Infrared optical system

An infrared optical system comprising:

• a spherical lens group having an image-forming function as a whole and including one or more spherical lenses to form an image of an incident light beam from a subject;
• an aperture diaphragm to limit transmission of the incident light beam through the spherical lens group;
• an aberration correcting plate disposed at a stage preceding the spherical lens group, having at least one aspherical surface, and to give an optical path length difference for compensating an aberration to be generated in the spherical lens group to the incident light beam;
• an aberration correcting plate attaching and detaching part to and from which the aberration correcting plate is attachable and detachable;
• and
• a lens barrel holding the spherical lens group, the aperture diaphragm, the aberration correcting plate, and the aberration correcting plate attaching and detaching part, wherein
• the spherical lens group has a structure maintaining image-forming capability regardless of attachment and detachment of the aberration correcting plate, and
• where power of the spherical lens group is denoted by φ, an F value of the spherical lens group is denoted by F, a maximum diameter of a spot diameter generated by the spherical lens group on an image surface is denoted by ε, and ε coincides with a pixel size of a detection element disposed on the image surface, a parameter β=φ′(1−φ′L′)−1 expressed by power φ′ of the aberration correcting plate and an interval L′ between the aberration correcting plate and the spherical lens group satisfies a relationship of |β[φ(β+φ)]−1|<εF.

An infrared optical system comprising:

• a spherical lens group having an image-forming function as a whole and including one or more spherical lenses to form an image of an incident light beam from a subject;
• an aperture diaphragm to limit transmission of the incident light beam through the spherical lens group;
• an aberration correcting plate disposed at a preceding stage of the spherical lens group, having at least one aspherical surface, and to give an optical path length difference for compensating an aberration to be generated in the spherical lens group to the incident light beam;
• an aberration correcting plate attaching and detaching part to and from which the aberration correcting plate is attachable and detachable;
• and
• a lens barrel holding the spherical lens group, the aperture diaphragm, the aberration correcting plate, and the aberration correcting plate attaching and detaching part, wherein
• the spherical lens group has a structure maintaining image-forming capability regardless of attachment and detachment of the aberration correcting plate, and
• where power of the spherical lens group is denoted by φ, an F value of the spherical lens group is denoted by F, a maximum diameter of a spot diameter generated by the spherical lens group on an image surface is denoted by ε, and ε coincides with a diffraction limit spot diameter of the spherical lens group, a parameter β=φ′(1−φ′L′)−1 expressed by power φ′ of the aberration correcting plate and an interval L′ between the aberration correcting plate and the spherical lens group satisfies a relationship of |β[φ(β+φ)]−1|<εF.