Diaphragm Of A Microscope: What Is It And How Can It Be Used?

What Is The Diaphragm Of A Microscope?

To summarize what a microscope does; it collects the light from a specimen and uses a combination of different lenses to magnify and control the view of the specimen.

The diaphragm of a microscope is arguably the most important part of the microscope when it comes to a key part of the microscope’s function, controlling the light.

While adjusting the lenses of a microscope massively impacts the quality of the final image produced, people sometimes overlook how important controlling the light source is to get the best quality final picture. 

Diaphragm Of A Microscope: What Is It And How Can It Be Used?

The diaphragm of a microscope works in conjunction with the condenser of the microscope.

While the condenser focuses on the light source, the diaphragm controls the amount of light that will pass through the condenser and eventually hit the subject.

This control of the light source can drastically change the quality of the final image depending on the quality of the specimen. 

The most commonly used type of diaphragm for use in most microscopes is the iris diaphragm, but there are other varieties that are sometimes used. 

Now you know the basics of what a microscope diaphragm is, let’s have a look at how it works and everything this vital part of the microscope controls and affects.

How Does The Diaphragm Of A Microscope Function?

To further specify what was stated in the introduction, the main role that the diaphragm fulfills in a microscope is changing the angular aperture of the cone of light that is made when the light source goes through the condenser before hitting the specimen.

This sentence is quite complex so breaking it down is the best way to understand its meaning.

The best way to understand the function of the diaphragm of a microscope is to see how the microscope functions without it being present.

So if there is no light source present the light will travel uninhibited through the condenser that will concentrate the light onto the specimen.

After this, the light from the specimen will be collected by the objective lens and magnified to give the final image.

While this seems fully functional there is a complete lack of control over the quality and quantity of light from the source.

This can cause very detrimental effects, for example, a lot of specimens used in microscopes can be sensitive to light and have the final image altered because of an uncontrolled light source, or the final image can just end up being too bright and very difficult to read properly.

Controlling the size of the cone of light produced by the light source is more important than it seems as if its size does not match the optimal numerical aperture (NA) of the objective lens that is being used to gather the light from the specimen, the quality of the final image produced will suffer. 

So to stop the chance of any of this happening, we use a diaphragm. Simply speaking the diaphragm will block out the desired amount of light only letting what you control through.

This allows you to adjust for light-sensitive specimens, alter the brightness of the final image, and match the necessary numerical aperture to get the best results from the objective lens.

Before going more in-depth into what the diaphragm controls, let’s have a look at the different types of diaphragms available for use.

What Are The Different Types Of Diaphragms Used In Microscopes?

Disc Diaphragms

This is one of the least utilized types of diaphragms, especially in more formal experiments, due to its simplistic design and lack of precise control.

Instead of carefully altering the level of light going through the condenser, this diaphragm is a bigger piece that has multiple preset sizes of holes to allow the light to pass through. 

While this works well if you do not need a massive level of control or are using an objective lens that has a numerical aperture matching one of the holes in this diaphragm if this is not the case this type of diaphragm is not the best choice.

This type of diaphragm is usually more affordable than other varieties, but this is due to the complete lack of control you will end up with.

However, if you are working informally with a microscope this could be a good beginner pick.

Aperture Iris Diaphragm

Aperture Iris Diaphragm

This is the most commonly used style of diaphragm used on microscopes and is named after its inspiration being the iris of the eye.

The iris of the human eye, contracts and retracts to change the amount of light it lets in to protect the retinas and allow the eye to produce the best quality image.

In the dark, the iris lets more light in to get the best image possible and when it is too bright they let less light in to have more control over the image.

The aperture iris diaphragm works very similarly, giving the user complete control over the amount of light let in with a design that opens and closes very similarly to a human iris.

Because of this more sophisticated design, these diaphragms are usually significantly more expensive than disc diaphragms. However, this is generally considered a worthwhile investment.

These diaphragms give complete control over the light hitting the specimen and allow the user complete control over the light hitting the condenser making it easy to match the numerical aperture of any objective lens if you are careful enough.

These types of diaphragms are also located much closer to the condenser with the condenser usually sitting directly on top of it. 

The aperture iris diaphragm also stands out for its ability to not just control the amount of light let in, but also to control the contrast. The contrast affects how much the light and dark will be different from each other in the final image produced.

If the contrast is higher you will be able to identify more details, especially between different colors.

However, if the specimen being identified is more transparent you will need to control this contrast more to get the optimal contrast.

Field Diaphragm

In contrast to the aperture iris diaphragm. The field diaphragm sits much closer to the light source that the microscope utilizes.

The field diaphragm works in a quite similar way to the aperture iris diaphragm, but it does have some important key differences. 

The field diaphragm controls the amount of light let into the condenser just like an aperture iris diaphragm but also controls how large the field of view you get of the final image is (ergo being named the field diaphragm).

This is easy to confuse with zooming in, but it is not the same, instead of changing the appearance of the produced image, it just adjusts the field of view covering unnecessary details. 

So this type of diaphragm can be used to control the amount of light let in like all diaphragms and is unique for changing the field of view, but it does not change the contrast of the light as the aperture iris diaphragm can.

This key feature has many uses and is often used in conjunction with an aperture iris diaphragm, so the user can have complete control over both factors. 

Compromises When Using A Diaphragm On A Microscope

Like with many of the other functions of the microscope, while you adjust one setting to improve the quality of the image in one way, this is usually at the detriment of another factor, and you have very little control over this trade-off.

Getting a balanced image including all the details which you desire is always an act of comprising to get the best results. 

For example, the more light you let onto the specimen, you will get less contrast resulting in less clearly visible details.

But if you prioritize this contrast you will be getting less light from the light source, limiting the light collected by the objective lens can lead to a poor quality final image with a grainy appearance. 

Because of this compromise, the process of properly focusing the image on a microscope can take an incredibly long amount of time.

Getting a good balance of contrast and light is one of the most important roles of the diaphragm.

You can also get a poor resolution final image if you do not balance the diaphragm well enough as too much light or not enough light can massively impact the final image.

If you are using both an aperture iris and field diaphragm the best way to get the best results is to not change the setting on either too quickly and focus on getting the best result with one before changing the other.

Final Thoughts

So now you know all the different roles the diaphragm of a microscope is responsible for, and what each different type of microscope diaphragm is used for.

With all the different factors controlled by the diaphragm, it can be difficult to manage all of them, but with time it can become a natural part of focusing a microscope.

Jennifer Dawkins