Optical vs Electron Microscope

The purpose of this article is to briefly demonstrate the difference between optical and electron microscopy of a knife blade.  The difference between “sharp” and “dull” will be measured in microns.  The difference between somewhat sharp and very sharp will be measured in fractions of a micron.  To directly “see” these difference with a microscope requires the ability to resolve features at those scales.  The best optical microscopes can resolve down to a few tenths of a micron (hundreds of nanometers) while the best Scanning Electron Microscopes can resolve down to a few nanometers.  While optical microscopes (and loupes) can provide a great deal of information about a blade, it is often indirect, and can be misinterpreted.   The key to using optical magnification effectively is to understand what can and cannot be seen and correlate what we do see with other observations and our knowledge of what can and does happen to the blade during the sharpening process.

In a previous article it was demonstrated that side-view images are of limited value in assessing the apex geometry.  Optical images are very difficult to obtain edge-on at any but the lowest magnifications.   In this article, I compare only side-view images simply to demonstrate where optical microscopy falls short of electron microscopy in this application, and where it can be misleading.

When we examine an object in a microscope, there are two factors that determine what we can see – resolution and contrast.

Resolution is essentially how small of an object we can see or “resolve” – magnification is typically used to quantify resolution, although this number is often misleading.

Contrast is generally more difficult to achieve, and the microscopist will usually employ a variety of sample preparation techniques to control and/or maximize the contrast.  The conditions of illumination and detection (and the design of the microscope) will significantly affect the contrast.

USB Microscope

The following series of images were taken from a single location on the bevel of a straight razor.  The first image is from a USB microscope, although a high quality example, one designed for longer working distance and larger depth of field than for the highest possible magnification.  The exposure was chosen to enhance the region near the apex, resulting in over-exposure further down the bevel.  This may not represent the best possible image that can be achieved with this type of microscope, but it is typical.

When imaging a reflective metal surface, it is necessary to avoid the direct reflection of light from the source into the camera (like taking a flash picture of yourself in the mirror).  This can be avoided by tilting the surface very slightly off normal; however, the light reflected and diffracted from the surface scratches will often dominate the image.  The exposure was set in this image to maximize the information near the apex; however the remainder of the bevel is over-exposed as a result.  In this image, it appears that the bevel is heavily and deeply scratched.

dinolite_usb

Image from a (Dino-Lite AD7013MTL) USB microscope at “90x magnification.”

Optical Inspection Microscope

The following set of images were captured with a very high quality optical inspection microscope.  There are other types of optical microscopes that can achieve somewhat higher resolution and that have other (possibly better) options for generating contrast. This particular microscope features relatively longer working distances (the object further from the objective lens) and larger depth of field (more of the bevel is in focus).

The image from the 10x objective has a comparable magnification to the image from the USB microscope; however, the features are far more clear and smaller features are resolved.  In this image we see that only a small fraction of the surface is scratched, where the USB image gave the impression that the scratches were deeper and more abundant that the actually were.  We also see that there is stropping residue on the blade.

10x-optical

Image from a professional quality optical microscope (Mitutoyo FS70) through the 10x the objective lens and captured with a research quality camera (Zeiss Axiocam ICC5).

20x-optical

Image from a professional quality optical microscope (Mitutoyo FS70) through the 20x objective lens and captured with a research quality camera (Zeiss Axiocam ICC5).

At the highest magnification, the ability to resolve features at the apex is limited by diffraction. We can resolve spots/particles that are approximately 1/2 micron in size.  Diffraction at the apex prevents us from resolving features below the 1 micron scale.

100x-optical

Image from a professional quality optical microscope (Mitutoyo FS70) through the 100x objective lens and captured with a research quality camera (Zeiss Axiocam ICC5).   We reach the limit of resolution at about 1/2 micron.

Scanning Electron Microscope (SEM)

The final set of images were again taken from the same area of the blade as the preceding images but with the SEM.  As with optical microscopy, preparation and imaging conditions will determine what we can see (and those are optimized in these images).

The high resolution and absence of diffraction in these images allows us to see the small foil-burr at the apex, which was not visible with the optical microscope.   These SEM images also provide a clear indication of the depth of the scratches on the bevel.  By any estimation, the scratches are much shallower than suggested by the optical images.

200x-sem

Scanning Electron Microscope (SEM) image at 200x magnification (relative to Polaroid size).

2000x-sem

Scanning Electron Microscope (SEM) image at 2,000x magnification.  The field of view is similar to the highest magnification optical image.

5000x-sem

Scanning Electron Microscope (SEM) image at 5,000x magnification.

10000x-sem

Scanning Electron Microscope (SEM) image at 10,000x magnification.

20000x-sem

Scanning Electron Microscope (SEM) image at 20,000x magnification.

The small foil-type burr is clearly visible at the highest magnifications.

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17 responses to “Optical vs Electron Microscope

  1. This reminds me of a thread I commented on a shave forum. The author called it “Encyclopedia of razor edge images”, and he took a side view image with a USB miocroscope after using various stones/hones in an effort to provide the reader with information about each stones/hones results.

    I read it and replied “these look all the same to me”. He was upset that I’d stated the obvious. I was surprised to see pages upon pages of replies from other forum members saying “wow, great work” etc..when I know that they weren’t learning anything new. I also mentioned that repeating the same action and getting the same result but thinking it’s different is the definition of insane. I was then banned for 3 months for “calling another member insane”.

    I hope that people who Google-search “razor images” or “magnified edge” end up here first rather than the above mentioned thread.

    This website is all you ever need and I’m so glad I found it.

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    • So you went to a straight razor honing forum and expected a rational discussion where constructive criticism is appreciated. You may want to re-evaluate who is the sane person there.

      It is very difficult for most people to comprehend the relevant dimensions. Even as someone who has used Angstroms and nanometers on a daily basis for more than 25 years, I was astonished by how subtle the measured differences are between various DE razor blades. One of the reasons I show images over a range of magnifications is to try to convey just how small these feature are.

      Liked by 2 people

      • The same member swears by the DMT 8000. He in fact advocates it with fury. In another thread, I debate with him about why the 8000 should be thrown in the garbage.

        After seeing his image thread, I then knew why he is hypnotized into thinking the 8000 DMT is great. Empathy ensued.

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  2. C’mon Todd, where is the entry on slurry honing I’ve been waiting for!? Still waiting to see how much the edges get convexed due to slurry honing, and whether some stones are worse than others. Haha, love reading your stuff man, it’s always interesting and informative. Keep up the great work!

    Liked by 1 person

  3. You are probably the first to ever make this (and your other articles) visual comparison available to sharp edge makers. It takes access to the instruments, an interest in sharp edges, your skill in operating the microscopes and your generosity in sharing the information. Thank you.

    I have tried various magnifying glasses, loupes and microscopes, but not an electron microscope. My favorite for looking at edges is a stereo microscope. Mine has high optical contrast and has 6x, 12x, 25x and 50x magnification options. I find that the 12x is the easiest to use, and very informative — particularly after seeing your high magnification SEM images. With your images in mind, the low power 12x gives a clue to what is on the bevel/edge. I use a bright light (quartz halogen machinists light) which shines from the side. I twist and turn the blade to reflect the light source. At 12x, I can easily detect a glinting foil burr from a 1k stone after edge trailing strokes. After an 8k stone, edge trailing stokes, the shiny edge is still visible. After a few strokes on the Mother’s polish strop, the edge “disappears” and the last few microns of the bevel go dark (no reflective scratches), even though the scratches on the bevel sparkle. This is a look I know from looking at Feather blades which appear to be sharpened with a roughly 4k stone and then polished on a strop which I now know is not a secondary bevel as might be seen from adding a layer of tape to a razor spine, changing the angle of the bevel near the edge.

    I have recently bought a Coticule. At 12x, the scratch pattern on the bevel looks very different from that off a 10k Gokumyo synthetic stone. The Coticule bevel looks “misty” and the Gok makes a very regular and crisp scratch pattern.

    Too sharp? I made a razor edge too sharp! The razor is worn and the edge angle too acute. I did not micro-convex the edge on your Mother’s polish denim strop. After stropping on a clean strop, the first two slices into the whiskers were very smooth, but the third already felt worse. A few laps on a clean strop allowed another slice or two. I returned to a 10k Gokumyo stone for a few laps, followed by the Mother’s denim strop, 0.25µm cBN strop and clean strop. This “dulled”, or micro convexed the edge enough to make it durable for a shave.

    Magnifying glasses, loupes and USB microscopes were very not helpful for me. My stereo microscope is a great informer. It’s simple, but only after seeing the complexity of your SEM images. Looking at the results of sharpening explains feeling the results.

    I, too, hope you will explain more about slurries. I have found that using an 8k slurry on a 10k stone is very helpful, thinning it out to finish, as used in a Coticule Dilucot method. I have also been experimenting with an Arkansas slurry, using a smaller piece of Arkansas to make the slurry.

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    • Overall, I find the stereo-zoom microscope to be the most useful for evaluating sharpening progress, usually at the lowest zoom setting. The ability to tilt the blade under the objective to see how the light plays off the surface can tell you most of what you need to know.

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    • This particular USB is in the range of $1k, but I expect more affordable versions would perform just as well for this type of imaging. This one is most useful as an alternative to an SLR camera with a macro lens.

      The optical microscope is in the range of $50k.

      Each of the SEM images requires approximately 3 minutes and the research rate is $55/hr.

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      • This stuff is BA!
        Will you take images for pay? Are you saying you can do several images within an hour? If so, about how many? A YT video of you using this would be awesome.

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      • These SEM images take about 60 seconds to collect. Including the time required to position the sample and make fine adjustments to the focus and contrast, it takes 2-3 minutes total per image.

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  4. Thank You for tese last updates, hyper interesting as usual!!!

    It is worth noticing that those very foil burrs could be spotted by the diffraction pattern with the optical equipment, and then confirmed with the SEM imaging!!

    Like

    • Yes, that’s one of the reasons I chose this particular blade edge for demonstration. The foil edge has some extra material beyond the nominal line of the apex, as well as some regions that are bent off center. Both of those cause some variation in the diffraction pattern. It will be more challenging to see this if there is debris or oil at the apex.

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  5. I wonder if we would still notice the foil burr from the diffraction pattern in a different case, if the foil burr would be not fragmented but continuos all edge long.
    I bet it is more easy to catch discontinuity, amplifyied by the diffraction interferences

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    • Foil burrs are usually misaligned, for example bent away from the last side that last touched the strop. It’s also quite rare that they are uniform in size along the length of the blade. I would expect they are often mistaken for micro-chips.

      We can always get more information by watching as we change the focus, or by tilting the blade.

      Like

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