Lens Image Aberrations in Feptem Techniques
In steel and iron metallography, Feptem (FER and Porous Transferred Electron Microscopy) technologies are used to study the microstructure and phases of materials through the transmission electron microscopy (TEM) technique. This process requires that a thin sample of specimen is placed between two electrodes and subjected to an electric current. As the electron beams pass through the sample, an image of the inside of the object is created on the viewing area of the microscope. The quality of the generated images is, however, dependent on the performance of the microscope’s lens assembly. In specific, lens image aberrations can be a major source of distorted images of the internal structure of the tested specimen.
Image aberrations are defects or variations of the original image caused by the inability of the lens’ optical components to accurately focus the light emanating from the viewfield on the image plane. Depending on the underlying causes, image aberrations can be classified into the categories of spherical aberration, chromatic aberration, astigmatism, coma, curvature of field and distortion.
Spherical aberration is one of the major types of optical aberrations that affect the quality of the images obtained from Feptem techniques. This phenomenon occurs when all of the beams originating from a single point in the object plane do not arrive at the same point on the image plane after passing through the magnetic lenses. This can happen when the lenses are not properly aligned at the appropriate optical distances. As a result, the image resolution on the image plane of the microscope is reduced due to the lower contrast between lens elements and the effect of overlapping of adjacent beams in the image plane causing blurring of the image.
Chromatic aberration is another important source of optical aberrations in Feptem images. This aberration occurs when different wavelength components of the light passing through the lens assembly arrive at different point on the image plane. This is due to the lenses not having a uniform refracting index for light of all wavelengths. This is especially problematic for Feptem technique as the electron beam being used for image formation typically has multiple wavelength components, resulting in severe distortions and degradation of images.
Astigmatism is another type of distortion occurring in Feptem images. This happens when principal points of focused light arrive at different points in two different principal planes. The principal planes are orthogonal to each other, hence any aberrations in one plane are likely to affect the other. This aberration results dominantly from aberrations of optical components such as lenses and allows the same amount of chromatic aberration.
Coma and curvature of field aberrations are yet other important aberrations affecting the quality of Feptem images. These are mainly caused by the lens components being positioned in incorrect angles or at incorrect distances. These aberrations cause the light in the object plane to spread out spherically when it passes through the lenses, resulting in the entire image being distorted and blurred.
Finally, distortion is the last type of aberration resulting from Feptem techniques. The origin of this defect lies in the centration errors of the objective lenses which leads to different magnification of image parts and further misalignment of the object spots in the image plane. This results in crooked lines, rotations, deformations and jagged edges of the images.
In general, aberrations in Feptem techniques act to degrade the imaging quality of the samples studied by this process. As such, it is vital for metallographers to be cognizant of these aberrations that can be introduced by the optical components of the microscope. This helps them to identify any possible sources of these aberrations and minimize their effects on the quality of the images generated. By doing so, they can ensure that the images generated are as accurate and representative of the sample’s internal structure as possible.
