A simple offset filter contrast technique.

[apochronaut]

One of the differences between the standard wattage version of some microscopes and the higher, usually 100 watt version is that the 100 watt version includes more flexible options for filtering the light before it arrives at the condenser. This was the case for all of the 100 watt infinity corrected AO scopes; the 20, 120 and 420. The 20 was equipped with two 5 place filter carousels, just under the condenser, the 120 and 420 had three 5 place filter carousels, located just forward of the illuminator collector lens. Certain high output remote illuminators also had this facility.
The 20 system was like a 2 circle venn diagram , with each circle containing 4 filters and one open port. These could be overlapped at will and any filter discs of choice could be installed. This nice built in accessory was employed for transmitted fluorescence on the 20.
The 120 and 420 each had 3 in-line filter wheels, with each carrying an open. They likewise can be ganged at will.

One technique that can be employed with these filtering systems is to partially offset or partially overlap filters. In this way, a kind of modified oblique lighting can be created.

There are 3 pictures below, each taken with differing filters and degrees of offset taken through a Diastar 420. The objective is 40X and the crop is 50%.

[MicroBob]

Hi Apochronaut,
nice balance of contrast, resolution and little background gradient!
I don't know much about american microscopes and have never heard of multiple filter wheels. This must be really useful if you work with different filter sets and can swap over between them with just a twist of some wheels.
The second image has some kind of grid in the background - is this an artifact from downsizing or is it present in the original image too?

Bob

[coominya]

I can't seem to see the differences apochronaut, what should I be looking for?

[apochronaut]

I should have included a conventional BF image in the group for comparison. It's a little demonstration of the possibilities of bi-polar oblique illumination. Being able to balance the illumination with an opposite offset, reduces the depth of the relief but provides less shadowing and a bit of a cleaner image. One of the filter positions is held by a light block, so there are in fact 48 different filter combinations available, and then the degree of offset and direction can be altered, with each position.

I am not sure what that grid effect is, MicroBob. It probably is an artifact of downsizing.

As far as the differences, Coominya. There isn't a lot but the third one does seem to have more relief and a slightly sharper image. These are essentially , modified BF images. For some reference, the little strips in the substrate are actually 1/2 um wide gliding bacteria and the dots are cocci.

[Hobbyst46]

In addition, multiple filter wheels provide the flexibility of changing the intensity for any color filter, by using one wheel for a range of ND filters and the other wheel for the color filters.
Single filter wheels have been popular in fluorescence studies involving periodical changes of the excitation or the emission wavelengths, under computer control.

@Apochronaut, if I may ask, what is are the distances between the filter wheels and the other optical elements ? I believe that the effects are stronger for low NA objectives than for the 40x?

[apochronaut]

It's a fairly large stand. The filter carousels are just forward of the illuminator collector lens and it is a further 7.5cm. to the collimating lens, then about another 5cm. to the field iris, 5cm. to the mirror and then bounced another 8cm. to the condenser aperture.

Regarding the relationship to N.A. I suppose depth of field would play somewhat of a role.

Here are a few shots at about 1.15 or so N.A. ( .90 dry condenser with a 1.32 N.A. objective). It's a 15 um diatom, in a pretty thick water sample; probably about .20. phase this time.

[zzffnn]

Nice work, apo.

Images of objective back focal planes would help me better understand how exactly you positioned combinations of filters for oblique effects.

[apochronaut]

I can see if I can take some afocal pics of the shape of the illumination beam. It works out a bit like a slit condenser, with the two filter ports, becoming like elongated elipses the more the carousel is rotated. Certain filters, shifted in the opposite direction to each other, create a central ellipse, which narrows, the more the filters are shifted away from the central axis. Too much and the slit lowers the N.A. too much and there is a resolution loss.

There has a degree of similarity to the PZO slit diaphragm condenser, used for certain types of Interference contrast on the Biolar Polarizing Interference microscope. It has a travelling slit, adjustable for width and height offset. Similarly, the Goerz 3-D condenser utilizes a variable slit.

[manu de hanoi]

quite unrelated but I just got beautifull results playing with a dichroic cube