Conoscopic illumination optical device with a hollow cone for an optical microscope and method of optical microscopy in conoscopy

A hollow-cone and point-focusing conoscopic illumination optical device for an optical microscope, comprising:

• illumination means comprising a point light source, said illumination means being able to generate a collimated incident light beam (10),
• an optical objective (6) having an optical axis (8, 8′) and an image focal plane (12), the optical objective (6) being arranged so as to receive a cylindrical incident light beam (50) and to form an image of the source at one point (18) in the image focal point (12),
• characterized in that the illumination optical device comprises:
• an optical system comprising a first conical lens (1) and a second conical lens (5), said optical beam being arranged on the optical path of the incident light beam between the illumination means and the optical objective (6),
• the first conical lens (1) being arranged so as to receive said collimated incident light beam (10) and to form a hollow-cone light beam (20) having, in a plane transverse to the beam axis, a light distribution comprising a dark central part and a bright ring part,
• the second conical lens (5) being arranged so as to receive said first hollow-cone light beam (20, 40) from the first conical lens (1) and to form a black-background cylindrical light beam (50) having, in a plane transverse to the beam axis, a light distribution comprising a dark central part and a bright ring part, and
• in that the optical objective (6) is arranged so as to receive said black-background cylindrical light beam (50) from the second conical lens (5), to form a hollow-cone light beam (60) and to focus said hollow-cone light beam (60) at one point (18) of micrometric size in the image focal plane (12).