brightfield

Brightfield microscopy is a mode of optical microscopy in which illumination passes through the specimen from below, and the image is formed by light that is transmitted, absorbed, or scattered by the sample. The background appears bright, while structures within the specimen may appear darker or colored depending on their absorption of light. Contrast in brightfield arises from differences in absorption, scattering, and refractive index among cellular components or materials.

Most biological specimens are prepared to enhance contrast, typically by applying stains that bind selectively to cellular structures. Unstained specimens can sometimes be observed if they have inherent contrast, but details are often faint. Brightfield is widely used for histology, cytology, and microbiology on prepared slides and simple, fixed samples. It is widely available and inexpensive, making it the default method in many clinical and teaching settings. For living cells, brightfield can be used but often provides limited contrast, so alternative techniques such as phase contrast or differential interference contrast are preferred for dynamic observation.

Equipment and techniques involve careful control of illumination and image formation. The condenser, field diaphragm, and objective determine brightness, field uniformity, and resolution. Köhler illumination is a common method to achieve even illumination and optimal contrast. The resolving power of brightfield is limited by the wavelength of light and the numerical aperture of the objective, in accordance with the Abbe limit. Overall, brightfield offers simplicity and compatibility with standard stains and slides, but it has lower contrast for transparent specimens and may require staining or alternative imaging modes to reveal fine structure.