How a scanning electron microscope image is built
A light microscope forms an image with lenses all at once. A scanning electron microscope (SEM) works differently: a finely focused electron beam is moved point by point across the sample in a raster, like lines of text on a page. At each position the beam generates signals, a detector measures them, and the electronics convert the measured intensity into the brightness of one pixel.
The image therefore exists only as a map of signal strength versus beam position. Magnification is set simply by how small an area the beam scans: scanning a smaller field onto the same number of pixels gives a higher magnification. Image quality depends on the probe size, beam current, accelerating voltage, the time the beam spends on each pixel (dwell time) and how many frames are averaged to reduce noise. If you want a refresher on the column, detectors and signals, see our overview of how a scanning electron microscope works.
Why are SEM images black and white?
SEM detectors count electrons, not colors. The signal carries information about intensity, which depends on topography, composition and orientation, but it carries no information about the visible wavelengths a surface would reflect. Visible light spans roughly 400 to 700 nm; the electrons used in an SEM have wavelengths in the picometer range, so the very idea of “true color” does not apply at the scales being imaged.
The result is a grayscale image, usually stored as 8-bit (256 gray levels) or 16-bit data. Bright pixels mean more electrons reached the detector from that spot, dark pixels mean fewer. Interpreting an SEM image is largely a matter of understanding why one area emits more signal than another.
Where SEM contrast comes from: secondary vs backscattered electrons
The two signals used for almost all imaging are secondary electrons (SE) and backscattered electrons (BSE). They come from different depths, are collected by different detectors and tell you different things about the sample.
| Property | Secondary electrons (SE) | Backscattered electrons (BSE) |
|---|---|---|
| Origin | Sample electrons knocked loose by the beam | Beam electrons scattered back out of the sample |
| Energy | Low, by convention below 50 eV (most only a few eV) | High, from 50 eV up to nearly the beam energy |
| Escape depth | Shallow, a few nanometers to some tens of nanometers | Much deeper, a significant fraction of the interaction volume |
| Main contrast | Topography, edges, surface texture | Average atomic number (composition), crystal orientation |
| Typical detector | Everhart–Thornley detector in the chamber, or in-lens/through-the-lens detectors | Solid-state or scintillator detector, often annular, under the pole piece |
| Best used for | Surface morphology, fractures, cells, particles | Phase distribution, inclusions, polished cross-sections, minerals |
Secondary electrons and topographic contrast
Because secondary electrons escape only from a thin surface layer, their yield depends strongly on the local angle between the beam and the surface. Where the beam hits a slope, more of the interaction volume lies close to the surface and more SE escape. Edges, ridges and small particles therefore appear bright; this is called the edge effect. Recesses and holes appear darker, partly because electrons emitted inside them are less likely to reach the detector.
Backscattered electrons and compositional contrast
The fraction of beam electrons that are backscattered rises with the atomic number (Z) of the material. In a BSE image, regions rich in heavy elements look bright and light-element regions look dark: a gold nanoparticle on carbon, a lead-bearing inclusion in a glass or a heavy-metal stain in a biological section stands out clearly. On polished crystalline samples, BSE images can also show channeling contrast, where grains of different orientation appear as different gray levels.
A common practical choice is to record both signals: SE for shape and BSE for composition. Comparing the two images of the same field quickly tells you whether a bright feature is bright because it sticks out or because it is made of something heavier.
Why do SEM images look 3D?
Two effects combine. First, the SEM has a very large depth of field. The beam converges at a small angle, so a wide range of heights stays acceptably sharp at once; at moderate magnification this depth can be far greater than with a light microscope at a comparable magnification. Rough surfaces such as pollen, insect eyes or fracture faces stay in focus from top to bottom.
Second, the side-mounted Everhart–Thornley detector collects more electrons from surfaces facing it, which produces shading similar to light coming from one direction. Our visual system interprets this shading as illumination and reads the image as a relief. Keep in mind that the impression can be misleading: a single SEM image does not measure height. Real 3D information requires stereo pairs taken at two tilt angles, or other techniques such as atomic force microscopy.
False color: how SEM images are colorized
The colorful SEM images in magazines and on posters are false-color (pseudocolor) images. Color is added after acquisition, for clarity or aesthetics, and does not reflect the real color of the object. The main approaches are:
- Manual segmentation: an operator or illustrator outlines features in image-editing software and paints each region, for example coloring bacteria differently from the cells they sit on.
- Lookup tables: gray levels are mapped to a color scale, which can make subtle intensity differences easier to see.
- Channel merging: SE and BSE images, or images from two detectors, are assigned to different color channels and combined.
- Elemental overlays: maps from energy-dispersive X-ray spectroscopy (EDS) are colored by element and laid over the SE or BSE image.
In scientific work, colorized images should be identified as such, the original grayscale data should be kept, and only adjustments applied uniformly to the whole image (brightness, contrast, gamma) should be used for figures where intensities are interpreted. Colors coming from an EDS overlay carry real information about composition; hand-painted colors carry only the illustrator's interpretation.
Reading scale bars, magnification and the data bar
The magnification printed on an SEM image is defined relative to a reference display or print size. The same file viewed on a phone, a monitor and a projector shows the same field at very different apparent sizes, so the number alone is not a reliable guide. The scale bar, or the horizontal field width, is what you should measure against.
| Data bar field | What it means | Why it matters |
|---|---|---|
| HV or kV | Accelerating voltage of the beam | Higher voltage penetrates deeper and gives more BSE signal; lower voltage shows finer surface detail and reduces charging |
| WD | Working distance from the lens to the sample | Affects resolution and depth of field |
| Detector (SE, BSE, in-lens) | Which signal formed the image | Tells you whether contrast is mainly topographic or compositional |
| Mag | Nominal magnification | Depends on display size; use only as a rough guide |
| HFW or scale bar | Width of the imaged field or a reference length | The basis for measuring feature sizes |
| Pixel size | Real length represented by one pixel | Sets the smallest feature that can be measured meaningfully |
For measurements, use the calibrated scale bar or pixel size, measure features in image-analysis software rather than by eye, and remember that a tilted sample foreshortens lengths in the tilt direction unless tilt correction is applied.
Common SEM image artifacts and how to spot them
Many surprises in SEM images are artifacts of imaging or preparation, not properties of the sample. The most frequent ones are:
- Charging: on non-conductive samples, electrons accumulate and deflect the beam and the emitted signal. Signs are abnormally bright or dark patches, horizontal streaks, image shifts during scanning and distorted shapes. Remedies include a thin conductive coating, lower voltage, low-vacuum or variable-pressure mode, and faster scanning with frame averaging.
- Beam damage: polymers and biological samples can shrink, crack or bubble under the beam, especially at high magnification.
- Contamination: hydrocarbon deposits build up where the beam has dwelled, so a dark rectangle often appears when you zoom out after imaging a small area at high magnification.
- Drift and vibration: slow drift stretches or skews features; vibration produces wavy edges or jagged lines.
- Astigmatism: fine features look stretched in one direction, and the direction flips as you go through focus. It is corrected with the stigmator.
- Coating artifacts: at very high magnification, the grain of a sputtered metal layer can be mistaken for sample texture.
- Preparation artifacts: shrinkage, collapsed structures and cracks from poor drying, or salt crystals from incomplete rinsing. Our guide to SEM sample preparation covers how to avoid them.
How to interpret SEM images step by step
- Read the data bar first: detector, accelerating voltage, working distance and scale.
- Decide whether the contrast is mainly topographic (SE) or compositional (BSE).
- Use the scale bar to estimate feature sizes before forming any hypothesis.
- Look for signs of charging, drift, contamination or beam damage, especially on insulating or soft samples.
- Check whether the image is colorized and, if so, whether the colors come from data (such as EDS) or from manual painting.
- Compare with a lower-magnification overview to make sure the field is representative of the sample.
- Where it matters, confirm conclusions with a second signal, a different voltage or a complementary technique.
Key takeaway: in an SEM image, brightness means signal strength, not color or height. Identify the detector, trust the scale bar rather than the magnification number, and treat any color as an annotation unless it comes from measured data.
Conclusion
Scanning electron microscope images are maps of electron signals recorded pixel by pixel. Secondary electrons reveal surface shape, backscattered electrons reveal composition, and the large depth of field together with directional detection gives the familiar 3D look. Once you know how to read the data bar, measure with the scale bar and recognize charging and other artifacts, SEM images become precise scientific evidence rather than just striking pictures.