where to place polarizing filter?

[janvangastel]

I made - for my Leitz Orthoplan - a filter slide with a 25 mm hole that fits into the slit between the eyepieces and the objectives - just under the black eyepiece holder- and glued a piece of polarizing foil on it. I placed another piece of polarizing foil on the light soure, below the condenser. So, I thought, the specimen is between the two pieces polarizing foil and should work as a polarizing filter. But it's not the case, it only becomes darker and rotating the piece of polarizing foil on the light source doesn't show any difference. It only works as it should when I hold a piece of polarizing foil between my eye and the eyepiece. I think there must be another, more elegant solution, but obviously I have not enough knowledge of how the Orthopolan is constructed. I can see the filter sliding getting into place when I look through the eyepiece while sliding the slide into the filter slit, so that's not where it goes wrong. I hope other Orthoplan users know how to do this.

[billbillt]

Hello,
The simple solution is to remember that one goes over the light source, and the other goes between the subject and the eye.. You are using linear polarizers aren't you?...

BillT

[janvangastel]

So my setup is right. I don't know if the polarization foil is linear or circular. There was no information about that on the webshop's website. I have asked the web shop owner.

[janvangastel]

The shop owner says the filter foil is a linear polariser. Maybe it is not clear te me what I should see. What I think I should see is, that, with the filter slide (with a piece of poloarization fillter foil) in place between objectives and eyepieces and then rotating the piece of foil on the light source (below the condenser), the lighting become darker and then, when rotating further, less dark again. What I see when rotating is that everything stays the same. When I have the slide in place and also a piece of foil on the light source (so one piece below and one piece above the specimen) and then rotate a third piece of foil above the one on source, it gets darker and lighter again. So I need three pieces of polarizing foil for this to happen. But maybe that's not what I should be looking for.

[75RR]

Two is all you need. Here is a diagram of the position to place the analyzer if the microscope is not fitted with one nor a dedicated place to put one.

The one below the head can be left in as it does not overly affect the light transmission.

[janvangastel]

This is exactly how I do it. Only difference is, that the top one (the one below the eyepiece head, the analyser) is on a filter slide. I removed the eyepiece head to have a look at it and the filter is nicely in place. The darkening when rotating the one on the light source (polarizer in the drawing) only takes place when I use a third one to rotate over the polarizer on the light source, but also with only two pieces of foil on the light source and non at all under the eyepiece head.

[75RR]

Remove both pieces and hold all of the polarizers up to the light and turn them. If any two work - darkening as you turn them - then use those.

[janvangastel]

All pieces I cut darken while rotating when looking at the blue sky and at the polarized light of a computer screen. But on the orthoplan this only workes with two pieces directly above the light source, where the polarizer is on the drawing and me looking through the eyepieces.

[75RR]

Try placing a piece under the head rather than in the filter slot. See if that works.

[Hobbyst46]

I wonder if you pasted any glue on the polarizer itself, who knows if the glue itself is not a polarizer.

[janvangastel]

No, there's no gluie on the filter foil.
I think I get it now. It now works with two pieces of foil, in the setup as shown in the drawing 75RR posted a couple of messages ago. I made a slide with powdered sugar and now I see all kind of nice and very clear collors which I do not see without the polarizer and analyser. I think that's what I should have been looking for and not (only) for darkening. Thanks all for your advises and your patience.

[janvangastel]

Here's a short video of the result: powdered sugar under polarized light.

[75RR]

Well done, you seem to have got it working nicely.

Here is a tutorial on Microscope Alignment for Köhler Illumination (sorry if I have mentioned it to you before) that is both fun and useful.

In this case you have to start each exercise by aligning the filament, in real life you would only have to do this once (if it were unaligned) - as long as you did not fiddle with the lighting too much afterwards.

http://zeiss-campus.magnet.fsu.edu/tuto ... flash.html

[janvangastel]

Thanks for the link. I am going to study it.

[janvangastel]

The only step in the Kohler procedure in the article I couldn't do is centering the filament. Reason is I can't see the filamant in the eyepiece or anywhere else.

[75RR]

In order to visualize the filament you would need a Bertrand Lens or a Phase Telescope.

Some microscopes have or can be configured to incorporate a Bertrand Lens, if not a Phase telescope will do.

Note that you will need one if you are doing Phase in order to align the annuli.

This is what they look like: https://www.ebay.co.uk/itm/Leitz-Wetzla ... :rk:4:pf:0

What is easy and useful to check is the filament focus.

The steps are:

1) Remove any opaque filters that may be in the light path.
2) Close the condenser iris completely
3) Place a mirror or slide under the condenser, angle it so that you can see the closed iris
4) You should be able to see the filament coils reflected on the condenser iris. (This is the Front Focal Plane)
5) If no filament coils are visible or they do not look sharp you need to move the light source either in or out until the filament coil is sharply focused.

Note: Your lamp house should contain a focusing lens, either the lens is designed to move or the bulb will. Either way it should allow you to focus the filament.

This is what it looks like:

[janvangastel]

OK, I can clearly see the diafragm in the tilted slide. I suppose the dark straight lines shown in your photograph it what I should see. On the back side of my lamp house is a lens that can be pulled further out (or pushed further inward). The microscope came with the lens all the way in. I have pulled the lens out in small steps and then back in again, but didn't see any coils or any other image of the lamp.

[75RR]

Check that you do not have a diffuser filter in the way.
A close-up photo of you lamp house would be helpful

Note: You could remove the lamp house and aim it at a wall about 3 meters away and try to focus the filament as sharply as possible. When you put it back it should not be too far off where you want it.

EDIT: Best to follow mrsonchus' instructions ... he is the man

[mrsonchus]

Hi Jan, as 75' suggested, there is a diffusion-filter in the way, the lens within the lamp-house is itself frosted! I refer to the 100 model lamp-house that takes a 100W halogen bulb, with a mirrored plastic reflector at the other side of the bulb, i.e. at the back-end of the housing.

The idea is to fully open the field iris, remove any filters, coloured, frosted or otherwise. Then move the reflector back and forth until an image of the filament is focused - however this cannot be achieved as the frosted lens within the lamphouse blurs any image and prevents a focus.... No problem, simply focus the blurred-bright patch that would have been the filament's image without the frosted lens... 'Focus' this bright spot to it's smallest brightest position by moving the reflector back and forth. Tracing-paper over the field iris light port is where Leitz say this focusing should be made.
You'll then have a bright spot and its weaker reflection visible, using the up/down and side/side wheels on the outside of the lamp-housing move them so that they are in-line (vertically centered if your filament is horizontal, horizontally centered if your filament is a vertical type). Once you have the filament and it's weaker reflection aligned use the adjuster not used to align them tothen bring them to almost touching with the other adjuster - giving effectively the biggest image of a nearly combined filament and it's reflection. Then put back any filters etc and view the light through an eyepiece-tube (eyepiece removed) and use the focus knob on the outside of the lamp-housing to basically 'blur the two bright spots together' to give an evenly-lit field of view.

Once done, only needs to be repeated if you change to a different type of bulb....

Here's a link to the instruction-book. Note the above method is for the 100 housing - the 100Z housing is different as it 'grabs the bulbs' from above and has different alignment controls...
The PDF of the --> 100 & 100Z instruction book

Hope this helps. I just ran through this on my own Orthoplan and changed the bulb while I was at it.

Here's the inside of my 100 model lamphouse,

ws_lightbox_cleaning (11).jpg (46.92 KiB) Viewed 13419 times

Images from instructions,

100 lamphouse.JPG (48.02 KiB) Viewed 13415 times

adjustments,

lamp instr.JPG (48.82 KiB) Viewed 13415 times

Remember though that only a bright-spot will be seen instead of a focused filament if, like mine, your 100Z lamp-house has a frosted condenser lens..... Kohler is still no problem to set up.

John B.

[janvangastel]

OK, I didn't know that lens was frosted, thanks. I followed your procedure. I understand how it works, but I don't know if I made it better or worse. The knobs on the lamp house do work, but not smoothly and I don't think they can move far enough. When I look in the eyepiece hole without an eyepiece, with the condenser open, I see an as good as round light dot, reasonably but not perfectly lit uniformly. With the condensor closed and in bright light I also see that. But when I dim the light I don't see a uniform round dot any more with closed condensor , but a number of small dots 'glued' together. I also seethat when I look in an eyepiece form a little distance.

[mrsonchus]

Hmm, there's one thing that's also needed to fill the ultra-wide field of view of the Orthoplan, the 'K4' auxiliary lens that fits into the bottom of the condenser to widen the light path.... I too had to find one of these, they're hard to find and not really well known-about....

A few images would be of enormous help here if you can.....


John B.

[janvangastel]

Do you mean images of the condensor?

[mrsonchus]

Do you mean images of the condensor?

Sorry Jan, no, I mean images of the inside and outside of the lamp-housing.

I'll go through the process tomorrow and make either a short video for you or a series of stage-by-stage images, once you 'get it' it'll never be a problem again, with or without the infamous K4 you will still be able to achieve Kohler.

Back soon my friend.
Oh yes, meanwhile have you downloaded the PDF of the manual for the lamp-housing, and is yours definitely the 100 and not the 100Z version, i.e. the 100 version is one that is the same inside as mine, as per the above images?
Also, before you begin, set the two knobs on the side of the housing roughly midway in their travel ranges, and turn the knob on the top of the housing all the way anti-clockwise rather than mid-range.
Yes, the knobs/mechanisms are quite rough-feeling and can seem a little 'old fashioned' but this is normal - they have to be robust and tolerant of the very high temperature around the halogen bulb - no place for oils, greases etc I suspect, just good-old threads, knobs and springs!

John B.

[janvangastel]

I made a photograph of my lamp house (see below). On the other side of the lamp house are two knobs for moving the front lens and for moving the lamp sideways. I think it's the same as yours. The spring to prevent the bulb from moving side ways broke when I removed the lamp some time ago, when I was looking for the cause of the light not working any more. That spring was heavy rusted. I think one of the reasons I could not get the lamp high enough when doing the Kohler job, was that I used a new bulb with a 3.5-4 mm lower positioned filament.

lamp house.jpg (185.47 KiB) Viewed 13328 times

[mrsonchus]

Hi Jan, yes that's the same 100 model as mine. Don't worry at-all about the loss of the two flat 'prongs' that hold the bulb upright - mine broke off also, with no consequences that I'm aware of. Having set the top knob all the way anti-clockwise before you start, the bulbs 'legs' will determine it's starting-height, that and the position of the filament within the glass envelope - usually more or less central.

Pull out the reflector from the end, look through at the bulb and make sure it's vertical, just make sure and tweak with your finger on the top of the bulb if you need to, and that the bulb's reasonable centered relative to the condenser glass.....

Back soon, John B.

[janvangastel]

John, I turned to up/down knob all the way anti clockwise, which is to the lowest position of the bulb. I set the bulb verical and - to the eye - centered in the lens. I also made a piece of paper with concentruc circles to place over the field lens (the lens under the condenser). When I shine a (not too bright) flashlite on it, I can easily see that the bright spot is off center to the top/left side. On the photograph the piece orf paper isn't well centered, so here it looks (a little bit) worse.

light.jpg (63.45 KiB) Viewed 13306 times

[mrsonchus]

Hi Jan, a quick observation, are you locking-on the housing fully by rotating the big lever (seen in your image to be in an upper position and so unlocked) to the lower position. Meanwhile I'm going to take some images and get back soon....

John B.

[janvangastel]

John, I assume you mean the black lever to lock the lamp house against the microscope stand. It is in the upper position indeed. I started a quarter rotation to the left and then rotated clockwise until I couldn't rotate further, which was in upper position. Should I start in a quarter rotational position on the other side (to the right) of the microscope and end in lower position? I can't feel any play between lamp house and microscope stand.

[mrsonchus]

Hi Jan, if there's no play and the housing is snug I think you're correct, sounds OK to me.

John B.

[janvangastel]

I played a bit more with the Kohler illumination. I followed the instructions of the lamp house article as good as I could, but whatever I tried, I could not see the two pins of the filament shown on the pictures. So what I did was focusing as good as I could on a piece of paper on the field lens.Then I centered the light patch as good as possible on that paper (marked with a centered circle) using the lamp house control buttons. Picture below. After that I looked into the eyepiece tube without an eyepiece for uniform illumination. I could see the specimen without the eyepiece. Uniformity of illumination seemed at least reasonably good to me. See small video with hand held camera

centered.jpg (73.09 KiB) Viewed 13129 times