dark field condenser NA

[mete]

I have the current Zeiss achromatic-aplanatic condenser including the dark field stop in the turret. The condenser is 0.9 H/BF, and 0.75 DF. The DF works fine on an objective with NA=0.5, but on an objective with NA=0.75, it does not work, some part of the view is dark other part is not etc. So a few questions:

- The dark field NA (0.75) is smaller than bright field (0.9). Is it because the maximum size of the light cone the condenser passes is as wide as NA=0.9, but then center has to be blocked, so there should always be some area left around the dark field stop ? So 0.75 represents the size of the block (hence the size of aperture of the objective?), and basically remaining 0.15 is where the scattered light is coming from ?

- Do you think it is normal I have this problem with an objective exactly at the same NA as the dark field ? I also checked it with auxiliary eyepiece and it is totally dark in the objective with NA=0.5 but a small amount of light is entering in the objective with NA=0.75. I tried a bit but I think it is not an aligning issue, but not 100% sure. Edit: I tried a bit more and I managed to center it enough to have a uniform dark background view but still auxiliary eyepiece shows a very thin halo around the dark stop, I guess because of this I do not have a proper DF (the edges are not sharp) at the limit of dark field condenser.

- If I make a darkfield stop, with the blocked area slightly larger than the one I currently have (lets say for NA=0.8, so the area scattered light will come from will be smaller), would it work for the objective having NA=0.75 ? Is there a more complicated calculation that has to be done ? My light source is pretty powerful so I dont think decreasing this area can affect the amount of light input negatively.

[apochronaut]

The reason your DF N.A. is .75 is that a D.F. condenser has two N.A. specifications ; a minimum and a maximum, representing the diameter of the outside ring of the cone of light and the diameter of the inside of the ring. The outside or maximum N.A. of the DF cone will be the same as the BF N.A. or .9., so the spec. on the condenser when used for DF is .75 - .90.

The minimum condenser N.A. for a DF condenser is the determining factor as to what the N.A. of an objective that will provide quality DF will be. It is usually about .20 lower than the condenser due to various variables causing scatter, so your finding of a .50 objective working well is predictable.

A light stop of .80, could in theory give you DF with a .75 N.A. objective but due to scatter in the sample , you are more likely going to get something akin to dusk, or what I call duskfield.

[mete]

Thanks for the explanation. It seems I have to try this, it would be nice to have DF at NA=0.75.

[apochronaut]

It is very difficult to achieve DF with DF stops above about .65. Theoretically possible but practically unlikely. The higher you raise the N.A. , the harder it is to center the stop. As well the larger the stop, the useful illumination circle is entirely in the uncorrected or more poorly corrected part of the condenser ; the periphery giving relatively poor performance. .If you are working with a well corrected condenser, at least that part of the picture might be o.k. but the better DF condensers tend to be better corrected condensers than even an achromat aplanat, especially at the periphery where the N.A. of condensers falls off.
Older AO DF condensers are still abundant and often go for 30-$40 . DF condensers are universal in optical performance, so if you can get any one to focus at the stage with a little adapting, even the really old ones will give you at least a 500 micron illumination circle, so will cover the field for 40X and up. They are mirror condensers with silvered mirrors. The dovetail comes off those older AO condensers with a couple of screws and they are quite convertible to other stands.
The version 214F made since roughly 1950 for the series 15/35, 2/4 and 10/20 will likely be a little more but even they can be cheap, super well made and quite convertible.

[Phill Brown]

Is the top lens of the condenser in contact or near as can be with the slide?
Is the condenser perfectly centred.
My experience differs slightly.
0.9n/a is a dry condenser.
Relatively low n/a, it is possible that under perfect conditions that DF is achievable at n/a .75 or less.
With an air gap/focusing for BF it will most likely not be possible.
With the condenser out of alignment it will likely not be possible either.
If you can get DF with 40x .65 you have near the limit for that.
Good luck anyway.

[mete]

Is the top lens of the condenser in contact or near as can be with the slide?

Not sure if you mean something else but I align Kohler first, so it is at the place it should be. But yes it is quite close to the slide.

Is the condenser perfectly centred.

I realized it was not perfect (although it was working with NA=0.5 objective) when I asked the question first but then I centered it as good as possible. That is where I still see a very slight light entering with the auxiliary scope, for NA=0.75 objective.

My experience differs slightly.
0.9n/a is a dry condenser.
Relatively low n/a, it is possible that under perfect conditions that DF is achievable at n/a .75 or less.
With an air gap/focusing for BF it will most likely not be possible.
With the condenser out of alignment it will likely not be possible either.
If you can get DF with 40x .65 you have near the limit for that.
Good luck anyway.

I dont use the objective with NA=0.65 normally, but I mounted it and tried, and there is no problem with DF. I dont have one between 0.65 and 0.75, so do not know about 0.70 etc.

[mete]

It is very difficult to achieve DF with DF stops above about .65. Theoretically possible but practically unlikely. The higher you raise the N.A. , the harder it is to center the stop. As well the larger the stop, the useful illumination circle is entirely in the uncorrected or more poorly corrected part of the condenser ; the periphery giving relatively poor performance. .If you are working with a well corrected condenser, at least that part of the picture might be o.k. but the better DF condensers tend to be better corrected condensers than even an achromat aplanat, especially at the periphery where the N.A. of condensers falls off.

I thought DF is always made with a DF patch in the condenser, I wasnt aware of special DF condensers before asking this. I learned now.

Older AO DF condensers are still abundant and often go for 30-$40 . DF condensers are universal in optical performance, so if you can get any one to focus at the stage with a little adapting, even the really old ones will give you at least a 500 micron illumination circle, so will cover the field for 40X and up. They are mirror condensers with silvered mirrors. The dovetail comes off those older AO condensers with a couple of screws and they are quite convertible to other stands.
The version 214F made since roughly 1950 for the series 15/35, 2/4 and 10/20 will likely be a little more but even they can be cheap, super well made and quite convertible.

I will check, thanks.

[apochronaut]

DF condensers are generally in two categories , dry and immersion. The dry ones are simpler in design, more approximating the design of a BF condenser often but with a pecisely fixed stop. The immersion condensers are quite different with most using a combination of reflection and refraction, rather than just refraction. The reflectors can be sphericsl, paraboloid or cardioid.and some use plano as well.

DF condensers have two N.As, a maximum and minimum, represented physically by the outer and inner border of the illumination cone. The outer N.A. of the condenser is the determining factor of the theooretical resolving capability and the inner N.A. is the determining factor of both the practical resolving capability and thus the upper N.A. limit possible. The absolute theotretical limit of the N.As. of a dry DF condenser is .90 - 1.0. This would allow for it's use up to .85 under ideal conditions. I have never seen such a condenser, nor have I ever seen verified DF at .85 with a dry condenser. In practice, most dry DF condensers are .70 or .75 - .90. giving good performance at .65 N.A.. With immaculate cleanliness and a discreet uniform sample free of amorphous material, immersed debris and odd reflective surfaces, one might get to .70 with a commercial high quality dry condenser or a stopped achromat condenser. It is difficult with aqueous samples and not really necessary, when high quality immersion condensers are available so cheaply on the second hand market. With a little ingenuity and actually less than is required to make an easily centered stop, one can adapt most DF condensers to any microscope, UNLESS they are too tall to begin with. Making up for lack of height is easy, which is why those old AO/Spencer DF condensers are so trick. They even have a thread in condenser section that adjusts vertically up to 10mm.

There is another specification outside of the N.A. that bears notice when it comes to DF condensers.
All condensers have a limitation in their illumination circle, restricting their use with objectives lower than a threshold magnification. This is tied to the N.A. of the condenser modified by it's design. An average BF condenser for instance has an illumination circle that can cover about 2000 microns, making it useful for 10X objectives up to 100X objectives. Manufacturers have used various methods to increase the illumination circle of BF condensers, from aux. condenser lenses, to the temporary removal of the top high N.A. lens , to having a flip top lens, to widening the condenser lenses themselves to reformulating the lens pack at some expense to produce a widefield condenser. These all work for BF condensers but most are either impractical for DF condensers due to performance issues or due to excesive cost. DF condensers are expensive enough as it is
DF condensers pass much less light than BF condensers , so the designs minimize light loss but also concentrate the light into a minimal circle. Thus the condensers that can illuminate high magnification and thus higher N.A. objectives well, do not have a large enough illumination circle for low magnification objectives and few options exist with which to increase the size of the circle. For this reason DF condenser use has typically involved 2 condensers : one with a broad enough image circle to illuminate 10 and 20X objectives and as well sometimes 4 or 5X, or 40-45X. Stops fitted into a BF condenser were first used but as time went on , the difficulty in achieving medium resolution with stops was glaring and low N.A. dry DF condensers evolved.
An immersion condenser was always required for high resolution as was and is, very bright illumination. Some systems utilize aux. lenses or convertible lenses to get around the two condenser dilemma.

The neatest one of those is the AO/ Reichert toric DF condenser that uses a toric aux. lens in advance of s spherical mirror oil condenser to increase the illumination field. I use one of those from 10X .30 right through to 100X 1.32 with perfect illumination of all fields.
Another system uses DF top lens caps on a single condenser. Each cap includes a DF stop and lensing system that adjusts the N.A. and field coverage accordingly.
Another system and a very expensive one uses a vertically moving stop plus alternate top lens caps.
And I think there is a system that uses two aux. lenses under the condenser : one low and one high. My mind's eye sees Leitz for that?

There is another system that works well for low N.A. and magnification DF.. That involves using 100X and 40X phase diaphragms as DF stops for 10X and 4X objectives. I don't know if that works for all phase systems but it does for the infinity one, I use. A thread in replacement DF oil condenser in lieu of the optical phase condenser can then be used in the same condenser mount for the 40X and 100X DF. A little cumbersome but it all takes place in the same condenser housing plus 4 phase settings available too.

[Phill Brown]

Says turret condenser na 0.9 which I took as Zernike dry.
.75 would make sense as the DF limit.
Any gap between the condenser and the slide with reduce the effectiveness.
Anyone who focuses for BF Kohler and switches to DF is welcome to do that.
It doesn't work, and it's pointless but each to their own.

[Hobbyst46]

... There is another system that works well for low N.A. and magnification DF.. That involves using 100X and 40X phase diaphragms as DF stops for 10X and 4X objectives. I don't know if that works for all phase systems but it does for the infinity one, I use...

The 100X phase diaphragm serves as DF stop for the 10X objective on the finite, 160mm Zeiss (West) standard as well. That option benefits from the ease of centration of the phase anulus. Relative to a fixed stop on (or below) a simple BF condenser.

[apochronaut]

Says turret condenser na 0.9 which I took as Zernike dry.

What is a " Zernike dry" condenser?

[Phill Brown]

Says turret condenser na 0.9 which I took as Zernike dry.

What is a " Zernike dry" condenser?

Really?
It's not oil at 0.9na unless you really like oil.
If it has a turret with choices I'm trying to picture something that isn't Zernike.
Either way it's Zeiss not AO.

[apochronaut]

I'm trying to figure out the use of the term Zernike as applied to a condenser. You haven't explained it.
Fritz ZernIke invented the physics of phase contrast and an application. His prototype was crude and included phase annuli in a rotating wheel up around the eyepieces, not at all like the system we know now. There are no extant examples of so called Zernike phase contrast. It is all just phase contrast. . Zernicke was a Jew and by the time he made his invention , it was a very good ticket to extending his longevity, especially if offered to Zeiss, who made liberal use of free labour during the Nazi era. History has not been kind to Zeiss in terms of it's employee relations during the Third Reich. Leitz on the other hand were more egalitarian in their relations with the many who they ended up sheltering.
Zeiss didn't do much with phase contrast, just made some changes to shift the components to locations we are famIliar with today. No doubt they were preoccupied with making periscopes, binoculars and gunsights.
From 1946 and on the refinement of phase contrast took place and by 1948 about 8 or 9 companies had functional systems. Many required manual changing of the phase diaphragms including Zeiss and a few had the diaphragms in a rotating carousel. The first such patent for a phase diaphragm carousel that I have come across was in 1948 , granted to American Optical ( I can dig up the patent if you like but it was already linked to on this forum). It is the diaphragm that makes phase work, not the condenser and the rotating group of diaphragms while a convenience is not essential for the function of phase contrast.
The condenser on the other hand, for phase, is not unlike any other microscope condenser, so there is really no Zernike condenser. He just applied a standard microscope condenser. It just requires a front focus at the diaphragm plane and can be dry or oil , whichever the condenser is designed for and any condenser can work. In fact , due to the fact that like DF, phase works efficiently at lower N.A.s, an Abbe condenser can be used well for phase, better than for DF and as long as it is designed to focus critically on the phase diaphragms it can be made to work well.

[Phill Brown]

Take it up with the industry.
Probably best to keep it separate from Kohler.
Personally I have no problem with DF.