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Viewing Microstructures of Materials with the Optical Microscope . Elizabeth Merten and Fumio Ohuchi University of Washington, Seattle Department of Materials Science & Engineering. Processing. Microstructure. Properties. Performance. Key Concepts.
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Elizabeth Merten and Fumio Ohuchi
University of Washington, Seattle
Department of Materials Science & Engineering
Microstructures of a material relate to:
Microstructures for metallic samples can be resolved into individual crystal “grains”
This phenomena can easily be observed in utilizing various characterization tools.
One such tool is the Light Optical Microscopes (LOM)
* variety of grain shape and size may be observed
Optical microscopes are typically utilized for:
Optical microscopes allow us to magnify samples from 1-100X
Micrographs of a Brass sample as well as a schematic of a basic transmitted light
microscope are shown on the following slides
*Reflected light microscope is similar but has the light source entering from above, through the viewing optics
**Details for this process can be found in MatEd module “Brass Hardness”
reflected light microscope has light source entering from above
Figure 1: Basic Optical Microscope*
*Image taken from Wikipedia :http://en.wikipedia.org/wiki/File:Optical_microscope_nikon_alphaphot_%2B.jpg
Figure 1: Micrograph of brass as received, 50x Figure 2: Micrograph of brass 40% cold rolled, 50 x
Can you see the elongation of the grains on the processed sample?
Shortest distance between two points on a specimen that can still be distinguished by the observer
Resolving power of microscopes are the most important feature
Objective Lens—magnifies the object
Eyepiece enlarges the image (does not add to resolution)
Available resolution depends on
*shorter wavelengths are capable of resolving details to a greater degree than are the longer wavelengths
Maximum angle is 90º, so max (sinµ) = 1
n = index of refraction
n (air) = 1.0003
n (water) = 1.33
n (oils) = up to 1.515
Maximum magnification is about 1000 NA, or
Figure 2: Diagram of an angular aperture*
*Image taken from: http://www.microscopy.fsu.edu/primer/anatomy/numaperture.html
Utilizing higher magnification objectives or eye pieces often displays degraded images and poor resolution (similar to missing pixels when a photo is enlarged too much)
Higher magnification also reduces the depth of focus (also called depth of field)
Figure 3: Depth of Focus for a Single Lens *
*Image taken from: http://ion.asu.edu/descript_opt.htm
Visible range of light is 390-760 nm
Electron microscopes utilize electrons
The main concept is that the resolution of the image is directly related to the useful magnification of the microscope and the perception limit of specimen detail.
Wikipedia: Optical Microscope image
Numerical Aperture image
Depth of Focus image