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