There are several optical principles including image formation, magnification, resolution, brightness/contrast, and depth of field. Image formation deals with how light is reflected off of a sample through a array of lenses to produce an image that can be recognized by the human eye or transferred to a digital format. Magnification is the process of making a image larger by a certain scalar quantity, total magnification depends of the focal length and image distance. Resolution is the ability to differentiate between closely spaced features and is controlled by diffraction effects. Resolution can be improved by decreasing the wavelength of light (use blue light), increasing the refractive index of the medium or the aperture semi-angle (using oil-immersion lenses). Brightness is the intensity of the light that lights up the sample while contrast is the change in intensity of the image. Depth of field is the range of ...
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...liquid is comprised of dendrites and the partially melted zone under the weld material exhibits columnar dendrites which are to be expected as the temperature gradient drives theses dendrites to grow in an orthogonal direction from the partially melted zone to the center of the weld bead. Also, the grain size in these areas are relatively small making for a fine grain structure which is likely due to the fast cooling rate since these are the uppermost regions of the weld. The heat affected zone exhibited grain grown which is due to the amount of heat added by both the TIG and CMT processes and since this region is under the rest of the weld it takes longer to cool which allows the grains to grow. Overall, VLM allowed many characteristics of this welding technique to be observed rather easily which is why it can be a very useful materials characterization technique.
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