What Is an Epifluorescence Microscope?

An epifluorescence microscope is a fluorescence microscope in which the excitation light travels down through the objective lens to the specimen, and the same objective collects the fluorescence that comes back. Researchers, core-facility staff and lab managers use it to see fluorescently labelled cells, tissue and molecules. This page explains the light path, the parts, the upright and inverted stand types, and the routine checks that keep an instrument producing clean images.

What an epifluorescence microscope is

Nikon’s MicroscopyU describes the geometry directly: in epi-fluorescence, the objective acts as the condenser for the excitation light and also collects the emitted light, with the illuminator sitting between the viewing tubes and the nosepiece. The same source calls epi-fluorescence illumination “the overwhelming choice of techniques in modern microscopy” (Nikon MicroscopyU). In everyday use the term usually refers to a widefield fluorescence microscope built this way.

What is a fluorescent light microscope?

“Fluorescent light microscope” is a common name for a fluorescence microscope: it illuminates a labelled specimen with excitation wavelengths and forms an image only from the longer-wavelength light the fluorophores emit. A filter set separates the two. See the fluorophores guide for the labels themselves.

What is the difference between fluorescence and epifluorescence microscopy?

Fluorescence microscopy is the technique; epifluorescence describes how the excitation light is delivered, namely through the objective rather than through a separate condenser on the opposite side of the specimen. In practice, “epifluorescence microscope” usually means a widefield fluorescence microscope of this design. For the next step up in optical sectioning, see confocal vs fluorescence microscope.

How the light path works

The excitation light is far brighter than the signal. Nikon states it is typically several hundred thousand to a million times brighter than the emitted fluorescence, so the microscope must block it: “In a properly configured microscope, only the emission light should reach the eye or detector” (Nikon MicroscopyU). The job is done by a filter cube with three elements:

• Excitation filter: passes only the wavelength band that excites the chosen fluorophore. Band-pass filters are named by centre wavelength and bandwidth (Thermo Fisher).
• Dichromatic mirror (dichroic): a 45-degree beamsplitter that reflects the excitation light toward the specimen and passes the longer-wavelength emission (Nikon MicroscopyU).
• Emission (barrier) filter: a band-pass or long-pass filter that removes leftover excitation light before the detector. Unwanted light that gets through shows up as background fluorescence, and a filter set captures the peak of a fluorophore’s emission, not all of it (Thermo Fisher).

Thermo Fisher notes that in an epifluorescence microscope these three components are usually housed together in one cube matched to a fluorophore, with filter wheels or turrets as the alternative.

Parts of an epifluorescence microscope

Part	Role in the light path	Source
Light source	Mercury burners (50-200 W) and xenon arc lamps (75-150 W) are the arc sources described; lasers are essential in scanning confocal microscopy and are also used for TIRF	Nikon MicroscopyU
Filter cube, turret or slider	Holds the excitation filter, dichroic and emission filter; stands typically carry 4 to 6 cubes	Nikon MicroscopyU; Thermo Fisher
Objective lens	Delivers excitation light to the specimen and collects the emitted light	Nikon MicroscopyU
Eyepiece or detector	Receives only the filtered emission light	Nikon MicroscopyU

Objective choice affects both brightness and resolution; see the objective selection guide.

Upright vs inverted epifluorescence stands

Epifluorescence describes the illumination geometry, not the stand type, so it can be built on an inverted stand; the inverted design is summarised below.

Stand	Objective position	Key difference	Source
Inverted	Below the stage; the optical path is directed up toward the eyepieces	Culture vessels such as Petri dishes are observed from below; vessel bottoms are much thicker than coverslips, so longer working distance objectives are needed; the stage is fixed in Z and focus moves the objective	Nikon inverted microscope glossary

What is an inverted microscope?

It is a microscope with the objective under the stage and the condenser above the sample, which lets live cells in culture vessels be observed from below. Details are in the inverted microscopy guide.

What epifluorescence can and cannot do

• Fluorescence cannot resolve structures below the diffraction limit, but it can detect fluorescing molecules smaller than that limit (Nikon MicroscopyU). Techniques that go beyond the limit are covered in super-resolution microscopy.
• Applications Nikon lists for fluorescence include TIRF, FRAP/FLIP and FRET.

Routine checks for lab staff

• Match the cube to the fluorophore. The excitation and emission bands must fit the label’s spectra (Thermo Fisher).
• Limit light exposure on live cells. Excited fluorophores can generate reactive species that harm living cells. Evident’s Knowledge Hub describes limiting exposure time, lowering excitation energy, neutral density filters and antifade reagents as ways to reduce photobleaching, noting they also reduce the measurable signal. See photobleaching and phototoxicity in live-cell imaging.
• Confirm the stand suits the vessel. Culture vessel bottoms are much thicker than coverslips, so cell-culture observation on an inverted stand needs longer working distance objectives (Nikon glossary).

Common mistakes

• Treating “fluorescence” and “epifluorescence” as different techniques rather than a technique and its illumination geometry.
• Using a filter set that is not matched to the fluorophore, then attributing the high background to the sample.
• Imaging live cells at full excitation intensity for long periods.

FAQ

Can you see bacteria at 40x magnification?

For a standard brightfield microscope, usually not well. A Biology LibreTexts microbiology lab manual states that viewing bacteria generally requires the 100x oil immersion objective with a 10x eyepiece, because most bacteria are small (it gives a range from about 1 µm to over 100 µm), and that a 100x objective with a numerical aperture of 1.25 has a resolving power of 0.22 µm at 550 nm (Biology LibreTexts, checked 2026-11-07). That source does not test the 40x objective specifically. Fluorescence labelling is a separate route: a fluorescently labelled cell can be detected even when its detail is not resolved.

Is an epifluorescence microscope the same as a confocal microscope?

No. Nikon notes lasers have become essential in scanning confocal microscopy and are also used for TIRF, whereas the mercury and xenon arc lamps described above are the sources it describes for widefield epi-fluorescence. Compare the two in confocal vs fluorescence microscope.

Sources

All sources checked 2026-11-07.

• Nikon MicroscopyU, Introduction to Fluorescence Microscopy: https://www.microscopyu.com/techniques/fluorescence/introduction-to-fluorescence-microscopy
• Thermo Fisher Scientific, Using Filters to Capture Your Signal (Molecular Probes School of Fluorescence): https://www.thermofisher.com/sg/en/home/life-science/cell-analysis/cell-analysis-learning-center/molecular-probes-school-of-fluorescence/fundamentals-of-fluorescence-microscopy/Using-filters-to-capture-your-signal.html
• Nikon Healthcare, Cell Image Lab glossary, Inverted microscope: https://www.healthcare.nikon.com/en/ss/cell-image-lab/glossary/inverted-microscope.html
• Evident (Olympus) Knowledge Hub, Introduction to fluorescence: https://evidentscientific.com/en/microscope-resource/knowledge-hub/techniques/fluorescence/fluorescenceintro
• Biology LibreTexts, Ohio State University Microbiology Lab, 1.3 Microscopy: https://bio.libretexts.org/Courses/Ohio_State_University/Microbiology_Lab_SP25/01:_Lab_1/1.03:_Microscopy