Ultra compact inverse telephoto optical system for use in the ir spectrum

An infrared imaging optical system for focusing infrared radiation on an infrared detector, the optical system comprising:

• a front lens group having a negative optical power, wherein the front lens group receives an infrared radiation and comprises a first front lens with at least one aspherical surface and a second front lens with at least one aspherical surface;
• an intermediate lens group that receives the infrared radiation from the front lens group, wherein the intermediate lens group comprises a first intermediate lens with at least one aspherical surface, a second intermediate lens with at least one aspherical surface, and a third intermediate lens with at least one aspherical surface;
• and
• a rear lens group having positive optical power, wherein the rear lens group receives the infrared radiation from the intermediate lens group, and wherein the rear lens group comprises a first rear lens with at least one aspherical surface, a second rear lens with at least one aspherical surface, and a third rear lens;
• wherein the imaging optical system has a stop between the rear lens group and a focal plane located at said infrared detector, and wherein the ratio of the physical length to the effective focal length is less than about 3.0.