Köhler illumination

Köhler illumination is a type of special lighting used in microscopes. It is used to make sure that the light entering the microscope is evenly distributed, so that the sample being examined looks clear and sharp. It is done by using a special type of lens, called an "apochromat" lens. The lens has two pieces that look like circles with a hole in the middle, and it is placed so that the light passes through the two pieces, which spread it out evenly. This ensures that the light is evenly lit. Köhler illumination makes it much easier for scientists to see the details of a sample, which is important for making accurate observations.