Role of Condenser Spherical Aberration on Image Quality

The need to raise the condenser from the height setting set for Koehler illumination with the 9X objective for fully filling the rear focal plane of the 40X objective and an additional adjustment upward needed for darkfield illumination is clear evidence of spherical aberration. I decided to image the same field using the LOMO aplanatic condenser, both with the field diaphragm imaged just outside the field of view and fully open and the aperture diaphragm at the end of the light-guide source. I found that the image with the diaphragm fully open, see Figure 18, had better contrast than the brightfield image in Figure 15 taken with the prototype condenser. The image taken with the field diaphragm just outside the field of view had the highest contrast as expected, see Figure 19. These results stress the importance of the condenser being corrected for spherical aberration (aplanatic). The wave optical treatment of image formation and resolution assumes that the object be illuminated with spherical or plane wave fronts. This is not the case for illumination with spherical aberration. The wave front phase relationship for proper destructive and constructive interference to form the image is altered by the spherical aberration. 

click image to enlarge (165K) Figure 18

click image to enlarge (160K) Figure 19

The late Edward P. Herlihy, who was a fellow of the Royal Microscopical Society and Vice-President of the Quekett Microscopical Club, lamented the almost universal use of the Abbe condenser, which has an aplanatic aperture far below its claimed aperture value⁵. Herlihy notes that only the aplanatic aperture is useful for microscopy and that the achromatic condenser is far superior. Dr. Walter McCrone has stated in his requirements for a good polarized light microscope that the condenser be aplanatic⁶. Barry Ellam’s article in The Amateur Diatomist notes that the Abbe condenser can be satisfactorily used with darkfield stops⁷. Barry notes that there are problems caused by the lack of correction for spherical aberration in brightfield. He notes that these difficulties can be easily overcome by use of annular illumination (renamed COL by Paul James) long favored by diatomists for resolving the most difficult specimens, especially with a green filter. 

Effects of Condenser Spherical Aberration at Rear Focal Plane of a High NA Objective

My first exposure to the problems caused by condenser spherical aberration were while attempting to demonstrate for John Delly the imaging of the interference figure from a Mylar film using my modified Monolux microscope¹. He showed how to fill the rear focal plane of the 60X 0.85 NA objective with much more of the figure by significantly raising the condenser from the position giving Koehler illumination with the 10X objective and then fully opening the field diaphragm. The resulting images were included in my student microscope article and are reproduced in this article along with ray diagrams recently done for this article that indicate what is occurring. 

I first set up proper Koehler illumination for the 10X objective using a tissue. I then swung in the 60X 0.85 NA objective and reduced the field diaphragm opening just outside the field of the 60X objective. I then replaced the tissue section with a thin film of polyester. John realized that the rear focal plane was not being fully filled with illumination even when the aperture diaphragm was fully open shown in Figure 20. He suggested raising the condenser, which brought a ring of illumination at the outer edge of the rear focal plane along with an inner dark zone and a center bright spot shown in Figure 21. He then suggested fully opening the field diaphragm along with inserting the analyzer.

click image to enlarge (215K) Figure 20

click image to enlarge (263K) Figure 21

click image to enlarge (90K) Figure 22