However, in practical microscopy, the state of the condenser to obtain adequate image contrast is very important. Without this correction - even if theoretically resolvable - the structure (based upon aperture consideration alone) remains invisible. The fact is that the image contrast is directly affected by the correction of the condenser.

We will try to show that only a rather highly corrected condenser system permits the use of theoretically desirable condenser apertures. The substage condenser in a microscope for transmitted light - according to the principle of Köhler illumination - performs a two-fold function:
First, it transilluminates the specimen with parallel light in a cone of high aperture. This can be seen in Figures 7a and 7b.

Fig.7a shows a pencil of transilluminating light originating from the front focal plane of the condenser parallel to the optical axis.
Fig.7b shows a similar parallel pencil of light which originates at the periphery of the aperture stop. Although this pencil of light passes through the specimen at a high angle of incidence, it is parallel in itself. The sum total of all pencils of light originating from each area in the plane of the aperture diaphragm forms a cone of high aperture light. The front focal plane of the condenser is conjugate to the plane of the light source. An elarged image of the light source is projected into this conjugate plane to serve as a substitute for the light source.
Fig.8 shows how this is achieved by means of a collector lens.

The second function of the condenser according to the Köhler principle is to project an image of the field diaphragm sharply into the specimen plane. This is shown in Fig.9.

The field iris is usually mounted at some distance from, and below the aperture stop. Its image restricts the illuminated specimen area; this eliminates glare and light scattered from areas not in the field of view. Correction and quality of this reduced field diaphragm image in the object (specimen) plane have a profound influence upon the final quality and resolution.

The aperture stop regulates the angle of the cone of transilluminating light, or the "aperture of the illumination". It adjusts image contrast and resolution, and should under no circumstances be used to adjust light intensity.

The original formulation of the Abbe theory of image resolution assumed not a fully opened, but a rather narrowly closed aperture diaphragm. The theory explains that the image of two closely neighbored, transilluminated object points will show them as being two distinct points, or being "visually resolved", when their distance apart is not less than:

where: 'lambda' is the operational wavelength, and 'nAobj' is the objective's numerical aperture [1].

The two diffraction disks will then be seperated by an intensity dip of approximately 20%, and this is visually required to recognize a seperation.

Abbe also discussed the case of a narrowly closed, but highly de-centered aperture stop. Under these conditions of transillumination with a high angle of incidence, or under high aperture, the same 20% intensity dip in the elementary image of the two close object points may be obtained when the points are seperated by only a distance of approximately:

If one now transilluminates a specimen with a fully opened aperture stop, the intensities of all the elementary (intermediate) images will be superposed. Only the high aperture light, however, will produce an elementary image that resolves the fine detail under study.

Unfortunately the combined, superposed intensities of all the other, non-resolving, or structure revealing elementary images are so much stronger that in most cases it is not possible to maintain the required contrast in the resolving elementary image. The fine detail in the object would have to have very high inherent contrast. It is, therefore, rarely possible to reach the theoretical limit of resolution with such a specimen as they occur in practice. The discussion of the effect of high aperture illumination, the need for, and the desirability of high condenser apertures shows quite clearly that, by definition, microscopic resolution is a function of the image contrast attained. In brightfield microscopy this is naturally lower than the object contrast; the contrast loss being detemined by the contrast transfer capablities of the optical system of the microscope.

The empirical equation usually given for microscopic resolution:

implies such a high degree of object contrast that the latter is not the limiting factor. Most biological preparations, however, do by far not even approach such high inherent contrasts, It is, therefore, in practical microscopy almost always neccessary to close the aperture stop to some extent so that the image attains sufficient contrast.
One will rarely work with illuminating apertures of much more than 0.9, even when an immersed condenser top element is employed. High condenser aperture is, therefore, not necessarily of such advantage in routine work as is occasionally assumed. The demand for condensers with high apertures is probably to be explained by the fact that high aperture condensers were the best corrected systems.

The optical correction of the condenser has a very substantial effect upon what is generally described as "image crispness", clarity or "definition". These are not terms that one would use in quantitative analysis of the system, but they convey the microscopist's direct visual impression. The condenser correction directly affects image contrast, and with this, the attainable resolution - especially when Köhler illumination is employed.

An ideally corrected condenser should project a sharp, reduced image of the field stop into the object plane; and it should transilluminate the object with strictly parrallel pencils of light up to high apertures. This requires the following measures:
  - spherical aberration correction
  - chromatic aberration correction
  - correction for chromatic difference of spherical correction (Gauss aberration)
  - fulfillment of the sine-condition [2]
Systems in which the chromatic difference of the spherical aberration is eliminated, and which fulfill the sine-condition are called "aplanatic systems".

Spherical correcton of a condenser is the minimum requirement. Without it the different lens zones of the condenser produce images of the field diaphragm plane at different focal distances; the high aperture rays form an image much closer to the condenser than the central rays. The result is an indistinct image of the field diaphragm and low contrast in the final image of the specimen. It is not recommended to use a spherically uncorrected condenser for photomicrography. Condensers with an aspherical correction only may be used for b/w photography, provided that a strict monochromatic green filter is employed. Chromatic correction is necessary for a condenser used in color photomicrography. Chromatic abberation produces not one, but a sequence of colored images of the field diaphragm plane, one behind the other at different focal distances from the condenser. But even achromatic condensers are rather sensitive with respect to improper focusing. A condenser position only a few hundred microns below or above the prescribed setting will, in color photography, invariably produce a strong color hue in the background, and a definite lack of image definition. The apparent shift in the color temperature due to incorrect condenser focus is much more pronounced than usually results from an incorrect setting of the light source transformer (color variation with lamp current). Too high a condenser focus causes a bluish, too low a focus a yellow cast in the background.

The best systems are achromatic-aplanatic condensers. Systems of this type have spherical aberrations corrected not only for one color but for a wider range of the spectrum; simultaneously the sine-condition is fulfilled. The sine-condition in short demands the following:
  The condenser projects a reduced image of the field diaphragm into the object plane. In a spherically corrected system the different lens zones will all project this image at the same focal distance. The different lens zones may, however, produce images with different lateral magnifications, or as it would be the case here, different lateral reduction ratios. The result is an indistinct image. When the sine-condition is fulfilled, all lens zones in the condenser, the high aperture zones as well as the central ones, produce a field diaphragm image with one and the same reduction ratio.

In the discussion of desirable condenser qualities and specifications the question of condenser aperture receives great attention. Practical experience as well as theoretical considerations, however, have shown that the degree of condenser correction has a decisive influence upon final image quality and attainable resolution. A high degree of condenser correction, therefore, is the prerequisite of the use of high condenser apertures, and should by all means be a primary consideration.

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