Ideal Location for Illumination Filters

Here you can discuss different microscopic techniques and illumination methods, such as Brightfield, Darkfield, Phase Contrast, DIC, Oblique illumination, etc.
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BrianBurnes
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Joined: Sat Feb 08, 2020 5:05 pm

Ideal Location for Illumination Filters

#1 Post by BrianBurnes » Tue May 26, 2020 4:12 pm

Hi all,

I'm currently experimenting with inserting a transparent LCD into the light path of my microscope to get a programmable light filter---similar to a darkfield stop/rheinberg filter, just with liquid crystals.

What I'm not sure about is where to best mount the panel to get the best optical results, and before I start designing 3D printed mounts I'm curious to learn about the tradeoffs. Traditionally one would insert a DF filter directly under the condenser iris, but that location is a bit awkward in this case because of the bulk of the electronics/cables on the moving condenser rack. Much more convenient is laying the LCD directly on top of the field lens; however, that is further down the optical path, and it's unclear to me how that would affect the image quality.

While I'm not sure how common the LCD adaption is, for those who have experimented with darkfield, what are the tradeoffs of inserting a DF stop directly under the condenser versus putting it further below, possibly all the way down to the field lens? Does the filter even do anything useful at that point? What happens to the image as the DF stop is moved?

dtsh
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Joined: Wed May 01, 2019 6:06 pm

Re: Ideal Location for Illumination Filters

#2 Post by dtsh » Tue May 26, 2020 4:34 pm

I'm not optical expert, but I would think directly below the condenser where filters/stops are usually placed would be most effective.

PeteM
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Location: N. California

Re: Ideal Location for Illumination Filters

#3 Post by PeteM » Tue May 26, 2020 6:37 pm

Interesting idea.

I'd also think placement as near as possible to the condenser aperture diaphragm would be typical. However, might be worth an experiment (black paper cutouts etc .) to see what happens at the field aperture diaphragm. These are sometimes located near the bottom field lens.

Another question will be if the glass scatters too much light. Could also be that it will introduce polarization. Final question will be if the LED, when dark, is opaque enough for good darkfield. All seem like questions been answered with a very temporary setup before making a final fit.

BrianBurnes
Posts: 37
Joined: Sat Feb 08, 2020 5:05 pm

Re: Ideal Location for Illumination Filters

#4 Post by BrianBurnes » Tue May 26, 2020 6:59 pm

Good idea about the paper cutouts, I'll try that.

The LCD will almost certainly not be suitable for "real" DF due to lacking contrast, but it should be good enough to simulate different multicolored filters (including gradients and the like). Additionally, after removing the top polarizing layer off of the liquid crystal the LCD becomes essentially a programmable polarizer, which could be fun to play with - I'm not sure what happens when different parts of the light cone (or different wavelengths) are polarized differently, and it would be fun to try.

Zuul
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Location: California

Re: Ideal Location for Illumination Filters

#5 Post by Zuul » Tue May 26, 2020 8:01 pm

LCD's need polarizing filters to work. You can't simply remove them and expect it to behave the same way. Here is an example:

https://www.youtube.com/watch?v=ydRZkLK7S-w

BrianBurnes
Posts: 37
Joined: Sat Feb 08, 2020 5:05 pm

Re: Ideal Location for Illumination Filters

#6 Post by BrianBurnes » Tue May 26, 2020 9:36 pm

LCDs need a final polarizing filter to modulate transparency, but that filter doesn't have to sit on the display itself - it can be in the microscope head instead (i.e. the analyzer), in which case light passes through the sample first. The LCD then serves as a (programmable) bottom polarizer in a polarizing microscope. What interests me about this is that polarization doesn't have to be the same over the entire light cone, but it can be modulated in space and color (for an RGB display). I'm not sure whether this will give any useful results, but it seems interesting to try.

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