Abstract

The residual absorption of materials for use in handling of carbon dioxide laser radiation causes radial thermal gradients in rim cooled lenses. A model is presented, which considers the effects of thermal expansion, the varying refractive index over the ray height, and ray bending due to the radial index gradient. The last effect is predominant in most practical lenses. The model has been verified experimentally using lenses with 1% to 2% absorption, and agreement between measured and calculated focal shift within 10 percent has been reached.

The effect has practical consequences for the design of high performance lens systems, because the thermal gradient also affects the spherical aberration. As one may expect, that lens absorption gradually increases over the lifetime, a design should be optimized for a somewhat higher temperature gradient than would be found in new lens elements. A design example is discussed.

© 1994 Optical Society of America