
r
0
l
2NA
(17:1)
For example, with a central wavelength of l ¼ 550 nm and an NA of 1.4, the
lateral resolving power is about 0:2 mm. This diffraction limit, however, can be
surpassed by using structured illumination [1, 2]. In theory at least, a SIM
microscope could approach unlimited lateral resolution [3].
When three-dimensional objects are examined, we must also consider reso-
lution along the optical axis. If the optical axis is along the z direction, the
in-focus region (the depth of field, DOF) is given approximately by
Dz
field
nl
NA
2
(17:2)
where n is the index of refraction of the medium in which the object is immersed
(Chapter 2). As with lateral