Imaging confocal microscopy (ICM) and focus variation (FV) are two of the most used technologies for 3D surface metrology. Both methods rely on the depth of focus of the microscope objective, which depends on its numerical aperture and wavelength of the light source to compute an optical section. In this paper we study how several methods of structured illumination microscopy affect the metrological characteristics of an areal optical profiler. We study the effect of the projection of different structured patterns, the sectioning algorithms, and the use of high and low frequency components onto the optically sectioned image. We characterized their performance in terms of system noise, instrument transfer function and metrological characteristics such as roughness parameters and step height values.
Confocal microscopes are widely used for areal measurements thanks to its good height resolution and the capability to
measure high local slopes. For the measurement of large areas while keeping few nm of system noise, it is needed to use
high numerical aperture objectives, move the sample in the XY plane and stitch several fields together to cover the
required surface. On cylindrical surfaces a rotational stage is used to measure fields along the round surface and stitch
them in order to obtain a complete 3D measurement. The required amount of fields depends on the microscope’s
magnification, as well as on the cylinder diameter. However, for small diameters, if the local shape reaches slopes not
suitable for the objective under use, the active field of the camera has to be reduced, leading to an increase of the
required number of fields to be measured and stitched. In this paper we show a new approach for areal measurements of
cylindrical surfaces that uses a rotational stage in combination with a slit projection confocal arrangement and a highspeed
camera. An unrolled confocal image of the cylinder surface is built by rotating the sample and calculating the
confocal intensity in the centre of the slit using a gradient algorithm. A set of 360º confocal images can be obtained at
different heights of the sample relative to the sensor and used to calculate an unrolled areal measure of the cylinder. This
method has several advantages over the conventional one such as no stitching required, or reduced measurement time. In
addition, the result shows less residual flatness error since the surface lies flat in the measurement direction in
comparison to field measures where the highest slope regions will show field distortion and non-constant sampling. We
have also studied the influence on the areal measurements of wobble and run-out introduced by the clamping mechanism
and the rotational axis.
KEYWORDS: 3D metrology, Imaging systems, 3D scanning, Confocal microscopy, 3D metrology, Manufacturing, Optics manufacturing, Microscopes, 3D modeling, Interferometry, Inspection, Optical testing, Astatine, Image fusion, Data fusion, Signal to noise ratio, Sensors, Cameras, Objectives, 3D image processing
The most common optical measurement technologies used today for the three dimensional measurement of technical surfaces are Coherence Scanning Interferometry (CSI), Imaging Confocal Microscopy (IC), and Focus Variation (FV). Each one has its benefits and its drawbacks. FV will be the ideal technology for the measurement of those regions where the slopes are high and where the surface is very rough, while CSI and IC will provide better results for smoother and flatter surface regions. In this work we investigated the benefits and drawbacks of combining Interferometry, Confocal and focus variation to get better measurement of technical surfaces. We investigated a way of using Microdisplay Scanning type of Confocal Microscope to acquire on a simultaneous scan confocal and focus Variation information to reconstruct a three dimensional measurement. Several methods are presented to fuse the optical sectioning properties of both techniques as well as the topographical information. This work shows the benefit of this combination technique on several industrial samples where neither confocal nor focus variation is able to provide optimal results.
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