Quality Magazine

Stereomicroscope Solutions

July 31, 2007
The optical concept of Stereo Explorer is shown here. Source: Leica Microsystems Inc.


For decades, companies have used stereomicroscopes for 3-D stereoscopic visual inspection and imaging as quality control tools. The stereomicroscope offers countless uses and benefits for examining the surfaces of a sample for consistent quality, but a stereomicroscope’s inability to measure in 3-D has always been a challenge.

A stereomicroscope provides a large visual depth of focus. However, direct 3-D measurements are not possible throughout the extended depth of focus. In order to acquire quantitative 3-D surface analysis, companies have had to look to alternative devices to supplement the stereomicroscope, or apply other technologies to achieve this level of precise analysis. This limitation has posed a challenge in terms of time constraints, budget and results.

New 3-D analysis capability in stereomicroscopy is available. Conventional stereomicroscopes now transform into true 3-D measurement devices with full analytical capability to generate profile, roughness, area and volumetric measurements.

3-D Stereo Reconstruction

Unlike the light microscope, a stereomicroscope has a very large depth of focus. Up until recently, a true 3-D analysis package for stereomicroscope quality control was not commercially available. With the development of the stand-alone software package, this has changed. It calculates surface data from stereoscopic images, and allows a microscope operator to acquire this data by simply capturing a stereo image pair.

Two high-resolution digital CCD cameras attach to the stereomicroscope and acquire an image of each optical beam path of the stereomicroscope. One of the advantages as compared to other approaches is that the operator can still access the optical paths for visual inspection while using the instrument. The use of high-resolution cameras results in accurate, detailed 3-D models that provide sufficient data for precise quantitative analysis.

Conventional stereoscopic approaches to analysis require complex optical setups in order to provide acceptable results. The new approach reduces the complexity of the configuration because the software performs automatic offset calculation and geometric correction. This calculation produces accurate, robust, 3-D surface reconstructions. For example, with this method the relative height accuracy is more than 3%, and a fully automatic calculation can be performed in less than 100 seconds.

A 3-D model of solder paste is visualized with Stereo Explorer. Source: Leica Microsystems Inc.

Quantitative Measurement

After the software determines the sample’s surface information, 3-D data can be visualized and analyzed. The number and range of analytical capabilities comprise numerous calculations. The profile analysis module allows height profile extraction along operator-defined paths and can be used to calculate relative height measurements. All established roughness, profile and waviness measurements can be performed, as well as various statistical calculations. The area analysis module similarly calculates parameters such as the ratio of the true-to-projected area, which is also called the bearing area curve. The volume analysis module enables direct volumetric measurements relative to freely definable polygon lines. All analysis routines are designed in a modular, intuitive manner, and fully support Windows reporting conventions.

Applications

This technology can be applied in any quality control situation where analysis of surface structure is important. This may include documentation of the roughness of microscopic samples for affinity calculations, the analysis of fractured surfaces from mechanical parts, or the volume of impressions and uprisings of formed surfaces.

The technology can establish a direct link between the microscope image and the 3-D measurement capability. This eliminates the need to find the area of interest using a supplemental device as the image and 3-D surface data coexist. Some applications include fracture analysis, materials development, drug discovery and production, semiconductor manufacturing and life science research.

New advances in image processing and computer technology allow measurement of 3-D surface parameters directly in stereomicroscope images. Profile, area and volumetric calculations can be performed in reconstructed images. Therefore, the sample does not require examination by other 3-D measurement devices to ensure quality; instead all analysis is performed by existing microscopes retrofitted with the technology. This technology can reduce a company’s investment cost for optical inspection, while improving convenience and performance. The operator can easily export all of the measured values, 3-D models and other parameters to standard files, which can then integrate with reports. This technology for microscopic quality control is becoming a new standard for advanced optical inspection. NDT