Basic Principles of Infinity Optical Systems

Abstract

Infinity corrected microscopes have become the industry standard thanks to their ability to add optical components to the image forming beam path without causing image aberrations. The parallel (infinite) light rays exiting such objectives are focused by a tube lens onto the intermediate image plane. However, it is important to note that each microscope manufacturer’s infinity concept is proprietary. Mixing infinity objectives from different manufacturers will result in incorrect objective magnifications and strong lateral chromatic aberrations.

Key Learnings:

Infinity corrected optical systems allow optical components to be inserted into the “infinity space” without causing image problems.
The tube lens in an infinity corrected microscope is crucial in focusing parallel light rays to the intermediate image plane and also often correcting lateral chromatic aberration.
Mixing infinity objectives from different manufacturers is not possible.

A cross-sectional illustration showing the beam path of an optical microscope with condenser, specimen plane, infinity objective, infinity space, tube lens, intermediate image plane, and eyepiece.

Benefits of Infinity Corrected Optical Systems

Infinity corrected microscopes have been the industry standard since the 1970s. Although infinite optical beam paths have long been popular (e.g. in polarized and reflected light microscopes) by integrating intermediate infinity spaces (e.g. by using a “Telan lens” system), the modern infinity optics have strong advantages also in transmitted light contrasting methods or reflected light fluorescence: It becomes possible to insert optical components, such as beam splitters, emission filters, DIC sliders, and intermediate tubes into the optical path above the objective.

This can be done without introducing spherical aberration, astigmatism, the need for focus correction, or causing other image problems. In a classical finite optical system, the light passing through the objective converges, so adding components (e.g. beam splitters) to such beam paths would cause image quality issues. The situation is significantly different in modern infinity corrected optical systems, where the objective itself can produce parallel rays. By adding a tube lens, these rays are focused into the intermediate image plane where the microscopic image is formed.

The illustration depicts a typical beam path with condenser, specimen, infinity objective, infinity space, tube lens, intermediate image plane, and eyepiece. The parallel rays of light rays exiting the objective are focused by the tube lens onto the intermediate image plane. This is where the eyepiece field diaphragm is also located. For a given optical design/manufacturer, the distance between the tube lens and the intermediate image plane (= focal length of the tube lens) is exactly defined. Common values are f= 164.5 mm, 180 mm, 200 mm, and 250 mm respectively. As each of these infinity systems is incompatible with any other, mixing infinity objectives from different manufacturers will at the very least result in incorrect objective magnification, but often also introduce strong chromatic aberrations.

Tube Lens Focal Length and Chromatic Aberration Correction Are Important in Infinity Corrected Microscopes

The tube lens in a given infinity corrected microscope has a specific focal length and color behavior. Its focal length is sometimes called the “reference focal length” and ranges from typically 164.5 to 250 mm, depending on the manufacturer. The lateral chromatic aberration – often referred to as “CVD”, German=“Chromatische Vegroesserungsdifferenz” – in an infinity system is corrected either by the objective alone or by the tube lens and objective together. Any residual longitudinal chromatic aberration is bound to the color correction grade of the objective (achromat, semi-apochromat or apochromat).