The maximum slope that a microscope objective can measure is an important parameter characterizing the measurement capability of 3D optical microscopes. It is one of the most important criteria for selection of appropriate optical topography measuring instruments for areal surface texture measurements and the setting as well. In this article, a method is proposed using optically smooth spheres for characterization of the maximum measurable slope by optical topography measuring instruments with different objectives. The material measure and the measurement procedure are described and the method for the calculation of the measured sphere radius, the maximum measurable local slopes and characterization of the homogeneity of the slope transfer function within the FOV of the objective measured by a confocal microscope are presented.
Optical surface topography measuring instruments are used more and more widely for surface quality control in industry by enabling fast, areal and non-destructive surface topography measurements. However, due to the complexity of the interaction between the surface properties to be measured and the measuring system, their capability to accurately reproduce topographical features of a surface under test is quite often questionable. To understand and investigate the topographic measurement accuracy of different optical surface topography measuring instruments, a physical measurement standard has been developed at PTB which is intended to be used to determine the metrological characteristics of surface topography measuring instruments such as topographic spatial resolution and topography fidelity. The physical standard, fabricated by a diamond turning process, containing nine sinusoidal structures with different amplitudes from 50 nm to 10 μm and varying spatial wavelengths from 2.6 μm to 82.8 μm, is suitable for the characterization of optical instruments with different magnification and numerical apertures. The design of the chirp structures, including the wavelength series, the smallest wavelength for different amplitudes, the slope distribution and the layout are detailed in this paper. The tool path for accurately positioning the cutting tool in fabrication is also described. First measurement results of the instruments response in terms of the features’ aspect ratio, slopes and curvatures, the homogeneity of the field of view of a confocal microscope are presented.
The topography fidelity TFi indicates the accuracy of the estimation of the real surface, describes the instrument influenced deviation of a measured topography image and depends on the interaction of the surface topography with the instrument. To understand and investigate the topography fidelity of optical surface measurement instruments, and interference microscopes in particular, an analytical model based on the fraction of the total illumination criteria and a numerical model based on Richards-Wolf theory are used to characterize the topography fidelity of 3D optical microscopy. As reference artefacts step-like micro-structures with varying spatial frequency and therefore different aspect ratios are numerically investigated with the aforementioned models. To testify the feasibility of the numerical analysis, a commercial white light interference microscope has been employed to measure these reference artefacts. The relationship between the measured heights and the spatial frequency of the samples under investigation are detailed in this paper. The aspect ratio influences on measurement results predicted by the simulation models and the agreements with the experimental results are investigated and reported in detail.
Imaging Confocal Microscopes (ICM) are highly used for the assessment of three-dimensional measurement of technical surfaces. The benefit of an ICM in comparison to an interferometer is the use of high numerical aperture microscope objectives, which allows retrieving signal from high slope regions of a surface. When measuring a flat sample, such as a high-quality mirror, all ICM’s show a complex shape of low frequencies instead of a uniform flat result. Such shape, obtained from a λ/10, Sa < 0.5 nm calibration mirror is used as a reference for being subtracted from all the measurements, according to ISO 25178-607. This is true and valid only for those surfaces that have small slopes. When measuring surfaces with varying local slopes or tilted with respect to the calibration, the flatness error calibration is no longer valid, leaving what is called the residual flatness error.
In this paper we show that the residual flatness error on a reference sphere measured with a 10X can make the measurement of the radius to have up to 10% error. We analyzed the sources that generate this effect and proposed a method to correct it: we measured a tilted mirror with several angles and characterized the flatness error as a function of the distance to the optical axis, and the tilt angle. New measurements take into account such characterization by assessing the local slopes. We tested the method on calibrated reference spheres and proved to provide correct measurements. We also analyzed this behavior in Laser Scan as well on Microdisplay Scan confocal microscopes.
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