An important role in microscopy is how the sample is illuminated. Illumination methods are broadly classified into transmitted illumination and incident illumination. The appropriate method needs to be selected according to your specimen and purpose.
Illuminates the back surface of the sample. This method is suitable for biological observation, such as colorless, transparent cells, and is used by general biological microscopes. There are two types of transmitted illumination: brightfield and darkfield:
- - Brightfield illumination
- A general method to observe samples by illuminating the back surface to make it transparent. Against a bright background, it projects the parts darker than the background.
- - Darkfield illumination
- Illuminates the back surface, as with the brightfield illumination, but interrupts direct light to make the outline of the sample shine against a dark background. This method is suitable for observation of low-contrast cells and requires a darkfield condenser.
Illuminates the front surface of a sample. This method is useful when observing 3D objects, such as materials and other industrial samples, as well as opaque samples. Stereoscopic microscopes generally utilize this type of lighting.