Page 70 - Fundamentals of Light Microscopy and Electronic Imaging

DESIGNS AND SPECIFICATIONS OF OBJECTIVE LENSES 53

DESIGNS AND SPECIFICATIONS OF OBJECTIVE LENSES

Achromats are red-blue corrected (meaning for wavelengths at 656 and 486 nm). Spher-
ical correction is for midspectrum yellow-green light at 540 nm. These objectives give
satisfactory performance in white light and excellent performance in monochromatic
light, and are quite suitable for low magnification work at 30–40 and lower. They are
also much less expensive than more highly corrected lens designs (Fig. 4-9).
Fluorite or semiapochromat lenses contain elements made of fluorite or fluorspar
(CaF ) or synthetic lanthanum fluorite—materials giving low color dispersion (Fig. 4-9).
2
Corrections for color dispersion and curvature of field are easily applied. The combina-
tion of good color correction, extremely high transparency (including to near UV light)
and high contrast makes them favorites for immunofluorescence microscopy, polariza-
tion and differential interference contrast (DIC) microscopy, and other forms of light
microscopy. The maximum obtainable numerical aperture (NA) is about 1.3.
Apochromats are expensive, highly color-corrected designs suitable for color pho-
tography using white light (Fig. 4-9). These lenses are red-, green-, blue-, and dark
blue–corrected for color, and are corrected for spherical aberration at green and blue
wavelengths. This design tends to suffer some curvature of field, but is corrected in
plan-apochromatic lenses. The high degree of color correction makes them desirable
for fluorescence microscopy, since various fluorescence wavelengths emitted from a
multiple-stained specimen are accurately focused in the same image plane. It is also
possible to obtain very large NAs (up to 1.4) with this lens design, making them desir-
able for low light applications such as dim fluorescent specimens. Newer designs are
now transparent to near UV light, making them suitable for fluorescence microscopy
involving UV-excitable dyes.
A summary of the characteristics of some commonly used objective lenses is pro-
vided in Table 4-1.

Planapochromat Achromat and
fluorite
Figure 4-9
Objective lens designs. Two popular lenses for fluorescence microscopy are shown.
Apochromatic lenses may contain 12 or more lens elements to give bright, flat images with
excellent color correction across the visual spectrum. Fluorite lenses have fewer lens
components and produce sharp, bright images. These lenses exhibit excellent color
correction and transmit UV light.