Dark field microscopy

Ah. Dark field microscopy!

It's probably the most accessible technique that still is absolutely awesome for a wide array of materials and biological samples. In the simplest arrangement, a black disc is positioned at the center back focal plane. In simple terms, this filters out the transmitted light. Only light that is scattered by the sample will give contrast. The image background is a spooky black and parts of sample will have a stand-out white.


Using dark-field microscopy is a tiny bit double edged though.

Firstly, you will see absolutely everything in that water. A clean microscope and clean slides helps immensely. Secondly, the contrast from out-of-focus elements can be distracting, especially in thicker water samples.
With that said, it's one of those techniques that REALLY makes your eyes pop out of the socket. All that life… wow!

The best news is that the only thing you need is a condenser with a dark-field stop. In the simplest case, you can make a stop yourself, if your condenser has some way of inserting a mask at the back focal plane. It's also pretty light hungry, so the brighter the lamp on your microscope, the better. Personally, I really think dark-field microscopy should be enjoyed in the binoculars. While doing so, it helps immensely to kill the ambient light in the room.



So… what are you waiting for! Go try it out.

Another cool thing about dark-field microscopy is that it is not limited to regular compound microscopes. I also have a stereo microscope with a dark-field base. Even in transmission electron microscopy you also do the same thing, by inserting a stop at the back-focal plane.

For those of you that loves mathematics: the back-focal-plane represents (more or less) the fourier transform of the image plane. By inserting a black disc, we effectively filter out all the low-spatial frequencies of the image. So in a sense, it's something like a high-pass filter for audio, but for images.