Introduction

An earlier article on ModernMicroscopy.com, Machining a Darkfield Insert for the Olympus BH2 1.25 NA Condenser, noted that the darkfield inserts were made for Peter Cooke (MICA, Chicago, IL) to be used to teach high-resolution dispersion staining in his advanced microscopy classes using Olympus BH2 microscopes. Peter has asked whether there is a better condenser design that will reduce the time and effort needed to switch between brightfield and darkfield. This has not been an issue for my student microscopes with external fiber-optic illumination systems because the darkfield stop is inserted at the end of the light-guide, which serves as the light source. This is not feasible for microscopes such as the Olympus BH2 with built-in illumination systems. My second student microscope is a modified LOMO Biolam (Multiscope) with more capability in transmitted light than my modified Monolux microscope¹. This added capability includes the phase contrast upgrade for the Multiscope, an aplanatic 1.40 NA condenser, and a ball-bearing rotary stage. The LOMO phase contrast condenser design seemed to provide the basis for a prototype to demonstrate an answer to Peter’s need for rapid change between brightfield and darkfield when the darkfield stop must be near the aperture diaphragm of the condenser. The phase contrast condenser allows this rapid switch only for the 10X objective, which needs the phase annulus for the 100X objective to achieve darkfield. The open position in the annulus wheel provides brightfield with an aperture diaphragm. I was not satisfied with the 0.8 NA condenser optics when used for brightfield with the 40X 0.65 NA objective. I found that the LOMO 1.25 NA Abbe condenser is able to achieve acceptable Koehler illumination with the 40X objective. My idea was to replace the phase contrast annuli in the centerable wheel with a set of darkfield stops and use the 1.25 NA Abbe lens in place of the 0.8 NA lens. Chris Vander Tuuk of LOMO America was generous enough to donate a base of the LOMO phase contrast condenser for building a prototype condenser. Since the top lens can be unscrewed from the top of the 1.25 NA condenser, my plan was to use this configuration with a suitable stop in the wheel with the 4X objective.  McCrone Microscopes and Accessories donated a diatom test plate used to demonstrate the performance of the prototype condenser. Oblique illumination is obtained with this condenser by decentering the stop and setting the aperture diaphragm at almost the full NA of the objective. Annular illumination, recently named circular oblique lighting (COL) by Paul James, is achieved by using a smaller stop so that the rear focal plane of the objective shows a narrow ring of illumination at just below the maximum NA of the objective².

Prototype Condenser Mounted in the Modified Biolam Microscope

Figure 1 shows my modified Biolam microscope fitted with the prototype condenser shown in Figure 2. The one-piece LOMO bracket shown in Figure 3, with a slot for the annulus wheel and an upper threaded ring for the condenser lens, was replaced with a two-piece design shown in Figure 4. The aperture diaphragm in the lower part of the bracket is now centerable while viewing the objective rear focal plane before final tightening of the two socket-head cap screws. Note that a male thread needed to mount the LOMO 1.25 NA Abbe lens assembly has replaced the female thread in the one-piece bracket.

click image to enlarge (240K) Figure 1

click image to enlarge (117K) Figure 2

click image to enlarge (81K) Figure 3

click image to enlarge (114K) Figure 4

The wheel, before the phase annuli were replaced with darkfield stops, is shown in Figure 5. (Close-up examination of the center of the wheel will show a partially completed detent spring needed to replace the spring accidentally broken in a failed attempt to disassemble the center pivot assembly.) All of the annuli were removed and the openings for them bored out to contain the darkfield stops shown mounted in the wheel in Figure 6. Observation of the rear focal planes of the objectives was of critical importance for determining the stop size just sufficient to give a good dark ground as well as to achieve proper centering of the stop using the centering screws shown in Figure 4. These operations were done using a 25X Klein loupe slipped over the eyepiece as shown in Figure 7. The short eyepoint of this loupe, fabricated from a 30 mm stereo microscope 25X eyepiece, does not permit digital image recording with my Nikon CoolPix^® 995 camera. This image recording can be done with this microscope using its drawtube end-mounted 1X objective shown in my online article in Micscape³. That article also contains operating ray diagrams along with close-up views of the system components including the dovetailed attachment for the analyzer and polymer wave-plates, and the mating drawtube holder.

click image to enlarge (211K) Figure 5

click image to enlarge (161K) Figure 6

click image to enlarge (236K) Figure 7