A condenser quest.

[mrsonchus]

..............

Chromatic aberration, whether it be created in the illumination beam or introduced by inadequately corrected elements within the image beam makes it difficult to realize the theoretical resolution of the entire system. Light that is separated into elements with chromatic differences in magnification cannot be utilized to resolve fine detail.

It doesn't matter whether the N.A. is low or high, chroma distorts the image nonetheless, however, imaging through a lower N.A. system can often visually sustain chromatic distortion more , because the details being sought are gross, whereas those being sought with higher N.A. are fine and easily obscured by chroma.

I'm no expert, but this explanation makes the most sense to me - very informative thread, thanks for posting.

John B.

[apochronaut]

In general, an incompletely corrected optical component will introduce chromatic aberrations that deteriorate image quality. What I found interesting, reading the Wikipedia entry "condenser", is that Zeiss was quite late in producing achromatic condensers for their microscopes, and did that only in about 1870... apparently because aberrations are especially problematic for high NA objectives (conforms with Murphy's Law )...

I dont get it on the theoretical standpoint, the objective will chop off all light that has NA too large. As long as within the NA of the apochromatic objective you have all 3 colors you'll be fine. The color intensity distibution may vary a bit but you shouldnt get color fringes regardless of the condenser being plain or corrected as long as condenser NA for all colors is > objective NA

I guess I have been wrong about this.. I thought this was a method to match objectives to condensers..

Some of the posts are jumbling N.A. with chromatism. The two performance parameters are distinct but the latter can limit the former's potential.
The more highly corrected the condenser is, the less the off axis rays break up into chromatic differences of magnification. Thus the condenser can be brought to a more correct focus. In terms of condenser performance, a well corrected lower N.A. condenser will perform better than a poorly corrected higher N.A. condenser, especially if being used dry as many commonly are, nowadays and plan objectives and objectives capable of a large image circle are being used.

Condenser element diameter also plays a role in condenser performance. If a two element simple condenser were to be made with exceptionally wide elements, it's performance over the needed coverage area could be very excellent and very close to chromatically correct.

Some manufacturers ; PZO comes to mind but there are others I would presume, produced condensers with larger diameter glass elements. This not only allowed for a wider coverage of the field but also a wider central area of correction free from peripheral chroma, allowing for a larger well corrected central area of the illumination beam while still using a 2 element design. AO did the same thing but since the condenser in question was an aspheric 3 element design the purpose was not to overcome peripheral chroma but to increase the field coverage for low magnification objectives and avoid the necessity for a supplemental condenser lens.

[billbillt]

Apo,

Thanks for the explanation....

BillT

[mrsonchus]

Apo', very interesting, thanks again for the info.

John B.

[manu de hanoi]

Thanks all for the clarifications, though I never paid much attention to the koelher focus. If the koelher was focused on the slide we'd see an image of the light source imprinted on the specimen right ? So it has to be out of focus anyways right ?

Condenser element diameter also plays a role in condenser performance. If a two element simple condenser were to be made with exceptionally wide elements, it's performance over the needed coverage area could be very excellent and very close to chromatically correct.

let's size a condenser for 100x oil immersion:
the obj shows : NA =1.25
from here :

I get f number =0.3
for ex a lens with 5cm diameter would need a focal length of 5*0.3=1.5 cm
That doesnt seem possible with a single lens, anyone knows how to find proper parameters for 2 lenses ? I have a few lenses around to play with....

[MichaelG.]

On the matter of chromatic correction of Condensers:

From a Gillett & Sibert brochre

IMG_1962.PNG (282.82 KiB) Viewed 6803 times

MichaelG.

[Hobbyst46]

@MichaelG
Very relevant information.

[apochronaut]

Usually, there is a broader range of types available from a company. This is probably only a partial list from Gillet and Seibert.

The general list of usual designs goes like this. I don't know of any company that offered all of these options at one time. Usually about 3 or 4 options is the norm, plus some very specialized designs for very specific applications.

1.2 or more commonly a 1.25 abbe oil immersion
1.25 abbe aspheric or abbe aplanat oil immersion
1.3 or sometimes an abbe 1.4 N.A. oil immersion
1.3 or 1.4 abbe aplanat oil immersion( not too common)
.90 or sometimes a .95 achromat aplanat
1.25 achromat or achromat aplanat oil immersion( not too common)
1.30 achromat aplanat oil immersion
1.40 achromat aplanat oil immersion

Some companies in the past offered a condenser body designed with an R.M.S. thread. Baker, Spencer and Leitz are ones I know about. This allowed for critical matching of the condenser and objective, as well as the fitting in of specialized condensers, such as a DF condenser with an R.M.S. thread. Baker, made one of those. The Spencer cat. # 322 condenser mount, the most common condenser mount found on their series 3 and 5 research stands had a 1.30 achromat aplanat R.M.S thread condenser fitted. Some brochures, show the possibility of using for instance a 20X .65 N.A. apochromat objective as a condenser for a 20X .65 apochromat objective, in the same mount.

[MichaelG.]

Usually, there is a broader range of types available from a company. This is probably only a partial list from ...

Quite so
I carefully selected these two items from the page, because the text nicely supports the point we were discussing.

MichaelG.