Interactive Tutorials - Spinning Disk Fundamentals

Interactive Tutorials

Basic Microscopy

Infinity Optical System Basics

Infinity-corrected microscope optical systems, which have overtaken the microscope market, are designed to enable the insertion of auxiliary optical devices, such as vertical illuminators, filter cubes, and intermediate tubes, into the optical pathway between the objective and eyepieces without introducing spherical aberration, requiring focus corrections, or creating other image problems. In a fixed tube length finite optical system, light passing through the objective converges at the image plane to produce an image. The situation is significantly different for infinity-corrected optical systems where the objective produces a flux of parallel light wavefronts 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.

The primary optical components of an infinity system are the objective, tube lens, eyepieces, and camera projection port. The specimen is located at 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. A portion of the light reaching the objective emanates from the periphery of the specimen, and enters the optical system at oblique angles, advancing diagonally (but still in parallel bundles) toward the tube lens. All of the light gathered by the tube lens is then focused at the intermediate image plane, and subsequently enlarged by the eyepiece.

In a finite optical system of fixed tube length, light passing through the objective is directed toward the intermediate image plane (located at the front focal plane of the eyepiece) and converges at that point, undergoing constructive and destructive interference to produce an image. The situation is different for infinity-corrected optical systems where the objective produces a flux of parallel light wavetrains imaged at infinity (often referred to as infinity space, and labeled in the tutorial window), which are brought into focus at the intermediate image plane by the tube lens. It should be noted that objectives designed for infinity-corrected microscopes are usually not interchangeable with those intended for a finite (160 or 170 millimeter) optical tube length microscope and vice versa. Infinity lenses suffer from enhanced spherical aberration when used on a finite microscope system due to lack of a tube lens.

The tube length in infinity-corrected microscopes is referred to as the reference focal length and ranges between 160 and 200 millimeters, depending upon the manufacturer. Correction for optical aberration in infinity systems is accomplished either through the tube lens or the objective(s). Residual lateral chromatic aberration in infinity objectives can be easily compensated by careful tube lens design, but some manufacturers choose to correct for spherical and chromatic aberrations in the objective lens itself. This is possible because of the development of proprietary new glass formulas that have extremely low dispersions. Other manufacturers utilize a combination of corrections in both the tube lens and objectives.

Contributing Authors

Rudi Rottenfusser - Zeiss Microscopy Consultant, 46 Landfall, Falmouth, Massachusetts, 02540.

Tadja Dragoo and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.

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