NA greater than 1

Malcolm Storey · Post by **Malcolm Storey** » Sat Oct 17, 2015 3:02 am

100x oil immersion objectives are typically marked NA 1.3. Without using an oil immersion condenser the NA is always less than 1.0 due to diffraction at the slide lower surface and the angle subtended by the condenser exit pupil.

How about using a slide with a ground glass under-surface? The Kohler illumination would then be reduced to a simple light concentrator, but the subject would be infused with diffuse light from pretty close to the full hemisphere.

I've never seen slides with one side ground/etched, but you do often see them with an etched area at one end for writing on. Unfortunately I can't lay my hands on one at the moment...

Beatsy · Post by **Beatsy** » Sat Oct 17, 2015 6:57 am

Wouldn't a piece of white paper between slide and light source produce the same effect? Possibly oiled in place to ensure light entering from all angles

Pau · Post by **Pau** » Sat Oct 17, 2015 7:44 am

You can test the idea yourself but I strongly suspect that it won't work very well.

I've seen a simple frosted glass acting as condenser in very cheap and bad children microscopes, and many stereos use a ground glass for transillumination (good only for low power)

Zeiss West made a more ellaborated approach with the Lucigen condenser based in the same principle but it was publicited because the easy of use, not for high resoluiton in reseach microscopes.
http://www.science-info.net/docs/zeiss/ ... andard.pdf

In any case, having a condenser NA wider than the objective NA will not provide any advantage, despite what the literal interpretation of the Abbe's formula could suggest.

Malcolm Storey · Post by **Malcolm Storey** » Sat Oct 17, 2015 11:37 am

Beatsy: agreed, paper oiled to underside would be equivalent - although a little messy!

Pau: Was aiming to raise the condenser NA to closer to that of the objective, rather than trying to exceedt it.

Too high an NA can be useful reduce contrast in very contrasty subjects at low mag.

Anyway I've ordered some frosted glass slides on eBay so I'll let you know!! Apparently they're used for cell cultures (or something - couldn't find any details, just the sales patter).

ChrisR · Post by **ChrisR** » Sat Oct 17, 2015 1:50 pm

Tipp-Ex might be good. Dilute with the thinners they provide, to adjust opacity. With a ringing table and a paintbrush you could give it a circular, graded profile..

phil m · Post by **phil m** » Sun Oct 25, 2015 11:54 am

In any case, having a condenser NA wider than the objective NA will not provide any advantage, despite what the literal interpretation of the Abbe's formula could suggest.

Many condensers are stamped 1.2, which I have found puzzling because they are integrated into systems no different than those with 1.25 N.A. condensers. It would seem that there is an admission at work here that in practical use, matching the objective's N.A. exactly does not happen for one of several reasons.

If this is true, then using a 1.3 or 1.4 N.A. condenser could possibly raise the N.A. to the optimum and improve the performance of a 1.25 N.A. objective slightly.

I did some tests using dry condensers, with higher N.A. dry objectives.

I did these with 5 different condensers : an abbe 1.25, abbe aspheric 1.25, achromat .90 , achromat 1.4 and a home built 4 element air spaced condenser that appears to have an N.A. greater than the 1.4 achromat when both are tested dry, the only one I have compared it to. These were all in mounts made for a specific microscope and the 4 of them, aside from the DIY were in fact made for that microscope.

The test objectives were a 40X .70 planfluorite and a 60X .80 planachro, both dry and I was looking primarily for differences in resolution.

The results were a little surprising because they indicated that the 1.4 N.A. achromat might have a lower N.A. when dry, than one would expect. It really was no better than the 1.25 abbe aspheric and the .90 achromat was the pick of the litter for the commercially made condensers. The abbe 1.25 clearly lagged behind, indicating that it was having difficulty in meeting the demands of the .70 objective, by a little. The DIY condenser fared very well and was about equal to the .90 achromat but seems a little brighter. For this reason it turns out to be a more comfortable condenser to use with higher N.A. dry objectives.

I should test these all with oil immersion objectives and then again as immersed condensers with dry objectives and immersed condensers with immersed objectives.

Pau · Post by **Pau** » Sun Oct 25, 2015 1:05 pm

The results were a little surprising because they indicated that the 1.4 N.A. achromat might have a lower N.A. when dry, than one would expect. It really was no better than the 1.25 abbe aspheric and the .90 achromat was the pick of the litter for the commercially made condensers. The abbe 1.25 clearly lagged behind, indicating that it was having difficulty in meeting the demands of the .70 objective

Yes, little surprising: Without immersion the actual maximum NA of any condenser will be about 0.9, and a 0.7 objective will not profit of higher than 0.7 NA condenser. The uncorrected Abbe has too much aberrations to profit its NA.
Very interesting that you has been able to make a well working DIY condenser.

Malcolm Storey · Post by **Malcolm Storey** » Sun Oct 25, 2015 2:20 pm

I bought some so-called frosted slides ("SuperFrost") but they just had a layer of white plastic paint on one end for writing on. Anyway, I gave it a go using the frosted part of the slide (frosting down) with the 100x oil immersion obj and photographed the result. Running the image through Gimp/Wavelet Transform showed there was a tiny bit more resolution than with a standard slide. To the eye, the image looked heavier - thin lines had become thicker. It was the same subject (Russula fungus spores in Melzer's iodine), but of course not the same slide prep. so not directly comparable.

I'm trying to borrow some etched slides to try next cos I think they'll give a better scatter and less light lost to reflection.

phil m · Post by **phil m** » Sun Oct 25, 2015 3:00 pm

This was with a dry condenser?

Malcolm Storey · Post by **Malcolm Storey** » Sun Oct 25, 2015 3:24 pm

Yes, a dry condenser. That was the point really.

phil m · Post by **phil m** » Mon Oct 26, 2015 5:53 am

It seems the N.A. of the condenser is at it's maximum when it is precisely focused. When used dry, there is a point where the N.A., as measured by brightness, reaches a maximum very close to the slide but then reduces slightly with a closer approach to the slide. The air gap increases the focal distance. The same is true with an oiled condenser, where a specific thickness of oil produces the optimum N.A. Thus, certain research microscopes employing very high N.A. apochromats, had a coarse and fine focus controlling the condenser.
There can be no possible increase in N.A.above the condenser's optimum but the light could possibly be dispersed more evenly, so the condenser might function better with low power objectives and provide a more even background for photography,with higher N.A. objectives, especially with an uncorrected abbe condenser.

If you could determine the point of focus with a lower n oil( cooking oil, 1.46-7) than immersion oil, in order to compensate for the slight air gaps, you would encounter using a solid filter and then measure that condenser to slide gap, you could replace that oil with a white dispersion filter, milk plastic, paper or whatever works of the same dimension under the slide. Racking the condenser top lens right against it would give close to an ideal homogeneity for the condensers function, the air gaps would be very tiny, and if you liked what you achieved, you could find or cut a small piece of the dimensional dispersion filter you used and cement it to the top of the condenser, or just use it dry, whichever worked best. You could test the cement idea by first using oil.

Malcolm Storey · Post by **Malcolm Storey** » Mon Oct 26, 2015 7:57 am

Thanks for your thoughts but none of these address the limiting factor that refraction at the air surface reduces the cone of illumination. What I'm trying to do is use a diffuser bonded to the lower surface of the slide (and so optically part of it) to create a larger angle than is possible with an air gap - the same as happens when you oil the condenser to the slide.

My Olympus condenser isn't designed to be oiled so I don't want to risk getting fluid into it or dissolving the cement. Olympus supply another model of condenser for oiling to the slide.

phil m · Post by **phil m** » Mon Oct 26, 2015 2:07 pm

By introducing numerous small angles onto the lower surface of the slide, there will be a net loss of light. Changing the lower plane surface to one of numerous random angles,pits and bumps, will create areas where more diffraction, more internal reflection,more oblique reflection and more absorption will occur.

Malcolm Storey · Post by **Malcolm Storey** » Mon Oct 26, 2015 2:21 pm

Getting sufficient brightness isn't a problem. But the resulting illumination will come from all angles so should make full use of the N.A. 1.30 of the oil immersion 100x objective.

phil m · Post by **phil m** » Mon Oct 26, 2015 5:54 pm

Have a look here. http://micro.magnet.fsu.edu/primer/anat ... nsers.html