Page 71 - Fundamentals of Light Microscopy and Electronic Imaging

54 LENSES AND GEOMETRICAL OPTICS

TABLE 4-1. Characteristics of Selected Objective Lenses a
M Type Medium WD NA d min DOF B
(n) (mm) ( m) ( m)
5 Achromat 1 9.9 0.12 2.80 38.19 0.1
10 Achromat 1 4.4 0.25 1.34 8.80 0.4
20 Achromat 1 0.53 0.45 0.75 2.72 1.0
25 Fluorite 1.515 0.21 0.8 0.42 1.30 6.6
40 Fluorite 1 0.5 0.75 0.45 0.98 2.0
40 Fluorite 1.515 0.2 1.3 0.26 0.49 17.9
60 Apochromat 1 0.15 0.95 0.35 0.61 2.3
60 Apochromat 1.515 0.09 1.4 0.24 0.43 10.7
100 Apochromat 1.515 0.09 1.4 0.24 0.43 3.8
a The magnification (M), type of lens design, refractive index (n) of the intervening medium (air or
immersion oil), working distance (WD), numerical aperture (NA), minimum resolvable distance (d), depth
of field (DOF), and brightness (B) are indicated. Terms are calculated as: wave-optical depth of field,
2
2
n /NA ; brightness in epi-illumination mode, 10 NA /M . Resolution and depth of field are discussed in
4
4
Chapter 6.

Special Lens Designs

Other performance characteristics such as working distance, immersion design, and UV
transparency are optimized in special lens designs:

• Long working distance lenses allow focusing through thick substrates (microscope
slides, culture dishes) or permit the introduction of devices such as micropipettes
between the specimen and the lens. The working distance is the distance between
the surface of the front lens element of the lens and the surface of the coverslip. In
contrast, conventional oil immersion lenses have short working distances that may
be as small as 60 m.
• Multi-immersion and water immersion lenses are used for examination of speci-
mens covered with medium in a well chamber, and in some cases, can be placed
directly on the surface of a specimen such as a slice of tissue. These lenses usually
contain a focusable lens element to correct for spherical aberration.
• UV lenses made of quartz and other UV-transparent materials support imaging in
the near-UV and visible range (240–700 nm).

Markings on the Barrel of a Lens

The engraved markings on the barrel of the objective describe the lens type, magnifica-
tion, numerical aperture, required coverslip thickness if applicable, and type of immer-
sion medium (Fig. 4-10).