Claudiu Giusca

Determination of the transfer function for optical surface topography measuring instruments—a review

A significant number of areal surface topography measuring instruments, largely based on optical techniques, are commercially available. However,implementation of optical instrumentation into production is currently difficult dueto the lack of understanding of the complex interaction between the light and the component surface. Studying the optical transfer function of the instrument can help address this issue. Herea review is given of techniques for the measurement of optical transfer functions. Starting from the basis of a spatially coherent, monochromatic confocal scanning imaging system, the theory of optical transfer functions in three-dimensional (3D) imaging is presented. Further generalizations are reviewed allowing the extension of the theory to the description of conventional and interferometric 3D imaging systems. Polychromatic transfer functions and surface topography measurements are also discussed. Following presentation of theoretical results, experimental methods to measure the optical transfer function of each class of system are presented, with a focus on suitable methods for the establishment of calibration standards in 3D imaging and surface topography measurements.

Calibration of the scales of areal surface topography-measuring instruments: part 1. Measurement noise and residual flatness

In this paper, we present methods for determining the measurement noise and residual flatness of areal surface topography-measuring instruments. The methods are compliant with draft international specification standards on areal surface texture. We first introduce the international standards framework and then present current methods based on averaging and subtraction to isolate the measurement noise and residual flatness from the sample surface topography. These methods are relatively difficult to apply and time consuming in practice. An alternative method is presented based on thresholding and filtering techniques. This method is simple to apply in practice. Traceability and measurement uncertainty are discussed.

Calibration of the scales of areal surface topography measuring instruments: part 2. Amplification, linearity and squareness

Methods for determining the amplification coefficient, linearity and squareness of the axes of areal surface topography measuring instruments are presented. The methods are compliant with draft international specification standards on areal surface texture. A method of calibrating the z-axis scale according to the guidelines given in surface profile specification standards, which is applied to areal measurements, is first presented. Then a method of calibrating the scales of the x and y axes using cross grating artefacts, and which is not based on pitch measurement, is introduced. A method for extending the calibrated range of the z-axis scale, which uses multiple overlapped measurements of a step height artefact, is also discussed.

Development and characterization of a new instrument for the traceable measurement of areal surface texture

Modern manufacturing industry is beginning to benefit greatly from the ability to control the three-dimensional, or areal, structure of a surface. To underpin areal surface manufacturing, a traceable measurement infrastructure is required. In this paper, the development of a new traceable instrument for the measurement of areal surface texture is presented. The instrument uses a two-axis coplanar air-bearing slideway to move the measured surface beneath a stylus probe. The motion of the slideway is measured using linear and angular interferometers. The key to the new instrument is a novel probing system incorporating a cylindrical air-bearing guideway and an electromagnetic system to maintain a constant stylus force on the surface. The deflection of the stylus is measured using a differential plane mirror interferometer thereby minimizing the effect of any error motion in the metrology frame. The uncertainties of the instrument are calculated using a Monte Carlo approach and are evaluated to be 5 nm in the z axis and 16 nm in the x and y axes (all at k = 2). The results are given for the instrument and are compared to results from a traceable profile measuring instrument and a coherence scanning interferometer.

Calibration of the scales of areal surface topography measuring instruments: part 3. Resolution

Calibration of the scales of areal surface topography measuring instruments requires testing of the resolution. Several designs of artefact that allow testing of the resolution of such instruments are currently available; however, analysis methods need to be developed to provide comparable results. A novel method for determining the lateral resolution of areal surface topography measuring instruments is presented. The method uses a type ASP (star-shaped) material measure. To demonstrate the validity of the method, the resolution of a phase shifting interferometer was determined based on the ISO definition of the lateral period limit. Using the proposed method, the type ASP material measure, which is often used to judge qualitatively an instruments resolution, can be used to quantitatively estimate the resolution of instruments using the topography data.

Surface texture measurement for additive manufacturing

The surface texture of additively manufactured metallic surfaces made by powder bed methods is affected by a number of factors, including the powders particle size distribution, the effect of the heat source, the thickness of the printed layers, the angle of the surface relative to the horizontal build bed and the effect of any post processing/finishing. The aim of the research reported here is to understand the way these surfaces should be measured in order to characterise them. In published research to date, the surface texture is generally reported as an Ra value, measured across the lay. The appropriateness of this method for such surfaces is investigated here. A preliminary investigation was carried out on two additive manufacturing processes—selective laser melting (SLM) and electron beam melting (EBM)—focusing on the effect of build angle and post processing. The surfaces were measured using both tactile and optical methods and a range of profile and areal parameters were reported. Test coupons were manufactured at four angles relative to the horizontal plane of the powder bed using both SLM and EBM. The effect of lay—caused by the layered nature of the manufacturing process—was investigated, as was the required sample area for optical measurements. The surfaces were also measured before and after grit blasting.

Advances in engineering nanometrology at the National Physical Laboratory

The National Physical Laboratory, UK, has been active in the field of engineering nanometrology for a number of years. A summary of progress over the last five years is presented in this paper and the following research projects discussed in detail. (1) Development of an infrastructure for the calibration of instruments for measuring areal surface topography, along with the development of areal software measurement standards. This work comprises the use of the optical transfer function and a technique for the simultaneous measurement of topography and the phase change on reflection, allowing composite materials to be measured. (2) Development of a vibrating micro-CMM probe with isotropic probing reaction and the ability to operate in a non-contact mode. (3) A review of x-ray computed tomography and its use in dimensional metrology. (4) The further development of a metrology infrastructure for atomic force microscopy and the development of an instrument for the measurement of the effect of the probe?surface interaction. (5) Traceable measurement of displacement using optical and x-ray interferometry to picometre accuracy. (6) Development of an infrastructure for low-force metrology, including the development of appropriate transfer artefacts.

Towards geometrical calibration of x-ray computed tomography systems - A review

Industrial x-ray computed tomography (XCT) is seen as a potentially effective tool for the industrial inspection of complex parts. In particular, XCT is an attractive solution for the measurement of internal geometries, which are inaccessible by conventional coordinate measuring systems. While the technology is available and the benefits are recognized, methods to establish the measurement assurance of XCT systems are lacking. More specifically, the assessment of measurement uncertainty and the subsequent establishment of measurement traceability is a largely unknown process. This paper is a review of research that contributes to the development of a geometrical calibration procedure for XCT systems. A brief introduction to the geometry of cone-beam tomography systems is given, after which the geometrical influence factors are outlined. Mathematical measurement models play a significant role in understanding how geometrical offsets and misalignments contribute to error in measurements; therefore, the application of mathematical models in simulating geometrical errors is discussed and the corresponding literature is presented. Then, the various methods that have been developed to measure certain geometrical errors are reviewed. The findings from this review are discussed and suggestions are provided for future work towards the development of a comprehensive and practical geometrical calibration procedure.

New method development in prehistoric stone tool research: evaluating use duration and data analysis protocols.

Lithic microwear is a research field of prehistoric stone tool (lithic) analysis that has been developed with the aim to identify how stone tools were used. It has been shown that laser scanning confocal microscopy has the potential to be a useful quantitative tool in the study of prehistoric stone tool function. In this paper, two important lines of inquiry are investigated: (1) whether the texture of worn surfaces is constant under varying durations of tool use, and (2) the development of rapid objective data analysis protocols. This study reports on the attempt to further develop these areas of study and results in a better understanding of the complexities underlying the development of flexible analytical algorithms for surface analysis. The results show that when sampling is optimised, surface texture may be linked to contact material type, independent of use duration. Further research is needed to validate this finding and test an expanded range of contact materials. The use of automated analytical protocols has shown promise but is only reliable if sampling location and scale are defined. Results suggest that the sampling protocol reports on the degree of worn surface invasiveness, complicating the ability to investigate duration related textural characterisation.

Confocal laser scanning microscopy and area-scale analysis used to quantify enamel surface textural changes from citric acid demineralization and salivary remineralization in vitro

OBJECTIVES This paper investigates the application of confocal laser scanning microscopy to determine the effect of acid-mediated erosive enamel wear on the micro-texture of polished human enamel in vitro. METHODS Twenty polished enamel samples were prepared and subjected to a citric acid erosion and pooled human saliva remineralization model. Enamel surface microhardness was measured using a Knoop hardness tester, which confirmed that an early enamel erosion lesion was formed which was then subsequently completely remineralized. A confocal laser scanning microscope was used to capture high-resolution images of the enamel surfaces undergoing demineralization and remineralization. Area-scale analysis was used to identify the optimal feature size following which the surface texture was determined using the 3D (areal) texture parameter Sa. RESULTS The Sa successfully characterized the enamel erosion and remineralization for the polished enamel samples (P<0.001). SIGNIFICANCE Areal surface texture characterization of the surface events occurring during enamel demineralization and remineralization requires optical imaging instrumentation with lateral resolution <2.5 μm, applied in combination with appropriate filtering in order to remove unwanted waviness and roughness. These techniques will facilitate the development of novel methods for measuring early enamel erosion lesions in natural enamel surfaces in vivo.