Help: does Koehler illumination requires a field lens in microscope?

[osterport]

Hello

I recently got a Motic BA400 part, when I tested it, the image quality is not even as good as BA300. When I disassemble it, it's quite doubtful that the field lens ( compared to standard Keohler system ) is missing. Do you know if the field lens in the picture is a must?

[Jose Antonio Mena]

optiphot optical trail.pdf

(192.84 KiB) Downloaded 115 times

Hi:

I Think that no, not all Koeler microscopes have that lense, each model have different optical trail; but for real Köler ilumination, it mustn´t be a difuser in the optical trail, you must focus the ligth source (bulb filament, discharge tube of a flash, led array...) in the FFP of the condenser (or the condenser iris position).
Regards

JA

[apochronaut]

The origin of Köhler illumination is intricately tied to the development of the tungsten coiled filament which is a poor illuminator for a microscope. Previous illuminators produced a broader more even illumination wavefront. A tungsten filament is tiny, even when magnified and focused by a field lens.
The previous post is correct in that each microscope iillumination system is somewhat different but for Köhler there must be a method of focusing and defocusing the image of the filament at the field plane, so there must be a lens somewhere to do that. There are what are called , modified Köhler illumination systems that place the lens after the field plane whereas in a classic system, it is before it. In some modified Köhler systems there is a diffuser.
Where the previous post is incorrect is that because the filamrnt is so small, in order to align Köhler properly, you need to defocus the filament image, not focus it.
The only time you focus the filament image is to center it when a new bulb is installed. Subsequent adjustment of Köhler is to defocus the filament image in order to produce as even an illumination field as possible.
There is also usually a field iris at the field plane. This is not really part of a Köhler illumination system. It is an added convenience though, since it allows for an adjustment of the illumination intensity and by focusing on it, the beam can be centered.
A correctly adjusted system then involves a perfectly focused and centered field iris and a perfectly defocused and centered filament image in the same plane.

[osterport]

optiphot optical trail.pdf

Hi:

I Think that no, not all Koeler microscopes have that lense, each model have different optical trail; but for real Köler ilumination, it mustn´t be a difuser in the optical trail, you must focus the ligth source (bulb filament, discharge tube of a flash, led array...) in the FFP of the condenser (or the condenser iris position).
Regards

JA

Thanks JA. My trouble is, comparing to Motic BA300, the light comes to condenser is quite narrow and also comes with serious color chromatic aberration.

[osterport]

And it resulted in different image quality ( see the blue color aberration on diatom on the lefthand side )

[osterport]

optiphot optical trail.pdf

Hi:

I Think that no, not all Koeler microscopes have that lense, each model have different optical trail; but for real Köler ilumination, it mustn´t be a difuser in the optical trail, you must focus the ligth source (bulb filament, discharge tube of a flash, led array...) in the FFP of the condenser (or the condenser iris position).
Regards

JA

I read this pdf, and then put in a diffuser like this, the image quality is much improved, but i'm not sure if that's the right place.

[Jose Antonio Mena]

Hi again:

The Optiphot and Labophot (Both Nikon) have a diffuser after the lamphouse condenser (the big curvture lense close to the bulb; in the optiphot that diffuser is removeable); I understand that as Apochronaut said, it is for get more uniform illumination. In my experience, in the optiphot, without the diffueser you get more contrast in the image, but a quite few uniform illumination; in the optiphot manual it´s written that you have to focus the bulb fillament in the place of the condenser iris (as it´s written in othe microscopy books that I have from german autours). You have put the diffuser before the condenser lens of the lamp-house... I think that it´s good try and check to put it after...

In the optiphot (expensive lineup model), I have seen that the lense (field lense lbeled in the PDF) in the basis of the microscope is a doublet lense (see the diagram that I attached; I understand that it´s for correct the chromat aberration of the illumination system). The labophot, cheaper, have a simple lense... So with your microscope could be the same: have a better o worst corrected illumination system, more complex or more symple...

I understand that the Koeler principle is: that the filament plane, the Front Focal Plane (iris plane) of the condenser,and back focal plane of the objective must be conjugated optical planes (you must see them focused togeter); as the field iris plane and slide plane must be cojugates too.

Regrds

[PeteM]

. . . I read this pdf, and then put in a diffuser like this, the image quality is much improved, but i'm not sure if that's the right place.
[/quote]

You'll want the diffuser after the heat filter (and the Optiphot already has a place for it). At some point it may suffer thermal shock and crack if placed so near the bulb.

[apochronaut]

Diffusers always reduce contrast, that's why they are not generally regarded as a first choice option. However, in a system where budgetary requirements are prominent, in order to keep a microscope within a certain competetive price range and compactness, they are used in lieu of a more classic Köhler system, which requires more space.
Originally Köhler illumination was based on a remote illuminator, which is quite distant from the object plane. Condensers were infinity corrected . Early microscopes that applied the Köhler principle to a microscope with an integrated illuminator were mostly larger , with rear entry pumpkin lamps and an illuminator path closer to the original pattern.. The systems in more compact microscopes benefit from a diffuser but the tradeoff is a potential reduction in contrast.

[osterport]

Hi again:



I understand that the Koeler principle is: that the filament plane, the Front Focal Plane (iris plane) of the condenser,and back focal plane of the objective must be conjugated optical planes (you must see them focused togeter); as the field iris plane and slide plane must be cojugates too.

Regrds

This is the part I don't quite understand. The condenser can be raised up or pulled down according to the use cases, for instance, DF would require very high position of condenser. I don't see any mechanism that Kohler parts can adjust the focus of the filament image.

I put a paper on the top of condenser holder, the filament is well focused. Well, when I raised the condenser holder, the image does not change much though, but I believe there must be some minor differences.

[osterport]

Diffusers always reduce contrast, that's why they are not generally regarded as a first choice option. However, in a system where budgetary requirements are prominent, in order to keep a microscope within a certain competetive price range and compactness, they are used in lieu of a more classic Köhler system, which requires more space.
Originally Köhler illumination was based on a remote illuminator, which is quite distant from the object plane. Condensers were infinity corrected . Early microscopes that applied the Köhler principle to a microscope with an integrated illuminator were mostly larger , with rear entry pumpkin lamps and an illuminator path closer to the original pattern.. The systems in more compact microscopes benefit from a diffuser but the tradeoff is a potential reduction in contrast.

This is a right place to learn and try Kohler illumination! My trouble is, the filament is pretty well focused here, there are two issues:
1. The size of the image is really not big, that's why the light path is quite narrow ( see the right side ), where we would have trouble in some cases.

2. There are some color aberration at the end of this image, which would result in c.a. in the final objective image as well. When we put a diffuser in the light path, see the right hand side, it's much better.

Filament image comparison on condenser Iris wo and w diffuser.jpg (45.76 KiB) Viewed 2244 times

I'm just wondering, maybe a field lens can improve all these by enlarging the filament image so much that only the central part of it is delivered to condenser, no red fringe

[apochronaut]

Normally, you put a piece of tissue over the illuminator window to act as a projection screen, obtain a filament image, then center it and defocus it. There seems to be a centering issue with the left hand system pictured.,
There must be a way of centetirg the filamrnt image and as well precisely overlap the images of the filament and it's reflector image, Once done and centered, you then defocus the filament image to fill the illuminator window.
If you can see a shatply outlined image of the filament you have critical illumination, not Köhler illumination.

[osterport]

Normally, you put a piece of tissue over the illuminator window to act as a projection screen, obtain a filament image, then center it and defocus it. There seems to be a centering issue with the left hand system pictured.,
There must be a way of centetirg the filamrnt image and as well precisely overlap the images of the filament and it's reflector image, Once done and centered, you then defocus the filament image to fill the illuminator window.
If you can see a shatply outlined image of the filament you have critical illumination, not Köhler illumination.

Thanks apochronaut! The image quality is pretty much improved.

[osterport]

Then I compared with vs. without diffuser when the bulb is properly centered:

left with diffuser: right without diffuser

Diatom image comparison w vs. wo diffuser.jpg (69.32 KiB) Viewed 2151 times

looks like they are almost the same, and if we take a closer look, it seems diffuser will slight cause resolution drop, but very little. The little circles marked in red are squeezed almost connected, but we can still see the gap between circles on the right hand side.

Anyway I don't need a diffuser any more. Thanks all for the help.

[Hobbyst46]

In a complete Kohler setup, there is one more degree of freedom - the distance between the lamp and the collector optics. On my very old microscope for example, which dates from the 1950's, that distance can be adjusted by sliding the lamp housing slightly in or out, until the filament image is defocused (the housing is then locked in place by means of s small screw).
Also on my microscope, the front collector lens is lightly frosted. That improves Kohler illumination but is not essential. A frosted glass piece as thick as shown in the photos above will reduce the beam brightness a lot. Perhaps a more humble matt surface can suffice.

[apochronaut]

That's what focusing and defocusing the filament means.

[osterport]

In a complete Kohler setup, there is one more degree of freedom - the distance between the lamp and the collector optics. On my very old microscope for example, which dates from the 1950's, that distance can be adjusted by sliding the lamp housing slightly in or out, until the filament image is defocused (the housing is then locked in place by means of s small screw).
Also on my microscope, the front collector lens is lightly frosted. That improves Kohler illumination but is not essential. A frosted glass piece as thick as shown in the photos above will reduce the beam brightness a lot. Perhaps a more humble matt surface can suffice.

It's the first time that I learned that the lamphousing can move, can you share the model name of your old microscope?

[Hobbyst46]

In a complete Kohler setup, there is one more degree of freedom - the distance between the lamp and the collector optics. On my very old microscope for example, which dates from the 1950's, that distance can be adjusted by sliding the lamp housing slightly in or out, until the filament image is defocused (the housing is then locked in place by means of s small screw).

It's the first time that I learned that the lamphousing can move, can you share the model name of your old microscope?

Zeiss Standard GFL. On Zeiss Standards, there is a round port on the rear side of the base. The lamp housing is inserted through the port. The depth can be adjusted to achieve Koehler.
In some other models, the bulb position within the housing can be adjusted.

[apochronaut]

Most microscope illuminators had adjustments built into the illuminator until the 1980s when "pre-focused" bulbs arrived on the scene. On good systems, the bulb filament could be moved in xy and z and rotated. On some , the x and y were accomplished via adjusting the first surface mirror and the z via a focusing lens.
Rotation is necessary in order to maximize the bulb output and to even out the filament image across the field. The bulb reflector casts a slightly hazy image of the back of the filament, which by rotating the bulb can be framed between the coils of the primary filament image, spreading the illumination across the field more evenly.
Some instruments used threaded adjustments and some used a spring grip on the socket so that it could be adjusted by the fingers and stay put.