Infinity-Corrected Optical Systems

In modern research-grade microscopes equipped with infinity-corrected optical systems, the objective no longer projects the intermediate image directly into the intermediate image plane. Instead, the objectives are designed so that light emerging from the rear aperture is focused to infinity, and a second lens, known as the tube lens, form the image at its focal plane. Light rays exiting the infinity-focused objective lens are collimated, so that beamsplitters, polarizers, Wollaston or Nomarski prisms, vertical illuminators and other components requiring a parallel beam can be easily introduced into the optical pathway. After passing through these auxiliary optical devices, the parallel light beam is made to converge and form an image by the tube lens. Unlike the situation with fixed-tube optics, magnification of the intermediate image in infinity optical systems is calculated by the ratio of focal lengths between the tube lens and objective. Because the focal length of the tube lens varies between 160 and 250 millimeters (depending upon the manufacturer), the focal length of the objective can no longer be assumed to be 160 millimeters divided by its magnification.