Brightfield microscopy

Bright-field microscopy is the simplest of all the optical microscopy illumination techniques. Sample illumination is transmitted (i.e., illuminated from below and observed from above) white light, and contrast in the sample is caused by attenuation of the transmitted light in dense areas of the sample. Bright-field microscopy is the simplest of a range of techniques used for illumination of samples in light microscopes, and its simplicity makes it a popular technique. The typical appearance of a bright-field microscopy image is a dark sample on a bright background, hence the name.

The light path of a bright-field microscope is extremely simple, no additional components are required beyond the normal light-microscope setup. The light path therefore consists of:

Bright-field microscopy may use critical or Köhler illumination to illuminate the sample.

Bright-field microscopy typically has low contrast with most biological samples, as few absorb light to a great extent. Staining is often required to increase contrast, which prevents use on live cells in many situations. Bright-field illumination is useful for samples that have an intrinsic color, for example chloroplasts in plant cells.

Bright-field illumination, sample contrast comes from absorbance of light in the sample

Cross-polarized light illumination, sample contrast comes from the rotation of polarized light through the sample

Dark-field illumination, sample contrast comes from light scattered by the sample

Phase-contrast illumination, sample contrast comes from interference of different path lengths of light through the sample

...
Wikipedia