Experiments with Cross Polarization and Fibers

In the published papers, it’s clear that a variety of flavors of the Foldscope are being developed to allow for specialized lighting or contrast-enhancement. Some will require additional components, but one in particular seems accessible and easy to adapt the “vanilla” brightfield Foldscope. Polarized light microscopy is attainable by introducing two, perpendicularly oriented polarizing filters into the microscope system. One must be placed over the light source and the other at a location after the objective lens but before the imaging sensor. Here is my setup using a Samsung Galaxy Note 4 and Foldscope:


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Linear polarizing film is easy to come by and cheap (example here) especially considering how little is needed to cover the lenses of the Foldscope and smartphone camera. It would be easy enough to mount  such filters directly on the black plastic strips used to hold the magnets and lenses, but I’m sure the team has already thought of that!

This addition of filters cuts down on illumination intensity, so imaging is aided if we swap the included LED light source with something brighter. It’s not necessary, but given smartphone cameras’ struggles under low-light situations, we might expect better image quality (i.e. detail not obscured by aggressive noise reduction) if we can offer the sensor more light to work with. When properly aligned, cross-polarized filters in the system will extinguish all background illumination. It helps, then, to use some manual exposure controls in your camera app– we don’t want it averaging the scene to calculate exposure.

Next I needed a subject. I’ve got a dog, and it’s winter, so I needed to look no further than my sweater for some dog hair samples. Fibers of all kinds exhibit birefringence, the characteristic of exhibiting multiple refractive indices. Here are some of my initial explorations!

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Refer to this illustration that identifies the anatomy of a hair follicle and see if you can spot the structures in my images. These were taken with the 140x lens, as I didn’t seem to get the high mag lens in my kit.

I have also been experimenting with easy ways to mount specimens. I started out with glass slides, cover slips, and some oil. This method of making slides can offer improved image quality and resolution, but because it’s a temporary mount and relies on suspending the sample in oil, it gets caught up in the Foldscope stage mechanism. After making a mess, I switched to the supplied paper slides and clear tape. This works, but as you can see in the above images, the tape itself exhibits birefringence! Hence the colorful backgrounds. Not a bad side effect visually; it also offered proof that I was in fact getting the intended result with my filters.

Here’s an example of a non-polarized dog hair:


And finally, a few blue synthetic fibers from my sweater:


These are just the start of my polarized light experiments. Once the filters are in place, usage is exactly the same as when making brightfield images: no additional adjustments or controls. Fiber analysis for forensics often employ polarized light microscopy, among other applications. I saw another user working with chemical crystal formations– that’s where I think I’ll be taking my polarized Foldscope next!

Let me know what you think!



3 Comments Add yours

  1. laksiyer says:

    Get that old aspirin out and into the polarized foldscope.. fantastic.

  2. Manu Prakash says:

    So wonderful to see this. Beautiful..


  3. Niramay Gogate says:

    I liked this idea very much! This technique will be quite helpful while observing micro organisms; because they have billions of organic compounds and many of them are optically active.

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