BRIGHTFIELD MICROSCOPE: FEATURES, PARTS, FUNCTIONS - BIOLOGY - 2025

The bright field microscope or light microscope is a laboratory instrument used for the visualization of microscopic elements. It is a very simple instrument to use and it is also the most used in routine laboratories.

Since the appearance of the first rudimentary microscope created by the German Anton Van Leeuwenhoek, microscopes have undergone innumerable modifications, and not only have they been perfected, but different types of microscopes have also emerged.

Bright field optical microscope and visualization of the microscopic field with this type of microscope. Source: Pixabay / Pxhere

The first brightfield microscopes were monocular, so it was observed through a single eye. Today microscopes are binoculars, that is, they allow observation through the use of both eyes. This feature makes them much more comfortable to use.

The function of the microscope is to magnify an image many times until it can be seen. The microscopic world is infinite and this device allows it to be explored.

The microscope consists of a mechanical part, a lens system and a lighting system, the latter powered by an electrical power source.

The mechanical part consists of a tube, the revolver, the macro and micrometric screws, the stage, the carriage, the holding clamps, the arm and the base.

The lens system consists of the eyepieces and objectives. While the lighting system consists of the lamp, the condenser, the diaphragm and the transformer.

characteristics

The light or bright field microscope is very simple in its design, since in this case there are no light polarizers, or filters that can modify the passage of light rays as occurs in other types of microscopes.

In this case the light illuminates the sample from the bottom up; this passes through the sample and is then concentrated on the chosen objective, forming an image that is directed towards the eyepiece and that stands out in a bright field.

Since brightfield is the most widely used type of microscopy, other types of microscopes can be adapted to brightfield.

The microscope consists of three well-defined parts:

• The lens system responsible for enlarging the image.
• The lighting system that provides the light source and its regulation.
• The mechanical system that comprises the elements that provide support and functionality to the lens and lighting system.

Brightfield microscope parts

Source: amazon.es

-Optical system

Eyepieces

Monocular microscopes have only one eyepiece, but binoculars contain two. They have converging lenses that enlarge the virtual image created by the lens.

The eyepiece is made up of a cylinder that joins perfectly with the tube, allowing light rays to reach the magnified image of the objective. The eyepiece consists of an upper lens called an ocular lens and a lower lens called a collecting lens.

It also has a diaphragm and depending on where it is located it will have a name. The one between the two lenses is called the Huygens eyepiece, and if it is located after the two lenses it is called the Ramsden eyepiece, although there are many others.

The magnification of the eyepiece ranges between 5X, 10X, 15X or 20X, depending on the microscope.

Through the eyepieces the operator will observe the image. Some models come with a ring on the left eyepiece that is movable and allows image adjustment. This adjustable ring is called a Diopter ring.

The objectives

They are in charge of increasing the real image that comes from the sample. The image is transmitted to the eyepiece enlarged and inverted. The magnification of the objectives varies. Generally a microscope contains 3 to 4 objectives. Named from lowest to highest magnification are magnifying glass, 10X, 40X, and 100X.

The latter is known as an immersion objective because it requires a few drops of oil to be used, while the rest are known as dry objectives. By turning the revolver you can go from one objective to another, always starting with the one with the lowest magnification.

Most lenses are imprinted with the manufacturer's marking, field curvature correction, aberration correction, magnification, numerical aperture, special optical properties, immersion medium, tube length, focal length, coverslip thickness, and color code ring.

Usually the lens has a front lens located at the bottom and a rear lens located at the top.

Goals. Source: Szőca TamásTamasflex

-Lighting system

Lamp

The lamp used for optical microscopes is halogen and they are generally 12 Volts, although there are more powerful ones. It is located at the bottom of the microscope, emitting light from the bottom up.

Condenser

Its location varies according to the microscope model. It consists of a converging lens that, as its name indicates, condenses the light rays towards the sample.

This can be adjusted by means of a screw and depending on the amount of light that needs to be concentrated, it can be raised or lowered.

Diaphragm

The diaphragm acts as a regulator of the passage of light. It is located above the light source and below the condenser. If you want a lot of lighting it opens and if you need little lighting it closes. This controls how much light will pass through the condenser.

Transformer

This allows the microscope lamp to be powered by a power source. The transformer regulates the voltage that will reach the lamp

-Mechanic system

The tube

It is a hollow black cylinder through which the light beams travel until they reach the eyepiece.

The revolver

It is the piece that supports the objectives, which are attached to it by a thread and at the same time it is the piece that allows the objectives to rotate. It moves from right to left and from left to right.

Coarse screw

The coarse screw allows the objective of the specimen to be moved closer or further away with grotesque movements of the stage vertically (up and down or vice versa). Some models of microscopes move the tube and not the stage.

When focus is achieved, do not touch anymore and finish looking for sharpness of focus with the micrometer screw. In modern microscopes the coarse screw and the micrometer screw come with a graduation.

Microscopes that have the two screws (macro and micro) on the same axis are more comfortable.

Micrometer screw

The micrometric screw allows extremely fine movement of the stage. The movement is almost imperceptible and can be up or down. This screw is necessary to adjust the final focus of the specimen.

Platen

It is the sample placement part. It has a strategically located hole to allow light to pass through the sample and the lens system. In some models of microscopes it is fixed and in others it can be moved.

The car

The cart is the piece that allows the entire preparation to be covered. This is extremely important, since most analyzes require the observation of at least 100 fields. It allows you to move from left to right and vice versa, and from front to back and vice versa.

The holding pliers

These allow to hold and fix to the slide so that the preparation does not roll while the carriage is moved to travel the sample. It is located on the platen.

Arm or handle

It is the place where the microscope must be grasped when it is going to be moved from one place to another. This joins the tube to the base.

The base or foot

It is the piece that gives stability to the microscope; It allows the microscope to rest in a specific place without risk of falling. The shape of the base varies according to the model and brand of the microscope. It can be round, oval or square in shape.

Features

The microscope is highly useful in any laboratory, especially in the hematology area for the analysis of blood smears, red blood cell count, leukocytes, platelets, reticulocyte count, etc.

It is also used in the urine and feces area, both for the observation of the urinary sediment and for the microscopic analysis of the feces in search of parasites.

Also in the area of ​​cytological analysis of biological fluids, such as cerebrospinal fluid, ascitic fluid, pleural fluid, joint fluid, spermatic fluid, urethral discharge and endocervix samples, among others.

Likewise, it is very useful in the area of ​​bacteriology, for the observation of Gram stains of pure cultures and clinical samples, BK, India ink, among other special stains.

In histology it is used for the observation of thin histological sections, while in immunology it is used for the observation of flocculation and agglutination reactions.

In the research area it is very helpful to have a microscope. Even in areas other than health sciences, such as geology for the study of minerals and rocks.

Advantage

The brightfield microscope allows a good perception of microscopic images, especially if they are stained.

Microscopes that use light bulbs are easier to use and much more comfortable.

Disadvantages

It is not very useful for observing unstained samples. It is necessary that the samples are colored to be able to observe the structures with greater definition and thus they can contrast with the bright field.

It is not useful for the study of sub-cellular elements.

The magnification that can be achieved is less than that achieved with other types of microscopes. That is, when using visible light, the magnification range and resolution are not very high.

Microscopes that use mirrors require good external lighting and are more difficult to focus.

References

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4. Valverde L, Ambrosio J. (2014). Techniques for visualizing parasites through microscopy. Medical parasitology. 4th Edition. Editorial Mc Graw Hill.
5. Arraiza N, Viguria P, Navarro J, Ainciburu A. Manual of microscopy. Auxilab, SL. Available at: pagina.jccm.es/