Widefield Fluorescence Microscopy
Principle, Applications, and ibidi Solutions

What Is Widefield Fluorescence Microscopy?

Widefield fluorescence microscopy, often referred to as epifluorescence microscopy, is one of the most widely used imaging techniques in modern cell biology. This method allows researchers to visualize specific cellular structures by labeling them with fluorescent molecules known as fluorophores.

When illuminated with light of a defined excitation wavelength, fluorophores emit light at a longer wavelength, enabling the researcher to detect labeled structures with high contrast against a dark background. This approach makes it possible to study the localization, dynamics, and interactions of proteins, organelles, and other cellular components in both fixed and living cells.

Because the entire field of view is illuminated simultaneously, widefield microscopy enables rapid image acquisition. It is therefore particularly well suited for live cell imaging, time-lapse experiments, and high-content screening.

What Is the Principle of Widefield Fluorescence Microscopy?

The fundamental principle of fluorescence microscopy is based on the photophysical properties of fluorophores, which absorb and emit light at different wavelengths. Fluorescent molecules absorb photons at a specific excitation wavelength and subsequently emit photons at a longer emission wavelength due to energy loss during the excited-state relaxation process.

In a widefield microscope, the entire field of view is illuminated uniformly. The excitation light first passes through an excitation filter, which selects the wavelength range suitable for exciting the fluorophore. The filtered light is then reflected by a dichroic mirror toward the objective lens and focused onto the specimen.

Upon excitation, the fluorophores in the sample emit fluorescence in all directions. A portion of this emitted light is collected by the objective lens. Because the emitted fluorescence has a longer wavelength than the excitation light, the dichroic mirror now acts as a transmission element. The fluorescence then passes through an emission filter, which blocks residual excitation light and isolates the desired signal before detection.

Schematic of a widefield setup showing an excitation and emission light path. The excitation filter transmits a narrow wavelength band, which is reflected by the dichroic mirror and focused on the sample. The red-shifted emission passes through the dichroic mirror and emission filter before reaching the detector.

What Are the Key Components of a Widefield Fluorescence Microscope?

A typical widefield fluorescence microscope consists of several essential optical elements:

Excitation Light Sources

Widefield microscopes use either a broadband white light source (e.g., mercury or xenon arc lamps) combined with bandpass filters, or LED light engines providing spectrally separated excitation bands.

Optical Filters

• Excitation filter: Transmits only the desired wavelengths needed for fluorophore excitation.
• Dichroic mirror: An optical component with wavelength-dependent reflective and transmissive properties. In a microscope setup, it reflects the excitation light toward the specimen while transmitting the emitted fluorescence.
• Emission filter: Isolates fluorescence emission and blocks unwanted wavelengths.

Detectors

Highly sensitive cameras, such as sCMOS or CCD cameras, are typically used. These detectors enable high-speed imaging and quantitative fluorescence measurements.

Which Samples Can Be Imaged With Widefield Fluorescence Microscopy?

Widefield fluorescence microscopy works best for thin samples: Because the entire sample volume is illuminated, fluorophores both inside and outside the focal plane are excited simultaneously. As a result, only thin specimens, such as monolayers of cultured cells or thin tissue sections, can be imaged with high clarity. Thicker samples typically require physical sectioning.

Widefield fluorescence microscopy images of a single layer of adherent cells. Human umbilical vein endothelial cells (HUVEC) cultured in the µ-Slide I Luer and stained after fixation. Colors indicate VE-cadherin (cyan), Connexin 43 (yellow), F-actin (red), and nuclei (blue). Scale bar: 25 µm.

Widefield fluorescence microscopy image of a tissue section. Mouse intestinal formalin-fixed paraffin-embedded (FFPE) tissue section stained in the sticky-Slide Tissue. Cell nuclei are labeled with DAPI (blue), F-actin with Phalloidin-iFluor 488 (green), and collagen type I with Alexa Fluor 647 (red). Scale bar: 100 µm.

What Are the Main Applications of Widefield Fluorescence Microscopy in Cell Biology?

Widefield fluorescence microscopy is a fundamental tool in biological research, enabling the visualization of cellular structures and molecular processes with high specificity and comparatively low phototoxicity.

Common applications include immunofluorescence imaging, where antibody-based labeling is widely used to detect proteins within cells. Another key application is the imaging of fluorescent proteins in expression cell lines, where genetically encoded fluorophores such as GFP, mCherry, or YFP allow visualization of protein localization and dynamics in living cells.

Widefield microscopy is particularly well suited for live cell imaging, enabling the observation of dynamic biological processes such as cell migration, vesicle trafficking, cytoskeletal remodeling, and mitosis. Because the entire field of view is recorded simultaneously, this technique supports rapid time-lapse imaging or high-content imaging.

What Is the Resolution of Widefield Fluorescence Imaging?

The lateral resolution of widefield fluorescence microscopy is limited by diffraction and typically ranges from approximately 200–300 nm, depending on the numerical aperture of the objective and the emission wavelength.

Because fluorescence originating from multiple focal planes contributes to the recorded image, thick specimens often appear blurred due to the accumulation of out-of-focus light. In contrast, thin samples (such as adherent cultured cells) generally allow imaging with high clarity. However, even adherent cells typically exceed 1 µm in thickness, which approaches the axial resolution limit of widefield systems and can therefore lead to residual out-of-focus background.

Computational methods such as image deconvolution can improve image quality by estimating and redistributing out-of-focus light based on the system’s point spread function (PSF), thereby enhancing contrast and apparent resolution.

What Are the Advantages and Limitations of Widefield Fluorescence Microscopy?

Widefield fluorescence microscopy offers several important advantages, such as a fast image acquisition, a high sensitivity due to camera detection, relatively low phototoxicity and compatibility with live cell imaging.

The main limitation of widefield fluorescence microscopy is the detection of out-of-focus fluorescence, which reduces image contrast, particularly in thicker specimens. Widefield systems provide limited optical sectioning capabilities, exhibit lower axial resolution than confocal microscopy, and therefore produce blurred images in tissues and three-dimensional samples.

FAQs