COL with polarized filters

[osterport]

From some articles I learned COL could bring good contrast and resolution, but the stop in the middle is just black and white. Then I made a polarized filter to act as a stop, which is adjustable.

Here is the effect without and with filter: ( flower pistil cut )

[osterport]

This is how it looks like. The there is a fixed angle between the two filters so that the light through the two of them can be adjusted simultaneously.

[osterport]

This is the diatom with polarized filter

[zzffnn]

Nice work. Have you compared:

1) COL with a totally opaque (totally black) central mask + polarized peripheral light outside of the central mask

with

2) your COL with pol filter as central mask [of the same size as 1) ] and plain peripheral light?

Do you have a clear preference between 1) and 2) ?

2) seems more difficult to DIY make than 1). And if maximum contrast and 3D relief is your goal, 1) may work better? Unless you want to use 2) to reduce halo (shadow) effects?

In my experience, I prefer using plain darkfield up to objective NA of 0.75. Above objective NA of 0.75, I start to use COL using the above method 1).

I see that your mask is not precisely made. I am not able to make a perfectly circular mask with precision down to 2mm (diameter) either.

In my experience, however, 3mm difference in mask diameter can distinguish an optimal COL central mask from a not-so-optimal one.

[osterport]

Nice work. Have you compared:

1) COL with a totally opaque (totally black) central mask + polarized peripheral light outside of the central mask

with

This is the comparison: both allows narrow light at the fringe, left side polarized filter, right dark stop, 100%

diatom with polarized filter vs dark stop extreme light.jpg (153.48 KiB) Viewed 2436 times

My polarizer allows up to 0.1% light transmit through, you can see from the whole picture:

diatom with polarized filter vs dark stop extreme light overview.jpg (85.32 KiB) Viewed 2436 times

From my point of view, totally dark stop gives you good 3D view, but the contrast is so high that you may miss some details. Polarized filter allows some light come in, the contrast may not be favorable. At least polarizor brings more flexibility. I may try better polarizer material ( 10000:1 ), which would be closer to dark stop.

[osterport]

diatom with polarized filter vs dark stop extreme light 200%.jpg (158.87 KiB) Viewed 2433 times

If we zoom further in, we will see the depth of view with polarized filter is much less than dark stop.

[apochronaut]

I have found DF to be surprisingly 3 D, even at high apertures. Right now I am playing with pol + Göerz slit condenser which has DF caps up to 1.2 N.A
Fusion of DF and pol

Your images and techniques are very good.

[zzffnn]

From the diatom overview photo, I am guessing your dark center stop is slightly too big than optimal (that is why center of the photo is much darker).

Were your dark center mask about the same size as the pol center stop? If they were of the same size and both being slightly too big, then I agree that pol center mask would be preferred. However for simplicity, I would probably rather use a smaller dark center instead and polarize the peripheral illumination to reduce glare.

[osterport]

From the diatom overview photo, I am guessing your dark center stop is slightly too big than optimal (that is why center of the photo is much darker).

Were your dark center mask about the same size as the pol center stop? If they were of the same size and both being slightly too big, then I agree that pol center mask would be preferred. However for simplicity, I would probably rather use a smaller dark center instead and polarize the peripheral illumination to reduce glare.

The dark center stop is roughly the same as pol center stop, but pol center mask allows more light to get in.

Yes, the dark center stop is too big just in a comparison. Normally I would put it as 3/4 of the view. In that case, the difference is not much.

[zzffnn]

It may be beneficial in theory, to have a completely opaque mask blocking 3/4 view, and then split the rest 1/4 into two equal parts of 1/8 each:

part #1 = 1/8 blocked by tunable pol filter, and

the outside most part #2 = 1/8 completely open to light

However, each objective would need a precisely made filter. In my hands, cutting to pol filter to precise diameter size is not very easy.

How do you cut your pol filters precisely, osterport?

[osterport]

It may be beneficial in theory, to have a completely opaque mask blocking 3/4 view, and then split the rest 1/4 into two equal parts of 1/8 each:

part #1 = 1/8 blocked by tunable pol filter, and

the outside most part #2 = 1/8 completely open to light

However, each objective would need a precisely made filter. In my hands, cutting to pol filter to precise diameter size is not very easy.

How do you cut your pol filters precisely, osterport?

about 2*1/8, can you draw a picture, I don't understand.

I cut the filter with this tool, but it still takes patience.

Weixin Image_20240228172131.jpg (106.85 KiB) Viewed 2296 times

[apochronaut]

Your filter cutting looks o.k. to me but in these types of lighting adaptions geometry and materials selection are as important as precision. I am wondering if you have tried varying the slit width much and used rotated assymetry ? It seems that in some ways you are making a modified diy version of the PZO or Pluta slit condenser or possibly the Göerz condenser both of which use or can use polarizing filters in combination with an adjustable slit, assymetry and rotation.
In the case of the Pluta condenser the slit is symmetrically or assymetrically adjustable for width over a full range from almost completely closed to wide open and for length, variable in shape and symmetry of geometry over most of the field. A symmetrically 360° rotating polarizer is optionally situated below the condenser. No facility to gang a DF condenser with it is indicated in any literature but since the condenser lens pack is a standard 1.3 condenser, one could replace that with a DF condenser. The Pluta slit condenser yields high quality interference contrast, which was widely used professionally in Poland and elsewhere, mostly in Europe.The full use of the system utilizes a polarizer and analyzer.
The Göerz condenser is similar but employs a slit of fixed width wedged between two half moons of differential gradients. The condenser lens pack can be longitudinally slid along the slit over about 35% of the field to establish an oblique condition and the whole can be rotated against a simple fixed polarizer disc or any other type of transmitted partial barrier resting in a swing in filter tray underneath. The rotation in concert with the differential plates can create a sort of COL condition. The Göerz condenser further employs a cholce of parabolloid DF projecting top lenses instead of the standard BF top lens, so DF can be added to the system from low magnification up to about N.A. 1.0. The Göerz condenser has a less storied track record than the Pluta condenser, and was perhaps pitched more at an amateur crowd. One of the gaps in any literature or reports assesing the Göerz condenser is that the kit did not include a polarizing filter but is nicely equipped to use a simple one. It is the condenser that rotates, not an elaborate custom polarizer on an akehurst slide as is the case with the Pluta condenser. Adding a polarizing filter and possibly analyzer to the Göerz's natural function changes it's capability considerably.

Just stuff detailing some commercial working systems of similar concept to yours that might give you some further ideas.

[Saul]

https://www.photomacrography.net/forum/ ... 12#p269212

[osterport]

Your filter cutting looks o.k. to me but in these types of lighting adaptions geometry and materials selection are as important as precision. I am wondering if you have tried varying the slit width much and used rotated assymetry ? It seems that in some ways you are making a modified diy version of the PZO or Pluta slit condenser or possibly the Göerz condenser both of which use or can use polarizing filters in combination with an adjustable slit, assymetry and rotation.
In the case of the Pluta condenser the slit is symmetrically or assymetrically adjustable for width over a full range from almost completely closed to wide open and for length, variable in shape and symmetry of geometry over most of the field. A symmetrically 360° rotating polarizer is optionally situated below the condenser. No facility to gang a DF condenser with it is indicated in any literature but since the condenser lens pack is a standard 1.3 condenser, one could replace that with a DF condenser. The Pluta slit condenser yields high quality interference contrast, which was widely used professionally in Poland and elsewhere, mostly in Europe.The full use of the system utilizes a polarizer and analyzer.
The Göerz condenser is similar but employs a slit of fixed width wedged between two half moons of differential gradients. The condenser lens pack can be longitudinally slid along the slit over about 35% of the field to establish an oblique condition and the whole can be rotated against a simple fixed polarizer disc or any other type of transmitted partial barrier resting in a swing in filter tray underneath. The rotation in concert with the differential plates can create a sort of COL condition. The Göerz condenser further employs a cholce of parabolloid DF projecting top lenses instead of the standard BF top lens, so DF can be added to the system from low magnification up to about N.A. 1.0. The Göerz condenser has a less storied track record than the Pluta condenser, and was perhaps pitched more at an amateur crowd. One of the gaps in any literature or reports assesing the Göerz condenser is that the kit did not include a polarizing filter but is nicely equipped to use a simple one. It is the condenser that rotates, not an elaborate custom polarizer on an akehurst slide as is the case with the Pluta condenser. Adding a polarizing filter and possibly analyzer to the Göerz's natural function changes it's capability considerably.

Just stuff detailing some commercial working systems of similar concept to yours that might give you some further ideas.

Thanks apochronaut! It's bit hard for me to understand the two condensers, do you have pictures of them?

[osterport]

https://www.photomacrography.net/forum/ ... 12#p269212

I read the posts and watched the video, it is quite impressive! Do you have pictures of diatom or other stuff? I think oblique illumination can pretty much improve resolution and contrast.

[apochronaut]

Your filter cutting looks o.k. to me but in these types of lighting adaptions geometry and materials selection are as important as precision. I am wondering if you have tried varying the slit width much and used rotated assymetry ? It seems that in some ways you are making a modified diy version of the PZO or Pluta slit condenser or possibly the Göerz condenser both of which use or can use polarizing filters in combination with an adjustable slit, assymetry and rotation.
In the case of the Pluta condenser the slit is symmetrically or assymetrically adjustable for width over a full range from almost completely closed to wide open and for length, variable in shape and symmetry of geometry over most of the field. A symmetrically 360° rotating polarizer is optionally situated below the condenser. No facility to gang a DF condenser with it is indicated in any literature but since the condenser lens pack is a standard 1.3 condenser, one could replace that with a DF condenser. The Pluta slit condenser yields high quality interference contrast, which was widely used professionally in Poland and elsewhere, mostly in Europe.The full use of the system utilizes a polarizer and analyzer.
The Göerz condenser is similar but employs a slit of fixed width wedged between two half moons of differential gradients. The condenser lens pack can be longitudinally slid along the slit over about 35% of the field to establish an oblique condition and the whole can be rotated against a simple fixed polarizer disc or any other type of transmitted partial barrier resting in a swing in filter tray underneath. The rotation in concert with the differential plates can create a sort of COL condition. The Göerz condenser further employs a cholce of parabolloid DF projecting top lenses instead of the standard BF top lens, so DF can be added to the system from low magnification up to about N.A. 1.0. The Göerz condenser has a less storied track record than the Pluta condenser, and was perhaps pitched more at an amateur crowd. One of the gaps in any literature or reports assesing the Göerz condenser is that the kit did not include a polarizing filter but is nicely equipped to use a simple one. It is the condenser that rotates, not an elaborate custom polarizer on an akehurst slide as is the case with the Pluta condenser. Adding a polarizing filter and possibly analyzer to the Göerz's natural function changes it's capability considerably.

Just stuff detailing some commercial working systems of similar concept to yours that might give you some further ideas.

Thanks apochronaut! It's bit hard for me to understand the two condensers, do you have pictures of them?

I will do some detailed photos in the next few days.