You have definitely heard about such a method of studying living bacteria, blood and other biological samples as dark-field microscopy.But how familiar are you with this method? Do you know what its advantage is,how it works, and most importantly, what requirements are put forward for its implementation? In this article, we tried to set out in as much detail as possible the answers to many questions that may arise not only for an ordinary reader, but also for a laboratory assistant beaten by experience. View here for more info: dark field microscopy

Scope of the dark field method

First, it should be said that the dark field method can be implemented both when working in transmitted light and in reflected light. And,secondly, we note that dark-field microscopy is often used as a budget alternative solution to phase contrast microscopy.

So, a wide practical application of the dark field method in transmitted light received in many modern laboratories of medical institutions,clinics, pharmaceutical companies. So in the obtained native preparations using this method it is possible to detect the causative agents of syphilis,relapsing fever, leptospirosis, and also to study the mobility of bacteria.Biological fluids of humans, animals and plants, diatoms, living aquatic microorganisms, small insects, uncolored bacteria, and yeast are ideal"biological" candidates for research under dark-field illumination.In the field of colloidal chemistry and mineralogy, mineral and chemical crystals, colloidal particles, thin sections of polymers and ceramics are used as non-biological samples for dark-field microscopy.

What is the dark field method based on?

To begin with a little understanding of the essence of the dark-field microscopy method, you can turn your gaze to the sky. Paradoxically,but astronomy is excellent for explaining the dark field method in the microcosm. The world is really great!

We can see the stars shining in the sky only in the dark,"black" sky, after sunset, when the sun's rays do not illuminate the sky. Although, when the Sun is at its zenith, the stars do not go anywhere,they are still in the sky, they are simply invisible to our eyes, since the brightness of the Sun suppresses the faint light from the stars. But during a total solar eclipse, when the Moon passes between the Earth and the Sun,"blocking" its light, we manage to see the stars. So we can say that the ability to see the stars, first of all, is due to the sharp contrast between their weak light and the black background of the night sky.

How the dark field method works. Method essence

Once again, we draw attention to the fact that, unlike the traditional method of observation in a bright field, the studied specimen is illuminated by a hollow cone, that is, oblique rays from the side that do not fall into the lens. Thus, only those rays that have been scattered by various parts of the sample enter directly into the microlens. And it is precisely these details of the preparation that will ultimately be visible through a microscope against a black background.

Types of optical systems for dark field condensers

The most popular modern mirror condensers are the paraboloid condenser and a cardioid condenser. Moreover, most often for work in a dark field, the base condenser is replaced by a paraboloid condenser, which is a truncated plan-convex paraboloid lens with an opaque overlap (blocker of light rays) in the center of the lens and a mirror (aluminized, silvered) lateral inner surface of the condenser (wall). A stop block installed in the center impedes the course of the central rays and forms a black (dark) field of view.