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 betwen 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.

Introduction to Infinity Optical Systems - Over the past 10 years, the major microscope manufacturers have largely all migrated to the utilization of infinity-corrected optical systems in both research-grade biomedical and industrial microscopes. 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.

Tube Lens Focal Length - As the focal length of the tube lens is increased, the distance to the intermediate image plane also increases, which results in a longer overall tube length. Tube lengths between 200 and 250 millimeters are considered optimal, because longer focal lengths will produce a smaller off-axis angle for diagonal light rays, reducing system artifacts. Longer tube lengths also increase the flexibility of the system with regard to the design of accessory components. This interactive Java tutorial explores the effect of tube lens focal length on the angle of off-axis light rays in microscopes with infinity-corrected optical systems.

Objective Magnification in Infinity Optical Systems - Infinity-corrected microscope optical systems are designed to enable the insertion of auxiliary devices, such as vertical illuminators and intermediate tubes, into the optical pathway between the objective and eyepieces without introducing spherical aberration, focus corrections, or creating other image problems. In a finite optical system, light passing through the objective converges at the image plane to produce an image. The situation is quite different for infinity-corrected optical systems where the objective produces a flux of parallel light wavetrains imaged at infinity, which are brought into focus at the intermediate image plane by the tube lens. This tutorial explores 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 regrading how microscopes are configured to take advantage of infinity optics.