US Patent 7843634 Automatic focus detection device and microscope system having the same

An AF dark field illumination device and an objective lens are moved in unity. A tissue sample in a Petri dish is illuminated by infrared light incident with an oblique angle from an LED light source. The casting of the infrared light causes scattered light from the tissue sample. A part of the scattered light passes through the objective lens, thereby forming a dark-field microscope image, which is captured by a CCD camera. The sharpness of the dark-field microscope image is dependent upon the position of the objective lens. The position of the objective lens at which the dark-field microscopic image exhibits the highest sharpness is determined to be the focus position.