Dark Field Microscopy

Hello readers ^_^

Hope you all are studying well.

Today let's discuss Dark field microscopy. There are lot of different microscopes and microscopic techniques. We will try covering most and if possible all of them in coming time.

What is Dark field microscopy?

Dark field microscopy as the name suggests is contrast to bright field microscopy. It is an easier and cheaper alternative when it comes to observing samples in phase contrast microscopy. It is mostly used when bright field microscopy is unable to resolve the low contrast organelles.

Another quality that makes it special is that the unstained samples can be observed. SO the original properties can be observed without hampering it. Pigment producing organisms are tough to observe though and some false colors maybe seen in such samples. 

The unstained samples appear bright against a dark or absolutely black field. In absence of any sample the entire field appears black, hence the name dark field microscopy.

How does it works?

As we know that in bright field microscopy, the sample to be viewed is placed directly above the light source which gives it abundance illumination. In contrast to this, dark field microscopy is prevented from any direct illumination. In technical terms this is called as filled cone of light and hollow cone of light. In dark field microscopy, the light coming from the light source is blocked by a stop. A stop is simply any disc sort of a thing which is placed above the condenser to block the light. The disc should be placed close to the condenser. Due to this disc 90% of the light is blocked and only the thin outer ring of light is allowed to reach the condenser which focuses the light towards the sample. When a sample is on the stage, the light at the apex of the cone strikes it. The image is made only by those rays scattered by the sample and captured in the objective lens. The image appears bright against the dark background. Thus Darkfield microscopy reduces the amount of light entering the lens system of a microscope in two ways. First, the stop blocks the center of the beam of light that would otherwise fill the objective lens. Second, only the light which is scattered by the specimen and enters the objective lens is seen. 

Here's a diagram that might help you understand better.

PS: You can draw this in exam (without color) :p

How is it a cheaper alternative?

Because you can use anything as a stop. A circular black sheet or a tape can be used as a stop. Even a coin or any circular opaque material can be put above the condenser. The only precaution one needs to take that the stop is not blocking the entire light. The highest quality darkfield microscopes are equipped with specialized costly condensers constructed only for darkfield application.

Can I Optimize the microscope? If yes, then how?

Yes one can definitely optimize the microscope according to the  sample to obtain the best image possible by adjusting the light intensity, by varying the distance between the stop and the condenser and by changing the diameter of the stop.

Pros and Cons?

Dark field Microscopy as mentioned earlier is cheaper alternative.

It helps us to visualize live bacterial cells as well as mounted cells and tissues. 

It is ideal to visualize unstained or transparent samples.

It is more useful to observe the outer structure, boundaries and morphology rather than the inner.

Requirement of a very thin sample film is one of the major disadvantage.

Special care is reqiured to ensure that the entire unit is totally dust free as they can cause false diffraction of light.

Intense source of light is required.

That's all for this one.
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So finally a post without getting grilled by me. Good, I think you are slowly learning to behave.
Yeah, duh.