Spectral analysis of the finish of polished optical surfaces

Surface finish is a critical factor in many high performance optical components with major limitations arising from scattering due to topographic surface roughness. Scattering theory shows that the relevant statistic of the residual surface height fluctuations is their power spectral density. In this paper, methods of estimating such spectra from surface profile measurements are described and are illustrated by the analysis of mechanical stylus measurements of several polished surfaces which have nanometer height fluctuations over the surface wavelength range of about 10⁰–10³ μm.     

Assessment of surface finish on bulk scattering materials: A comparison between optical laser stylus and mechanical stylus profilometers

The accuracy of autofocusing laser stylus measurement systems based on the compact disk pick-up technique has been discussed in recent years, in particular for surface roughness measurement in the submicron-micron range. In this article we present a thorough comparison between an optical laser stylus measurement system and a Talystep mechanical stylus profilometer. Sixteen surfaces having root mean square roughnesses from 0.002 μm to 3 μm for cut-off lengths <0.8 mm have been used in the study. Both bulk scattering (paper, white ceramic) and surface scattering (metal sheet) types of samples were included. Our study shows that the laser stylus exaggerates the surface profile, in particular on surfaces having laterally small but steep local slopes. For the paper surfaces the laser stylus data were on average off by a factor of two for a cut-off length of 0.68 mm.     

A new optical technique for roughness measurement on moving surface

A new optical technique which allows the roughness of moving surfaces to be determined was developed. The new technique which is called the dark/bright ratio (DBR) method utilizes the combined effects of speckle and scattering phenomena. The roughness of surfaces is inferred from the dimensions of the recorded dark or bright area in the speckle pattern. Although it is a relative method, it has great potential to be used for in-process measurement and automation owing to the simplicity of both its principle and required optical set-up. The new technique has also been proved to have large measuring range and with high precision. The principle of this technique and the set-up of the measuring system are described. Experimental results for both static and dynamic conditions, which were compared to those obtained using the traditional stylus technique, were found to be in good agreement. The reliability of the new technique in obtaining roughness data of surfaces under various speed conditions (from 0 to 0.017 m/s) was validated.     

Microtopographic analysis of turned surfaces by model-based scatterometry

Turned surface profiles can be divided into a periodic and a random component. The periodic component is a function of the tool shape and the feed rate. The random component is caused by machine vibrations. To monitor the turning process, the periodic component is the essential global microtopographic feature which reacts sensitively to tool wear and tool scars. A measuring method evaluating the periodic component of turned surfaces is developed based upon the model-based scatterometry. Using an optimized scattering geometry, the model-based scatterometry leads to an intensified mapping of the microtopographic surface features in the angular distribution of scattered light. The profile parameters measured with the introduced scattering technique agree well with the values obtained by stylus measurements. Hence, in the range of precision engineering (R_a≤1.5 μm) the mean profile of the dominant periodic component can be measured optically without scanning and without resorting to comparator standards.     

Characterization of high-quality surfaces by Nomarski microscopy and light scattering

Supersmooth mirror substrates for laser-gyro applications were investigated. Bare samples made of BK7 glass, fused silica, and different silicon wafers were examined. Nomarski differential interference contrast microscopy was used to characterize the quality of the surfaces using a specially selected microscope objective with ultralow internal light scatter. The microscope system resolved a root mean square roughness in the range of 0.05 nm. Photographs of these supersmooth surface structures are presented. To obtain additional information, the total integrated light scattering of the bare substrates was measured. An Ulbricht integrating sphere with a diameter of 0.125 m was used for this purpose. Unwanted light scatter by the rear side of the transparent substrate was blocked using a special diaphragm. A specially developed polishing process that yields surfaces with a roughness of about 0.1 nm and probably lower is discussed.     

Design of an optical probe for testing surface roughness and micro-displacement

This paper presents a practical monitoring tool for measurements of surface roughness and micro-displacement. An optical probe of the methods based on light scattering for measuring surface roughness and optical triangulation for measuring micro-displacement is described. The proposed technique allows evaluation of surface roughness and micro-displacement of specimen by using just one device. The theoretical models of surface roughness and micro-displacement measurements have been established for the probe. The measuring principles applied in the design are described in detail and the validity of the design is demonstrated by experimental evaluations. The experimental results show that, for specimens with surface roughnesses R_a in the range from 0.005μm to 0.1 μm, micro-displacement measurements in the linear range of ± 300μm can be obtained.     

Comparison of optical and stylus methods for measurement of surface texture

Optical methods are increasingly used for measurement of surface texture, particularly for areal measurements where the optical methods are generally faster. A new Working Group under Technical Committee (TC) 213 in the International Organization for Standardization is addressing standardization issues for areal surface texture measurement and characterization and has formed a project team to address issues posed by the optical methods. In this paper, we review the different methods of measuring surface texture and describe a classification scheme for them. We highlight optical methods and describe some of their characteristics as well as compare surface-profiling results obtained from three optical methods with those obtained from stylus profiler instruments. For moderately rough surfaces (Ra?≈?500 nm), roughness measurements obtained with white light interferometric (WLI) microscopy, confocal microscopy, and the stylus method seem to provide close agreement on the same roughness samples. For surface roughness measurements in the 50 to 300 nm range of Ra, discrepancies between WLI and the stylus method are observed. In some cases the discrepancy is as large as about 75% of the value obtained with the stylus method. By contrast, the results for phase shifting interferometry over its expected range of application are in moderately good agreement with those of the stylus method.     

Uncertainty analysis of roughness standard calibration using stylus instruments

A stylus instrument was characterized and calibrated, including a dynamic calibration of the probe. This stylus instrument was used to calibrate ten roughness standards for six surface roughness parameters. The sensitivity of each parameter of each standard to such measurement conditions as stylus geometry, measurement force, cut-off wavelength, and so forth was determined experimentally. These results were used for an uncertainty evaluation of each parameter for each roughness standard. It is shown that the manufacturers’ specification for the stylus instrument (2% uncertainty in roughness parameters) is approximately correct for the most commonly used samples and parameters, but the uncertainty may range from 0.03% (for sinusoidal profiles) to 100% (for very fine surfaces), depending upon the standard and parameter to be calibrated.     

Measurement of surface roughness on rough machined surfaces using spectral speckle correlation and image analysis

A speckle pattern is formed when a rough surface is illuminated with coherent light. The properties of this pattern can be used in the calculation of roughness parameters. Spectral speckle correlation (SSC) is a technique applicable to the measurement of roughness on rough machined surfaces. This paper presents the SSC obtained from measurements on specimens with a surface roughness in range R_a = 0.5–5 μm. The measurement results correspond to reference measurements made using a stylus instrument, Form Talysurf.     

Characterization of micromachined mirror surfaces from high speed diamond fly cutting

The measurement of the surface finish of diamond “fly-cut” mirrors is discussed. By considering each surface as a series of superimposed sinusoidal gratings, the surface spectral density function (SDF) was calculated from measurements of light scattered from an He-Ne laser beam. The SDF allows examination of the roughness as a function of spatial frequency from which the r.m.s. roughness is calculated. The scattering results are compared with other methods of assessing the surface finish of the mirrors. Results from Talystep measurements compare favourably with those from the lightscattering technique for the smooth copper mirrors, the best of which had a total r.m.s. roughness of 33 Å. The apparatus developed is described. Problems encountered with stylus methods, interferometry and scanning electron microscopy are reported.     

On the Use of Silicon Rubber Replica for Surface Topography Studies

Silicone rubber is often used to obtain replica of surfaces which are hard to study directly. Here we discuss under what conditions engineering stylus measurements can be used to obtain the correct surface roughness from the replica. We also show how thin vertical slices of the silicone rubber can be used to obtain the surface roughness power spectra for the long-wavelength roughness on even very rough surfaces, such as road surfaces, which may be hard to probe directly with standard optical or stylus instruments.     

Laser optical roughness measurement

A method for non-contacting profile assessment and roughness measurements of engineering surfaces is presented. The well known effect of focusing a light beam on the surface is used, at the same time compensating for the effect on the measuring results of varying material colours by appropriately processing the measuring signal. The sensor and the transducer are connected with the data-processing unit of a stylus instrument, which provides for a simple procedure of recording the roughness profile and calculating standard roughness parameters from it. The measuring results thus obtained agree well with the results of traditional stylus instruments.     

Comparison of tribological parameters of surfaces determined by the stylus method and by the immersion method of holographic interferometry

The investigation of surface roughness by the use of the improved immersion method of holographic interferometry is reported in this paper; the results obtained for some engineering surfaces are given.The application of the holographic microscope, specially designed imersion cell and transparent replicas that reproduce details of the differently machined engineering surfaces allows us to obtain contour maps of the surface asperities with intervals between contour lines of about 1 μm.The contour maps were used for determining the tribological parameters of the surface which were compared with those obtained by the use of stylus methods.It is shown with a few examples that the roughness parameters determined from the contour maps have greater values than those obtained by the profilometry.     

光学表面X射线异常散射现象

研究了用掠入射散射法测量光学表面散射分布实验中存在的异常散射现象。首先,介绍了实验装置,并用原子力显微镜(AFM)测量了样品的表面粗糙度, 给出了工作波长为0.154 nm时不同样品在不同掠入射角下的表面散射分布。然后,分析了异常散射角与临界角的关系。最后,对影响散射强度的因素进行了分析。实验结果表明：当掠入射角大于临界角时能观测到光学表面的异常散射现象。在波长一定的情况下,异常散射角与样品材料有关,与掠入射角和表面粗糙度无关;异常散射角略小于临界角,误差变化为-8.6%~-0.9%。另外,镜像反射强度随着入射角和表面粗糙度的增大而迅速减弱,但异常散射强度与镜像反射强度的比值（峰值比）反而随着掠入射角或表面粗糙度的增大而增大,其比值在0.012~2.667变化。结果证明样品的材料和表面形貌是影响异常散射分布的两个重要因素。     

Surface roughness classification using image processing

Surface roughness is an important factor in determining the satisfactory functioning of the machined components. Conventionally the surface roughness measurement is done with a stylus instrument. Since this measurement process is intrusive and is of contact type, it is not suitable for online measurements. There is a growing need for a reliable, online and non-contact method for surface measurements. Over the last few years, advances in image processing techniques have provided a basis for developing image-based surface roughness measuring techniques. Based upon the vision system, novel methods used for human identification in biometrics are used in the present work for characterization of machined surfaces. The Euclidean and Hamming distances of the surface images are used for surface recognition. Using a CCD camera and polychromatic light source, low-incident-angle images of machined surfaces with different surface roughness values were captured. A signal vector was generated from image pixel intensity and was processed using MATLAB software. A database of reference images with known surface roughness values was established. The Euclidean and Hamming distances between any new test surface and the reference images in the database were used to predict the surface roughness of the test surface.     

A controlled-force stylus displacement probe

A novel design of displacement profiler with a controllable stylus force is presented. It provides highly controlled conditions for contact measurement of, for example, small step heights or surface roughness. Incorporating an electromagnetic force actuator and force feedback control, the profiler provides electronically selectable contact force in the range of 0.01–10 mN and gives a constant static and dynamic loading. In a typical configuration, it has a range of a few micrometers with a discrimination to better than 1 nm at a bandwidth higher than that of a conventional stylus instrument.     

利用功率谱密度函数表征光学薄膜基底表面粗糙度

光学表面的表面粗糙度通常利用两个传统参数方根粗糙度σ和相关长度l来进行表征。主要就如何引入功率谱密度函数(PSD)表征表面微观形貌进行了初步研究。说明了一维和二维功率谱密度(PSD)函数的数学计算方法、PSD函数的物理意义,同时给出了PSD函数与传统的表面评价指标σ和l之间的关系。利用不同仪器对多种样品进行测试,在分析比较测试结果的基础上,总结了利用PSD函数评价光学表面粗糙度的优点。功率谱密度函数作为一个全面的光学表面评价参数,正得到越来越广泛的重视和应用。     

Reliability of roughness measurements using contact stylus instruments with particular reference to results of recent research at the Physikalisch-Technische bundesanstalt

Even today international comparison measurements of surface roughness show differences of 40% and more, as they did more than 10 years ago. However, within the framework of the Deutscher Kalibrierdienst, differences occurring in comparison measurements do not exceed 5%. At the end of 1982, a project of the European Communities was started for the purpose of also carrying out comparison measurements yielding small differences. The prerequisites are the following: clearly defined surface roughness parameters including details of the measurement conditions required and calibration and testing of the contact stylus instruments before the comparison measurements are made.In recent years the contacting process, the interaction between stylus and surface, has been the subject of very thorough investigations at the Physikalisch-Technische Bundesanstalt. Elastic and plastic deformations of the surface were examined. The parameters of influence are the geometry of the stylus tip and of the surface under examination, the properties of the materials, the static and dynamic measuring forces and the rate of feed. Calculation methods for optimizing these parameters are stated in order to keep the resulting measurement errors as small as possible. A new contactless measuring method is described which is based on the evaluation of scanning electron microscopy stereo pairs. An error analysis makes precision measurements possible which up to now could not be carried out at all. Examples examined have shown that sometimes, particularly on finely ground surfaces, measurements with contact stylus instruments on surfaces with R_z values below 1 μm can be very significantly in error.     

An analysis of the reference lines of the surface profile and its true replica

The roughness of surfaces normally not accessible by conventional stylus instruments is often measured by a replica technique. The replica surface is the mirror image of the original surface. This paper deals briefly with an analysis of the reference lines used in practice for roughness measurements from surface profiles of original as well as replica surface. A statistical analysis of surfaces produced by different processes and of their true replicas is reported.     

Roughness measurement with a laser scanning analyser

In the laser scanning analyser a laser beam is reflected from a rotating polygonal mirror so as to scan the workpiece surface at high speed, where it is again reflected into a photodetector which measures the reflected intensity. If the surface is rough, in addition to the specular reflectance the diffuse reflectance due to diffraction is measured as a function of the off-specular angle. In effect the system when used in this mode is a glossmeter which measures the optical properties of a section through the surface and is thus highly suitable for comparison with a stylus instrument. Experiments are described in which reflectance measurements on surfaces finished by various machining processes are compared with stylus roughness measurements. It is found that reflectance measurements are sensitive both to roughness and to the orientation of the lay. Reflectance measurements are found to vary as the average roughness and as the fourth power of the profile slope. There is no correlation with curvature and no evidence of the effect of longer surface wavelengths. There is some evidence that two different scattering mechanisms are involved. Possible applications of such a real-time non-contacting roughness measuring system are described.