The core difference
The difference is where the light comes from, and so what it can show you.
- A transmitted light microscope shines light through the specimen, usually from a lamp and condenser below the stage. The objective collects the light that comes out the other side. This only works if the specimen is thin enough, and clear enough, for light to pass through.
- A reflected light microscope shines light onto the specimen from above, on the same side as the objective. The objective collects the light that bounces back. This is also called incident light or epi-illumination. It works for opaque objects that light can’t pass through.
So transmitted light shows the inside of thin specimens, and reflected light shows the surface of solid ones.
Transmitted light microscopes
Most “biological” compound microscopes, from student models to research instruments, are transmitted light microscopes. Light from the base passes through a condenser, which focuses it onto the slide. The specimen absorbs, bends or scatters part of it, and the objective forms an image from what gets through.
Typical specimens:
- Prepared slides and stained tissue sections
- Pond water, protozoa, algae, bacteria
- Blood films, cheek cells, plant epidermis
- Thin rock sections and crystal preparations
Because the light passes through the whole thickness of the specimen, preparations need to be thin, often a single cell layer or a section only a few micrometers to tens of micrometers thick.
Reflected light microscopes
When the specimen is opaque, such as a coin, a polished metal, a mineral grain, a circuit board or a fabric surface, transmitted light simply doesn’t reach the objective. Reflected light microscopes solve this by illuminating from above.
Coaxial (through-the-lens) illumination
Compound reflected light microscopes, often sold as metallurgical or materials microscopes, send light down through the objective itself. A half-silvered mirror or beam splitter in the body directs light from a side-mounted lamp down through the objective onto the specimen. The reflected light travels back up through the same objective to the eyepieces.
This “epi” design is what lets reflected light work at high magnification, where the objective sits so close to the specimen that there’s no room to shine a lamp in from the side. Flat, polished surfaces look bright, while scratches, pits, grain boundaries and inclusions scatter light away and look dark.
Oblique and ring lights on stereo microscopes
Most stereo microscopes are mainly reflected light instruments, but they light from outside the objective. Gooseneck spotlights, LED ring lights around the lens, or a top lamp shine down at an angle (see stereo microscope parts). Long working distances leave room for this. Raking light from one side emphasizes texture, while a ring light gives even, shadow-reduced lighting.
This is the type of reflected light most amateurs use, for insects, coins, stamps, minerals, electronics and small repairs.
Comparison table
| Transmitted light | Reflected light | |
|---|---|---|
| Light path | Through the specimen, from the opposite side to the objective | Onto the specimen, from the objective’s side |
| Specimens | Thin, transparent or translucent | Opaque, solid or thick |
| What you see | Internal structure, density, stains | Surface texture, color, scratches, grain boundaries, flaws |
| Preparation | Thin mounts, sections, smears, coverslips | Often none; metallography needs polishing (and often etching) |
| Typical instrument | Biological compound microscope | Metallurgical compound microscope; most stereo microscopes |
| Main fields | Biology, medicine, education | Materials science, metallurgy, geology (ores), electronics, forensics |
| Hobby examples | Pond life, prepared slides, crystals | Coins, minerals, insects, circuit boards, fabrics |
Contrast techniques in each mode
Many contrast techniques exist in both transmitted and reflected versions.
Transmitted light:
- Brightfield, the default
- Darkfield, which blocks direct light so specimens glow on black
- Phase contrast, for living transparent cells
- DIC, for relief-like images of transparent specimens
- Polarized light, for crystals, minerals and fibers
- Rheinberg illumination, a colored variant of darkfield
Reflected light:
- Reflected brightfield: flat surfaces bright, features dark
- Reflected darkfield: a ring of angled light, so scratches and edges glow while flat areas go dark. It’s useful for finding surface defects
- Reflected polarized light: used for some ores and anisotropic metals, and to reduce glare
- Reflected DIC: shows very small height differences on polished surfaces
- Epi-fluorescence: strictly speaking, most modern fluorescence microscopes use reflected-light geometry. Excitation light comes down through the objective and the fluorescence travels back up, even when the specimen itself is a thin slide
Phase contrast is essentially a transmitted light technique, because it relies on light passing through a transparent specimen.
Microscopes that do both
Some microscopes offer both illumination paths:
- Dual-light student and hobby microscopes have a lamp below the stage for slides and a small lamp above for opaque objects. The top light usually comes in at an angle rather than through the objective, so it works best at low magnification.
- Combined metallurgical microscopes have both coaxial epi-illumination and a transmitted light base. They’re used for semitransparent materials, thin sections of ores, and electronics with both opaque and transparent layers.
- Stereo microscopes with transmitted light bases add a lit stage from below, often with a darkfield option, for small aquatic organisms and translucent specimens.
Which do you need?
- Mostly slides, pond water and cells? A transmitted light compound microscope.
- Mostly coins, minerals, insects, stamps and electronics at low power? A stereo microscope with top lighting.
- Polished metals, ore sections or wafers at high magnification? A reflected light (metallurgical) compound microscope with coaxial illumination.
- A bit of everything at low power? A stereo microscope with both top and bottom lights, alongside a basic compound microscope for slides. Our stereo vs compound comparison helps you decide.
For a closer look at what reflected light is used for in and out of microscopy, see what reflected light is used for.
Tips for reflected-light work
- Keep the specimen surface perpendicular to the objective for coaxial illumination. Tilted surfaces reflect light away and look dark.
- Control glare with a polarizer on the light and an analyzer on the objective, or use a diffuser over a ring light.
- Use raking light from a low angle to bring out tool marks, engraving and surface texture.
- Polish samples properly for metallography. Coaxial illumination shows every scratch left from preparation.
- Don’t use coverslip-corrected objectives without a coverslip for critical work. Metallurgical objectives are normally designed to be used without a coverslip, while biological objectives usually expect one. Using the wrong type can reduce image quality at high magnification.