Oil immersion condenser

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Hon-shimeji
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Oil immersion condenser

Post by Hon-shimeji »

Hi!

Can anyone tell me when it is appropriate to use oil immersion on the condenser top lens? I guess first of all the condenser must be designed for it, but why don't you always use a oil immersion condenser when you use oil immersion objectives? Wouldn't the resulting image be better?

JohnyM
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Post by JohnyM »

Resolution equation consist of:
Wavelenght / objective NA + condenser NA

So when you need to achive higher resolution, you oil your condenser.
Condenser NA cannot exceed Objective NA.

enricosavazzi
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Post by enricosavazzi »

In practical terms, you must use oil on the condenser top lens when the objective NA is higher than about 1.0, and you want to fully utilize it. This also means, in practice, that the objective is designed for immersion too.

I am not too sure whether it is best to use oil or water on the top condenser lens when using a water immersion objective and water between cover glass and objective. With a darkfield condenser designed for oil, for example, it might be good to use the full NA 1.4 of the condenser (which is achievable with oil on the top condenser lens, but not with water) even if the objective itself only has NA 1.1 or thereabout. This should guarantee that the maximum amount of light illuminates the subject and is available to be refracted or diffused by the subject.
--ES

iconoclastica
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Post by iconoclastica »

JohnyM wrote:Resolution equation consist of:
Wavelenght / objective NA + condenser NA
This is resolution according to Abbe. However, Abramovitch says that "Failure to use oil will restrict the highest numerical aperture of the system to 1.0, the highest obtainable with air as the imaging medium", or restriction by the weakest shackle in the chain. So which one is true?
--- felix filicis ---

JohnyM
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Post by JohnyM »

It's not a big secret, that air immersed glass is limited to 0.95
Abbe already knew that.

enricosavazzi
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Post by enricosavazzi »

iconoclastica wrote:
JohnyM wrote:Resolution equation consist of:
Wavelenght / objective NA + condenser NA
This is resolution according to Abbe. However, Abramovitch says that "Failure to use oil will restrict the highest numerical aperture of the system to 1.0, the highest obtainable with air as the imaging medium", or restriction by the weakest shackle in the chain. So which one is true?
Both, actually. It depends on the type of subject and illumination.

If you look at light that passes through the subject without being refracted, diffracted or diffused away from its original path, then the smallest aperture in the train of optics is the limiting one. This can be either the objective NA or the condenser NA. This type of subject is typically observed with transmitted illumination.

If you look at light that reaches the objective after being refracted, diffracted or diffused by the subject, then if you use a condenser with higher NA than the objective, more light reaches the subject, and therefore more light can be refracted, diffracted or diffused into the objective. This is particularly true of darkfield illumination, where all light that reaches the objective has been deviated by the subject into the objective.

The "extra" light from a higher NA condenser contributes to illumination intensity, I think it is easy to agree about this. Now the question is whether this "extra" light also contributes to image resolution.

My two cents is that the objective neither knows nor cares about the "extra" NA of the condenser (which cannot directly enter the objective), and therefore that image resolution is still limited by the objective NA. But it is quite possible that highly oblique light reveals details of the subject not observable with a more axial illumination, so the "extra" light produces a qualitatively different image, potentially with more detail. We have all seen, on this site, examples of how COL (Circular Oblique Lighting) reveals detail not visible with more traditional illumination. The principle is similar.

Edit: On the other hand, if the condenser NA is lower than the objective NA (for example if the objective is oiled but the condenser is not), then light refracted, diffracted or diffused by the subject enters the objective with a wider cone than light that is only transmitted through the subject. In this case, the objective "sees" an illumination with a somewhat higher NA than the one provided by the condenser. This may well be what Abramovitch refers to with his formula. If so, it is not an exact formula, but only a rough approximation that depends on the type of subject.
--ES

pbraub
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Post by pbraub »

Hi, maybe this is of relevance within this discussion:

I have read about a clever way to higher NA illumination (than observation) for superresolution. There is a paper out which does this with a off the shelf LCD as light modulator in the condenser back focal plane. I would like to replicate this some day (but there is so much to do...).

Guo et al. Microscopy illumination engineering using a low-cost liquid crystal display
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4354584/

Zheng et al. Wide-field, high-resolution Fourier ptychographic microscopy
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4169052/

Guo also has a couple of other papers with ingenious low-cost solutions for microscopy.

Kind regards
Peter

Hon-shimeji
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Post by Hon-shimeji »

Thanks for the input! I have a Olympus BH2-UCD condenser with NA 0.9, so oil immersion wouldn't be improving the image then. But what if I used the NIC/PHASE NA 1.4 condenser designed for BH-series? Is this made for oil immersion? It should be I guess.

viktor j nilsson
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Post by viktor j nilsson »

Yes, I'd say that it's safe to assume that any condenser with a NA higher than 1.0 will be designed to be used with oil.

enricosavazzi
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Post by enricosavazzi »

The Olympus stand-alone condensers for brightfield illumination with NA 1.4 are made for oil immersion. There is also a darkfield stand-alone condenser for oil immersion.

If it is a "Universal" condenser, for oil immersion it needs an U-TLO top lens or equivalent , while the top lens U-TLD is for air and reaches only NA 0.9 or 0.95 if I remember correctly. The oil lens has a flat top surface, the air lens has a concave surface.
--ES

Pau
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Post by Pau »

pbraub wrote:Hi, maybe this is of relevance within this discussion:
Not sure about it, but the linked papers are very interesting
Pau

Pau
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Re: Oil immersion condenser

Post by Pau »

Hon-shimeji wrote: Can anyone tell me when it is appropriate to use oil immersion on the condenser top lens? I guess first of all the condenser must be designed for it, but why don't you always use a oil immersion condenser when you use oil immersion objectives? Wouldn't the resulting image be better?
Hon-shimeji wrote:Thanks for the input! I have a Olympus BH2-UCD condenser with NA 0.9, so oil immersion wouldn't be improving the image then. But what if I used the NIC/PHASE NA 1.4 condenser designed for BH-series? Is this made for oil immersion? It should be I guess.
A 0.9 condenser is not designed for oil, likely it will not perform well and oil could penetrate inside the lens damaging it.

Oiling the oil condenser for low NA objectives makes no sense. With high NA dry objectives you could get a small improvement in some cases because a 1.4 condenser is highly corrected and with a dry objective you're only using the central part of its aperture, but in general avoiding the hassle of oil cleaning for little if any improvement is to be recommended.

The only case when it could be advisable is when you use DIC condenser prisms designed for oil condenser top lens, not sure
Pau

JohnyM
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Post by JohnyM »

Dic prisms are magnification dependant. Oil / dry makes no difference to them.
Ie: i have 1.4 oil DIC condenser with oil dic prisms that work just as well with top lens oiled or dy.
But when i change top lens to 0.9 prisms stop prodicing usable DIC image. I need to change prisms as well.

Pau
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Post by Pau »

Thanks for pointing it, Johny
My DIC system is atypical (to say the least) and with it I get better DIC oiling the condenser than using it dry with a 50/1.0 oil and a 40/0.95 dry objectives
Pau

iconoclastica
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Post by iconoclastica »

pbraub wrote:Guo et al. Microscopy illumination engineering using a low-cost liquid crystal display
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4354584/
Gosh, that is offering a solution before the need was spoken! Only a few days ago I started wondering if this might be a better idea than sawing a slit in my condenser to insert filters and stops.

I wondered if the polarization would become an issue, and apperently it is, with a fixed (non-rotating) analyzer.
--- felix filicis ---

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