Basic Principle of Infinity Optical Systems

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.
  

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.

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).