Patent ID: 6906872

Claim:
A lens module for a thin scanning device, said lens module to be disposed on an optical path from an original to an image sensor, the optical path including a chief ray at zero-field propagating from a zero-field of the original to a zero-field of the image sensor, a chief ray at full-field propagating from a full-field of the original to a full-field of the image sensor, said lens module to be disposed between the original and the image sensor such that the chief ray at zero-field and the chief ray at full-field form an angle greater than 30 degrees therebetween at a side between said lens module and the image sensor, the optical path having a total length measured from the original to the image sensor, said lens module forming an object distance with the original, the total length being less than 1.5 times the object distance, an object height of the original when imaged on the image sensor being defined as an image height, the total length being less than five times the image height, said lens module comprising, in sequence from the original to the image sensor along the optical path: a first lens having a focal length and a shape factor with identical signs, and disposed to form the object distance with the original; a second lens having a focal length with a sign opposite to that of the focal length of said first lens, and a shape factor with a sign opposite to that of the shape factor of said first lens; a stop having a center and disposed such that the chief ray at zero-field and the chief ray at full-field pass through said center of said stop; a third lens with a positive focal length and a positive shape factor; a fourth lens with a positive focal length and a negative shape factor; and a fifth lens with a negative focal length and a positive shape factor; one of said first, second, third, fourth, and fifth lenses having a largest diameter and two opposite surfaces, said two opposite surfaces having curvature centers with identical signs; a ratio of a diameter of said stop to that of said fifth lens being less than 0.5.