Correct position of darkfield/oblique patch stop in relation to condenser?

[scottmudge]

I recently purchased an Amscope T670. The condenser it came with has no swing-out filter holder. Instead, it has an inset circular depression on the top of the LED assembly (directly above the field diaphragm) into which filters can be placed. It came with 2 glass filters (green and blue) which can be set in this spot (diameter of 45mm).

This filter spot does not match the tutorials for inserting DIY darkfield patch stops into the optical system which I've seen online. In those tutorials, the patch stop is inserted into a filter holder which is coupled to and swings out of the condenser itself.

In my case, the filter location is uncoupled from the condenser (and relatively far away). I've 3D printed a few patch stops of various diameters and placed them in the filter location above the LED assembly, and adjusting the height of the condenser does allow me to focus the patch stop at the focal plane of the objective, and achieve a dark-field effect. However, the results are sub-par compared to the purpose-made dry darkfield condenser (with the darkfield mask printed directly on the lens). However, this may simply be due to having an incorrect patch stop diameter for the objective I'm using (40x, NA 0.65).

My question: is there an ideal/correct z-axis location for a darkfield patch stop in the optical system? My intuition is telling me that having it as close to the condenser lens as possible is what you'd want, but is having the patch stop uncoupled from the condenser (and thus being able to focus the patch stop by adjusting the height of the condenser relative to the patch stop) going to achieve a similar/desired effect?

Or should I design some sort of filter holder for my condenser assembly? There's not a lot of clearance between the condenser and condenser holder (maybe 0.8-1.0 mm), so inserting a patch stop at this location would be difficult.

[blekenbleu]

I recently purchased an Amscope T670. The condenser it came with has no swing-out filter holder.

That sucks; its manual mentions filters...
Meanwhile, T670's provided condenser reportedly has a fairly common 37mm circular clamp mount;
this 37mm condenser has a swing-out filter holder: https://www.ebay.com/itm/324507501487

.. about 50mm overall height:

[scottmudge]

That sucks; its manual mentions filters...

To be fair, it *does* come with filters, they just intend them to go here:



Which in a way is more convenient than a swing-out filter holder. But I'm just not sure about the optical properties of putting a patch stop there.

I don't think the linked condenser will work, the set-screw locking the condenser in place is meant to seat against the lowest ~5mm of the condenser (where the filter holder is on the one you linked). On the T670 condenser, it has a sort of concave v-groove flange all around the lower edge of the condenser, which the set-screw seats into.

[Hobbyst46]

That sucks; its manual mentions filters...

To be fair, it *does* come with filters, they just intend them to go here:
...

The marked position is designed only for filters (e.g.polarizer) that completely pass the entire diameter of the illumination beam.
The ideal place for a patch-stop is as close as possible to the condenser aperture. It sort of replaces the aperture, since it blocks the beam and allows peripheral illumination only.

[scottmudge]

That sucks; its manual mentions filters...

To be fair, it *does* come with filters, they just intend them to go here:
...

The marked position is designed only for filters (e.g.polarizer) that completely pass the entire diameter of the illumination beam.
The ideal place for a patch-stop is as close as possible to the condenser aperture. It sort of replaces the aperture, since it blocks the beam and allows peripheral illumination only.

Thanks! Yeah after experimenting more last night, I agree. I am able to achieve a darkfield effect by focusing the patch stop at the focal plane of the objective, but there is poor contrast and a lot of stray light coming in, compared to the darkfield condenser. I'm curious if collimating the light coming out of the LED and placing a patch stop in the path of the collimated light will reduce stray light getting in, but for now I'll try putting the patch-stops closer. I can't find a small enough collimator anyway. If I print them thinner I think I can slip them in between the condenser holder and the condenser.

[apochronaut]

Patch stops are often positioned at a location that is physically practical but optically impractical and a number of factors can contribute to the achievement of dusk field, rather than dark field. As the N.A. of your intended DF objective increases, so does the level of precision required of the condensing system, aperture occluder and their optical interface and a key factor in reducing stray peripheral light is how well focused the edge of the occluder is. An ill focused occluder is pretty heavy with diffraction and ray scatter and interference, so in order to overcome the glow of such a fuzzy border, you need a wide separation between the condenser minimum N.A. and the objective minimum N.A. , usually no less than .20 and as much as .30 under poorly precise arrangements and those with any reflecting or diffracting dust, film, debris or eccentricity. This means that at .65, you need a condenser working at a minimum of about .75 if all is nice and tidy, or .80 and above if not : .85 being more likely.

There are only 2 ideal locations for the stop. One of them occludes the illuminator and the other occludes the condenser. Obviously , occluding the condenser is best because that is how most good dedicated DF condensers work. That is how the low N.A. add on DF stage condensers of the past worked, as well as some precision DF caps, where the condenser top lens is either replaced or covered by a DF cap. Those can even be oil DF stops.

When occluding the illuminator, the degree of out of focus becomes very important. Phase diaphragms for instance are a form of DF occluder that interferes with the illuminator beam and are placed precisely at the condenser front focal plane thus the collimation of all three of the illuminator, condenser and object focal planes relative to each other as a DF stop position is necessary in order to achieve minimum acceptable separation between the condenser and objective N.A.s. The lower the N.A. of the objective while using the same condenser, allows for a wider variance in the conjugation of those focal planes.