What is confocal microscopy?
Confocal microscopy is a fluorescence imaging technique that uses a pinhole in front of the detector to block light coming from above and below the plane of focus. The result is a sharp optical section: a thin slice through a thick specimen, recorded without physically cutting it. Stacking many such sections produces a three-dimensional data set.
The principle was advanced by Marvin Minsky, who built a stage-scanning instrument in 1955 and patented the idea in 1957. Modern instruments look very different, with lasers, scanning mirrors and sensitive detectors, but they still rely on the same idea of spatial filtering described in his patent.
Confocal microscopes are standard tools in cell and developmental biology, neuroscience and pathology, and they are also used in materials science and microchip inspection. If you are new to light microscopy, our guide to phase contrast microscopy covers a simpler technique for unstained cells.
Confocal microscope principle: how the pinhole works
In a conventional widefield fluorescence microscope the whole field is illuminated at once, so fluorescence from out-of-focus planes reaches the camera as haze. In thick samples, such as embryos or tissue slices, that glare can bury the detail you want to see.
A confocal microscope solves this with two conjugate focal points. Light is focused to a spot in the specimen, and the emitted fluorescence is focused back onto a pinhole placed in a plane conjugate to the focal plane. Light from the focal spot passes through the pinhole; light from above or below the focal plane arrives defocused and is mostly blocked. That is where the name comes from: illumination and detection share the same focus.
Choosing the pinhole size
Pinhole diameter is usually expressed in Airy units (AU), the size of the central spot of the diffraction pattern. Leica Microsystems describes 1 AU as a practical compromise rather than a law of nature:
- Above 1 AU, more out-of-focus light gets through and optical sectioning gets worse; with a very large pinhole the system behaves like a widefield microscope.
- Below 1 AU, lateral resolution improves, by about 30% at the theoretical limit of a zero pinhole, but optical sections become only about 25% thinner than at 1 AU.
- Closing the pinhole also throws away signal, so very small pinholes need bright samples and low-noise detectors.
Confocal laser scanning microscopy step by step
Because a single pinhole sees only one point at a time, the image has to be built up by scanning. In confocal laser scanning microscopy (CLSM or LSCM), computer-controlled mirrors driven by galvanometers steer the laser across the field, and a photomultiplier tube (PMT) or similar detector records the intensity pixel by pixel.
- Excitation: a laser line matched to the fluorophore is focused by the objective to a diffraction-limited spot.
- Emission: fluorescence returns through the objective and is separated from the laser light by a dichroic mirror and filters.
- Spatial filtering: the emission is focused onto the pinhole, which rejects most out-of-focus light.
- Detection: the detector converts the light that passes into an intensity value for one pixel.
- Raster scanning: the mirrors move the spot line by line until the whole frame is recorded.
- Z-stack: the focus is stepped through the specimen and the process repeated, giving a series of optical sections for 3D reconstruction.
Optical section thickness is set by the objective and the pinhole, not by a microtome. Changing the pinhole diameter changes how thick each section is, which is why the pinhole setting belongs in every confocal methods section.
Point scanning vs spinning disk confocal
Point scanning is flexible but relatively slow, and every point receives intense illumination. Spinning disk confocal microscopes take a different route: a rapidly rotating disk with many small pinholes illuminates and reads thousands of points almost simultaneously, and the image is recorded by a camera. This allows video-rate imaging of moving samples.
| Feature | Point-scanning (CLSM) | Spinning disk |
|---|---|---|
| How the image is formed | One laser spot rastered pixel by pixel by mirrors | Many pinholes on a rotating disk scan in parallel |
| Detector | PMT or similar point detector | Camera |
| Speed | Slower, especially for large z-stacks | Video rate, dozens of frames per second |
| Photobleaching per image | Higher | Lower; each point gets less cumulative light |
| Pinhole | Adjustable to the sample | Usually fixed by the disk |
| Thick or dense samples | Better rejection of haze | Light from neighbouring pinholes can add blur |
| Typical use | Fixed and thick specimens, quantitative 3D work | Live cells, fast intracellular dynamics, development |
The efficiency difference has been measured. In a 2005 study in the Journal of Microscopy, Wang, Babbey and Dunn compared a Yokogawa CSU10 spinning disk system with several single-point scanning confocals. The spinning disk reached the point scanners' maximum signal-to-noise ratio at about one-fifteenth of the photobleaching rate. The authors attributed part of the advantage to the CCD camera and noted that point scanners are limited at high speed by fluorescence saturation under intense illumination.
Confocal vs widefield: resolution and trade-offs
A common misconception is that confocal microscopy dramatically increases resolution. Nikon's MicroscopyU describes the gain in lateral and axial resolution as marginal; the real benefit is the removal of out-of-focus glare, which raises contrast and makes optical sectioning and 3D imaging possible. Resolution remains far below that of electron microscopy; for nanometre-scale surface detail see our explainer on how a scanning electron microscope works.
Photobleaching and phototoxicity
Focusing laser light into a small spot comes at a cost. Fluorophores fade with repeated exposure, and intense light can damage or kill living cells; early laser scanning confocals were notably wasteful with laser energy. Practical ways to reduce the light dose include:
- using the lowest laser power that gives an acceptable signal-to-noise ratio;
- limiting the number of z-sections and time points to what the question needs;
- choosing bright, photostable fluorophores;
- switching to spinning disk for long live-cell experiments.
Applications of confocal microscopy
Confocal microscopes are used wherever fluorescently labelled structures must be seen in three dimensions or separated from background haze:
- Cell biology: localisation of proteins and organelles in fixed or living cells, often with several labels at once.
- Developmental biology: immunofluorescently labelled embryos, where widefield blur grows as cell numbers increase.
- Live-cell imaging: intracellular transport and other fast processes, usually on spinning disk systems.
- Tissue and pathology: 3D reconstruction of thick sections and quantitative measurements.
- Materials science: surface inspection of microchips and other engineered materials.
Conclusion
Confocal microscopy works because a pinhole rejects out-of-focus light, turning a blurred fluorescence image of a thick specimen into crisp optical sections that can be stacked in 3D. Point-scanning systems give the most control and the cleanest sections in thick samples. Spinning disk systems trade some flexibility for speed and a much gentler light dose, which makes them the usual choice for live cells.