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Section 9: Understanding Microscope Objectives
More information on infi nity corrected objectives can be found at www.edmundoptics.eu/objectives
Figure 9.1 Figure 9.2
Sensor or Eyepiece
Image Plane
Microscope Tube Lens
Objective
Achromatic
Triplet
Achromatic
Doublet
Group
Meniscus
Hemispherical
136 +44 (0) 1904 788600 | Edmund Optics® targets Microscopy allows users to view samples that cannot be resolved by
the human eye. Microscopy can be segmented into three fi elds: optical,
electron, and physical scanning probe microscopy. Optical microscopy,
the focus of this article, relies heavily on properties known
as diff raction and refraction. Optical microscopy is further segmented
into a number of techniques: brightfi eld, darkfi eld, phase contrast,
diff erential interference contrast (DIC), fl uorescence, and confocal
based systems. Each technique has a number of similarities and differences;
the type of objective used in the system addresses many of
these diff erences.
Microscope objectives are grouped under two conjugate types: fi nite
and infi nite (infi nity corrected).
In a fi nite conjugate optical design, light from a source (not at infi nity)
is focused down to a spot. For a microscope, the image of the
inspected object is magnifi ed and projected onto the eyepiece (or
sensor if using a camera). The particular distance through the system
is characterized by either the DIN or JIS standard; all fi nite conjugate
microscopes are either one of these two standards. These types
of objectives account for the majority of basic microscopes, such as
general inspection or assembly systems. Finite conjugate designs are
used in applications where cost and ease of design are major concerns.
Additionally, these objectives are typically used for brightfi eld
techniques only.
In an infi nite conjugate, or infi nity-corrected optical design,
light from a source placed at infi nity is focused down to a small spot
(Figure 9,1). In an objective, the spot is the object under inspection
and infi nity points toward the eyepiece (or sensor if using a camera).
This type of modern design uses an additional tube lens between the
object and eyepiece to produce an image.
Though this design is more complicated than the fi nite conjugate,
it allows for the introduction of optical components such as fi lters,
polarizers, a nd beamsplitters into the optical path. Fluorescence microscopy
applications typically use this type of design. Infi nity corrected
objectives often incorporate multi-element designs and correct
for a number of optical errors such as fl atness, chromatic aberration,
spherical aberration, and polarization.
The qualities of the objective and eyepiece determine how well the
system performs. Understanding quality correction is extremely important
when deciding on the type of objective to use. Quality corrections
are often denoted on the objective itself to allow the user to
easily see the design of the objective in question. There are typically
three levels of chromatic and spherical aberration corrections: achromatic,
fl uorite, and apochromatic (apochromatic and achromatic
are seen in Figure 9.2) . Achromat objectives are among the simplest
and least expensive objectives. Color correction occurs at the red and
blue wavelengths, while spherical aberrations are accounted for at a
green wavelength. The limited corrections make these objectives better
suited for general and monochromatic applications. Fluorite (or
fl uor) objectives are constructed using advanced glass types containing
the mineral fl uorite (or fl uorspar), hence the name, or now often
with synthetic materials instead. The diff erent glass types used in
fl uorite objectives allow for improved chromatic and spherical aberration
corrections for at least 2 colors. They also often feature higher
numerical apertures and resolving powers than achromat objectives,
delivering brighter images and better contrast in color imaging applications.
Apochromat objectives provide the highest level of corrections.
Their chromatic aberration corrections are for at least 3 colors
(red, blue and green) and the spherical aberration corrections are for
at least 2 to 3 colors, making them ideal for white light applications.
They tend to have longer WDs and the highest numerical apertures.
Note that many of the newer high-performance fl uorite and apochromat
objectives often have even more corrections, correcting for 4 or
more colors chromatically and 4 colors spherically. All three objective
designs, however, suff er signifi cantly from distortion and fi eld curvature,
which worsen as objective magnifi cation increases.
Field curvature is a type of aberration present when the off -axis image
cannot be brought to focus in a fl at image plane, resulting in a
blurred image as it deviates from the optical axis. There are two levels
of quality designation for fi eld curvature correction: plan (or planar)
and semi-plan (or semi-planar and microplan). Objectives with
no fi eld curvature correction have a fl at fi eld in the center 65% of the
image. Semi-plan objectives feature 80% of the fi eld appearing fl at.
Plan objectives achieve the best overall correction and display better
than 90% of the fi eld fl at and in focus.
Section 9.1: Types of Objectives
Object
Infinite Conjugate Space Where
Additional Optical Components
Such as Filters and Beamsplitters
Can be Added
Achromatic
Doublet
Achromat
Meniscus
PCX
Figure 9.1: A basic infi nity corrected
objective confi guration.
Figure 9.2: Apochromatic (top)
vs. achromatic (bottom)
objective design.
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