Charging Artifacts
Reads the saturated bright and dark regions of a scanning electron micrograph and asks whether they look like genuine charging. A non-conductive specimen accumulates electrons under the beam and produces irregular, organic saturated zones with rough boundaries. Geometric saturated shapes, such as filled circles or rectangles, are the mark of a drawn cover or an edit, and a total absence of saturation or an excess of it is also recorded. The shape judgment is area-weighted, so a single large drawn region stands out among small organic spots. It works on the pixels alone, with no model.
Technical description
M8 is a deterministic screen for the saturation signature of scanning electron microscopy. When the electron beam scans a poorly conducting specimen, charge builds up and locally deflects or enhances the signal, producing saturated bright or dark patches whose boundaries are irregular and organic because they follow the physics of charge accumulation. A manipulator who covers an inconvenient feature, by contrast, tends to draw a regular shape, a filled circle or a rectangle, with a smooth, geometric boundary. M8 counts the saturated pixels, and when saturation is present it extracts the saturated regions as contours and measures their shape regularity with circularity. Three signals are scored: a total absence of saturation, geometric saturated shapes, and excessive saturation. The image must be at least 64 by 64 pixels, or the indicator returns a zero score and records that it was skipped.
How it works
Pixels above 250 or below 5 are counted as saturated, and the saturated ratio is their fraction of the image. If there are no saturated pixels at all, an absence score of 2.0 is added, because a real micrograph of a non-conductive sample usually shows some charging.
When saturation is present, the bright and dark saturated pixels are thresholded into a binary mask and their external contours are found. For each contour above a minimum area, the circularity is computed from the isoperimetric shape factor
circ = 4 pi A / P^2,
where A is the contour area and P its perimeter; this equals one for a perfect circle and falls toward zero for an elongated or ragged shape. The contour circularities are combined into an area-weighted mean, weighting each region by its area, so that the regularity judgment is driven by the substantial saturated zones, where circularity is both meaningful and reliable, and a single large drawn region stands out among many small organic spots rather than being averaged away. An area-weighted circularity above 0.7 marks geometric shapes and adds a shape score of 2.5, a value below 0.4 marks organic shapes and adds nothing, and an intermediate value is ambiguous and adds 1.0. Finally, a saturated ratio above 0.1 adds an excessive-saturation score of 1.0. The three contributions are summed and capped at 5.0. Findings report the absence of saturation, geometric or ambiguous shapes, and excessive saturation. The metadata records the saturated ratio, the area-weighted mean circularity, the saturated-pixel count, and the contour count.
Score thresholds
| Score | Meaning |
|---|---|
| 0 to 1 | Saturated regions are present and organic, consistent with genuine charging. |
| 2 to 3 | One anomaly: no saturation at all, an excess of saturation, or ambiguous saturated shapes. |
| 4 to 5 | Geometric saturated shapes, alone or with an absence or excess of saturation. Consistent with a drawn cover or an edited region. |
Why this matters
Charging is a defining, hard-to-fake feature of scanning electron microscopy on non-conductive specimens. The physics is well characterised: the balance between incoming beam electrons and emitted secondary and backscattered electrons determines whether a surface charges positively or negatively, and the resulting fields distort the image into the irregular bright and dark artefacts that microscopists work hard to suppress [1]. Their boundaries are organic because they are governed by surface conductivity and geometry, not by a drawing tool, so a saturated region with a smooth circular or rectangular boundary is out of place. Quantifying that boundary regularity is a classical shape-analysis problem, and the circularity that M8 uses is the isoperimetric shape factor introduced by Cox to grade the roundness of particles, equal to one for a circle and smaller for any irregular outline [2]. The forensic relevance follows from the prevalence of figure manipulation in the literature and the share of cases showing signs of deliberate alteration [3], together with the principle that erasing or covering content in a micrograph is misconduct even when it is locally seamless [4]. By reading the presence and the shape of saturation, M8 turns a physical quirk of electron imaging into a check on whether the bright and dark zones were formed by charge or by an editor.
Limitations
The absence-of-charging cue is the weakest, because charging is not universal: a conductive specimen, a sputter-coated sample, a low-vacuum or environmental SEM, or a well-tuned acquisition can legitimately show little or no saturation, so a clean image is flagged only weakly and the signal should be read as a prompt for review rather than proof. The shape cue assumes a manipulator draws a regular outline; a cover with a deliberately irregular boundary evades it, and conversely a genuine charging spot that happens to be compact can raise the circularity. Circularity from a digital contour is biased by the staircasing of the boundary, which the area weighting mitigates by favouring large contours but does not remove. The saturation thresholds are fixed, so an image with a different dynamic range is read less reliably. The thresholds are directional rather than exact. The spatial uniformity and presence of the sensor noise level is handled by indicator M6, so M8 stays on the saturation regions and their shape.
Theoretical background
M8 rests on the electron physics of imaging a non-conductor. In a scanning electron microscope each landing primary electron either is absorbed or causes the emission of secondary and backscattered electrons; on a conductor the excess charge drains away, but on an insulator it accumulates, building a local potential that deflects the beam and modulates the emission. The visible result is saturation: regions that bloom bright or collapse dark, with boundaries set by the specimen's surface conductivity and topography and therefore irregular and organic. This is a stochastic, physically driven pattern, not a designed one, so its outlines have low circularity and vary from spot to spot. A drawn cover added in editing is the opposite: a deliberate, smooth, high-circularity shape placed to hide content. The isoperimetric shape factor separates the two, because among all shapes of a given area the circle has the smallest perimeter and hence the highest circularity, so a regular drawn region scores near one while an organic charging artefact scores well below it. Weighting by area ties the judgment to the substantial regions, where both the physics and the shape measure are reliable, letting M8 read whether saturation arose from charge or from a tool.
References
- Cazaux J. Recent developments and new strategies in scanning electron microscopy. Journal of Microscopy. 2005;217(1):16-35. DOI: 10.1111/j.0022-2720.2005.01414.x
- Cox EP. A method of assigning numerical and percentage values to the degree of roundness of sand grains. Journal of Paleontology. 1927;1(3):179-183. https://www.jstor.org/stable/1298056
- Bik EM, Casadevall A, Fang FC. The Prevalence of Inappropriate Image Duplication in Biomedical Research Publications. mBio. 2016;7(3):e00809-16. DOI: 10.1128/mBio.00809-16
- Rossner M, Yamada KM. What's in a picture? The temptation of image manipulation. The Journal of Cell Biology. 2004;166(1):11-15. DOI: 10.1083/jcb.200406019