Most people who have survived a biology class know what a light field microscope is. This type of scope uses bright field illumination, meaning it floods the specimen with white light from the condenser without any interference. Thus the specimen shows up as a dark image on a light background (or white field if you will).
This type of unit works best with specimens that have natural color pigments. The samples need to be thick enough to absorb the incoming light; so staining is usually paired with this type of microscope.
Yet what if the specimen is light colored or translucent, like the plankton on the right? It certainly won't stand out against a strong white background. Additionally, some specimens are just too thin. They cannot absorb any of the light that passes through them, so they appear invisible to the user. This is where the concept of dark field illumination comes in!
Rather than using direct light from the condenser, one uses an opaque disk to block the light into just a few scattered beams. Now the background is dark, and the sample reflects the light of the beams only. This results in a light colored specimen against a dark background (dark field), perfect for viewing clear or translucent details.
On a grand scale, the same thing happens every day when you look up at the sky. Do the stars disappear when it's light out? Of course not! They're still there, their brilliance blotted out by the mid-day sun.
If you're still having a hard time visualizing this concept, think of a dusty room with the light on and the door open. You may feel the dust affecting your breathing, but you probably won't see it flying through the air.
Now turn off the light and close the door to just a sliver, while leaving the light on in the adjacent room. If you look at that sliver of light coming through the door, you'll see all sorts of dust motes suspended in it. You're employing a similar principle when you use dark field illumination!
Dark field microscopes are used in a number of different ways to view a variety of specimens that are hard to see in a light field unit. Live bacteria, for example, are best viewed with this type of microscope, as these organisms are very transparent when unstained.
There are multitudes of other ways to use dark field illumination, often when the specimen is clear or translucent. Some examples:
Dark field microscopy makes many invisible specimens appear visible. Most of the time the specimens invisible to bright field illumination are living, so you can see how important it is to bring them into view!
No one system is perfect, and dark field microscopy may or may not appeal to you depending on your needs.
Some advantages of using a dark field microscope are:
Some of the disadvantages are:
Below are contrasting examples of dark field (left) versus bright field (right) illumination of lens tissue paper. Note how they both create a different style of image.
Admit it, by now you're curious to check out your own dark field! You can create one with minimal time and effort. Just read on...
You don't need to buy a huge expensive set-up to experiment with dark field illumination.
To create a dark field, an opaque circle called a patchstop is placed in the condenser of the microscope. The patchstop prevents direct light from reaching the objective lens, and the only light that does reach the lens is reflected or refracted by the specimen. Easy enough, right?
If you want to make a dark field microscope you'll first need a regular light microscope. Below is your full list of "ingredients":
Now use the following steps to make your patchstop:
Now use your patchstop to turn a light field unit into a dark field microscope:
You did it!
Thanks to Windtrader for this original guide. You can read it here on Ebay.
As you can see, a dark field microscope can let users see specimens in a whole new way, bringing those into focus that don’t stand out under intense light. Using dark field illumination can open up a whole new view of microscopy.
The first picture of the plankton was taken by Uwe Kils and is from Wikipedia under the GNU Free Documentation License.
The second picture of caffeine crystals, the third and fourth pictures of tissue paper, and the last picture of a microscope setup were taken by Richard Wheeler and are from Wikipedia under the GNU Free Documentation License.