In microscopy, the objective lenses are the optical elements closest to the specimen. The objective lens gathers light from the specimen, which is focused to produce the real image that is seen on the ocular lens. Objective lenses are the most complex part of the microscope due to their multi-element design. It is this complexity that makes the objectives the most important components of the device.
Objectives lenses greatly vary in design and quality. As such, they can be roughly classified based on:
In general, objective lenses are responsible for:
The differences in microscopy methods can
largely be attributed to the different types of objective lenses used.
Objective lenses classified according to microscopy methods include:
Reflected dark field objectives - Have a special construction that consists of a 360 degree hollow chamber that surrounds the centrally located lens element
Differential interference contrast (DIC objectives) - Uses stain-free optical elements and relies on the action of Nomarski prisms (or Wollaston prism) which influence optical path differences between sheared light beams at the rear focal plane.
Fluorescence objectives - designed with quartz and special glass with high transmission from ultraviolet to the infrared regions.
Phase contrast objectives -These types of objectives are divided in to several categories depending on construction and neutral density of internal phase ring. These include; dark low objectives (DL) Dark low low objectives (DLL) Apodized dark low objectives (ADL) Dark medium objectives (DM) Bright medium objectives (BM).
Essentially, objective lenses can be categorized in to three main categories based on their magnification power. These include: low magnification objectives (5x and 10x) intermediate magnification objectives (20x and 50x) and high magnification objectives (100x).
Apart from the differences in their magnifications, objective lenses are also different on how they are used. For instance, with a high magnification lens (100x) immersion oil is often used to obtain high resolving power. This is not the case with lower magnification objectives.
Essentially, with regards to chromatic aberration correction, there are two main levels of correction. These include the achromatic ad apochromatic. Achromatic objectives are the simplest, least expensive and most common objectives used. These objectives are designed to correct for chromatic aberration in both the red and blue wavelengths. They are also corrected for spherical aberration in the green wavelength.
The main weakness with this type of objective is that there is limited correction when it comes to chromatic aberration as well as the lack of a flat field of view. These issues reduce objective performance of these objective lenses. These lenses are particularly well suited for monochromatic applications. With apochromatic objectives, there is higher precision. These objectives are chromatically corrected for red, blue and yellow.
With apochromatic objectives, there is also spherical aberration correction for two and three wavelengths in addition to a higher numerical aperture and long working distance. Because of their better design, apochromatic objectives are ideal for white light applications.
The refractive objectives are the most common objectives. With refractive objectives, light is bent (refracted) by the optical elements, which are designed in a manner that reduces back reflections thereby improving the overall light passing through. These type of objectives are often used in applications that require resolution of highly fine details. For refractive objectives, designs may range from two elements in the basic achromatic objectives to fifteen elements in plan-apochromatic objectives.
As for reflective objectives, the typically use reflective/mirror based design. While this objectives may not be as common as refractive objectives, they can overcome a number of problems found in the design of refractive objectives.
For instance, the design of reflective objectives incorporates a primary and secondary mirror system that helps in magnify and relay the image. With this system, the reflective objectives avoid the similar aberration experienced in refractive objectives given that light is reflected any metallic surfaces. With reflected objectives therefore, no additional designs are necessary to overcome aberrations. On the other hand, reflective objectives also have an advantage in that the produce higher light efficiency and better resolving power, which is excellent for fine detail imaging.
Here, the system is largely dependent on mirror coating rather than the glass substrate. Lastly, reflective objectives have an advantage over refractive objectives in that they allow for working deeper into either the ultra-violet or infrared spectral regions given that they use mirrors.
Specifications of any objectives are listed on the body of the objective. It is important to understand what the labeling means if one is to select the right objectives for their intended purpose.
Objective standard - Such objective standards as DIN or JIS will be listed on the body of the objective depending on the type of standard. This shows the required specification present in the system. For instance, the DIN, which is the most common standard, has 160mm distance from objective frange to the frange of the eyepiece while JIS has 170mm distance.
Magnification - On the objective, this is usually denoted by an X next to a numeric value (100X, 10X etc). On the other hand, objectives will also have a colored band around the circumference of the objective that indicates the magnification of the objective. For instance, a yellow band around the objectives (lower part of the objective) indicates that it is a 10x objective.
Numerical aperture (NA) - numerical aperture refers to the function of focal length and entrance pupil diameter. This is usually labeled next to the magnification of the objective (1, 1.30 etc) A large numerical aperture (more than 1) means that that immersion oil may have to be used given that the highest NA that can be achieved without immersion oils (in air) is NA of 1. This labeling is therefore important in that it directs the user on how to use the objective for better quality images.
Cover slip thickness - denoted by a number (such as 0.17mm) the cover slip thickness is labeled on the objective to note the type of cover slip that should be used. A cover slip changes the way light is refracted from the specimen. Therefore, it is important to ensure that the right cover slip is used in order to produce good quality image.
Quality correction - Quality correction such as achromatic, apochromatic, plan and semi-plan are often denoted on the objective in order to show the design of the objective. Plan and semi-plan objectives (also referred to as microplan, planar or semi-planar) correct for field curvature. Field curvature often results in blurred images and correction for this helps produce good quality images. Whereas plan objectives correct better, allowing for better display (over 90 percent) of field flat, semi-plan objectives produce about 80 percent.
Today, there are different types of microscopes intended for different applications. The techniques will largely depend on the type of objectives used, given that different types of objectives provide different results. For this reason, it is important to have a good understanding of the different types of objectives, their strengths and weaknesses as well as the type of specimen they are ideal for.
For instance, where as reflective objectives have better features that make
them superior to refractive objectives, users will also realize that they are
both well suited for different applications. Therefore, having a good
understanding of the different types of objectives is important if the user is
to have a good viewing experience.
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