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Natural Science Forum / Physics / Optics / December 2005features visible at low magnification are not "clear" at high magnification
Dear microscopist, I think i have a relatively simple problem for the experts in optical microscopy. I'm looking at indentation in hard ceramics [opaque material] using simple optical microscope with x10,x20,x50,x100 using yellow light, observing in reflection mode. The problem I have is at low magnification x10 around the indents a ring like feature is visible, however at high magnification x50 or x100 the features is not visible. The lateral size of the indents ranges from 30um to 200um. measurements by different techniques (AFM) the vertical depth of indents range from 2um - expected 15ums. The ring like features ranges in height from few(10-50) nanometers to about 1ums. I would appreciate comments, literature, references etc. explaining this effect. since, images can not be posted, i would gladly give images for indpeth response. thanking you for the help. Warm regards Sanjiv What type of microscope are you using? Stereozoom or compound microscope? Does your EPI illuminator support Koehler type illumination? What does it mean 10x, 20x, 100x? Please list the resolution, not the magnification since magnification is not too relevant to microscopists. Resolution and contrast are the bread and butter. For instance, if you increase magnification by just increasing the magnification of your oculars you will NOT increase resolution. Hence an image that appears crisp and sharp at lower magnification suddenly appears washed out. Gregor > Dear microscopist, > [quoted text clipped - 23 lines] > Warm regards > Sanjiv Just in case: http://www.microscopyu.com/articles/formulas/formulasindex.html and especially http://www.microscopyu.com/articles/formulas/formulasresolution.html Gregor > What type of microscope are you using? Stereozoom or compound microscope? > Does your EPI illuminator support Koehler type illumination? What does it [quoted text clipped - 34 lines] >> Warm regards >> Sanjiv I can't tell you much about the "why's", but here is a trick about "how to make it visible": you just vaporize a very thin layer of carbon with a vacuum sputtering machine, such as used by SEM and EMP techniques. Your indentation will become neatly visible under reflected light microscopy. J.J. > Dear microscopist, > [quoted text clipped - 23 lines] > Warm regards > Sanjiv >I can't tell you much about the "why's", but here is a trick about "how to >make it visible": you just vaporize a very thin layer of carbon with a >vacuum sputtering machine, such as used by SEM and EMP techniques. Your >indentation will become neatly visible under reflected light microscopy. > >J.J. I do this all the time. However, I use a coating of Au/Pd. The coating makes a big difference when viewed optically. Under SEM, it makes little difference. Gary Gaugler, Ph.D. Microtechnics, Inc. Granite Bay, CA 95746 916.791.8191 gary@microtechnics dot com >>I can't tell you much about the "why's", but here is a trick about "how to >>make it visible": you just vaporize a very thin layer of carbon with a [quoted text clipped - 6 lines] >coating makes a big difference when viewed optically. Under SEM, >it makes little difference. As a note, this technique is used for examinining stress cracks, separatation fractures and other issues that do not show up monumentally under SEM. The thin (80A) coating really makes a difference. Gary Gaugler, Ph.D. Microtechnics, Inc. Granite Bay, CA 95746 916.791.8191 gary@microtechnics dot com I suppose the boundary of the indentation becomes more 'stretched' with higher mag, therefore less easy to detect. Compare with the more contrast rich image from a lower mag eyepiece. HTH, Rene. > Dear microscopist, > [quoted text clipped - 23 lines] > Warm regards > Sanjiv Sounds like its time to use reflected dark field. You might have to upgrade your microscope. Kevin Cunningham SMS Kevin is correct. But you may not have to have darkfield objectives. The basis of epi-darkfield is lighting at an oblique angle from the side. You may be able to duplicate the effect by turning off the main light source and using another light source aimed at the specimen from an angle outside the objective. Aaron >> Dear microscopist, >> [quoted text clipped - 29 lines] >Kevin Cunningham >SMS >Dear microscopist, > [quoted text clipped - 23 lines] >Warm regards >Sanjiv If you can tolerate destructive action on your sample, sputter coating with 60A Au/Pd will make the specimen look quite different under LM. Gary Gaugler, Ph.D. Microtechnics, Inc. Granite Bay, CA 95746 916.791.8191 gary@microtechnics dot com Hi Gary, Doubtless you are correct. However, in this case, I get the feeling that the equipment to sputter coat the specimen is just as out-of-reach as the darkfield epi- objectives. I wonder if another type of coating might do the same thing, say a thin coating of paint or perhaps a dilute carbon balck suspension. Aaron >>Dear microscopist, >> [quoted text clipped - 32 lines] >916.791.8191 >gary@microtechnics dot com >Hi Gary, > [quoted text clipped - 5 lines] > >Aaron Aaron: The coating is only a few tens of Angstroms thick. Another type??? I don't think so. Too bad, however. The ability of the sputter coat to adhere to and enhance morphology is quite unique and stunning. A regular specimen w/o coating is likely to be near impossible to evaluate. With just a thin coating, the whole scene changes. This is a key tool in forensics analysis. In this case, forensics includes metallurgical failure. Alumina is a big reflector. The coating stops this and yields many LM data points. Then, in the SEM, even more is found. After that, image processing produces even more data...if it is there to be found. If not coated, VPSEM EBSD can show grain anomalies that otherwise would never be found or noticed. The challenge is to sort out what the EBSD data says. Not always trivial. Just for the record, one must not coat alumina with Au/Pd or other metals to get good EBSD patterns. This will not happen. Thin C is preferred. The other option is VPSE. Gary Gaugler, Ph.D. Microtechnics, Inc. Granite Bay, CA 95746 916.791.8191 gary@microtechnics dot com |
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