Bioscience Instruments : Product Overviews
CFI60 Optics Overview

 


  Why Nikon has chosen the CFI60 200/60/25 specification for the Biomedical Microscope
  • Technical framework for the design of the new Nikon Eclipse Series of CFI60 Infinity Optics for Biological Microscopes




Introduction
When the typical microscopist speaks about Infinity Optics, they may have this image of a dream optical system that can do anything. Some say performance increases when you use a microscope with an infinity optical system. So their reasoning is if it's not an infinity optical system, it is not performing at a high level. Are all manufacturers really trying hard to make this happen and to meet the expectations of users? It is true that infinity optics significantly improves system flexibility, but is infinity optical performance always superior to finite optical systems?

Nikon's CFI60 optical design team faced this proposition head on. They thoroughly studied the advantages and disadvantages of other manufacturers' systems, and found an optimum balance between optical performance and system flexibility. This document will help you understand why an infinity optical system, for biological applications, sets new performance standards incorporating a tube lens with a focal length of 200mm, an objective with a parfocal distance of 60mm, and an objective thread size of 25mm.

1. Why is the focal length of the tube lens 200mm?

In a finite optical system, after light from an object passes through the objective, it is directed toward the primary image plane (located at the eyepiece focus point) and converges there (Figure 1).

FIGURE 1

FIGURE 2

FIGURE 3
In an infinity optical system, however, light becomes a flux of parallel rays after passing through the objective and does not converge until after passing through the tube lens (Figure 2).

This does not mean that an infinite distance can be obtained after light passes the objective (up to the tube lens). After passing through the objective, light from an object on the optical axis moves parallel to this axis along the optical path. Light coming from the periphery of the object forms a flux of parallel rays and advances at a diagonal angle to the optical axis (Figure 3).

Because of this, there are occurrences where these rays of light can no longer be captured by the tube lens if the location of the tube lens is too far from the objective. This causes the image around the edges of the field of view to become dark or blurred, preventing the microscope from performing at its full potential. The term Infinity Optics simply means that light becomes a flux of parallel rays after passing through the objective, not that an infinite space is available inside the optical system.

If we are going to adopt infinity optics in order to further develop the microscope, we will need to increase the distance between the objective and tube lenses, as well as increase system flexibility. To lengthen this distance, we reduced the angle of the flux of parallel rays outside the optical axis. It is generally thought that a longer focal length for the tube lens will accomplish that, but this length has limitations.

The magnification (mo) of the objective in an infinity optics microscope is obtained using the formula: mo = tube lens focal length (ft)/objective focal length (fo) (Figure 2). If the focal length of the tube lens is lengthened, the distance to the image plane (at the eyepiece) would also increase with the longer focal length of the objective. Naturally, this makes the size of the microscope larger. With this in mind, the conclusion reached was that a focal length of 200mm would be the most appropriate for the tube lens.

The focal lengths adopted by other manufacturers are as follows:
Zeiss:160mm
Olympus:180mm
Leica:200mm


To obtain a same-size image from an object far from the optical axis, the longer focal length of the tube lens produces a smaller angle of light against the optical axis. The light rays do not spread out so the distance between the tube lens and the objective can be increased greatly enhancing the potential for system flexibility (Figure 4).

(Figure 4)


This design has certain optical advantages. As shown in Figure 5, when tube lenses of 160mm and 200mm focal lengths are compared, the 200mm lens produces a flux of off-axis light rays with a smaller angle. In this context, light rays passing through the phase ring in a phase contrast attachment, the DIC prism in a Nomarski DIC attachment, or the dichroic mirror in an epi-fluorescence attachment, produce smaller shifts between light elements parallel to the optical axis and those diagonal to it, so their accessories work more efficiently. This is a big optical advantage, and also a primary factor contributing to an improved level of contrast in epi-fluorescence microscopy.

(Figure 5)


2. Why is the parfocal distance of the objective 60mm?

Once the tube lens focal length was set to 200mm, the parfocal distance of the objective had to be increased from the standard 45mm.

As explained in Section I, the focal length of the objective is also increased in order to preserve the same magnification, and since 45mm does not provide optimum space in this design, a high-quality image cannot be obtained. In practice, the CF N Plan Apo 60X oil with a mechanical tube length of 160mm, believed to be the ultimate in finite objectives, is crowded with lenses in a limited space of 45mm. When this finite system is replaced with an infinite system and the objective is divided into an objective and a tube lens, the focal length of the tube lens becomes the equivalent of approximately 150mm. On this basis, we can calculate numerically for the optical performance which surpasses that of the finite system as follows:

The finite system objective parfocal distance = 45mm; for a tube lens focal length of 150mm, the infinite system objective parfocal distance = x; and the tube lens focal length = 200mm; in solving this proportion, if 45 : 150 = x : 200, then x = 60mm. Therefore, if the tube lens focal length = 200mm, the optimum objective parfocal distance has to be 60mm.

Zeiss, Olympus and Leica, have set the objective parfocal distance in their infinity optics systems to 45mm:

Zeiss: For 45 : 150 = x : 160, x = 48mm
(45mm is thus 3mm too short.)
Olympus: For 45 : 150 = x : 180, x = 54mm
(45mm is thus 9mm too short.)
Leica: For 45 : 150 = x : 200, x = 60mm
(45mm is thus 15mm too short.)
These results show that it is impossible for microscopes produced by other manufacturers to exploit the full potential of their objectives.

Since the working distance (WD) also increases to match the longer objective focal length, manufacturers who use a parfocal distance of 45mm are at a disadvantage in their inability to utilize the longer working distance by Nikon.

Using the Plan Apo 60X oil (N.A. 1.40) objective as a comparison, we see W.D.s by manufacturer as follows:

Nikon: 0.21mm
Olympus: 0.10mm
Zeiss: 0.09mm
Leica: 0.06mm


This shows that there are differences in ability to accommodate various types of specimens, as well as ease of operation.

Low-power lenses demand a specific size. If the magnification of the objective is 1X, the "mo = ft / fo" formula used in Section I shows that the focal length of the objective and that of the tube lens would have to be the same.

In Nikon's case, in order to perfect a tube lens of 200mm, a parfocal distance of 45mm would leave too little space in the design. By increasing this distance to 60mm, a magnification of 1X is obtainable and thanks to this revolutionary change, an objective with a magnification as low as 0.5X has been achieved.

The lowest magnification offered by Zeiss and Olympus is 1.25X, and Leica, 1.6X. None of them have produced a 1X objective yet.



3. Why use a 25mm objective thread size?

When the focal length of the tube lens is increased, the focal length of the objective must also increase. There is a limit to the objective pupil diameter (effective diameter remaining after the limits of the objective thread size), so a high numerical aperture (N.A.) cannot be obtained. Thus the N.A. of low-power lenses is critically affected.

At present, other manufacturers use a 20.32mm thread size, but as mentioned above, Nikon uses 25mm and is able to attain high N.A.

Originally, the brightness of photo lenses (F) was expressed with the formula:

F = f / D [f: lens focal length; D: effective diameter]

Since the N.A. of a microscope corresponds to the F value of a photo lens, the brightness can be expressed with the formula:

F = 1 / (2N.A.)

The effective diameter needed to achieve a desired N.A. can thus be found using this formula. In other words, the size of the pupil on an objective (effective diameter on the exit side) is expressed as:

D = 2N.A.x f

For example, to find the effective diameter of the CFI Plan Apo 4X (N.A. 0.2), objective with the highest (brightest) N.A.; given that the objective focal length is 50mm and where the focal length of the tube lens is 200mm, the following calculation is made:

D = 2 x 0.2 x 50 = 20mm

This shows that the conventional thread size of 20.32mm physically cannot be used.

The following table shows the various pupil diameters required for designing 4X objectives with N.A. 0.2 based on tube lens focal length, by manufacturer.


Company Tube Lens FL 4X Obj. FL Pupil Diam.
Zeiss 160mm 40mm 16mm
Olympus 180mm 45mm 18mm
Leica 200mm 50mm 20mm


In comparing the N. A. of the Plan Apo 4X:
Zeiss N.A. 0.16
Olympus N.A. 0.16


Leica has no applicable product (though the objective thread size is 25mm). Thus we can see that no other manufacturer even comes close to Nikon's N.A. of 0.20, which is the highest in the industry.

As shown, to obtain a high numerical aperture, a low-magnification objective requires a large pupil diameter. The longer the focal length of the tube lens, the greater the necessity to enlarge the thread size on the objective. Nikon has solved this problem by choosing a 25mm thread size for the CFI Infinity Optics system.



4. In conclusion

We trust these explanations accompanied by specific examples have helped you to understand why a tube lens of focal length 200mm is considered optimal for use in an infinity optical system and why higher optical specifications can be obtained with an objective parfocal distance of 60mm and a thread size of 25mm.

Though JIS and other conventional standards have been followed for mechanical dimensions, the adoption of infinity optics itself has necessitated a sacrifice in compatibility with conventional systems.

Thus, rather than be bound by conventional dimensions, Nikon felt that its true task was to create products that users need in today's cutting edge microscopy techniques. Innovations in engineering, manufacturing, quality control, inspection and production, have all contributed to the advent of Nikon's CFI60 series of optical systems.




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