Patent ID: 6999243

Claim:
An a thermalized zoom lens assembly comprising a plurality of lens elements disposed along an optical axis that passes through an aperture for viewing a scene to a focal point of the zoom lens assembly, said lens elements comprising in a sequence along said optical axis in a direction from the scene to the focal point: a first surface of a first lens element, said first lens element having a positive power and a second surface opposite said first surface; a first surface of a second lens element, said second lens element having a negative power and a second surface opposite said first surface; and a first surface of a third lens element, said third lens element having a positive power and a second surface opposite said first surface, where said first and third lens elements are comprised of a firm material that differs from a second material that comprises said second lens element, where a change in refractive index for a change in temperature (dn/dT) of said first material is less than a change in refractive index for a change in temperature of said second material by a predetermined amount, and where both of said second surface of said first lens element and said second surface of said third lens element are diffractive surfaces.