Last revised: January 8, 2025

Reviewed by: Scott Orlosky, consulting engineer

Biological microscopes are used to study organisms and their vital processes. Microscopes used in this field range widely, from relatively simple optical microscopes to very advanced imaging systems used in cell research, forensic medicine, and state-of-the-art high resolution molecular studies. The most common configurations of biological microscopes are student, benchtop, and research.

Student microscopes are the smallest and least expensive type of microscope. They are capable of advanced techniques and documentation even though they are for student use.

Benchtop microscopes are used in various industries like textiles and animal husbandry. Benchtop microscopes can do many techniques but are limited by the amount of techniques they can be used for at one time.

Research microscopes are large, weighing in the range of 30 kg to 50 kg. This mass is composed of complex optical, mechanical, and electronic systems. They may use multiple cameras, large specimens, and the widest range of simultaneous techniques.

Applications

Biological microscopes can use one of many types of technologies. The most common biological microscopes are compound microscopes used for viewing very small specimens such as cells, pond life samples, and other microscopic life forms; inverted microscopes, which are better for looking through thick specimens, such as dishes of cultured cells, because the lenses can get closer to the bottom of the dish where the cells grow; and stereomicroscopes, which are great for dissecting as well as for viewing fossils and insect specimens. Other technologies include acoustic and ultrasonic microscopes, microwave microscopes, fluorescent microscopes, laser or confocal microscopes, polarizing microscopes, portable field microscopes, scanning electron (SEM) microscopes, scanning probe or atomic force microscopes (SPM/AFM), and transmission electron microscopes (TEM).

The magnification of biological microscopes is the ratio of the size of an image to its corresponding object. This is usually determined by linear measurement. Resolution is the fineness of detail in an object that is revealed by an optical device. Objectively, resolution is specified as the minimum distance between two lines or points in the object that are perceived as separate by the human eye. Subjectively, the images of the two resolved points must fall on two receptors (rods or cones), which are separated by at least one other receptor on the retina of the eye. Field of view is defined as the extent of the visible image field that can be seen when the microscope is in focus.

Types

Biological microscopes can come in one of many types of eyepiece styles. These include monocular, binocular, trinocular, or dual head. A monocular eyepiece has one objective and one body tube for monocular vision. Binocular microscopes are fitted with double eyepieces for vision with both eyes. The purpose in dividing the same image from a single objective of the usual compound microscope is to reduce eyestrain and muscular fatigue, which may result from monocular, high-power microscopy. These types of microscopes are also used for stereoscopic vision, which allows for depth perception of the sample. Trinocular microscopes are fitted with a vertical tube at the top and regular binocular eyepieces at 30 degrees. The vertical tube is often used for a digital camera or a second observer. A dual head has one vertical eyepiece lens and a second eyepiece off the side at 45 degrees (so that two people can view the sample at one time, or one person and a camera). Important features in specifying biological microscopes include a digital display, mechanical stages, oil immersion lenses, fine focus, computer interfaces, and image analysis processing software.

Biological Microscopes FAQs

How do the optical properties of lenses affect the resolution and clarity of images in biological microscopy?

Resolution

Resolution is defined as the fineness of detail in an object that can be revealed by an optical device. It is specified as the minimum distance between two points that can be perceived as separate by the human eye.

The resolution of a microscope is influenced by the wavelength of light and the numerical aperture (NA) of the lens. The formula for resolution is given by

Resolution, or Resolving Power (Rp) = 0.61 x λ / NA

Where:

0.61 = scaling factor

Λ = wavelength of light

NA = Numerical aperture (The lens’ ability to gather light and resolve fine detail)

Numerical Aperture (NA)

Increasing the NA improves resolution but also poses challenges in correcting aberrations and achieving long working distances simultaneously.

Lens Design and Coatings

Customizing lenses to reduce aberrations and using specialized coating techniques can enhance resolution and clarity by minimizing light reflection and scattering.

Achromatic objectives correct chromatic aberration, improving image quality by ensuring that different wavelengths of light are focused at the same point.

Types of Lenses

Immersion objectives enhance resolution and numerical aperture by using a fluid medium between the lens and the specimen.

APO (or apochromatic) objective lenses correct optical imperfections, ensuring precise color reproduction and delivering sharper, more detailed images.

Magnification

Achieving high magnification must be balanced with resolution. The magnification of infinite conjugate objectives is determined by the focal lengths of the objective and the tube lens.

In an infinite conjugate design, the diverging light from a spot is made parallel. In a finite optical design, the light from a spot is focused into another spot with the aid of a couple of optical elements.

These factors collectively determine the quality of the images produced by biological microscopes, affecting both resolution and clarity.

What is the role of numerical aperture and its impact on resolution?

The numerical aperture is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. It is defined by the equation:

Numerical Aperture (NA) = n×sin(θ)

Where:

n = refers to the refractive index of the medium between the objective front lens and the specimen and

θ = the half-angle of the maximum cone of light that can enter or exit the lens.

The resolution of a microscope is directly related to the numerical aperture. A higher NA allows for a smaller resolution value, meaning finer details can be distinguished.

Increasing the NA improves the resolution, allowing for more detailed imaging. However, this also presents challenges such as correcting optical aberrations and maintaining a long working distance.

Lenses with higher NA are often designed with specialized coatings and materials to minimize aberrations and enhance image clarity. Immersion objectives, which use a fluid medium between the lens and the specimen, can further increase the NA and improve resolution.

High NA lenses are desired for applications requiring detailed imaging, such as biological microscopy, where distinguishing fine cellular structures is essential.

What is the role of optical coatings in enhancing image clarity?

Optical coatings play a significant role in enhancing image clarity in biological microscopy.

Minimizing Light Reflection and Scattering

Optical coatings are applied to lenses to reduce light reflection and scattering. Reflections can cause glare and reduce the contrast of the image, while scattering can lead to a loss of detail. By minimizing these effects, coatings help produce clearer and more detailed images.

Enhancing Optical Quality

Techniques such as ion-assisted deposition are used to apply coatings, which contribute to superior optical quality. These coatings help in achieving better light transmission through the lenses, creating clear and bright images.

Correcting Aberrations

Coatings can also aid in correcting optical aberrations, such as chromatic aberration, where different wavelengths of light are focused at different points. By addressing these imperfections, coatings ensure that images are sharper and colors are more accurately reproduced.

Improving Field of View

Specialized coatings can enhance the field of view by ensuring that the entire observable area is clear and free from distortions. This is particularly important in applications requiring a wide field of view without compromising image clarity.

What are the challenges of correcting aberrations with high NA lenses?

Correcting aberrations in high numerical aperture (NA) lenses presents several challenges due to the complex interplay between optical properties and design constraints.

Aberration Correction

High NA lenses are designed to gather more light and resolve finer details, but this also increases the likelihood of optical aberrations such as spherical and chromatic aberrations. Correcting these requires advanced lens design and manufacturing techniques.

Design Complexity

The design of high NA lenses is inherently more complex because they must balance multiple factors, including resolution, working distance, and aberration correction. This complexity can lead to increased costs and manufacturing difficulties.

Material and Coating Requirements

High NA lenses often require specialized materials and coatings to minimize aberrations and enhance image clarity. Techniques like ion-assisted deposition are used to apply coatings that reduce light reflection and scattering, but these processes can be technically demanding and costly.

Working Distance

Achieving a high NA often reduces the working distance of the lens, which can limit its practical applications. This trade-off requires careful consideration in the design phase to ensure that the lens can be used effectively in its intended application.

Field of View and Distortion

High NA lenses must also maintain a clear and undistorted field of view. This requires precise engineering to ensure that the entire observable area is free from distortions like field curvature and barrel or pincushion distortions 

How do immersion objectives enhance numerical aperture?

Immersion objectives enhance numerical aperture (NA) by using a fluid medium between the lens and the specimen. Here's how this process works and its impact on resolution:

Fluid Medium Immersion objectives utilize a fluid, such as oil or water, to fill the space between the objective lens and the specimen. This fluid has a higher refractive index than air, which effectively increases the numerical aperture of the lens system.

Increased Numerical Aperture

By using a fluid with a higher refractive index than air, the NA is increased, allowing the lens to gather more light and resolve finer details.

Enhanced Resolution

A higher NA results in improved resolution, as explained earlier by the resolution equation. With a higher NA, the resolution value decreases, meaning that the microscope can distinguish finer details in the specimen 

Applications

Immersion objectives are particularly useful in applications requiring high-resolution imaging, such as detailed cellular studies in biological microscopy.

What are the types of optical coatings used in microscopy?

Optical coatings are used to enhance image clarity in microscopy by improving light transmission and reducing unwanted reflections and aberrations. Here are some types of optical coatings used in microscopy:

Anti-Reflective Coatings

These coatings are applied to lens surfaces to minimize light reflection and maximize transmission. By reducing reflections, they enhance image contrast and clarity, which improves detail in imaging.

Ion-Assisted Deposition Coatings

This technique is used to apply coatings that contribute to superior optical quality. It helps in achieving better light transmission through the lenses, creating clear and bright images.

Chromatic Aberration Correction Coatings

Coatings that correct chromatic aberration ensure that different wavelengths of light are focused at the same point. This correction is vital for accurate color reproduction and sharper images, especially in applications like fluorescence microscopy. 

Biological Microscopes Media Gallery

References

GlobalSpec—Microscopes Information

GlobalSpec—Optical Assemblies

GlobalSpec—Metallurgical Microscopes Information