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.
The basic optical components of an infinity system are the objective, tube lens, and the eyepieces. For observation, a specimen is placed near the front focal plane of the objective, which gathers light transmitted through or reflected from the central portion of the specimen and produces a parallel bundle of rays projected along the optical axis of the microscope toward the tube lens.
Uncover the effect of tube lens focal length (short and long) on the angle of off-axis light rays in microscopes with infinity-corrected optical systems in this interactive java tutorial.
Explore how changes in tube lens and objective focal length affect the magnification power of the objective in infinity-corrected microscopes.
Selected Literature References
Gathered from our vast library of literature on optical microscopy, the reference materials listed in this section are an excellent source of additional information on infinity-corrected optical systems. Included in this section are references to review articles, original research reports, and book chapters that discuss various aspects of the theory and applications regarding how microscopes are configured to take advantage of infinity optics.