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:
Primary image formation
Determine the quality of
the image produced
The total magnification
The overall resolution
Classification based on Microscopy Method
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.
Classification based on Aberration Correction
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.
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.
Refractive and Reflective Objectives Lenses
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
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
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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