Introduction to Microscopes and Objective Lenses

July 3, 2023

A microscope is an optical device designed to magnify the image of an object, enabling details indiscernible to the human eye to be differentiated. A microscope may project the image onto the human eye or onto a camera or video device. 

Historically microscopes were simple devices composed of two elements. Like a magnifying glass today, they produced a larger image of an object placed within the field of view. Today, microscopes are usually complex assemblies that include an array of lenses, filters, polarizers, and beamsplitters. Illumination is arranged to provide enough light for a clear image, and sensors are used to ‘see’ the object.

Although today’s microscopes are usually far more powerful than the microscopes used historically, they are used for much the same purpose: viewing objects that would otherwise be indiscernible to the human eye.  Here we’ll start with a basic compound microscope and go on to explore the components and function of larger more complex microscopes. We’ll also take an in-depth look at one of the key parts of a microscope, the objective lens.

Compound Microscope: A Closer Look

While a magnifying glass consists of just one lens element and can magnify any element placed within its focal length, a compound lens, by definition, contains multiple lens elements. A relay lens system is used to convey the image of the object to the eye or, in some cases, to camera and video sensors.

 A basic compound microscope could consist of just two elements acting in relay, the objective and the eyepiece. The objective relays a real image to the eyepiece, while magnifying that image anywhere from 4-100x.  The eyepiece magnifies the real image received typically by another 10x, and conveys a virtual image to the sensor. 

There are two major specifications for a microscope: the magnification power and the resolution. The magnification tells us how much larger the image is made to appear. The resolution tells us how far away two points must be to  be distinguishable. The smaller the resolution, the larger the resolving power of the microscope. The highest resolution you can get with a light microscope is 0.2 microns (0.2 microns), but this depends on the quality of both the objective and eyepiece.

Both the objective lens and the eyepiece also contribute to the overall magnification of the system. If an objective lens magnifies the object by 10x and the eyepiece by 2x, the microscope will magnify the object by 20. If the microscope lens magnifies the object by 10x and the eyepiece by 10x, the microscope will magnify the object by 100x. This multiplicative relationship is the key to the power of microscopes, and the prime reason they perform so much better than simply magnifying glasses. 

In modern microscopes, neither the eyepiece nor the microscope objective is a simple lens. Instead, a combination of carefully chosen optical components work together to create a high quality magnified image. A basic compound microscope can magnify up to about 1000x. If you need higher magnification, you may wish to use an electron microscope, which can magnify up to a million times. 

Microscope Eyepieces

The eyepiece or ocular lens is the part of the microscope closest to your eye when you bend over to look at a specimen. An eyepiece usually consists of two lenses: a field lens and an eye lens. If a larger field of view is required, a more complex eyepiece  that increases the field of view can be used instead. 

Microscope Objective

Microscope objective lenses are typically the most complex part of a microscope.  Most microscopes will have three or four objectives lenses, mounted on a turntable for ease of use. A scanning objective lens will provide 4x magnification,  a low power magnification lens will provide magnification of 10x, and a high power objective offers 40x magnification. For high magnification, you will need to use oil immersion objectives. These can provide up to 50x, 60x, or 100x magnification and increase the resolving power of the microscope, but they cannot be used on live specimens.

An microscope objective  may be either reflective or refractive. It may also be either finite conjugate or infinite conjugate.  

Refractive Objectives

Refractive objectives are so-called because the elements bend or refract light as it passes through the system. They are well suited to machine vision applications, as they can provide high resolution imaging of very small objects or ultra fine details. Each element within a refractive element is typically coated with an anti-reflective coating.

A basic achromatic objective is a refractive objective that consists of just an achromatic lens and a meniscus lens, mounted within appropriate housing. The design is meant to limit the effects of chromatic and spherical aberration  as they bring two wavelengths of light to focus in the same plane. Plan Apochromat objectives can be much more complex with up to fifteen elements. They can be quite expensive, as would be expected from their complexity.

Reflective Objectives

A reflective objective works by reflecting light rather than bending it. Primary and secondary mirror systems both magnify and relay the image of the object being studied. While reflective objectives are not as widely used as refractive objectives, they offer many benefits. They can work deeper in the UV or IR spectral regions, and they are not plagued with the same aberrations as refractive objectives. As a result, they tend to offer better resolving power. 

Microscope Illumination 

Most microscopes rely on background illumination such as daylight or a lightbulb rather than a dedicated light source. In brightfield illumination (also known as Koehler illumination), two convex lenses, a collector lens and a condenser lens,  are placed so as to saturate the specimen with external light admitted into the microscope from behind. This provides a bright, even, steady light throughout the system. 

Key Microscope Objective Lens Terminology

There are some important specifications and terminology you’ll want to be aware of when designing a microscope or ordering microscope objectives. Here is a list of key terminology.

Numerical Aperture

Numerical aperture NA denotes the light acceptance angle. Where θ is the maximum 1/2 acceptance ray angle of the objective and n is the index of refraction of the immersive medium, the NA can be denoted by

NA = n x sin θ

Both the resolving power and light gathering ability of the objective are determined by the numerical aperture. 

Field of View

The field of view (FOV) of a microscope is simply the area of the object that can be imaged at any given time. For an infinity-corrected objective, this will be determined by the objective magnification and focal length of the tube lens. Where a camera is used the FOV  also depends on sensor size.

Working Distance

The working distance of a microscope is defined as the free distance between the objective lens and the object being studied. Low magnification objective lenses have a long working distance. 

Immersion Medium

While most microscope objectives are designed to work with air between the objective and cover glass, objectives lenses designed for higher NA and greater magnification sometimes use an alternate immersion medium. For instance, a typical oil immersion object is meant to be used with an oil with refractive index of 1.51.

Parfocal Length

The parfocal length of a microscope is defined as the distance between the object being studied and the objective mounting plane.

Optical Aberration Corrections

The optical performance of an objective is dependent largely on the optical aberration correction, and these corrections are also central to image quality and measurement accuracy. Objective lenses are classified as achromat, plan achromat, plan semi apochromat, plan apochromat, and super apochromat depending on the degree of correction.

Microscope Lenses from Avantier Inc

At Avantier we produce high quality microscope objectives lenses, ocular lenses, and other imaging systems. We are also able to provide custom designed optical lenses as needed. Chromatic focus shift, working distance, image quality, lens mount, field of view, and antireflective coatings are just a few of the parameters we can work with to create an ideal objective for your application. Contact us today to learn more about how we can help you meet your goals.

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