Optimized Condenser-less dark field LED lighting

A forum to ask questions, post setups, and generally discuss anything having to do with photomacrography and photomicroscopy.

Moderators: rjlittlefield, ChrisR, Chris S., Pau

Linden.g
Posts: 401
Joined: Mon Apr 06, 2009 6:36 pm
Location: PA USA
Contact:

Optimized Condenser-less dark field LED lighting

Post by Linden.g »

I thought I would try out a condenser-free darkfield set-up on an Olympus BH2 BHS as described by Kevin Webb https://www.researchgate.net/publicatio ... 1p-q2nILyg. I placed two sets of LED ring lights, one between the stage and stage plate mount and one around the condenser swing top. I was trying to span the full width of the NA range. You can see the results below. The lower ring worked well for SPlanF2x, Splan 4x, Splan Apo 10x, Splan 20x. I tried to push the NA range up to 1.3 with the upper ring for D Plan Apo 20x UV, D Plan Apo 40x UV, Splan Apo 100x. I couldn't achieve dark field at the 100x. Lots of spiky flair as you go to the upper ring. All single shots. Unfortunately my diatom test slide came apart during the test.


Image

Image

Image

Image

Image

Image

Image

Image

Image

Image
Last edited by Linden.g on Sat Apr 04, 2020 11:35 am, edited 1 time in total.

rjlittlefield
Site Admin
Posts: 23561
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:

Post by rjlittlefield »

Nice rig.

One issue I see is that each of those LED's is quite small, so while you're getting illumination from all points of the compass, the whole setup is still "partially coherent". This makes it vulnerable to an assortment of interference effects, more likely at higher NA. I would not be surprised if that's the cause of your spiky flair.

To reduce this effect, you might try adding some diffusion material, separated by a few mm from the LED dies.

The cleanest darkfield I've ever gotten was illuminating through a ring of ping-pong ball. That was at much lower mag than you're using. I've never tried it at high mag.

--Rik

Lou Jost
Posts: 5942
Joined: Fri Sep 04, 2015 7:03 am
Location: Ecuador
Contact:

Post by Lou Jost »

Linden, that same paper inspired me to make a brightfield condenser-free setup using a half of a pingpong ball glued to the underside of a coverslip, with the whole assembly submerged, almost to the level of the coverslip, in a dilute solution of milk. I lit the milk with lasers. The milk decoheres the lasers and diffuses the light, and the pingpong ball creates a perfectly uniform light source similar in principle (though not as bright as) the LED hemisphere in that paper.

Smokedaddy
Posts: 1951
Joined: Sat Oct 07, 2006 10:16 am
Location: Bigfork, Montana
Contact:

Post by Smokedaddy »

Looks like nice dark field to me Linden for non stacked images. As you know, all that can be easily cleaned up with PS. Just stay away from the 100x. 8)

Linden.g
Posts: 401
Joined: Mon Apr 06, 2009 6:36 pm
Location: PA USA
Contact:

Post by Linden.g »

Thanks for the feedback. Rik, I took your advice and found a better LED light which gives a smooth diffuse light. It has helped to eliminate the flair spikes. The diatom images were taken with an SplanFl 2x and a Dplan Apo 40x UV. NA 0.85.

Image

Image

Image

rjlittlefield
Site Admin
Posts: 23561
Joined: Tue Aug 01, 2006 8:34 am
Location: Richland, Washington State, USA
Contact:

Post by rjlittlefield »

Excellent!

Can you provide a link to buy that better LED ring?

--Rik

Smokedaddy
Posts: 1951
Joined: Sat Oct 07, 2006 10:16 am
Location: Bigfork, Montana
Contact:

Post by Smokedaddy »

I absolutely knew you'd figure it out Linden. 8)

Linden.g
Posts: 401
Joined: Mon Apr 06, 2009 6:36 pm
Location: PA USA
Contact:

Post by Linden.g »

Thanks Jim and Rik, here you go https://www.ebay.com/itm/312733338679

zzffnn
Posts: 1896
Joined: Thu May 22, 2014 1:25 pm
Location: Houston, Texas, USA
Contact:

Post by zzffnn »

Great job, Linden!

How is your DF rig's diatom dotting ability at NA 0.85 40x?

If you can post a 100% crop on Pleurosigma angulatum, I can tell you how it compares with regular DF condener's dotting ability.

NA 0.85 is about as high as it gets for most oil DF condensers. At NA 0.85 DF, with a regular oil DF condensers, you should be able to resolve Pleurosigma angulatum to dots.

With a Leitz Heine variable DF / COL condenser (oiled) and a NA 0.95 LOMO 40x dry app objective, Frustulia rhomboides could also be resolved to dots. But Leitz Heine has a very slender and very large (variable) light cone.

I can gift you some diatom strew slides (message me if you want them). It is not cheap, nor convenient, to crash Klaus Kemp test slides :wink: I have been there.

Pau
Site Admin
Posts: 6051
Joined: Wed Jan 20, 2010 8:57 am
Location: Valencia, Spain

Post by Pau »

zzffnn wrote:...
NA 0.85 is about as high as it gets for most oil DF condensers. At NA 0.85 DF, with a regular oil DF condensers
...
With an oil 1.2/1.4 condenser you can get excellent DF with Na 1.0 oil objectives, at least in theory you could also do with NA 1.1 although I don't have such 1.1 objective.
But getting DF with a dry "condenser" and a 0.85 objective as Linden does is a big achievement, outside the range of typical DF dry condensers
Pau

Linden.g
Posts: 401
Joined: Mon Apr 06, 2009 6:36 pm
Location: PA USA
Contact:

Post by Linden.g »

Thanks for the feedback. Here are some crops. The first is with Splan 40x NA 0.7 and the second with Splan Apo 40x NA 0.95

Image

Image

Linden.g
Posts: 401
Joined: Mon Apr 06, 2009 6:36 pm
Location: PA USA
Contact:

Post by Linden.g »

I thought the Moiré pattern in these overlapping diatoms was interesting. 100% crop using Splan Apo 40x NA 0.95 with LED darkfield.

Image
Last edited by Linden.g on Mon Apr 13, 2020 8:15 am, edited 1 time in total.

zzffnn
Posts: 1896
Joined: Thu May 22, 2014 1:25 pm
Location: Houston, Texas, USA
Contact:

Post by zzffnn »

Those look great there, Linden! Congratulations!

You have some big Amphipleura species on your strew slide and a NA 0.95 Apo. Why not give it a try by zooming in onto pixel level (to see how far your LED rig can go)?

Frustulia rhomboides would serve as better resolution target there (being raised and closer to the limit), though I am not sure you have it there.


Pau:

My previous comment (regarding not easy to go over objective NA of 0.85 even with oil DF condensers) came from my actual hands-on diatom dotting experience.

I used paraboloid and cardioid oil DF condensers (all marked 1.2-1.4) made by Zeiss Jena, Bausch and Lomb, and American Optical, with various (Apo or achromat) objectives made before 1980.

My experience is that DF objective NA above NA 0.85 does not always work. Yes, I did try a NA 1.1 objective, an iris objective of NA 0.7-1.0, a NA 0.95 objective, a few objective DF funnel stops of around NA 0.85, and a NA 0.75 objective.

AO produced darkfield funnel stops for their oil objectives / oil DF condensers and limited NA to right around 0.85-0.9. That is because DF image quality deteriorates quickly, by flare / mount (subject) thickness / optical property; you may get excellent DF image with Frustulia, but flare badly with Surirella.

Maybe modern Zeiss West high NA plan apos would work much better in DF.

I don't use dry DF condensers at all, as my DIY darkfield stops work reasonably well up to objective NA 0.65.

Pau
Site Admin
Posts: 6051
Joined: Wed Jan 20, 2010 8:57 am
Location: Valencia, Spain

Post by Pau »

0.95 is definitively outside the dry DF condensers range (the maximum NA of a dry system is 0.95 and NA of the objective must be smaller than the inner NA of the illuminating hollow cone, so it seems to me exceptional.

zzffnn,
I don't use modern objectives for microscope work. I was referring to a Leitz NPL Fluotar 50/1.0 oel, a very nice immersion objective, because I don't have diaphragm bundled high NA objectives I can't test further but my experience with classic equipment fully agree with classic papers.
My DF condensers are Zeiss 0.8/0.95 and 1.2/1.4 "ultracondenser"
I my limited experience both are better corrected that the peripheral zone of BF condensers with dark stops. The dry one is very nice for 40/0.70 or 0.75 objectives
Pau

Post Reply Previous topicNext topic