Dark field illumination

Dark field illumination is a microscopy technique that enhances the contrast in unstained samples. It works on the principle of light scattering, but the design of the illumination optics is such that only scattered light enters the microscope lens while direct light is blocked. This results in a dark background with light objects, hence the term "dark field".

Principle

The principle of dark field illumination involves the use of a specialized condenser that scatters light off the specimen. The condenser is designed so that it forms a hollow cone of light. As the light passes through the specimen, it is scattered by the specimen into an even wider cone. The objective lens of the microscope is positioned so that it collects the light scattered by the specimen but misses the direct, unscattered light.

Applications

Dark field illumination is particularly effective for revealing features in transparent specimens that are invisible or hard to see in normal bright field illumination conditions. It is widely used in biology, medicine, and material science for studying the surfaces of objects or thin films, small particles, and defects in transparent materials.

Advantages and Disadvantages

The main advantage of dark field illumination is that it can make visible details that are not seen with other types of illumination. It is particularly useful for observing living organisms that would be damaged or killed by staining.

However, dark field illumination also has some disadvantages. It is not suitable for thick specimens because the depth of field is very shallow. In addition, it requires careful setup of the illumination system and the specimen must be very clean to avoid scattered light from dust particles.

See Also

• Bright field microscopy
• Phase contrast microscopy
• Differential interference contrast microscopy