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Scanning Electron Microscopy


Figure 6 : Slopes from shading of a detail from a euro coin. Image (a) shows the height map, obtained from the integration of slopes, in the false colors of the rainbow scale. Image (b) shows a 3D mesh derived from the height map. Image (c) shows the 3D mesh combined with the original SEM image. Here purple has been assigned to one pair of detectors and blue assigned to the other. Image (d) shows the same image but with yellow and red allocated to the two pairs of detector signals, producing a brass effect.


from the least square polynomial of the 2nd degree, that is, a function of type g(x,y) = ax²+ by² + cxy + dx + ey +f, where g(x,y) represents the gray level at the pixel (x,y) and the coeffi cients a to f give the best fi t between g(x,y) and the actual image pixels. T is subtraction fl attens the global shape of the function without changing the local contrast. T en, since the specimen material here is of a single phase and the surface is lighted from the right side, the gray level can be interpreted as -∂z/∂x (the slope from right to leſt ). Next this slope can be integrated, as for the four-quadrant detector in the previous example, producing a height map. From then on, the process is the same as for Figure 6 : a 3D display is generated, and the heights are color-coded using a false-color scale ( Figure 7b ). T is 3D reconstruction from a single SEM image can only be obtained under certain conditions. T e surface of the specimen must be of homogeneous composition and not overly porous. Again, the height values perceived using this technique do not have metrological value, but they may be useful in visual interpretation of the image.


Image segmentation and 3D


Figure 7 : SEM image of spots on a ladybug. (a) Original SEM image exhibiting image noise and a strong shadow effect at the left. Lighting homogeneity was restored by the software before beginning the reconstruction. (b) Same image rendered in 3D, using only the information contained in the fi rst image. Image courtesy of Chris Supranowitz, University of Rochester.


rendering . It is an advantage and a convenience to be able to work with a single image, particularly with images downloaded from the internet. T e “single image reconstruction” operator in MountainsMap uses diff erent methods depending on the image type to produce the 3D rendering, including shape-from- shading integration (ladybug example of Figure 7 ) and object-oriented segmentation (in Figures 8 and 9 ).


Figure 8 : Secondary electron SEM image of lanthanum hexoboride nanoparticles. (a) Original image, (b) height map, and (c) fi nal image. The latter resulted from operation of “single image reconstruction” in MountainMap™ SEM and shows a 3D effect with color rendering. Image courtesy of SkySpring Nanomaterials Inc.


16 www.microscopy-today.com • 2018 May


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