Biophotonics_Concepts_to_Applications

length have a greater NA (i.e., a greater light-gathering ability) and yield afiner
resolution of the sample.

Example 8.1Consider a series of objective lenses in which the half-angle of

the captured light cone varies from 7° to 60°. What is the range of numerical

apertures if the immersion medium is air?

Solution: From Eq. (8.1) with n = 1.00, as the angleαincreases from 7° to

60° (as the light cones grow larger) the numerical aperture increases from

0.12 to 0.87.

Example 8.2Consider a series of objective lenses in which the half-angle of

the captured light cone varies from 7° to 60°. What is the range of numerical

apertures if the immersion medium is oil with n = 1.51?

Solution: From Eq. (8.1) with n = 1.51, as the angleαincreases from 7° to

60° (as the light cones grow larger) the numerical aperture increases from

0.18 to 1.31.

Note: Objective lenses with a magnification range between 60x and 100x

typically are designed for use with immersion oil. In practice, most oil

immersion objectives have a maximum numerical aperture of 1.4, with the

most common numerical apertures ranging from 1.00 to 1.35.

8.1.4 Field of View

When looking into a microscope, it is useful to know the diameter of the viewed
field in millimeters measured at the intermediate image plane. Knowing this

D

D

D

Specimens

α = 8°

α = 20°

α = 60°

For NA = sin α

(a) NA = 0.14 for α = 8°

(b) NA = 0.34 for α = 20°

Objective lens (c) NA = 0.87 for α = 60°

Light cones

Focal

length f

(a)(b)

(c)

Fig. 8.5 The NA increases for short objective focal lenses

8.1 Concepts and Principles of Microscopy 241