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How to spot astigmatism in Scanning Electron Microscopy (SEM) images

You may have heard of astigmatism as a medical condition that causes visual impairment in up to 40% of adults [1], but how is this applicable to electron microscopy? First of all, let’s talk about what the word astigmatism, in fact, means: It is derived from the negative prefix ‘a’ (without) + ‘stigmat-’ (mark, or point, in Ancient Greek) + ‘ism’ (condition). In a perfect optical system, a lens has only one focal point, and is stigmatic. When the lens has more than one focal point, however, we refer to the lens as being astigmatic. This happens when the lens is elongated in either the sagittal (y-axis) or tangential (x-axis) plane, resulting in two focal points (= foci).

In electron microscopy, astigmatism arises due to imperfections in the lens system. At high magnification, the imperfections become more apparent, hampering the quality of your images. As a result, round objects might appear elliptical or out of focus. Fortunately, electron microscopes have a component called a stigmator, which is used to rectify the problem.

In this blog, I want to show you what astigmatism looks like. This example is ideal if you image metal spheres.

How to spot astigmatism in a scanning electron microscopy image

The image below shows the tin spheres, deposited on carbon, at low magnification (1500 ×, field of view 179 µm). There aren’t any obviously visible distortions on the image, but what happens if we increase the magnification?

Figure 1. At low magnification, astigmatism is not overly apparent.

When increasing the magnification to 50000 × (field of view 5.37µm), we notice that the tin spheres seem out of focus:

Figure 2. At high magnification, astigmatism becomes apparent.

Knowing that we should be able to see more detail when using a SEM, perhaps the image is simply out of focus? Let’s see what happens when we adjust the focus:

Figure 3. When out of focus, round objects become elongated on astigmatic images.

Figure 3 shows that when the image is astigmatic and out of focus, elongation of round objects occurs. Notice how the smaller spheres appear almost egg-shaped. In Figure 2, we see that an astigmatic, in-focus image simply appears to be out of focus.

How can I tell if my SEM image is astigmatic?

In the above examples (Figure 2 and 3), we see that when an image is astigmatic and out of focus, it becomes elongated. When astigmatic images are over- or under-focused, they become elongated in perpendicular directions. This means that you can test whether your image is astigmatic by over- and under-focusing it.

In the three images below, we show the process of testing whether an image is astigmatic or not. In Figure 4, the lab operator has obtained an image and wants to know if it is astigmatic.

Figure 4. To the untrained eye, this astigmatic image might just appear to be out of focus.

Not convinced that the best-quality image has been obtained, the microscope operator over- and under-focuses the image:

Figure 5. After over- and under-focusing the image, it is clear to the lab operator that the image was astigmatic, due to the visible elongation in perpendicular directions.

The microscope operator now knows that the microscope’s stigmator needs to be used to improve the quality of the image. After adjusting the stigmator in both X and Y directions, the operator again tests the image, and sees the following:

Figure 6. When an image is stigmatic, no elongation occurs when (A) under-focused or (C) over-focused. (B) When stigmatic and in focus the image is crisp.

Figure 6. When an image is stigmatic, no elongation occurs when (A) under-focused or (C) over-focused. (B) When stigmatic and in focus the image is crisp.

The operator is now ready to acquire beautiful images for a publication or report.

In summary, please consider the table below to see what your image will look like depending on how well it is stigmated and/or focused:

Table 1. SEM image quality depending on stigmation and focus.

1. Hashemi H et al. (2018) Global and regional estimates of prevalence of refractive errors: Systematic review and meta-analysis. Journal of Current Ophthalmology 30(1): 3–22.

Topics: Scanning Electron Microscope

About the author:
Willem van Zyl is Application Specialist at Thermo Fisher Scientific, the world leader in serving science. He is excited by analytical instruments that are accessible and user-friendly, and truly believes that a SEM image is worth a kazillion words.

For further information, application support, demo or quotation requests  please contact us on 01582 764334 or click here to email.

Lambda Photometrics is a leading UK Distributor of Characterisation, Measurement and Analysis solutions with particular expertise in Electronic/Scientific and Analytical Instrumentation, Laser and Light based products, Optics, Electro-optic Testing, Spectroscopy, Machine Vision, Optical Metrology, Fibre Optics and Microscopy.