The short answer
The illuminating parts of a microscope are the parts that produce light and deliver it to the specimen. The standard textbook answer is three:
- The light source: a built-in lamp, or a mirror on older and simpler microscopes.
- The condenser: lenses under the stage that focus the light onto the specimen.
- The diaphragm: an iris or disc under the stage that controls the cone of light.
Better microscopes add a collector lens, a field diaphragm, filters and a brightness control. All of these sit below the specimen. The objectives and eyepieces above it are the magnifying parts.
The illuminating parts in order
These are listed in the order light passes through them, from the base upward. For the complete route, including the lenses above the stage, see how a compound microscope works, following the light path.
Light source: lamp or mirror
Most current microscopes have an LED in the base. Older or lab models may use halogen or tungsten bulbs. Microscopes without a lamp use a tilting mirror to reflect light from a window or desk lamp. Mirrors often have a flat side for use with a condenser and a concave side for use without one; see which means mirror for more.
Collector lens
On better microscopes, a lens just in front of the lamp gathers its light into a usable beam. Basic microscopes may skip it.
Field diaphragm
An adjustable iris on the light port in the base. It sets the diameter of the lit area on the specimen. Closing it to just beyond the edge of the view reduces glare. It’s needed for Köhler illumination and is usually missing on student models.
Filters
Glass discs placed in the light path, either on the light port or in a holder under the condenser. Blue filters correct yellowish halogen light, green filters boost contrast, and neutral density filters dim the light without changing its color. See our guide to microscope filters.
Condenser
A lens system mounted under the stage, usually on a bracket that can be raised and lowered. It concentrates light into a cone that passes through the specimen and into the objective. It’s especially important at 400x and above.
Aperture (iris) diaphragm
Built into the condenser. Its overlapping blades open and close to change the angle of the light cone. That changes contrast, depth of field and resolution, not just brightness. It’s one of the most useful controls on the microscope; see how to set the iris diaphragm. Simple microscopes may have a disc diaphragm, a rotating wheel with holes of different sizes, instead.
Brightness control
A dial or slider, usually on the base, that adjusts lamp output. This, not the diaphragm, is the right control for making the image brighter or dimmer.
Summary table
| Part | Where it is | What it does |
|---|---|---|
| Lamp / illuminator | Base | Produces light |
| Mirror | Under the stage (instead of a lamp) | Reflects external light upward |
| Collector lens | In front of the lamp | Gathers light into a beam |
| Field diaphragm | Light port in the base | Controls the size of the lit area |
| Filters | Light port or condenser holder | Adjust color or intensity |
| Condenser | Under the stage | Focuses light onto the specimen |
| Aperture (iris) diaphragm | In the condenser | Controls the light cone angle: contrast vs resolution |
| Brightness control | Base | Adjusts lamp intensity |
What a basic microscope has vs a better one
| Part | Basic / student | Better lab or amateur |
|---|---|---|
| Light source | LED, or mirror | LED or halogen, with dimmer |
| Collector lens | Sometimes | Yes |
| Field diaphragm | Usually not | Often |
| Filters | Rarely | Filter holder or slot |
| Condenser | Fixed lens, or none | Focusable Abbe or better, often centerable |
| Diaphragm | Disc or simple iris | Iris in condenser |
How the illuminating parts work together
The light source supplies the light. The collector lens and field diaphragm shape it into a beam of the right size. The condenser focuses that beam into a cone through the specimen, and the aperture diaphragm sets how wide that cone is. Get these right and the objective can show all the detail it’s capable of. Get them wrong and even a good objective looks dull or blurry; if that’s happening, our troubleshooting guide can help.
For how to adjust them, including step-by-step Köhler illumination, see our guide to microscope illumination. For all the other parts, see our guide to the parts of a compound microscope.