US Patent 7585274 Optical system for endoscopes

An optical system for an endoscope can be used by being disposed in the distal end portion of the endoscope, the optical system having a plurality of illumination optical systems, which allow light emitted from a light source to diverge, and an observation optical system, in which at least one of the plurality of illumination optical systems satisfies the following formula (1):0≦−df(θ)/dθ≦0.025, 10°≦θ≦85°  (1),wherein θ denotes an angle of emission from a center of the illumination optical system, and f(θ) denotes a value obtained by standardizing, at an illuminance at a time when the angle θ of emission is 0°, an illuminance distribution (spherical light distribution characteristics) at a time when a spherical object is illuminated by the illumination optical system.