Phase Contrast condensers.

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apochronaut
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Phase Contrast condensers.

#1 Post by apochronaut » Wed Jan 18, 2023 8:46 am

Does anyone have links to or any hard copy information specific to phase contrast condensers? I have most of what AO published over the years but little else. I am interested in a definitive technical reason why oil immersion is not used with phase contrast condensers or if it has been used by some manufacturers. The topic seems to be avoided or overlooked in almost all literature I have found published or on line. I presume that oil , by altering the undeviated and deviated ratio causes an amplitude shift affecting contrast but that is all I can come up with.

Alexander
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Re: Phase Contrast condensers.

#2 Post by Alexander » Wed Jan 18, 2023 9:56 am

Leitz made at least one phase contrast condenser with immersion front element, the 403c. But it appears to be a pretty shy animal.
I do have a number of oil immersion objectives with phase rings in use.
25/0.75 Oel Phaco 2 Fluoreszenz
PL Apo 40/1.0 Oel Phaco 3
63/1.30 Oel Phaco 3
NPL 100/1.30 Oel Phaco 3
They are all made by Leitz for 170 mm tube length.

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Re: Phase Contrast condensers.

#3 Post by Phill Brown » Wed Jan 18, 2023 11:53 am

Must be an AO thing?
I have Watson microsystem 70 from 1965
Nikon on a labophot 2 and Euronext iscope all with PH 100x oil.
All are 1.25 with Zernike condenser.
Unless I'm missing the point for which I apologize in advance, I don't interchange PH between makers,
They would be specific optimised.
Nikon and Watson are 160mm RMS and mostly compatible optically with eyepieces and objectives.
The Watson is better made and doesn't have the acrylic lens element as in Nikon condensers.

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Re: Phase Contrast condensers.

#4 Post by Hobbyst46 » Wed Jan 18, 2023 12:25 pm

As I understand it, the question refers to the space between condenser front lens and the slide, regardless of the objective.

I am afraid the following is not the "definitive" answer to the question, it is sort of partial, indirect support at the most. But still.
There are several models of Zeiss Standard turret condensers (from the 1950-1960...) , at least one of which has an NA of 1.4. So it is expected to perform under oil immersion.
The text in the "Zeiss Optical System" publication by Zeiss (from those years) states that in most cases, "The front lens need not be connected to the specimen slide by means of an immersion liquid." (for reasons they describe).
This statement seems to apply to all illumination modalities, except darkfield that specifically requires oil immersion even for low NA objectives. But darkfield is irrelevant within this post.
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Re: Phase Contrast condensers.

#5 Post by patta » Wed Jan 18, 2023 1:09 pm

My take on why Oiled phase condensers are not common (never thought about before today):
The illumination, the phase annulus has NA much smaller than the objective: for an oil objective with NA 1.25, the annulus (the hollow cone illumination) is like 0.7-0.8, so there is no need to oil the condenser.
I have the Leitz Phaco 403a, it has one objective 100x 1.30; but its annulus has only NA~0.7 and is mounted in the normal dry condenser like for the other dry objectives; and works very well.

So, oil condenser is not needed; only some pervert manufacturer with fetish for high NA (see posts above) would have ventured in oil condenser with large NA annulus, given the significant extra hassle of oily condensers...

I think that a condenser, well designed, projects the phase annulus sharp and clear, whether it is an oil condenser with oil, or a dry condenser dry. While putting oil on a dry condenser will sure do some damage... It may be possible that an high NA annulus (like, 1.1) gives better performance than the smaller ones compatible with dry condensers (for example, thanks to higher NA illumination); or, other argument, an oil condenser may work better (it is sharper) at high NA like 0.9 that are otherwise borderline for a dry condenser (it isn't very sharp); as analogy, an oil objective with NA 0.95 gives nicer pictures than a dry one with same NA (and price).

There is also the issue of the thickness of the glass slide: like a coverslip of wrong thickness, the slide may cause aberrations on the illumination (hence the annulus is not projected sharply over the objective's phase ring, so the phase contrast isn't optimal); here the oiled condenser has a clear advantage, since the slide "disappears" in the oil; like a 40x 0.65 objective is sensitive to coverslip thickness, while oil objectives aren't.

Note that the annulus and the condenser do not make any "phase" effect; they just project an hollow cone of light, like darkfield; it is however important that this cone is neat and clean, so it hits precisely the phase ring in the objective (that, together with the specimen, do the "phase" effect).

I've checked the book of Payne, but couldn't find mention phase oil condensers. The above are personal guesses.
Last edited by patta on Wed Jan 18, 2023 2:57 pm, edited 1 time in total.

apochronaut
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Re: Phase Contrast condensers.

#6 Post by apochronaut » Wed Jan 18, 2023 2:52 pm

Hobbyst46 wrote:
Wed Jan 18, 2023 12:25 pm
As I understand it, the question refers to the space between condenser front lens and the slide, regardless of the objective.

I am afraid the following is not the "definitive" answer to the question, it is sort of partial, indirect support at the most. But still.
There are several models of Zeiss Standard turret condensers (from the 1950-1960...) , at least one of which has an NA of 1.4. So it is expected to perform under oil immersion.
The text in the "Zeiss Optical System" publication by Zeiss (from those years) states that in most cases, "The front lens need not be connected to the specimen slide by means of an immersion liquid." (for reasons they describe).
This statement seems to apply to all illumination modalities, except darkfield that specifically requires oil immersion even for low NA objectives. But darkfield is irrelevant within this post.
Thanks for your thoughts on that everyone but the last comment perhaps hits the nail on the head.
You are correct Hobbyst46, not objectives of which phase contrast oil immersion types are legion but condensers only is the post. It seems if the condenser is marked with an N.A. >1, then it is optimized for oil and oil needs to be used. If unmarked as to N.A. or marked as <1 it is dry. It is the condenser requirement not the technique that determines whether or not oil is required and any type of condenser can be used as long as it focuses accurately in both focal planes , has a wide enough field and sufficient N.A. : other qualities aside. I don't know that for sure but it begins to make sense. In other words, there is no prohibition against using oil on a phase contrast condenser unless it is prohibited by the condenser's design.

In microscopy up until a certain point in time, high N.A. dry condensers were non-existant or rare at least, so oil was the design and the use of oil on a condenser was expected. It was kind of like not needing instructions to dip your paintbrush in paint before use. As it became noted that more than a few users couldn't be bothered to oil the condenser, especially for objectives with an N.A. below 1, makers began offering better performing condensers when used dry because oil condensers are optimized for oil immersion. The beginning of the dry achromat aplanat condenser era for BF overlapped with the growth in commercial widespread use of phase, so earlier phase condensers were oil immersion but a degree of dry achromat phase condensers emerged over time, just as with BF . AO introduced a condenser threaded into phase contrast condenser housings with only a part or cat. # on them. It was a phase condenser but also could be used with the open port and low magnification DF. There wasn't a different housing it could be threaded into, so as yet in 1967, AO was still offering only oil immersion abbe or abbe aspheric condensers for BF and fluorescence . That seems to be their initial issue of a sub. 1.0 N.A. achromat aplanat but oddly targeted almost entirely for use with phase contrast. In attempting to figure out why that was after I wrote the initial post, I went over the brief section about condensers in Phase Microscopy ( Bennet, Osterburg, Jupnik and Richards).

There is no mention of oil or not in that section but a fair amount about spherical aberration and ray parallelism. Oblique rays are not a disastrous problem but play a small role in contrast reduction at the conjugate ray , deviated ray border due to ray overlap. The more parallel the rays the better the contrast, so it seems that over time it may have become clear to phase contrast designers that control of spherical aberration in the condenser was a more important engineering goal than N.A.. The condenser N.A. only needs to be equal to the objective N.A. but just as with BF , the sacrifice of a few sub microns of resolution can be made up for with improved contrast. Thus , a .90 achromat aplanat condenser will outperform a 1.25 N.A. oil immersion abbe condenser when used with all objectives under N.A. 1.0 and may do so as well up to a certain point with objectives over 1.0 because the uncorrected illumination image created by an abbe condenser between the condenser and the object contains lots of non parallel rays, lowering contrast.
Obviously, an oil immersion achromat aplanat would be superior yet but with an achromatic objective system the cost of such a condenser might over price the system. Phase fluorite or apochromats would need it and the cost presumably would be secondary to the performance.

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Re: Phase Contrast condensers.

#7 Post by patta » Wed Jan 18, 2023 4:30 pm

So weird, I thought of oiling the condenser as something exotic and innovative... instead it was the old standard!

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Re: Phase Contrast condensers.

#8 Post by Hobbyst46 » Wed Jan 18, 2023 11:13 pm

Apochronaut, Indeed the Leitz publication "Phase contrast equipment with the Heine condenser" gives very detailed information and description, yet oiling the condenser to the slide is only (very briefly) mentioned in relevance to darkfield, not phase contrast.
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Re: Phase Contrast condensers.

#9 Post by apochronaut » Thu Jan 19, 2023 1:15 am

Hobbyst46 wrote:
Wed Jan 18, 2023 11:13 pm
Apochronaut, Indeed the Leitz publication "Phase contrast equipment with the Heine condenser" gives very detailed information and description, yet oiling the condenser to the slide is only (very briefly) mentioned in relevance to darkfield, not phase contrast.
That's interesting. Oiling a DF condenser for above 1.0 N.A. is a necessity. If you don't oil, you get nothing ; well, you get something but it isn't DF, so the manufacturers were obliged to check that box. With phase you still get phase whether you oil immerse or not, it just becomes whether the unoiled phase condenser does the objective justice. A carefull comparison would be needed in order to tell. It appears that the manufacturers sought to leave it up to the microscopist's discretion. You sell more that way, especially when all of your competitors are as vague.

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Re: Phase Contrast condensers.

#10 Post by patta » Wed Jan 25, 2023 8:24 am

So summary:

- Condenser oiling has no direct effect on phase contrast
- In the 1950's the first Phase systems used oiled condensers just because that was the norm
- Later, high NA dry condenser came along, and today (I think) all commercial phase condensers are dry
- High NA oil objectives can be used with dry condenser because the illumination phase annulus has usually NA<1
- It is important for the phase condenser to be well corrected, for ex. achromat
- Oiled condensers may have some advantage at high NA or for contrast
- If you have an old phase system, plz try with oil vs. dry and report back, since we weren't able to find documentation from manufacturers.

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Re: Phase Contrast condensers.

#11 Post by apochronaut » Wed Jan 25, 2023 3:09 pm

patta wrote:
Wed Jan 25, 2023 8:24 am
Condenser oiling has no direct effect on phase contrast
The condenser doesn't create phase contrast, the diaphragm does, so however the condenser is configured to work is best.

quote=patta post_id=132191 time=1674635040 user_id=2269]
- In the 1950's the first Phase systems used oiled condensers just because that was the norm [/quote]

Beginning in 1942 with Zeiss and 1946 with AO with others following quickly.The condenser systems employed for BF were determined to be adequate but most objectives used were simple achromats, so the paralellism of the peripheral condemser rays were not particularly important.Obviously, achromat aplanat condensers would have been somewhat better but that would have escalated the cost of the system by possibly 150.00. It was common practice to oil condensers at the time, except for perhaps very low magnifications.

quote=patta post_id=132191 time=1674635040 user_id=2269]
- Later, high NA dry condenser came along, and today (I think) all commercial phase condensers are dry [/quote]

I wouldn't say all. Phase objectives with very high N.As would need an oiled condenser but it is common to see .90 achromat aplanats for phase. Some simple Chinese systems, where they have just a 1.25 abbe oil condenser use that for phase too. Obviously it would probably work better oiled for a 1.25 phase objective.

quote=patta post_id=132191 time=1674635040 user_id=2269]
-High NA oil objectives can be used with dry condenser because the illumination phase annulus has usually NA<1 [/quote]

Not sure about that. Only the conjugate part if the annulus is below 1. An objective rated for immersion with an N.A. over 1 still would benefit from a higher N.A. condenser but just as is the case with many modern infinity corrected BF systems, a better corrected condenser produces a full N.A.across it's field, wheras an abbe condenser has an N.A. hotspot, falling off at the periphery due to ray scatter.

quote=patta post_id=132191 time=1674635040 user_id=2269]
- It is important for the phase condenser to be well corrected, for ex. achromat [/quote]

Probably more important for higher N.A. objectives and better corrected ones. I have a phase system on a Diastar with a .90 achromat aplanat condenser. It works very well with the 100X 1.25 oil planachro dark phase objective. I really don't see much difference using a 1.4 N.A. oil achromat aplanat. However with either the 100X 1.30 planfluor or the 100X 1.32 planapo phase objectives I do see some difference and I take the time to use the 1.4 N.A. condenser.

quote=patta post_id=132191 time=1674635040 user_id=2269]
- Oiled condensers may have some advantage at high NA or for contrast [/quote]

An oiled condenser that is intended to be oiled will have better performance than when not oiled but a dry condenser will have better performance when dry. A .90 achromat aplanat will outperform an oiled 1.25 oiled abbe condenser with plan objectives. Not so much with non plan, if at all..
- If you have an old phase system, plz try with oil vs. dry and report back, since we weren't able to find documentation from manufacturers.

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