Section 10: Using Infi nity Corrected Objectives
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Section 10.1: Types of Objectives
Microscope objectives are typically grouped under two categories:
nite conjugate and in nite conjugate (in nity corrected).
In a nite conjugate optical design, light from a source (not at
in nity) is focused down to a spot. In the case of a microscope, the
image of the object under inspection is magni 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 nite conjugate microscopes are either one of these two standards.
These types of objectives account for the majority of basic microscopes,
such as systems used for general inspection or assembly purposes.
Finite conjugate designs are used in applications where cost
and ease of design are major concerns. Additionally, these objectives
are typically used for bright eld techniques only.
In an in nite conjugate, or in nity-corrected, optical design, light
from a source placed at in nity is focused down to a small spot. In an objective,
the spot is the object under inspection and in nity points toward
the eyepiece (or sensor if using a camera). This type of modern design
utilizes an additional tube lens between the object and eyepiece in order
to produce an image. Though this design is much more complicated than
its nite conjugate counterpart, it allows for the introduction of optical
components such as lters, polarizers, and beamsplitters into the optical
path (Figure 2). As a result, additional image analysis can be performed in
complex systems. For example, adding a lter between the objective and
the tube lens allows one to view speci c wavelengths of light or to block
unwanted wavelengths that would otherwise interfere with the setup.
Fluorescence microscopy applications utilize this type of design.
Microscopy, in its simplest form, allows users to view samples and
items that cannot be resolved by the human eye. The technical eld of
microscopy can be segmented into three separate elds: optical, electron,
and physical scanning probe microscopy. Optical microscopy, the
focus of this article, relies heavily on properties known as di raction
and refraction. Optical microscopy can be further segmented into a
number of techniques: bright eld, dark eld, phase contrast, di erential
interference contrast (DIC), uorescence, and confocal based systems.
Each technique has a number of similarities and di erences; the type
of objective used in the system addresses many of these di erences.
Another bene t of using an in nite conjugate design is the ability to
vary magni cation according to speci c application needs. Since the
objective magni cation is the ratio of the tube lens focal length to the
objective focal length, increasing or decreasing the tube lens focal length
changes the objective magni cation. Typically, the tube lens is an achromatic
lens with a focal length of 200 mm, but other focal lengths can
be substituted as well, thereby customizing a microscope system’s total
magni cation. In nity corrected objectives often incorporate multi element
designs and correct for a number of optical errors such as atness,
chromatic aberration, spherical aberration, and polarization. These objectives
can be used for almost all microscopy techniques.
In nity corrected objectives are often designed to address spherical
and/or chromatic aberration. Achromatic, apochromatic, planar, and
semi-planar objective designs each address a speci c optical need. Achromatic
objectives are among the simplest and least expensive objectives
that account for chromatic aberration. The correction occurs at the red
and blue wavelengths, and also account for spherical aberrations at green
wavelengths. Limited correction for chromatic aberration and lack of
atness in the eld make these suited for simple applications and entrylevel
users. Apochromatic objectives provide much higher precision and
are chromatically corrected for the entire visible spectrum. In addition,
they provide spherical aberration correction for two to three wavelengths
and tend to have higher numerical apertures, longer working distances,
and typically address eld atness/curvature issues by incorporating
semi-planar or planar designs. Figure 1 illustrates the internal structure
di erences between apochromatic and achromatic objectives.
Sensor or Eyepiece
Microscope Tube Lens
Infinite Conjugate Space Where
Additional Optical Components
Such as Filters and Beamsplitters
Can be Added
Figure 1: Setup for an In nity Corrected Objective System.
Figure 2: Basic In nity Corrected Objective Con guration.