PPT - Materials Education

Elizabeth Merten and Fumio Ohuchi
University of Washington, Seattle
Department of Materials Science & Engineering
Microstructures of a material relate to:
materials properties
mode of processing*
materials performance
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:
transparent samples (transmitted light microscopes)
metals such as brass, we must use a reflecting-type microscope*
Optical microscopes allow us to magnify samples from 1-100X
Metal surfaces require special polishing and etching in order to resolve grains**
Photographs of microstructures are often called “micrographs”
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”
Objective lens turret
Objective lens
Coarse adjustment knob
Fine adjustment knob
Stage for holding the
Light source
reflected light microscope
has light source entering
from above
Figure 1: Basic Optical Microscope*
Light adjustment
diaphragm, filters and
*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?
Image resolution
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
influences the ability of the system to distinguish between fine details
Objective Lens—magnifies the object
Eyepiece enlarges the image (does not add to resolution)
Available resolution depends on
 Magnification of the objective lens
 Uniform illumination of the sample
 Numerical aperture (ability of the lens to collect light and resolve
 Wavelength of light *
*shorter wavelengths are capable of resolving details to a greater degree than are the longer
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
Figure 2: Diagram of an angular aperture*
Maximum magnification is about 1000
NA, or
 1000x in air
 1500x in oils
*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)
Higher depth of focus
Shorter wavelengths
Low Magnification
Poor depth of focus
High Magnification
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
Resolving power of a light microscope is roughly 0.2 µmeters
Under typical conditions it is quite difficult to obtain an image down
to this resolution
Electron microscopes utilize electrons
Resolves images up to 10-10 meters!
Obtain images with increased magnification and depth of focus
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

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