Using a proprietary optical design program, Computer Optics Inc. (COI; Hudson NH; www.computeroptics.com) has produced an optically compensated infrared zoom (CIZ) lens consisting of only one moving part. Using aspheric elements and a lightweight mechanical apparatus, the optically compensated zoom lens is believed to be the only available infrared (IR) continuous zoom lens of its type, according to Jonathan S. Kane, vice president of engineering at Computer Optics.
The CIZ lens is designed to operate in the 3- to 5-µm spectral band with an aperture of less than ƒ/4.5, a length of less than 150 mm, a 4X zoom range covering focal lengths from 50 to 200 mm, and an angular field of 5° to 20°. This new lens has been tested with several cameras, including a Merlin camera from Indigo Systems (Goleta, CA; www.indigosystems.com).
Other companies, such as Thales (Alexandria, VA; www.thalesgroup.com), however, have produced 3- to 5-µm IR zoom lenses for specific projects. In addition, a few commercially available two-position lenses offer two distinct, as opposed to continuously varying, focal lengths.
"In optical-compensation afocal lens systems," says Arthur Cox, COI chief optical designer, "all the moving elements travel at the same rate by attaching them to a common control member." Spaced in front of, between, and behind the moving members are fixed elements, usually of opposite power. If there are two moving members (1 and 3) with respective powers π1 and π3 and two fixed members with powers π2 and π4,, then the separation between consecutive elements are d1 + x, d2 - x, and d3 + x, where d1, d2, and d3 refer to an arbitrary zero position of the moving elements.
If a (b, ß) trace is performed with ß1 = 1, where b is the slope and b1 is the height of the ray from the object to image plane, then these parameters result in a cubic equation that can be used to minimize the departure from afocality (see figure on p. 8). Should four separations be used, a quartic equation results. Under suitable conditions this equation has four real roots.
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"The fact that there are two roots, which lie between the roots giving the extremes of magnification, means that the residual departure from the exact afocal condition is substantially reduced to levels even lower than are obtained with the three separation system," says Cox.
"To establish suitable values of π1, d1, and so forth, to give the afocal condition at suitable magnifications, individual optical elements must be designed that operate at multiple positions," he adds. Since the material of these lens elements are fixed and cannot change as they move, fixed-focal-length computer programs are relatively useless for zoom-lens design. As a result, COI developed a proprietary zoom-lens optical-design program. For a description see: www.computeroptics.com.
