Optical microscope
Reflected light, bright fieldEmpty magnification
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Interactive SEM training
See how a scanning electron microscope turns one fine beam of electrons into a picture of a surface and a list of its elements.
Work through the lessons in order, or open the microscope and explore on your own. Each lesson sets up the instrument for you and explains what you see.
Learning path
Microscope
Free practice with every control: beam energy, detectors, focus, stage, tilt, measuring, point analysis, line scans and element maps.
OpenPreparation lab
Mount, polish, clean, etch and coat a specimen, see the predicted result, then load it into the microscope.
OpenReference
Glossary, fracture feature guide, EDX peak overlaps, image fault chart, shortcuts and the model notes.
OpenFor lecturers
- Present enlarges text and collapses the menu for a projector.
- Open a lesson and press Copy link to send the class straight to it.
- Each student's progress stays in their own browser; nothing to sign up for.
Lessons
Eight modules from first principles to failure analysis. Read the summary, test yourself, then start an investigation; the microscope is set up for you at each step.
Reference
Look things up while you work. The tables also link straight to an example in the microscope.
Glossary
- SE
- Secondary electrons: slow electrons knocked out of the top few nanometres of the sample. They show surface shape.
- BSE
- Backscattered electrons: beam electrons that bounce back out. Heavier atoms return more, so BSE shows composition.
- EDX / EDS
- Energy-dispersive X-ray spectroscopy: sorting the X-rays from the sample by energy to identify its elements.
- Interaction volume
- The region under the surface where beam electrons travel and lose energy: pear-shaped in light elements, closer to a hemisphere in heavy ones. Almost all signals come from inside it.
- Critical energy (Ec)
- The minimum energy needed to knock an electron out of a given inner shell. Below it, that X-ray line cannot form.
- Overvoltage (U)
- Beam energy divided by the critical energy. Practical analysis needs U of about 1.5 to 2 or more.
- Kα, Lα, Mα lines
- X-ray lines named after the shell that was emptied (K, L, M). Each element has its own set of energies.
- Bremsstrahlung
- The continuous background in a spectrum, from electrons slowing down near atomic nuclei. It ends at the beam energy.
- FWHM
- Full width at half maximum: the width of a spectrum peak, about 130 eV for a modern EDX detector at Mn Kα.
- HFW
- Horizontal field width: the real width of the area shown in the picture.
- WD
- Working distance: the gap between the objective lens and the sample surface.
- Dwell time
- How long the beam stays on each pixel. Longer dwell means less noise and a slower scan.
- Astigmatism
- An elliptical beam spot that smears the image in one direction. It is corrected with the stigmator.
- Hot-dip galvanizing
- Coating steel by dipping it in molten zinc at about 449 °C. The coating is a series of zinc–iron alloy layers (gamma, delta, zeta) topped by nearly pure zinc (eta).
- Charging
- Build-up of charge, usually negative, on an insulating sample. It causes bright streaks and drift, and negative charge also lowers the landing energy.
- Monte Carlo simulation
- Following many random electron paths to predict where electrons go and where signals are produced.
Fracture features
What each feature looks like and what it tells you. "Show me" opens an example in the microscope.
EDX peak overlaps
Lines closer together than the detector can separate, and how to confirm which element is present.
Image faults
Common causes of poor images. "Practise" starts a short diagnosis case.
Keyboard shortcuts
SpacePause or play
1Overview2SE picture3BSE picture4EDX map and spectrum
[Lower beam energy]Raise beam energy
−Zoom out+Zoom in
FAuto focusRRestart scan
TGuided tourEscResume scanning or exit tour
Model notes and references
What the simulator demonstrates
A raster links each beam position to one displayed pixel. Secondary electrons (SE, conventionally below 50 eV) are specimen electrons released by inelastic scattering; only those generated within a few nanometres of the surface escape, so SE images are dominated by topography, edges and detector geometry. Backscattered electrons (BSE) are beam electrons scattered back out of the specimen; their yield rises with atomic number, giving compositional contrast.
Characteristic X-rays follow inner-shell ionisation when the vacancy relaxes radiatively. Auger emission competes with X-ray emission and dominates for light elements. The beam energy must exceed the shell's critical excitation energy, and an overvoltage of about 1.5 to 2 or more is recommended in practice.
How the models work
Electron paths use a single-scattering Monte Carlo model: screened Rutherford elastic cross-sections with the Joy–Luo modified Bethe energy-loss expression, tracked to 0.5 keV. Compounds choose the scattering atom by cross-section. Backscatter fractions and depths shown are computed from these paths.
SE image contrast combines a secant-law tilt dependence, an edge term whose width scales with electron range, directional shading from the side-mounted detector and an SE2 component tied to backscattering. BSE contrast uses the Arnal tilt-dependent backscatter expression, mass-weighted for Al₂Cu, blurred by an amount that scales with electron range. Pixel noise follows counting statistics for the selected dwell time.
Fracture surfaces
Four fracture modes are generated procedurally: ductile overload in the cast Al–Si–Cu alloy, transgranular cleavage and intergranular fracture in a plain steel with MnS inclusions, and fatigue of a 6 mm aluminium bracket that initiated at a surface Al–Fe–Si particle. Fatigue striation spacing rises from about 0.15 µm near the origin to about 1.7 µm at the end of the fatigue zone. Fine features fade out when they become smaller than a pixel, as they would in a real low-magnification image. X-ray shadowing on rough surfaces is modelled as an energy-dependent loss for surfaces facing away from the EDX detector; tilt foreshortens the image and is corrected in the measuring tool.
Polymer fractography and preparation
The fractography method follows a failure-analysis sequence: examine all parts macroscopically, then by stereo and digital microscopy, then SEM; decide ductile or brittle, locate the origin, and judge crack speed and direction from rib markings, river markings, crack unions, bifurcation, secondary cracking and craze remnants. Polymer specimens are generated procedurally for polycarbonate (brittle overload, and slow crack growth with three edge origins) and polyethylene (ductile, stretched fibrils).
The preparation lab maps each choice onto the simulation: polishing sets scratch depth, relief and embedded SiC grit; skipping the final clean adds a hydrocarbon film and colloidal silica residue; etching grooves boundaries or recesses the matrix and raises surface oxygen; coatings add their own X-ray lines, absorb low-energy X-rays and add uniform backscatter; and ungrounded or uncoated insulators charge, which shifts and streaks the SE image and lowers the landing energy seen as a falling Duane–Hunt limit.
Simplifications
The specimen is a procedurally generated cast Al–Si–Cu microstructure with aluminium dendrites, eutectic silicon plates and Al₂Cu particles. It is illustrative, not a specific commercial alloy. The column drawing is schematic and the beam is exaggerated for visibility. Event rates are scaled for teaching.
The spectrum includes Al Kα, Si Kα, Cu Kα, Cu Kβ, Cu Lα and a small O Kα signal from a native surface oxide. Peak widths follow detector Fano statistics (129 eV at Mn Kα); Bremsstrahlung uses a Kramers form with simplified absorption and window losses. Matrix corrections, secondary fluorescence, escape and sum peaks, coating contributions and spatial mixing at phase boundaries are not modelled, so peak heights are not concentrations. The Monte Carlo view assumes a flat, homogeneous region and projects 3D paths onto a side view.
References
- Goldstein J. I. et al., Scanning Electron Microscopy and X-Ray Microanalysis, 4th ed., Springer, 2018.
- Joy D. C., Monte Carlo Modeling for Electron Microscopy and Microanalysis, Oxford University Press, 1995.
- Kanaya K. and Okayama S., J. Phys. D: Appl. Phys. 5 (1972) 43.
- Jansen J. A., Fractography: A Key Component of a Failure Analysis, The Madison Group.
- Vander Voort G. F., Metallography: Principles and Practice, ASM International, 1999.
- Echlin P., Handbook of Sample Preparation for Scanning Electron Microscopy and X-Ray Microanalysis, Springer, 2009.
- Newbury D. E., Mistakes encountered during automatic peak identification of minor and trace constituents in electron-excited energy dispersive X-ray microanalysis, Scanning 31 (2009) 91–101.
- Newbury D. E. and Ritchie N. W. M., Is scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS) quantitative?, Scanning 35 (2013) 141–168.
- ASM Handbook, Vol. 12: Fractography, ASM International, 1987.
- Hull D., Fractography: Observing, Measuring and Interpreting Fracture Surface Topography, Cambridge University Press, 1999.
- American Galvanizers Association: the HDG coating (layer iron contents) and Specifier's Guide (bath temperature)
- ASTM A123/A123M, zinc coatings on iron and steel products (minimum coating thickness), and ASTM B487, measurement of coating thickness by microscopical examination of a cross-section.
- Struers: metallographic preparation of zinc coatings
- NIST X-ray transition energies database
- JEOL SEM glossary
Compare instruments
The same spot on the same specimen, seen with a reflected-light optical microscope, a digital microscope and the SEM. Change the field of view and the focus to see where each instrument stops showing new detail.
Digital microscope
Optical lens, camera, focus stackingSEM
Secondary electrons, 15 kVWhen to use which
Numbers are for typical instruments; see the sources below| Optical microscope | Digital microscope | SEM | |
|---|---|---|---|
| Finest detail | Set by the wavelength of light: r = 0.61 λ / NA. About 0.25 µm at best, with an oil-immersion lens [1]; about 0.3 to 0.35 µm with a dry 100×/0.95 lens. | Uses optical lenses, so the same light-wavelength limit applies [1]. | About 0.6 to 3 nm in a secondary electron image at 15 kV [4]. |
| Depth of field | Very shallow at high magnification; it falls with the square of NA, about λ / NA² [2]. Rough fractures are only partly in focus. | Focus stacking combines images taken at different focus positions into one sharp image [6]; the same scan can measure the height at each point (focus variation) [7]. | About 100 times larger than an optical microscope, because the beam's opening angle is about 100 times smaller [5]. |
| Magnification limit | Useful up to about 1000 × NA; beyond that the picture gets bigger without new detail ("empty magnification") [3]. | The same optical limit applies [3]. | Detail continues far beyond the optical limit [4]. |
| Lighting and contrast | Bright-field reflected light suits flat polished sections. In steel, inclusions are examined unetched, but the other constituents need etching; Nital reveals ferrite grain boundaries in low-carbon steels [9]. | Coaxial light suits flat, reflective faces; ring light suits textured or irregular surfaces [8]. | Surface shape (SE), composition (BSE) and elements (EDX) from the same field. |
| Typical use | Phases, grain size and inclusions on prepared sections. | A first, non-contact look at parts and fractures: overall shape, beach marks, the origin, heights. | Fine fracture features such as dimples and most fatigue striations; chemistry of small particles. |
| Fatigue example | Beach marks stand out when the fracture is examined by eye or at low power [10]. Striations are much finer: once wider than about 0.5 µm they can be seen optically (they were first photographed with an optical microscope, in 1951), but smaller ones need the SEM [11]. | ||
Sources
- Nikon MicroscopyU: Microscope resolution
- Nikon MicroscopyU: Depth of field and depth of focus
- Nikon MicroscopyU: Useful magnification range
- JEOL SEM glossary: Secondary electron image
- JEOL SEM glossary: Depth of focus
- Vision Engineering: Benefits of focus stacking
- Focus variation; the method is standardised in ISO 25178-606
- Keyence: Coaxial illumination and Ring illumination
- Buehler Tech Note: Microstructure of ferrous alloys
- Keyence: Metallurgical failure analysis (fracture surfaces)
- Striation (fatigue), citing Zapffe and Worden (1951)
Images are simulated from the same specimen model as the microscope. Phase brightness in the optical and digital views is approximate, and they are shown in greyscale rather than true colour.
Microscope controls
Picture made from
Live picture
Built dot by dot as the beam sweepsSE detector
AlSiCu
Snapshots for comparison
Inside the microscope
Cutaway view, not to scale
Beam electrons
Secondary electrons (SE)
Backscattered electrons (BSE)
X-rays
What happens inside the sample
Electron paths, side view–bounce back out
–deepest typical reach
–X-rays made within
How one X-ray is born
Simplified shell pictureX-ray spectrum
Line scan profile