Simplified Variable amplitude contrast (VAC) setup & DIY HMC

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Sansub2
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Simplified Variable amplitude contrast (VAC) setup & DIY HMC

#1 Post by Sansub2 » Sat Feb 11, 2023 4:39 pm

While searching on the forum, I found VAC setup mentioned and how it is good on certain conditions. While doing further searching, found one earlier thread talking about how to create diy for VAC (link below). Even though you can get that done with a bit of tinkering, I thought I can make it simpler. My understanding of the double layer sandwiched together is to fully extinct the light at the center part and let the light go through at the oblique area. So, I adopted the variable oblique technique to add poliazed filter. In this setup, you can use it for all your objective by moving the slider left and right to increase or decrease the obliqueness ( it will only work if you have a slider as filter holder, not the swing out filter holder).

1. For polarizer, use the 3d movie theatre (real3d) glasses. It is already included in the ticket so it is free ☺️.
2. Will only work for slider filter holder. For people with swing out holder, if you have 3d printer, I have a 3d printable holder design to convert the swing out to slider. Please let me know.
3. Takes only 30mins to an hour to create it. Improvise if you don't have 3d printer (thick poster board from dollar store might work). Probably cost 1-2$.

Picture 1: Left one is as mentioned in the original link. Middle one is the simpler adaptation of the same. A 15mm hole cut in the rectangle sheet(2x), polarized film is sandwiched between them and stick together. Right one is polarizer goes on top of the light source.

Image20230209_145216 by San Sub, on Flickr

Picture 2: how you see it in the back focal plane of the objective (looking through the eye piece). You can set it left or right for oblique or fully centered.

Image20230209_145412 by San Sub, on Flickr


Original link:

http://www.microbehunter.com/forum/spec ... hod-vac-2/
Last edited by Sansub2 on Tue Feb 14, 2023 7:26 pm, edited 1 time in total.

ldflan
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Re: Simplified Variable amplitude contrast (VAC) setup

#2 Post by ldflan » Sun Feb 12, 2023 6:08 am

Proof is in the pudding, as they say. What do the results of your simplified VAC system look like?

I haven't made a VAC system, but I am planning on it and have thought it through. VAC is not a contrast enhancement system based on interference effects. It is really a very fancy oblique lighting effect.

As I understand it, the Strange VAC system creates a horizon of variable intensity illumination along 270 degrees of the circumference of the field of view, the result being similar to darkfield (or near darkfield) perimeter lighting condition, but with an oblique lighting effect in which the brightest light comes from one quadrant, while light from the other three quadrants is variable in intensity and can be tuned by rotating the polarizer at the light source. So the VAC is really a very fancy variable oblique lighting system, combined perhaps with some darkfield (perimeter lighting) effects.

Your version, as shown by the photo of the filter in position behind the objective, will only allow variation of light intensity in the left hand area by rotating the light source polarizer, right? So yours is, as you say, a much simpler oblique lighting system than the Strange VAC. If you think about it, what your system does is the equivalent to converting a simple condenser into one with a lateral displacement feature coupled with a light source dimmer. That seems like a very worthy result for a really small investment.

Interestingly, there does not seem to be any reason to use polarizing film rotated to extinction to make a near darkfield patch at the center of the 3/4 disk of linear polarizing film. You could use black construction paper for the central stop just as well, because rotating the polarizer at the light source won't change the extinction of the central patch. However, if you could rotate the polarizing film situated between the other two filters positioned at the extinction point, then the intensity of light coming through the central patch would also be variable as a result of an odd quantum phenomenon noted and demonstrated here:

https://www.youtube.com/watch?v=5SIxEiL8ujA

It may be that the VAC system varies the amplitude of light coming through the central patch as designed, and I am just not seeing that. I really need to put one together to be sure I understand how it all behaves...

Sansub2
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Re: Simplified Variable amplitude contrast (VAC) setup

#3 Post by Sansub2 » Sun Feb 12, 2023 2:20 pm

Ldflan,
Thanks for the detailed response. As you mentioned, yes, VAC seems to be fancy oblique. Atleast that is what I am getting as results. It varies the background light intensity uniformly to get better obliqueness. I didn't have better sample to demo it and make it "uuhh-wow" :D . That is why I didn't put sample image earlier. It is freezing outside and my sample culture pretty much dead. :(

Today I took some sample from my aquarium to give a simple demo. All were take with 10x. Tried my best to keep the video settings same (ev always changes as I don't have control over it). Videos are unedited.

1. Normal VAC2 as mentioned in the link. Oblique area is static. Once it is fully extinct, you get nice dark field if you open the condenser iris bigger.



2. Modified VAC2. You can vary the obliqueness to your liking. Based on my above test, I don't think nothing much going in the middle to lose anything. You can get dark field (oblique DF) if you open the condenser iris enough.
[Update] Sorry, you won't get oblique DF.



3. Regular oblique. Just demo purpose , a quick one.




Please let me know if you want me to do more tests. BTW, I am trying holfman slit and 3rd polarizer. Seems to give better results. Will post video later
Last edited by Sansub2 on Sun Feb 12, 2023 2:53 pm, edited 1 time in total.

Sansub2
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Re: Simplified Variable amplitude contrast (VAC) setup

#4 Post by Sansub2 » Sun Feb 12, 2023 2:50 pm

ldflan wrote:
Sun Feb 12, 2023 6:08 am

As I understand it, the Strange VAC system creates a horizon of variable intensity illumination along 270 degrees of the circumference of the field of view, the result being similar to darkfield (or near darkfield) perimeter lighting condition, but with an oblique lighting effect in which the brightest light comes from one quadrant, while light from the other three quadrants is variable in intensity and can be tuned by rotating the polarizer at the light source. So the VAC is really a very fancy variable oblique lighting system, combined perhaps with some darkfield (perimeter lighting) effects.
BTW, what you mentioned above is not what I understand and see on VAC. The original author clearly say the center area should be fully extinct. No light pass through. Only one quadrant is cut out to vary the intensity in that area to create variable intensity obliqueness.

What you explained is hoffman slit design. I tried it few day ago by combining the VAC and semi-Hoffman. I don't have 15 percent nd film(or any nd film for that matter). So, used full linear polarizer in the nose slit. Doing more experiments on it. Will post video when I get good results.

ldflan
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Re: Simplified Variable amplitude contrast (VAC) setup

#5 Post by ldflan » Sun Feb 12, 2023 2:58 pm

I probably didn't explain my understanding of VAC well. I do understand that the central patch stays black. I think it could however be varied as explained, if one wanted.

No, the cut out area of the main filter is not the variable area of light transmission. The area of variable light transmission is the 3/4 disk polarizing filter. The 3/4 perimeter can be tuned to extinction using the light source polarizer.

Neither of these are Hoffman modulation contrast. To get that, you need a polarizing band behind the objective (or inside it) to realize the interference effect.
Last edited by ldflan on Sun Feb 12, 2023 3:58 pm, edited 1 time in total.

ldflan
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Re: Simplified Variable amplitude contrast (VAC) setup

#6 Post by ldflan » Sun Feb 12, 2023 3:53 pm

Thanks for posting the videos. To my eye, the VAC2 setup seems to be giving a much better result than the modified system; the modified system's results seem very similar to normal oblique lighting. When you rotate the light source polarizer on the modified system, it pretty much seems to dim across the field, and you lose the oblique lighting effect fast. The original VAC system seems to do a better job with that, and the grey field looks a bit more like DIC or Hoffman modulation contrast (for what it's worth). It's interesting that the fibers in your video of the modified VAC system (and in the normal oblique shots) show a lot of chromatic aberration, but with the light reduced (DF patch and polarizer) in the normal VAC system, the visible chromatic aberration goes away. A side benefit, perhaps.

Sansub2
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Re: Simplified Variable amplitude contrast (VAC) setup

#7 Post by Sansub2 » Sun Feb 12, 2023 4:00 pm

ldflan wrote:
Sun Feb 12, 2023 2:58 pm

No, the cut out area of the main filter is not the variable area of light transmission. The area of variable light transmission is the 3/4 disk polarizing filter. The 3/4 perimeter can be tuned to extinction using the light source polarizer.

Neither of these are Hoffman interference contrast. To get that, you need a polarizing band behind the objective (or inside it) to realize the interference effect.

For the first part of your message, then both the original link and mine is not VAC as the 3/4th is not changing the light intensity. I couldn't find any reference to the VAC microcopy the verify what this method actually should do. The reference materials I read explans about VAC in general.if you have any reference, please do share.

For the 2nd part, yes, I am using 3 polarizers. See the effects for 10x. This is not circular polarizer. CP is too intense. This is in between liner and circular with the touch of oblique. In my quest to finding DIC closer alternative using cheap diy methods, this is the best so far. Looks like I need to have different slit size for different objective as 40x is a bit low in light intensity with the same path.


Please view it in full resolution. I will start another thread for it.

Sansub2
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Re: Simplified Variable amplitude contrast (VAC) setup

#8 Post by Sansub2 » Sun Feb 12, 2023 4:10 pm

ldflan wrote:
Sun Feb 12, 2023 3:53 pm
Thanks for posting the videos. To my eye, the VAC2 setup seems to be giving a much better result than the modified system; the modified system's results seem very similar to normal oblique lighting. When you rotate the light source polarizer on the modified system, it pretty much seems to dim across the field, and you lose the oblique lighting effect fast. The original VAC system seems to do a better job with that, and the grey field looks a bit more like DIC or Hoffman modulation contrast (for what it's worth). It's interesting that the fibers in your video of the modified VAC system (and in the normal oblique shots) show a lot of chromatic aberration, but with the light reduced (DF patch and polarizer) in the normal VAC system, the visible chromatic aberration goes away. A side benefit, perhaps.
It is possible that the difference in oblique size and the altered outer edge might be giving some additional effects.

Thanks for the feedback. :)

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Re: Simplified Variable amplitude contrast (VAC) setup

#9 Post by ldflan » Sun Feb 12, 2023 4:30 pm

I think you must be mistaken about the 3/4 perimeter not varying. Just rotate the condenser and light source filters against one another and you will see what I mean. The area that is cut out of the condenser filter cannot vary in intensity because only one polarizer affects the light that goes through that path. In the 3/4 perimeter, however, there are two polarizers capable of being brought to extinction.

I am intrigued to see how you assembled your HMC system. How did you place the polarizer band in or behind the objective? If there isn't one, you simply are never going to get any real interference contrast effects, only some interesting variations of oblique and polarized light. Are you adding a Berek waveplate or something to change the background colors?
Last edited by ldflan on Sun Feb 12, 2023 4:43 pm, edited 1 time in total.

Sure Squintsalot
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Re: Simplified Variable amplitude contrast (VAC) setup

#10 Post by Sure Squintsalot » Sun Feb 12, 2023 4:35 pm

This is a cool thread, and thanks for uploading your video tests; that's a lot of work.

Sansub, are you using an inverted microscope? What's your set-up look like for those videos?

Sansub2
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Re: Simplified Variable amplitude contrast (VAC) setup

#11 Post by Sansub2 » Sun Feb 12, 2023 5:37 pm

Sure Squintsalot wrote:
Sun Feb 12, 2023 4:35 pm
This is a cool thread, and thanks for uploading your video tests; that's a lot of work.

Sansub, are you using an inverted microscope? What's your set-up look like for those videos?
Thanks :) . Nothing much to do outside in the weekend, so trying things that I kept thinking of doing for sometime now.

My scope is soptop. It is rebranded Euromex oxion in the link below. Bought it used for cheap locally.

Oxion : https://www.optics-pro.com/bright-field ... pe/p,44125

Image20230212_121627 by San Sub, on Flickr

Sansub2
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Re: Simplified Variable amplitude contrast (VAC) setup

#12 Post by Sansub2 » Sun Feb 12, 2023 5:49 pm

ldflan wrote:
Sun Feb 12, 2023 4:30 pm
I think you must be mistaken about the 3/4 perimeter not varying. Just rotate the condenser and light source filters against one another and you will see what I mean. The area that is cut out of the condenser filter cannot vary in intensity because only one polarizer affects the light that goes through that path. In the 3/4 perimeter, however, there are two polarizers capable of being brought to extinction.
Tried to do it but it was hard as the alignment gets screwed up. I see what you are saying though. I think that is what is giving edge to the original VAC. It creates a nice glow in the image which is missing in my version.
I am intrigued to see how you assembled your HMC system. How did you place the polarizer band in or behind the objective? If there isn't one, you simply are never going to get any real interference contrast effects, only some interesting variations of oblique and polarized light. Are you adding a Berek waveplate or something to change the background colors?
Nothing fancy. I do have slit opening in the nosepiece. I am using another polarizer in there. Please see the 3 polarizers that I have used in the image below. All are home made using 3d printer and 3d movie theatre eyeglass (except the one in the middle. Kept it pure liner polarized film).


Image20230212_121841 by San Sub, on Flickr

Top one goes above the light source, middle one goes in the filter try and the bottom one goes in the nose slit. I know it is not directly above the objective as you see in the specialized objectives. Please not that the effects change based on the position of the 2nd filter(filter tray). Getting best effect if the direct light opening is just outside of the condenser NA. If you see it through eyepiece, you will only see the polarizer to the edge.


Currently doing another set of 3d printing to get the nose piece close to what is discribed in HMC sites. Going to take few tries though

ldflan
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Re: Simplified Variable amplitude contrast (VAC) setup

#13 Post by ldflan » Sun Feb 12, 2023 6:20 pm

I see! You are combining circular and linear polarization... at one edge fairly high up in the optical train. I have no clear idea what that is doing...

To get a real HMC effect, your condenser polarizer should be a reverse match for the slit version you made for the position above the objective. The two slit filters need to be aligned so that their edges come together... Not sure whether that alignment can happen without putting the filter much closer to the rear of the objective. I suspect it cannot.

Sansub2
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Re: Simplified Variable amplitude contrast (VAC) setup

#14 Post by Sansub2 » Sun Feb 12, 2023 6:34 pm

ldflan wrote:
Sun Feb 12, 2023 6:20 pm
I see! You are combining circular and linear polarization... at one edge fairly high up in the optical train. I have no clear idea what that is doing...
I am getting the gray and blue hue with just 2 proper linear polarizer( when I rotate the bottom one) without the nose filter. It only comes if the direct light opening is outside the objective NA.

The top one (in the nose piece), I am going to change it to use proper linear. Will post the results.
To get a real HMC effect, your condenser polarizer should be a reverse match for the slit version you made for the position above the objective. The two slit filters need to be aligned so that their edges come together... Not sure whether that alignment can happen without putting the filter much closer to the rear of the objective. I suspect it cannot.
That is going to be my next try. Based on what I read, it seems it is not going to be easy. That is the part I cannot match as the original design calls for 15 percent ND filter.
https://micro.magnet.fsu.edu/primer/te ... intro.html

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Re: Simplified Variable amplitude contrast (VAC) setup

#15 Post by Sansub2 » Sun Feb 12, 2023 7:21 pm

One more video on the original VAC. Just to avoid confusion, I used one linear polarizer at the light source and the VAC patch mentioned in the link in the first post. The patch goes to filter tray. Then the polarizer at the light source is turned to produce variable effects. Camera couldn't keep up with the intensity so I couldn't show full turning. It looks better when you see it directly through eye Piece.

Last edited by Sansub2 on Sun Feb 12, 2023 7:35 pm, edited 2 times in total.

ldflan
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Re: Simplified Variable amplitude contrast (VAC) setup

#16 Post by ldflan » Sun Feb 12, 2023 7:23 pm

I doubt that 15% transmission rating for the polarizing film is an absolute must. I would give it a try with whatever linear polarization film you can get.

The 1% transmission area of the Hoffman slit (marked on the website you reference as "D") can be omitted, at least at the objective rear. I know this because I recently acquired an early Olympus HMC system, and the objectives have only a piece of linear polarization film attached to the back flanges with a couple tiny dabs of shellac. The objectives have no 1% strip at all, while the condenser slits do. If you do want to make the "D" strip, perhaps it could be done by adding a rotated layer of film over that portion.

EDIT: This has errors in it. See below.
Last edited by ldflan on Sun Feb 12, 2023 10:18 pm, edited 1 time in total.

Sansub2
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Re: Simplified Variable amplitude contrast (VAC) setup

#17 Post by Sansub2 » Sun Feb 12, 2023 7:28 pm

ldflan wrote:
Sun Feb 12, 2023 7:23 pm
I doubt that 15% transmission rating for the polarizing film is an absolute must. I would give it a try with whatever linear polarization film you can get.

The 1% transmission area of the Hoffman slit (marked on the website you reference as "D") can be omitted, at least at the objective rear. I know this because I recently acquired an early Olympus HMC system, and the objectives have only a piece of linear polarization film attached to the back flanges with a couple tiny dabs of shellac. The objectives have no 1% strip at all, while the condenser slits do. If you do want to make the "D" strip, perhaps it could be done by adding a rotated layer of film over that portion.
Thanks ldfplan, that is good to know. Will give it a try with polarizer.

One question, in your HMC system, the condenser slits are diffent for different objective or same? Do you have any sample video for me to refer to?

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Re: Simplified Variable amplitude contrast (VAC) setup

#18 Post by ldflan » Sun Feb 12, 2023 7:41 pm

The condenser slits are different sizes for each objective. But there is a fair amount of tolerance - they don't seem to need to be exactly the right size to get some HMC effect. I'll take a few pictures of the condenser slits for you in a bit.

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Re: Simplified Variable amplitude contrast (VAC) setup

#19 Post by Sansub2 » Sun Feb 12, 2023 8:03 pm

Perfect. Thanks. Appreciate the response.

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Re: Simplified Variable amplitude contrast (VAC) setup

#20 Post by ldflan » Sun Feb 12, 2023 9:03 pm

I am glad you had me take some pictures, because it allows me to correct my faulty memory. Aging sucks.

Here is a set of pics.

https://www.yogile.com/6cg8k2fmeb9/21m/ ... =d5c26cfec

The condenser slits get larger as you go up in magnification, like DF patches.

It's the condenser slits that don't have a 1% strip of polarizing film, not the objectives as I thought and said above. You can see that they do have a metal piece that probably casts a shadow over the outer edge of the polarizing strip in the condenser turret, though.

I totally mis-remembered the objectives set up. The filter strips are in fact in the objectives, not at the back flange. They do have the 1% strip as well.

It might be that you will have to buy an HMC objective to make this work? It's possible to find orphaned ones fairly cheaply sometimes...

Sansub2
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Re: Simplified Variable amplitude contrast (VAC) setup

#21 Post by Sansub2 » Sun Feb 12, 2023 9:31 pm

Thanks ldflan for taking time to post the images. It gives me some ideas.

I am trying various lighting methods just for fun as I like the challenge. Gives me satisfaction at the end. I thought HMS seems simple enough to diy. :) . Posting sample videos help others make a decision to do diy ( just like I learn from others discussions).

BTW, you are right that the lambda that was in the nose piece slider is creating that dazzling colors including the gray and light blue which I liked the alot (bottom 2 are linear). Replacing it Linear polarizer (all 3 linear) only produced blue and purple. I liked the former better.

In your experience, did HMC produce colored output like DIC or only gray? Not much sample videos available in YouTube.

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Re: Simplified Variable amplitude contrast (VAC) setup

#22 Post by ldflan » Sun Feb 12, 2023 10:17 pm

I am not sure I understand your question correctly, so I will answer it two ways.

If the sample is colored or stained, then those colors remain in the HMC image (as in the case of DIC.)

HMC does not induce interference colors like DIC prisms can, so the field is always gray to white (assuming you aren't using a color filter at the light source).

Resolution however is best with a 545nm green interference filter (or thereabouts), making the image green to black.

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Re: Simplified Variable amplitude contrast (VAC) setup

#23 Post by Sansub2 » Mon Feb 13, 2023 1:17 pm

That is the information I was looking for. Thanks.

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Re: Simplified Variable amplitude contrast (VAC) setup

#24 Post by zzffnn » Mon Feb 13, 2023 1:43 pm

How does it look at higher NA of 0.65 or more, please?

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Re: Simplified Variable amplitude contrast (VAC) setup

#25 Post by Sansub2 » Mon Feb 13, 2023 4:23 pm

zzffnn wrote:
Mon Feb 13, 2023 1:43 pm
How does it look at higher NA of 0.65 or more, please?
Hi zzffnn,

Here it is. It is based on the original design in the link. The darkfield area is 18mm so I used the same path for both 10x and 40x(.75NA).

Please watch it at higher resolution if you can.


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Re: Simplified Variable amplitude contrast (VAC) setup

#26 Post by Sansub2 » Tue Feb 14, 2023 7:18 pm

Improvised something to do the diy HMC quickly. Here is the first version.

All the videos were unprocessed and taken quickly.
The photos were from the videos a d unprocessed

1. Top one - goes on top of light source.
2. Middle one - filter holder. Slit is for 40x
3. 2x poster board papers in the nose. The one with out the polarizer goes at the bottom. It creates the small black area and helps control the amount of polarizer file that sides on top of it. The 2nd paper slides on top the first paper to adjust and position the polarizer.

Image20230214_122848 by San Sub, on Flickr

What you see through the eye piece (objective back focal plane) without those papers.

Image20230214_122658 by San Sub, on Flickr

Sample videos:

HMC - fun starts at 15 second mark. 40x NA .75 objective



VAC2 for comparison


Oblique for comparison:
<Coming soon>

Photos:

DIY HMC
ImageVideoCapture_20230214-132912 by San Sub, on Flickr

Oblique:

ImageVideoCapture_20230214-132959 by San Sub, on Flickr

I know it is not pretty as DIC but the amount of details that you get even with this quick and dirty HMC is really great. If you have nose slit for slider, it is easy to make.

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Re: Simplified Variable amplitude contrast (VAC) setup & DIY HMC

#27 Post by ldflan » Tue Feb 14, 2023 7:58 pm

I would suggest setting up the HMC slits and leaving them alone. The effect is changed by rotating the light source polarizer, not moving the slits. When you view the slits through a phase telescope, after they have been properly aligned and brought together to overlap, you should see a black band, then a gray band, then a black band and that is all. Look up the user's guide and it shows how it should look. Honestly, I am not seeing much modulation contrast effect here. It's unmistakable when it pops in.

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Re: Simplified Variable amplitude contrast (VAC) setup & DIY HMC

#28 Post by Sansub2 » Tue Feb 14, 2023 9:53 pm

Ldflan, probably I didnt explain it clear. I didn't change the condenser slit after I set it. I am rotating the light source polarizer after everything is set. What I am moving is the 2 sheets in the nose to adjust the position to put them in place. I followed the information in the user guide below.

Thanks for the comment on output though. It helps me understand where I am with the result. Have to do more experiments. I suspect that the gray area might have some use. I am using polarizer as alternative.

https://dokumen.tips/education/microsco ... guide.html

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Re: Simplified Variable amplitude contrast (VAC) setup & DIY HMC

#29 Post by Sansub2 » Tue Feb 14, 2023 11:13 pm

Verified with phase telescope to make sure only the transparent area is covered by nose polarizer. It goes full extinction and create dark background at the highest turn. So it seems 3rd polarizer will not work. I will look for a gray/ND film.

BTW, even though it didn't provide desired result(whatever it is, couldnt find any sample video to baseline), I like the 3d effect though. It is really nice.

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Re: Simplified Variable amplitude contrast (VAC) setup & DIY HMC

#30 Post by Sansub2 » Sun Feb 26, 2023 10:16 pm

Got hold of ND Filter (12.5% light transmission, close to 15% mentioned everywhere). Changed my approach a bit. Instead of creating a slider as I tried before, I created an insert that goes into the objective backside. It gives better control. Tried it with various combinations ( 10% or 15% dark and gray area). Here is a comparison video and some pictures

HMC DIY V4




Oblique



HMC needs further tuning but I think it looks better than before. For the maximum modulation, it is becoming too dark most of the area.

Pictures:

Printed patches

Image20230226_165503 by San Sub, on Flickr

Tinker cad design

Image20230226_165642 by San Sub, on Flickr

How it looks in objective

Image20230226_165858 by San Sub, on Flickr


If anyone is interested in play around with the design, please let me know and I can post it in thingy verse.

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