High contrast for unstained, living specimens. Resolution Limited by the wavelength of light. Similar to bright field, but enhanced visibility of small structures. Applications Viewing stained cells, tissue sections. Viewing living, unstained bacteria, flagella, spirochetes.

TEM vs. SEM

Feature	Transmission Electron Microscope (TEM)	Scanning Electron Microscope (SEM)
Principle	Electrons pass through a very thin specimen.	Electrons scan the surface of a specimen.
Image Type	2D internal structure (ultrastructure).	3D surface topography.
Resolution	Very high (nanometer level), higher than SEM.	High (nanometer level), lower than TEM.
Specimen Prep	Ultra-thin sections, heavy metal staining, vacuum.	Coating with conductive material (e.g., gold), vacuum.
Magnification	Up to 1,000,000x or more.	Up to 100,000x or more.
Information	Internal structure, crystal defects, chemical composition.	Surface morphology, texture, elemental composition.
Applications	Cell biology, materials science (internal structure).	Surface analysis, forensics, entomology (surface details).

Transmission Electron Microscope (TEM) - Principles and Processes

Principle: A beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through. An image is formed from the electrons that are transmitted through the specimen, magnified and focused by electromagnetic lenses onto a fluorescent screen or a digital detector.
Key Components:
- Electron Gun: Generates a beam of high-energy electrons (e.g., tungsten filament, LaB$_6$ crystal).
- Condenser Lenses: Focus the electron beam onto the specimen.
- Specimen Stage: Holds the ultra-thin specimen in a vacuum.
- Objective Lens: Forms the first magnified image.
- Intermediate & Projector Lenses: Further magnify the image and project it onto the screen.
- Vacuum System: Maintains high vacuum to prevent electron scattering by air molecules.
- Detector: Fluorescent screen (for viewing) or digital camera (for recording).
Processes:
1. Electron Generation: Electrons are emitted from the electron gun.
2. Beam Focusing: Condenser lenses focus the electron beam onto a small, coherent spot on the specimen.
3. Specimen Interaction: Electrons pass through the specimen. Some are scattered (due to interactions with atomic nuclei or electron shells), others pass through unscattered.
4. Image Formation: Unscattered and forward-scattered electrons are collected by the objective lens to form an image. Scattered electrons are often blocked by an aperture, contributing to contrast.
5. Magnification: The image is successively magnified by intermedi