-precision and calibrated rotation and tilt axes ensure automated, repeatable and traceable measurement of form and roughness on the whole measurement object. Users are able to visualise and measure surface features such as diverse flank angles, thread pitch and undercuts.

With Real3D a component is measured at various rotation and tilt angles. Based on the registered true colour information of each measurement point, the single measurements are transformed into a joint coordinate measurement system. The single, overlapping measurements are then precisely merged into a complete 3D data set.

Alicona offers several options for Real3D measurements. The Advanced Real3D Rotation Unit is equipped with a motorised tilt axis and motorised rotation axes, whereas the Real3D Rotation Unit is based on motorised rotation axes and a manual tilt axis. Both models are used for full form measurement of typically round tools.

The fully motorised version can additionally be applied for the automatic measurement of cutting dies, micro-hole measurement and reverse engineering. Further, users are able to measure the trail and main edges of their drill, cutting miller or other tool in only one measurement circle. The compatibility of both units with a number of clamping systems allows precise and rapid interaction between processing and measurement. In addition, various adapters enable 360-degree rotation and components without rotational symmetry.

Alicona products are distributed and serviced in Australia and New Zealand by Met Optix.