Do the different oblique filters really do anything different?

Here you can discuss different microscopic techniques and illumination methods, such as Brightfield, Darkfield, Phase Contrast, DIC, Oblique illumination, etc.
Post Reply
Message
Author
Jerradin
Posts: 54
Joined: Thu Jul 30, 2020 9:48 pm

Do the different oblique filters really do anything different?

#1 Post by Jerradin » Tue Sep 15, 2020 1:06 am

I ordered a set of filters from an ebay seller, mostly for the darkfield filters sized correctly for my objectives. They also came with some oblique filters, though. A cut-out circle to the side, a crescent moon shape, a different moon shape that has a solid rim for some reason, and a double flower petal shape.

Do these have any practical difference? I can't really notice any through my eyepieces. they all have about the same effect - mostly darkfield at low objective power and mostly brightfield at high objective power, with a slight 3d effect in both cases. Do these filters have specialized uses I'm unaware of?

BramHuntingNematodes
Posts: 641
Joined: Tue Jan 21, 2020 1:29 am
Location: Georgia, USA

Re: Do the different oblique filters really do anything different?

#2 Post by BramHuntingNematodes » Tue Sep 15, 2020 3:56 am

Probably not but they're cheap!
1942 Bausch and Lomb Series T Dynoptic, Custom Illumination

MicroBob
Posts: 2366
Joined: Sun Dec 25, 2016 9:11 am
Location: Northern Germany

Re: Do the different oblique filters really do anything different?

#3 Post by MicroBob » Tue Sep 15, 2020 6:24 am

Hi Jerradin,
oblique lighting provides an asymmetric hollow light cone that leads to more contrast and resolution. The effect depends on the direcion, so you might have to turn the stop for the best effect. At first you should set up the bright field light so that the back lens of the objective is fully lit. Then check that the stop really is formed so that the maximum aperture is lit at least in one sector. Oblique and circular oblique are a bit fiddly to set up and to reproduce setups. I think that this is the reason it is not much used professionally. But they offer a good combination of contrast and resolution.

Bob

DonSchaeffer
Posts: 999
Joined: Sun Mar 22, 2020 10:06 am
Location: Winnipeg, Manitoba, Canada

Re: Do the different oblique filters really do anything different?

#4 Post by DonSchaeffer » Tue Sep 15, 2020 9:17 am

They generate side light to make shadows. You might want to try top stage side light.

Jerradin
Posts: 54
Joined: Thu Jul 30, 2020 9:48 pm

Re: Do the different oblique filters really do anything different?

#5 Post by Jerradin » Tue Sep 15, 2020 2:14 pm

I probably should have been clearer - what I'm wondering is whether there is any difference between the different oblique filter shapes. I can't seem to detect much of a difference except that larger shapes let more light in.

DonSchaeffer
Posts: 999
Joined: Sun Mar 22, 2020 10:06 am
Location: Winnipeg, Manitoba, Canada

Re: Do the different oblique filters really do anything different?

#6 Post by DonSchaeffer » Tue Sep 15, 2020 3:39 pm

I think there are diffraction differences. The smaller openings give a little more depth of field.

Greg Howald
Posts: 318
Joined: Tue Oct 20, 2020 6:44 am

Re: Do the different oblique filters really do anything different?

#7 Post by Greg Howald » Sat Oct 24, 2020 2:40 pm

Hi Don. I made a bunch of oblique filters when I made my reinberg filters. Most of them don't seem to do much. I have found that a good way to achieve oblique lighting is to swing open the filter holder and without a filter in it, slowly close the filter holder. You will achieve oblique lighting from different angles this way and the result can be effective as you block the light path at different areas.
Have fun.
Greg

Post Reply