Page 73 - Fundamentals of Light Microscopy and Electronic Imaging

56 LENSES AND GEOMETRICAL OPTICS

free of common aberrations, it is popular in fluorescence microscopy. The 40 /1.3 NA
fluorite lens is significantly brighter, but is less well corrected.

CONDENSERS

Imaging performance by a microscope depends not only on the objective lens but also
on the light delivery system, which includes the illuminator and its collector lens, and of
particular importance, the condenser. High-performance condensers are corrected for
chromatic and spherical aberrations and curvature of the focal plane (field curvature).
However, most of these aberrations are still apparent when using the Abbe condenser, a
very common condenser that is based on a two-lens design (Fig. 4-11). The three-lens
aplanatic condenser (indicating correction for spherical aberration and field curvature)
is superior, but still exhibits chromatic aberration. The highly corrected achromatic-
aplanatic condenser has five lenses including two achromatic doublet lenses, provides
NAs up to 1.4, and is essential for imaging fine details using immersion-type objectives.
These condensers are corrected for chromatic aberration at red and blue wavelengths,
spherical aberration at 550 nm, and field curvature. Such a condenser can be used dry
for numerical apertures up to 0.9, but requires immersion medium for higher NA val-
ues, although the condenser is commonly used dry even with oil immersion objectives.
Note, however, that for maximal resolution, the NA of the condenser must be equal to
the NA of the objective, which requires that both the condenser and the objective should
be oiled.

Abbe Aplanatic-achromatic

Figure 4-11
Two common microscope condensers. The Abbe condenser contains two achromatic doublet
lenses and gives very good performance for dry lenses of low to medium power. The
achromatic-aplanatic condenser is useful for lenses with NA 0.5, and is essential for oil
immersion lenses with high numerical apertures. For low NA performance, the top element
of this condenser can be removed. This condenser focuses light in a flat focal plane and is
highly corrected for the significant lens aberrations.

OCULARS

Oculars or eyepieces are needed to magnify and view the image produced by the objec-
tive. To make optimal use of the resolution afforded by the objective, an overall magni-
fication equal to 500–1000 times the NA of the objective lens is required. More
magnification than this gives “empty” magnification, and the image appears highly