Patent ID: 6898005

Claim:
In an illumination optical system for a microscope, comprising a light source, an illumination lens system that gathers light from said light source and directs the light toward a sample to be viewed, an object optical system positioned between said illumination lens system and said sample, the improvement of: making adjustable one of the light source position along the optical axis or the focal length of the illumination optical system so that illumination state changes from Koehler illumination to critical illumination, respectively, the following condition is satisfied: 0.15<|( a·f 2 )/( b·f 1 )|<0.5 where a is the diameter of the light source, b is the diameter of the microscope field of view, f 1 is the focal length of the illumination lens system, and f 2 is the focal length of the object optical system; and wherein said lens group includes a lens elements having a positive refractive power and a lens element having a negative refractive power which, on axis, are touching or spaced in air a distance of 1 mm or less.