QMT Features: September 2013
The future of laser scanning microscopy in surface inspection and metrology
For non-contact and non-destructive testing, optical profilometry is the ideal solution for surface inspection and metrology – demonstrated by the quarter dollar coin example on the front cover of this issue. Dr Miriam Schwentker of Olympus microscopy explores how, moving into the digital age, opto-digital microscopy offers  many advantages for quality assessment tasks throughout manufacturing.

Surface topology is an important property in many areas of manufacturing, and different applications each demand a defined level of surface roughness. While roughness dictates the interaction between two surfaces, irregularities also hold the potential to form nucleation sites for cracks or corrosion.

From studying how paint adheres to a vehicle’s surface, to understanding how an artificial hip interfaces with bone, surface roughness is a crucial factor in the performance of these products. The measure of surface roughness also provides insight into surface wear, which is particularly important in contact mechanics such as gears, tyres, or engine parts.

Many different approaches can be employed to evaluate surface roughness, and which to choose depends on considerations including sample size, the cost of the process, and the required accuracy and precision. Because every approach, from contact profilometry to optical techniques, has different advantages and disadvantages, which method to choose can be a complex decision. Nevertheless, the optical approach of 3D laser scanning microscopy is becoming an increasingly popular solution, with the Olympus LEXT OLS4100 microscope at the forefront of this technology.

Evolving methods for surface inspection - a rough guide
As a non-contact profilometry method, interferometry utilises optical waves to measure surface irregularities. This method is fully traceable but complex, and requires much training and expertise in optics and light phases. It is also very niche, limited to surface metrology of certain types of samples such as mirrored surfaces, and cannot produce a visual map or image of the surface to support quantitative data. Other specialised applications employ the techniques of atomic force microscopy (AFM) and scanning electron microscopy (SEM), for metrology on the nanometre scale. AFM is a type of scanning probe microscopy that scans a sample surface using a physical probe, while SEM scans the sample with a focused beam of electrons. However, these methods are of limited practicality in many industrial applications, being expensive and time-consuming. Only the most specialist applications require an atomic scale resolution, and the majority of tasks within industrial quality control benefit from more accessible and versatile methods.

Contact profilometry using the stylus-based surface roughness gauge is one such method, and tends to be the most common approach for surface inspection and metrology within industrial quality control. Whereas the contact nature of this method incurs several limitations, the approach of light microscopy overcomes these, affording several advantages. Used for rapid analysis throughout many industries, light microscopy has been a mainstay since the early 1900’s, enabling the first wave of precision engineering. Its success is reflected in its persistence as a core technique, and for surface inspection and metrology applications, 3D laser scanning microscopy is an ideal technique for non-contact and non-destructive testing.

The 3D confocal laser scanning microscope (CLSM), such as the Olympus LEXT OLS4100 really comes into its own as an optical profilometer, combining the ability to generate exceptionally clear and detailed optical images of the sample, with the non-contact capabilities of laser scanning technology. A CLSM only detects in-focus reflections from a single specified focal plane, and this can be used to generate height information. Using the CLSM as an optical profilometer, it is the laser that scans the surface, as opposed to the stylus in traditional contact profilometry, and this brings with it several major benefits. The diamond tip of a stylus dragging over the surface can scratch and damage softer materials. Furthermore, with adhesive samples, the stylus may be damaged when it pulls loose, making it impossible to achieve correct results.

Contact surface roughness gauges are also unable to measure contours less than the stylus tip diameter (a radius of 2 µm), whereas the CLSM can measure down to a radius of only 0.2 µm (Figure 1). With laser scanning, analysis takes only a few seconds, much faster than stylus-based systems. The CLSM also allows imaging through multiple layers, recognising each separate layer as a distinct focal plane along the Z-axis. Accurate inspection and measurement of each layer is possible, even when measuring the roughness and thickness of multiple transparent layers, for example a transparent resin layer over a glass substrate (Figure 2). This of course is impossible using traditional contact profilometry.

With these benefits, optical profilometry is becoming ever more widely used within industry, especially now that the Olympus LEXT OLS4100 conforms to the first ever geometric product specification (GPS) standard for 3D surface metrology, ISO25178. This new standard defines 3D surface texture parameters, and also describes associated specifications such as the measurement technologies, calibration methods and software. Furthermore, as the latest addition to the Olympus opto-digital range of microscope, this instrument has been designed specifically for industrial applications. 

Accessible inspection with opto-digital technology

As digital technologies become increasingly prevalent in many areas of our lives, the opto-digital concept follows this trend. Building upon the wide-spread success of light microscopy, the new category of microscope takes the best of both worlds from optical and digital technologies, for the efficient management of industrial quality control workflows.   

Opto-digital microscopes are based upon modern microscopy techniques, controlled and displayed through a digital imaging interface to achieve a simplicity that is intuitive to any operator, regardless of experience. Accurate inspection and measurement no longer depends on in-depth microscopy knowledge, due to several innovative features in both the system and software design of the opto-digital microscope.

Opto-digital CLSMs such as the Olympus LEXT OLS4100 afford the benefit of integrating confocal and optical observation methods on a single platform, to enable complete evaluation of a sample. Illumination from a white LED combined with detection from a CCD camera allows the whole surface of the sample to be acquired digitally as a 3D true-colour optical microscope image, which can also be viewed using different imaging modes such as inverse colour (Figure 3). This colour view supports data acquired simultaneously from 3D CLSM imaging and height information.  

Managing and reporting functions are also incorporated, with observation settings recorded and presented alongside results. Analysis and measurement information can therefore be rapidly fed back to R&D or production, or shared for training purposes. This rapid analysis and reporting in turn enables much quicker decision-making compared to traditional techniques, with opto-digital systems providing a complete investigation, measurement and reporting platform. If the situation is identified as particularly complex, specific techniques such as AFM and SEM can also be employed to complement this approach.

The opto-digital platform also incorporates several features for added versatility. For example analysing and documenting the surface properties of larger samples is possible with digital image stitching capabilities, allowing the construction of a single image formed of many individual captures (Figure 4). On the digital monitor, the image of the complete sample can then be viewed and measured in either 2D or 3D. Furthermore, this digital view also means a wide-field area map display of the sample under low magnification is always visible, which facilitates navigation around the sample view at higher magnifications.

So how are these benefits transforming the future of surface inspection and metrology?

The future of surface inspection and metrology

In recent years, microscopy techniques have been rapidly advancing in both performance and complexity. Opto-digital microscopy technology benefits from these developments, and makes them accessible for operators at every level of expertise, allowing them to fully focus on the inspection task. As such, opto-digital technologies are ideal for the management of complete industrial quality control workflows, streamlining the imaging, measurement and reporting stages of many tasks through rapid and intuitive operation.

Faster analysis enables faster decision making, and opto-digital presents a more efficient means of quality control, compared to traditional techniques. In more complex situations, opto-digital microscopy also presents a quick and cost-efficient starting point, indicating whether other more specialised analyses such as AFM or SEM are necessary, ensuring this investment of time and resources is fully justified.
Opto-digital technology has really taken hold within surface inspection and metrology, with the Olympus LEXT OLS4100 conforming to the first ever standard for 3D surface metrology. It is therefore conceivable that within the next few years opto-digital microscopy systems will become standard quality control tools within many industries, bringing industrial quality control into the digital era. l
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